1. 套磁信 I am an undergraduate student. I want to write an email to a Professor in MIT working on large language models to sell my experience and ask him if he is willing to recruit me as a PhD student in next year.

2. 催审稿 I am a graduate student. I want to write an email to the reviewer of Nature journal to ask him to accelerate the review process of my submitted paper.

Dr. Davide Esposito
Chief Editor
Nature Catalysis
No.: NATCATAL-24020271
Manuscript Title: Low-coordinate Cu1–N2 sites in single-atom catalyst for highly efficient propylene epoxidation with molecular oxygen
Dear Dr. Davide Esposito,
We would like to express our deep appreciation for the helpful suggestions and have carefully revised the manuscript following the comments. We have included additional discussions in response to the reviewers’ comments. This letter is our detailed point-to-point response to the reviewers. Additionally, an uploaded copy of the manuscript displays the revisions. We believe that the concerns raised by the reviewer have been fully addressed in our revised manuscript.



Reviewers' comments:

Reply to Reviewer #1:

Figure R1 (Supplementary Figure 35) | AIMD simulation results for Cu–N2 and Cu–N4 active sites at 550 K. (a) Energy fluctuations of the Cu–N2 active site in C3N4–L during a 30 ps AIMD simulation, along with its representative structure. (b) Energy fluctuations of the Cu–N4 active site in C3N4 during a 30 ps AIMD simulation, along with its representative structure. The atoms Cu, N, C, and H are represented by the orange, blue, grey, and white balls, respectively.


Supplementary Figure 11 | Characterizations of various catalysts. In situ DRIFTS of CO adsorption of Cu1–N2, Cu1–N3 and Cu1–N4 SACs.


Supplementary Figure 13 | Schematic illustration of the plausible formation mechanism of low-coordinated Cu1–N2 centres.

Supplementary Figure 20 | Catalytic performance of propylene epoxidation with O2. Product formation rate at different reaction temperatures over Cu1–N2 SAC. Reaction conditions: C3H6: O2: N2 = 10: 5: 85 vol.%, GHSV = 36,000 h-1, 50.0 mg catalyst.

General Comment: The authors demonstrated that they prepared Low-coordinate CuN2 active site in C3N4-L sheets, and the antithetical CuN3 and CuN4 actives sites in traditional C3N4 sheets through a pyrolysis based synthesis approach from similar classic beginning chemicals. More important, the CuN2 active site in C3N4-L possesses eminent catalytic performance for direct epoxidation of propylene into propylene oxide (PO) with molecular oxygen. Two important highlights in this study is the attained remarkable catalytic performance in the DEP reaction, including an impressive 78.0% selectivity for propylene oxide (PO) on the as-prepared sample, a high propylene conversion rate of 5.7% and the observed PO formation rate reaches 5.9 mmol gcat-1 h-1.
Generally, the result in this manuscript is very fascinating and its experiment data is very compelling. In contrast, the theoretical derivation at the active sites and the catalytic mechanisms need further to be clarified. However, there are some uncertainty in the deduction from the experiment results and the origin of structure activity. The responding data and the proposed active structure need to be clarified and confirmed before its final publication.
Response: We sincerely thank Reviewer #1 for their thoughtful and positive comments on the significance and quality of our work. We are pleased to hear that the result in this manuscript is very fascinating and its experiment data is very compelling. Futhermore, the responding data have been confirmed. To facilitate the review process, we have placed the revised figures at the end of this document in the order they appear in the main text. 

Comment#1. The work temperature for its eminent catalytic performance for direct epoxidation of propylene is 275 oC, hence the future work to check the AIMD stability at 550 K is a must. I hope the embedded positions for the CuN2 active site in C3N4-L and CuN4 active site in C3N4 sheets should be kept without obviously migration.
Response: Thank you for pointing out the importance of evaluating the stability of the active sites at elevated temperatures. We acknowledge the importance of evaluating the AIMD (Ab Initio Molecular Dynamics) stability of the Cu–N2 active site in C3N4–L and the Cu–N4 active site in C3N4 sheets at elevated temperatures, such as 550 K, to ensure their stability and performance under realistic conditions. We agree that this analysis is crucial for a comprehensive understanding of the catalytic performance.
To address this, we performed AIMD simulations using the NVT ensemble and controlled the temperature with a Nosé-Hoover thermostat at 550 K, employing a time step of 1 fs for a total simulation duration of 30 ps. The energy fluctuations during the simulation are presented in Figure R1. The structural stability of the active sites is illustrated by showing a representative structure after 20 ps of simulation.
Our results demonstrate that both CuN2 and CuN4 active sites remained stable during the simulation, with no significant migrations of the Cu atoms or active site distortions observed. This indicates that the Cu–N2 active site in C3N4–L and the Cu–N4 active site in C3N4 sheets exhibit strong structural integrity at elevated temperatures, which thereby supports their potential for practical catalytic applications under these conditions.
We appreciate your insightful comments and concur that this analysis makes a significant contribution to a comprehensive understanding of the thermal stability and robustness of these active sites.

Comment#2. The cutoff energy of 400 eV and the Hellman-Feynman forces at 0.05 eV/A is not enough in the VASP simulation to study the catalytic mechanism, I suggest the cutoff value is set at 500 eV and 0.02.
Response: Thank you for your valuable feedback regarding the computational parameters in our VASP simulations. As per your suggestion, we have raised the plane-wave cutoff energy from 400 eV to 500 eV, resulting in a more precise representation of the electronic wavefunction. In addition, we have tightened the Hellman–Feynman force convergence criteria for optimized structures from 0.05 eV/Å to 0.02 eV/Å, thus achieving a more precise optimization of the atomic geometries.
Although these adjustments led to minor changes in some of the calculated energy values, our main conclusions remain unchanged. Notably, for the Cu1–N2 catalyst, the rate-limiting step changed from TS1 to TS2 under the revised computational parameters. Meanwhile, the activation barriers for the second PO formation on Cu1–N2 and Cu1–N4 single-atom catalysts were still comparable, being 0.83 eV and 0.79 eV, respectively. Despite this, the energy barriers for the initial PO formation showed significant differences (0.56 eV vs. 1.86 eV). Furthermore, both of these barriers are lower than those for the α C–H bond abstraction on these catalysts.
We appreciate your insightful comments that have helped us improve the accuracy and reliability of our study. The manuscript has been updated accordingly with the revised results and discussions. If you have any additional questions or suggestions, please let us know.

Comment#3. The catalytic product of CO2 in these materials are very high, sometimes the percent is over 50%. The catalytic active sites haven't been mentioned in the manuscript. Meanwhile, the competition between CO2 and PO also is missing. Hence, the authors should give rational explanation for CO2 production.
Response: Thank you for the insightful comments from the reviewer. Our detailed and rational response to dispel the reviewers' doubts is provided in the following content.
As reported in most of the published literature and discussed in our manuscript, copper-based catalysts have been widely utilized in DEP reactions and exhibit exceptional catalytic performance. It is generally recognized that the Cu+ species possesses a high efficiency in DEP reactions with high PO selectivity because the oxygen species activated on Cu+ sites have relatively strong electrophilic properties, which is conducive to attacking the C=C double bond of propylene. However, the oxygen species activated on Cu2+ sites exhibit nucleophilicity, tending to attack the α C–H bond of propylene, leading to the formation of a deeply oxidized product (Refer to J. Mol. Catal. Chem. 228, 27–33 (2005); J. Catal. 268, 165–174 (2009)). In our study, we have characterized the oxidation states of Cu in Cu1–N2 SAC using XAS, XPS Cu 2p and Cu LMM characterizations in detail (Figs. 2c, d and Supplementary Figure 12). The results indicated that the oxidation state of Cu species was nearly +1 (XANES) and the Cu+:Cu2+ ratio in Cu1–N2 SAC was nearly 9:1. According to the above results, Cu1–N2 SAC possessed an intrinsic activity which was suitable for the production of PO with high selectivity.
Regarding the catalytic performance of DEP in our manuscript, we only presented the propylene conversion, the selectivity of various products and PO formation rate to evaluate the catalytic performance of the Cu-based catalysts. This led to misunderstandings among the reviewers. We sincerely apologize for our negligence and have conducted additional experimental analysis to better depict the catalytic performance. We have added the Supplementary Fig. 20 depicting the formation rates of various products including PO, CO2, acrolein, acetaldehyde and aceton at different reaction temperatures. It can be observed that from 200 oC to 275 oC, the formation rate of PO increased rapidly, while the formation rate of CO2 increased slowly., The selectivity of PO was primarily impeded by thermodynamic energy barriers at this stage, and the rate of complete oxidation was influenced by adsorption/desorption processes (Refer to J. Colloid Interface Sci. 611, 564–577 (2022)). Furthermore, the complete oxidation was more favorable according to the relevant data of thermodynamics (Refer to Appl. Surf. Sci. 355, 660–666 (2015); ACS Catal. 10, 13415–13436 (2020)). When the reaction temperatures were above 275°C, several factors contributed to the significant increase in CO2 formation. First, the excessively high reaction temperatures led to aggregation and oxidation of Cu species to Cu2O or CuO, which have been proved by the XRD and XPS Cu 2p characterizations (Supplementary Figs. 23b and 25a). The lattice oxygen species excited at high temperatures exhibit nucleophilicity, accelerating the extraction of α C–H bond of propylene, converting more C3H6 into by-product and further to CO2. Besides, the PO generated will be susceptible to isomerization at high temperatures for CO2 formation (Refer to J. Am. Chem. Soc. 111, 4553–4561 (1989); Surf. Sci. Rep. 76, 100524 (2021)). The carbon nitride, which serves as the support, can also promote the conversion of propylene into acrolein and CO2 at high temperatures (Supplementary Fig. 17). The generation rate of CO2 increased significantly due to these factors working together.
In summary, we believe that the Cu+–N2 species served as the real active-site for the production of PO, while Cu2+ favored the generation of CO2. Within a moderate temperature range, an increase in the reaction temperature was conducive to accelerating the efficient production of PO, compared to other products. However, the agglomeration/oxidation of copper species, the isomerization of PO, and the over-oxidation of propylene will be greatly exacerbated by the excessively high reaction temperatures.

Comment#4. There is no data on C3N4-L in supplementary Fig. 6. The author missed these data.
Response: Thank you for carefully reviewing our manuscript. We express our sincere apologies for our oversight. The correct content for Supplementary Fig. 6 should be “Physicochemical properties of carriers. (a) N2 adsorption/desorption isotherms and (b) pore size analysis of Cu1–N2, Cu1–N3 and Cu1–N4 SACs”. Besides, the relevant data on C3N4–L can be found in Supplementary Fig. 14h and i.

Comment#5. The simulation box with the integral structures must be presented in Computational models for (d) Cu1-N2 (e) Cu1-N3 and (f) Cu1-N4 SACs in supplementary Figure 10.
Response: Thank you for your detailed review and for highlighting the need to present the simulation box with the integral structures in our Computational Models section. Your attention to this crucial aspect of our study is greatly appreciated.
To address your concern, we have included the simulation boxes for the (d) Cu1–N2, (e) Cu1–N3, and (f) Cu1–N4 single-atom catalysts (SACs) in Supplementary Figure 10. This addition will ensure that the structural configurations and computational setup are clearly presented and can be accurately reproduced.
We appreciate your efforts in helping us refine our manuscript and anticipate any additional feedback you may have.

Comment#6. The peak for in situ DRIFTS of CO adsorption of Cu1-N2, Cu1-N3 and Cu1-N4 SACs in Supplementary Figure 11 seems not correct. Please double check these peaks and tagged with its vibrational modes from what bonding.
Response: We thank you for carefully reviewing our manuscript. Based on our rigorous academic approach, we have carefully checked and further retested this controversial measurement of all the samples to study the scientific results. Except the peaks around 2115 cm-1 and 2175 cm-1 assigned to gaseous CO in condition, an extra signal at 2054 cm-1 (Cu1–N2 SAC), 2057 cm-1 (Cu1–N3 SAC) and 2060 cm-1 (Cu1–N4 SAC) representing the CO linear adsorption (COL) on Cu atoms was found for all Cu based SACs. Besides, the absence of bridge bonded CO band in the range of 1700-1900 cm-1, ruling out the presence of multi-atom Cu species (Refer to Angew. Chem. Int. Ed. 62 (2023), e202217220, Angew. Chem. Int. Ed. 126, 4956–4961 (2014), Nat. Commun. 12, 6022 (2021), Appl. Catal. B Environ. 339, 123119 (2023), Chem. Eng. J. 146870 (2023), J. Catal. 405, 333–345 (2022), Appl. Catal. A Gen. 505, 334–343 (2015), J. Phys. Chem. C 113, 10689–10695 (2009) and J. Catal. 268, 367–375 (2009)). The in situ DRIFTS of CO adsorption on Cu1–N2, Cu1–N3 and Cu1–N4 SACs at 30 °C in our previous experiments showed only two peaks due to the CO adsorption time was too long and the CO concentration was too high, resulting in the overlap of the two peaks (around 2055-2060 cm-1 and 2115 cm-1). The revised manuscript has corrected the details of CO DRIFTS finally. Thanks again for the reviewer's comments and the relevant content has been corrected in the revised manuscript.

Comment#7. There some irrational expression or some errors are listed here:
1."with the Cu1-N2 SAC showing a significant number micropores compared to Cu1-N3 and Cu1-N4 "... in the end of Page 6.
2. "The presence of high- angle-shifted (002) peaks at 2θ of 27.5° was characteristic of carbon nitride supports." in the middle of Page 7. Here, the peaks at 2θ of 27.5° should be the obvious characteristic of pi-pi packing with ~ interlayer distance around 3.35 Å, since the angle at 2θ of 27.5° exists in C3N4 and graphite.
3. "revealed the co-existence of triazine and pyridine structures within the porous C3N4-L" in the middle of Page 11. In my understanding, the authors want to mention pyrimidine-containing fragments not pyridine fragments, since the novel aroma fragment is C4N2 nor C5N1.
4. For the paragraph "about the development of synthetic methodology about fabricated a SAC featuring low-coordinated Cu1-N2 anchored onto a C3N4–L framework via the ACAS technique." in page 11.1 suggest the author introduce a scheme to make it much clear in Supporting information.
Response: We greatly thank you for carefully reviewing our manuscript. We have carefully checked the entire manuscript and corrected the irrational expressions or errors in the revised manuscript.
1.According to the reviewer's suggestion, this part has been amended to "and the Cu1–N2 SAC demonstrated a significantly greater number of micropores compared to Cu1–N3 and Cu1–N4 SACs".
2.According to the reviewer's suggestion, this part has been amended to "The presence of high-angle-shifted (002) peaks at 2θ of 27.5° was characteristic of pi-pi packing with the interlayer distance around 3.35 Å, since the angle at 2θ of 27.5° exists in C3N4 and graphite".
3.According to the reviewer's suggestion, this part has been amended to "revealed the co-existence of triazine and pyrimidine-containing fragments within the porous C3N4–L".
4.According to the reviewer's suggestion, we have refined the scheme in Supplementary Fig. 13 to make the formation mechanism of Cu1–N2 SAC through the ACAS strategy clearer and more visualized. The schematic illustration included the amidation reaction, the self-condensation reaction, the metal-heteroatom coordination, and the dehydration reaction, etc. The following are the details.
Supplementary Fig. 13 specifically illustrates the synthetic steps for Cu1–N2 SAC utilizing the ACAS strategy. First of all, the hydrolysis of CuCl2 and the modifier L-cysteine creates an acidic medium that facilitates the hydrolysis of melamine. This step leads to the formation of Compound I through the reaction between L-cysteine and melamine. The abundant terminal –NH2 of melamine would coordinate with free Cu2+ species to form Compound II, as evidenced by the XRD characterization (Supplementary Fig. 14a). Secondly, during the solvothermal reaction process, melamine underwent further hydrolysis to generate cyanuric acid units. The in-situ formed cyanuric acid and melamine spontaneously assembled into a supramolecular structure through hydrogen bonding (Refer to Chem. Eng. J. 405, 126685 (2021); Nano Res. 11, 3462–3468 (2018)). Due to the in-situ modification of nitrogen-containing units, the low-coordinate Cu–N structures can form (Compound III) during the assembly process, which further stabilizes the anchoring of Cu–N species. After that, the cyanuric acid in Compound III released the ammonia gas and left melamine to self-condense into heptazine units (Compound IV) (Refer to ACS Appl. Mater. Interfaces 11, 10651–10662 (2019); Small 20, 2309032 (2024); Langmuir 30, 447–451 (2014)). Subsequently, the modified moiety within Compound III underwent cyclization to form Compound V by releasing NH3 and H2S molecules, followed by dehydration to generate Compound VI during pyrolysis. These steps were iteratively and periodically repeated, ultimately forming the low-coordinate Cu1–N2 SAC. Throughout these above processes, the assembly of the supramolecular structure and the formation of asymmetric melon configurations created favorable conditions for the formation of high-loading and low-coordinate copper single-atom sites.




Reply to Reviewer #2:


Fig. 3 | Catalytic performance of propylene epoxidation with O2. Product selectivity and C3H6 conversion at different reaction temperatures over (a) Cu1–N2, (b) Cu1–N3 and (c) Cu1–N4 SACs. Reaction conditions: C3H6: O2: N2 = 10: 5: 85 vol.%, GHSV = 36,000 h-1, 50.0 mg catalyst. (d) Performance comparisons of various IB group catalysts in propylene epoxidation including PO formation rate, PO selectivity and propylene conversion. The digital characters from 1 to 16 represent as Cu7Ce, LaCu0.5Mn0.5O3, Cl-RD-Cu2O, Ag-MoO3/ZrO2, Au/TS-1-KOH, Cu/SiO2, RD-Cu2O, VCe0.5Cu0.5-NaCl, (KAc)Cu/SiO2, K+-CuOx/SBA-15, Ag/CaCO3-NaCl, Ag/Y2O3-K2O/α-Al2O3, CuNP/C3N4-L, Cu1–N4 SAC, Cu1–N3 SAC and Cu1–N2 SAC, respectively. (e) Stability test over Cu1–N2, Cu1–N3 and Cu1–N4 SACs. Reaction conditions: C3H6: O2: N2 = 10: 5: 85 vol.%, T = 275 oC, GHSV = 36,000 h-1, 50.0 mg catalyst.


Supplementary Figure 5 | Characterizations of various catalysts. FT-IR spectra of C3N4, C3N4–L, Cu1–N2, Cu1–N3 and Cu1–N4 SACs.


Supplementary Figure 10 | Characterizations of various catalysts. EPR pattern of Cu1–N2, Cu1–N3 and Cu1–N4 SACs. 
 

Supplementary Figure 11 | Characterizations of various catalysts. In situ DRIFTS of CO adsorption of Cu1–N2, Cu1–N3 and Cu1–N4 SACs.


Supplementary Figure 20 | Catalytic performance of propylene epoxidation with O2. Product formation rate at different reaction temperatures over Cu1–N2 SAC. Reaction conditions: C3H6: O2: N2 = 10: 5: 85 vol.%, GHSV = 36,000 h-1, 50.0 mg catalyst.


Figure R1 | Catalytic performance. PO selectivity and C3H6 conversion over Cu1–N2 SAC. Gas type: C3H6 (0-75 min) mix gas (75-150 min, C3H6: O2: N2 = 10: 5: 85 vol.%). Reaction conditions: GHSV = 36,000 h-1, 50.0 mg catalyst, T = 275 oC.

General Comment: This article reports on epoxidation of propylene with molecular oxygen over Cu single atom catalysts comprised of Cu atoms dispersed in C3N4 prepared from pyrolysis of various precursors. This is an extraordinarily thorough article as pertains to characterization of the materials, and the computational aspects. The actual catalytic testing felt relatively limited compared to all of the characterization section, it reads at times like a materials science article rather than a catalysis article.
The article provides significant evidence that Cu atoms coordinated to only two nitrogen atoms are more reactive than Cu bound to 3 or 4 atoms, and Cu in nanoparticles. These Cu-N2 sites are active and selective for epoxidation, giving 78% PO selectivity at initial time on stream and around 6% propylene conversion. This selectivity decreases significantly in under 4 h, yet the article shows that the structure of the materials is relatively unchanged after reaction. So, it is unclear why selectivity changes, and the claims of high selectivity are relatively hindered by the limited stability of the selective material.
This is an interesting article, it's extraordinarily thorough in many aspects, and it could be appropriate for publication in Nature Catalysis after only relatively minor revisions.
Response: We thank you for the highly positive comment of “This is an extraordinarily thorough article as pertains to characterization of the materials, and the computational aspects” and “This is an interesting article, it's extraordinarily thorough in many aspects, and it could be appropriate for publication in Nature Catalysis after only relatively minor revisions”. It is common knowledge that the catalysts are at the core of most of catalytic reactions, determining the orientation of reaction pathways, product selectivity, kinetic processes, and more. In the past few years, a broad range of nanocatalytic materials have been achieved, which has improved the corresponding catalytic performance. Despite these advances, further development of nanocatalytic materials has been hindered by inefficient active site design, unclear active site properties, and the structure-performance relationship. Thus, the design, characterization, and optimization of the true active sites serve as the foundation for the further advancement of catalytic science. Among these aspects, regulating the fine coordination structure in atomically dispersed catalysts is of key importance. This is why we spared no effort to analyze the fine structure of the as-prepared catalysts, providing a solid foundation and specific approach for in-depth exploration of the catalytic reaction mechanism. Based on these, we have carefully revised our manuscript based on your comments.

Comment#1. Could the authors add some context for the claim of 5.9 mmol gcat-1 h-1 being an excellent PO formation rate? How does this compare to other rates reported in literature for this reaction? At what time on stream was this rate measured, and how stable is it with time on stream?
Response: We are thankful for the reviewer’s comments and admire the reviewer's rigorous academic attitude. 
The following is background supplementation for the DEP reaction. The DEP route has been considered as an ideal method for PO production because of the abundant availability and cost-effectiveness of molecular oxygen. However, during the DEP reaction process, both the α C–H in propylene and the PO produced have a high level of reactivity. Additionally, the DEP reaction is exothermic and the actual heat release for the DEP reaction with 50.0 mg Cu1–N2 SAC achieved about 270 mW tested via the C80 micro calorimeter technique. Additionally, other DEP catalysts exhibited more significant exothermic effects. The self-exothermic characteristics of the DEP reaction is unfavorable for the chemical equilibrium movement especially at high reaction temperatures, leading to an inevitable conflict between the reaction kinetics and thermodynamics. As a result, increasing the formation rate of PO at high propylene conversion remains an insurmountable challenge. As is the case with the DEP performance of the reported catalysts, both the PO formation rate and the propylene conversion remain at an extremely low level, severely impeding the industrialization process of this crucial catalytic reaction. 
Through extensive experimental exploration and theoretical studies, we ultimately identified the low-coordinated Cu–N single-atom center as the optimal active site for the DEP reaction. Furthermore, thanks to the implementation of our proposed ACAS strategy, a uniform loading of the Cu1–N2 sites on the surface of the carbon nitride has been achieved. Consequently, among all reported IB group metal catalysts, only Cu1–N2 SAC can achieve the ultra-high PO formation rate (5.9 mmol gcat-1 h-1) with high propylene conversion and high PO selectivity at relatively mild conditions at 275 oC. Considering the industrial value and separation costs, and choosing the reported DEP catalysts with ≥60% PO selectivity as a reference, the PO formation rate of 5.9 mmol gcat-1 h-1 over Cu1–N2 SAC has been increased by 1,073%~14,565%. The corresponding results have been presented in detail in Fig. 3d and Supplementary Table 6. This is a significant breakthrough in terms of the DEP catalytic reaction, which has great scientific and industrial value. The relevant content has been added in the revised manuscript.
As suggested, the stability results of the PO formation rate over Cu1–N2 SAC at 275 oC were provided in the revised manuscript. As shown in Fig. 3e, the PO formation rate over Cu1–N2 SAC exhibited steady performance. Over a 300-minute period test, the formation rate of PO for Cu1–N2 SAC remained at approximately 80% of its initial value. By extending the reaction time, only a slight decrease in the PO formation rate can be observed due to a slight increase of by-product acrolein selectivity. 

Comment#2. In the description of the FTIR spectra of the materials themselves, the shift in NH2 stretching vibration from 3165 to 3430 cm-1 seems remarkably large. What evidence do the authors have that this is from the same vibration, and have such large shifts been observed previously?
Response: We are thankful for the reviewer’s comments and admire the reviewer's rigorous academic attitude. 
The remarkable shift of –NH2 stretching vibration, moving from 3165 to 3430 cm–1, can be attributed to various synthesis methods especially the atmosphere during the pyrolysis process of carbon nitride carriers and Cu-based SACs. As shown in the Methods section in Supplementary information, the C3N4 and C3N4–L were preprared via a pyrolysis step in the muffle furnace, resulting in the introduce of water molecules from the air during the polymerization and subsequent cooling processes. This led to the appearance of a strong and wide absorption peak caused by intermolecular hydrogen bonds around 3000-3500 cm–1 in the FT-IR spectrum, specifically attributed to O–H bonds. Moreover, the C3N4 and C3N4–L exhibit a high degree of polymerization and a limited proportion of terminal –NH2 in the absence of Cu coordination, which leads to a weak signal of –NH2 stretching vibration in the FT-IR spectrum, causing the shift of IR peaks and shield by O–H bonds. By contrast, the Cu-based SACs were synthesized under a protective gas atmosphere, without the interference from complex components in the air. Besides, this above phenomenon can be also observed in other similar reported C3N4 materials (Refer to Bioresour. Technol. 372, 128677 (2023), Chem. Sci. 13, 754–762 (2022), and Sep. Purif. Technol. 308, 122955 (2023)).
With a rigorous scientific approach, a new batch of the carbon nitride carrier (termed as C3N4–L (5% H2/Ar)) have been synthesized by the same procedure to that of C3N4–L, except that the atmosphere was changed to 5% H2/Ar and re-tested the surface chemical property using FT-IR measurements. As shown in Supplementary Fig. 6, the presence and characteristic of the –NH2 stretching vibration on C3N4–L (5% H2/Ar) can be observed at nearly 3450 cm–1 in the FT-IR spectrum. After the single-atom Cu coordination, the vibrational spectral analysis also revealed a slight blue shift in the peaks corresponding to the stretching vibrations of the –NH2 groups, shifting from 3450 to 3430 cm–1. What's interesting is that the above phenomenon matches a blue shift in the bending vibrations of the triazine units shifting from 800 to 778 cm–1. These above conclusions correct the previously inadequate descriptions and provide credible evidence for the shift of the –NH2 group after the coordination anchoring of single-atom Cu species. 
We sincerely appreciate your comment again, and we have revised the manuscript to provide a clearer and more accurate expression.

Comment#3. In the discussion of the Raman data, how do the authors know that a material with many N vacancies necessarily has a Cu1-N2 configuration? Couldn't this just mean there are more sites with no Cu at all, and then the only sites with Cu have Cu-N4 coordination (a combination of Cu free sites and Cu-N4 sites seems similar in terms of number of N vacancies to the idea of Cu-N2 sites?)
Response: We thank you for carefully reviewing our manuscript. In order to explain the reviewer's confusion more scientifically, we have now added more experimental details in the modified manuscript, which makes our work more readable. 
The Raman pattern of Cu1–N2, Cu1–N3 and Cu1–N4 SACs (Supplementary Figure 9a) showed that the decrease of ID/IG ratio from Cu1–N4 SAC (ID/IG = 0.919) to Cu1–N3 SAC (ID/IG = 0.947) and to Cu1–N2 SAC (ID/IG = 0.980). Besides, a combination of C3N4 with Cu-free-site and Cu1–N4 SAC show a similar ID/IG ratio with Cu1–N4 SAC, indicating that the properties of Cu–N structure are the key determinants of its defect concentration. The relationship between the concentration of N vacancies and unpaired electrons was further confirmed by electron paramagnetic resonance (EPR) spectra (Supplementary Figure 9b). The Cu1–N4 SAC showed an extremely slight paramagnetic signal with a g value at about 2.004 to the unpaired electrons, suggesting that Cu1–N4 SAC exhibited a high degree of Cu–N coordination saturation with a handful of N vacancy. By comparison, the corresponding EPR peak intensity at g = 2.004 increases in Cu1–N3 SAC. Furthermore, the trend of EPR peak intensity variation became more evident in Cu1–N2 SAC, consistent with the Raman analysis in terms of the increase of N vacancies and the formation of low-coordinated Cu1–N2 single-atom sites. These results indicate that Cu1–N2 SAC has the highest defect concentration and the most unpaired electrons, thereby possessing the highest surface energy, which is conducive to the formation of unsaturated coordinated metal sites (Refer to Nat. Commun. 12, 6806 (2021)). 
In order to clearly reveal the relationship between the N vacancy and the Cu–N single-atom configuration, we re-tested the XAFS data and supplemented the quantitative fitting of the EXAFS at k-space. The relative results revealed that the copper species in Cu1–N2, Cu1–N3 and Cu1–N4 SACs all showed atomically dispersed, with Cu–N coordination numbers of 2.2, 2.8 and 3.7, respectively. Additionally, the ICP-OES, EDS and XPS results show that the SACs maintained the similar content of Cu and N species (Supplementary Figure 4 and Supplementary Table 1). These above results indicate a consistent coordination environment for the Cu single-atom sites within all as-prepared Cu based SACs. In conclusion, the difference in N vacancy between Cu1–N2, Cu1–N3 and Cu1–N4 SACs was directly derived from the difference in the coordination structure of Cu species. It can be seen that the ID/IG ratio of SACs is negatively correlated with the Cu-N coordination number.
We agree with the reviewer's suggestion that this is not direct evidence for demonstrating the coordination saturation of single atoms. To avoid improper guidance, we have made appropriate revisions in the revised manuscript, hoping to address the reviewers' concerns.


Comment#4. Were the CO peak centers the same for all samples? Doesn't it seem surprising that the CO vibration would not be sensitive to the proposed differences in Cu structure that apparently sufficient to change the electronics of adsorbed O2?
Response: Thank you for your important comment. Based on the rigorous academic attitude, we have carefully checked and further retested this controversial measurement of all the samples to study the scientific results. 
Except the peaks around 2115 cm-1 and 2175 cm-1 assigned to gaseous CO in condition, an extra signal at 2054 cm-1 (Cu1–N2 SAC), 2057 cm-1 (Cu1–N3 SAC) and 2060 cm-1 (Cu1–N4 SAC) representing the CO linear adsorption (COL) on Cu atoms was found for all Cu based SACs. It is worth noting that the COL peak has shifted to the lower frequency direction from Cu1–N4 SAC to Cu1–N3 SAC and to Cu1–N2 SAC. This is attributed to the difference of the oxidation state of Cu, which corroborates with the results of XANES and XPS. Moreover, the absence of bridge bonded CO band in the range of 1700-1900 cm-1, which rules out the presence of multi-atom Cu species (Refer to Angew. Chem. Int. Ed. 62 (2023), e202217220, Angew. Chem. Int. Ed. 126, 4956–4961 (2014), Nat. Commun. 12, 6022 (2021), Appl. Catal. B Environ. 339, 123119 (2023), Chem. Eng. J. 146870 (2023), J. Catal. 405, 333–345 (2022), Appl. Catal. A Gen. 505, 334–343 (2015), J. Phys. Chem. C 113, 10689–10695 (2009) and J. Catal. 268, 367–375 (2009)). The in situ DRIFTS of CO adsorption on Cu1–N2, Cu1–N3 and Cu1–N4 SACs at 30 °C in our previous experiments showed only two peaks due to the CO adsorption time was too long and the CO concentration was too high, resulting in the overlap of the two peaks (around 2055-2060 cm-1 and 2115 cm-1). In addition, the different intensity of CO linear adsorption peak also indicates that Cu1–N2 SAC contains a higher level of Cu+ species than Cu1–N3 and Cu1–N4 SACs. This indicates that the CO adsorption is sensitive to the Cu–N fine coordination structure, which is sufficient to activate the adsorbed oxygen and change their electronic properties. 
In order to further explore the above results, DFT calculations have also been carried out to analyze the Bard charge on the Cu atoms and the oxygen species. The Bard charge on the Cu atom in Cu1–N2 SAC is + 0.74 e, which is much lower than that of Cu1–N3 and Cu1–N4 SACs (+ 0.81 e and + 0.84 e, respectively). This indicates that the low-coordinate Cu1–N2 site is more likely to donate electrons. According to the Bard charge of activated intermediate oxygen species on copper species, the Bard charge of the intermediate oxygen species O2- shows obviously electrophilic compared with the Cu1–N3 sites, Cu1–N4 sites and traditional DEP catalysts (such as Cu nanoparticles or Cu2O), which tended to attack the electrically rich C=C double bond in propylene to generate PO. These results reflect the key role of isolated Cu1–N2 sites in changing the electronic structure of adsorbed O2 into electrophilic oxygen species. 
In the revised manuscript, the details have been corrected finally. Thanks again for the reviewer's comments.

Comment#5. What is the explanation for selectivity increasing with temperature? Typically, this increases rates of C-H scission/H abstraction in selective oxidation reactions, leading to decreased selectivity with increasing temperature. How do the authors explain the decreasing selectivity to PO with time on stream, despite all the characterization data that shows the materials are structurally stable during reaction?
Response: Thank you for the insightful comments from the reviewer. 
As reported in most of the published literature and discussed in our manuscript, copper-based catalysts have been widely utilized in DEP reactions and exhibit exceptional catalytic performance. It is generally recognized that the Cu+ species possesses a high efficiency in DEP reaction with high PO selectivity because the oxygen species activated on Cu+ sites have relatively strong electrophilicity, which is conducive to attacking the C=C double bond of propylene. However, the oxygen species activated on Cu2+ species exhibit nucleophilicity, tending to attack the α C–H bond of propylene, leading to the C-H scission/H abstraction and the formation of an over-oxidation product (Refer to J. Mol. Catal. Chem. 228, 27–33 (2005); J. Catal. 268, 165–174 (2009)). In our study, we have characterized the Cu oxidation states in Cu1–N2 SAC using XAS, XPS Cu 2p and Cu LMM characterizations in detail (Figs. 2c, d and Supplementary Figure 12). The results indicated that the oxidation state of Cu species was nearly +1 (XANES) and the Cu+:Cu2+ ratio in Cu1–N2 SAC was nearly 9:1. The above results indicated that Cu1–N2 SAC possessed an intrinsic activity that was suitable for the production of PO with high selectivity.
Regarding the catalytic performance of DEP in our manuscript, we only presented the propylene conversion, the selectivity of various products and PO formation rate to evaluate the catalytic performance of the Cu-based catalysts. This led to misunderstandings among the reviewer. We sincerely apologize for our negligence and have carried out additional experimental analysis to better display the catalytic performance. We have added the Supplementary Fig. 20 depicting the formation rates of various products including PO, CO2, acrolein, acetaldehyde and aceton at different reaction temperatures. It can be observed in Supplementary Fig. 20 that the formation rate of PO increased rapidly from 200 oC to 275 oC, while the formation rate of CO2 increased slowly. At this stage, the selectivity of PO was primarily hindered by thermodynamic energy barriers, while the rate of complete oxidation was influenced by adsorption/desorption processes (Refer to J. Colloid Interface Sci. 611, 564–577 (2022)). Furthermore, the complete oxidation is more favorable according to the relevant data of thermodynamics (Refer to Appl. Surf. Sci. 355, 660–666 (2015); ACS Catal. 10, 13415–13436 (2020)). Thus, the elevation in temperature from 200 oC to 275 oC exhibits tiny influence on adsorption/desorption processes, but instead promotes the activation of O2/C3H6 by Cu+–N single-atom sites. The experimental data indicate that the ratio of the formation rates of PO to CO2 increases approximately from 1.0 (200 oC) to 4.5 (275 oC), leading to an enhancement in PO selectivity. 
When the reaction temperatures was above 275°C, several factors contributed to the significant increase in CO2 formation. First, the excessively high reaction temperatures lead to aggregation and oxidation of Cu species to Cu2O or CuO, which have been proved by the XRD and XPS Cu 2p characterizations (Supplementary Figs. 23b and 25a). The lattice oxygen species excited at high temperatures exhibit nucleophilicity, accelerating the extraction of α C–H bond of propylene, converting more C3H6 into by-product and further to CO2. Besides, the generated PO will be susceptible to isomerization at high temperatures for CO2 formation (Refer to J. Am. Chem. Soc. 111, 4553–4561 (1989); Surf. Sci. Rep. 76, 100524 (2021)). The carbon nitride, serving as the support, can also promote the conversion of propylene into acrolein and CO2 at high temperatures (Supplementary Fig. 17). These factors collectively result in a substantial increase in the generation rate of CO2.
In summary, we believe that the Cu+–N2 species serves as the real active-site for the production of PO, while Cu2+ favors the generation of CO2. Within a moderate temperature range, an increase in the reaction temperature is conducive to accelerating the efficient production of PO, compared to other products. However, the excessively high reaction temperatures will significantly exacerbate the agglomeration/oxidation of copper species, the isomerization of PO, and the over-oxidation of propylene.
In the "Migration of Cu in the Cu1–N2 SAC" section, the EXAFS results demonstrate the durability of the Cu1–N2 coordination structure. Additionally, Supplementary Figs. 23, 24 and 25 also confirm the high dispersion and valence stability of the copper species in Cu1–N2 SAC. Concerning the decrease in PO selectivity over time on stream, the nature of the carbon nitride support caught our attention. As known, the carbon nitride material shows a two-dimensional layered characteristic with interlayer van der Waals forces. Prolonged exposure at high-temperatures and an oxygen-enriched environment leads to interlayer peeling and is easily oxidized. This was accompanied by a substantial reduction in BET surface area and the emergence of numerous pores, which reduces the structural order and stability of the support (Supplementary Table 4 and Supplementary Fig. 24a, Refer to Adv. Funct. Mater. 22, 4763–4770 (2012) and J. Colloid Interface Sci. 553, 530–539 (2019)). Defected and oxidized carbon nitride structures can increase the activation of molecular oxygen into nucleophilic oxygen species, leading to the formation of by-products and a decrease inPO selectivity (as demonstrated in Appl. Catal. B Environ. Energy 353, 124022 (2024)).
We sincerely appreciate your comment, and we have revised the manuscript to provide a clearer and more accurate expression. Thank you again for your helpful feedback.

Comment#6. The high selectivity initially that then decreases could result from formation of surface species that consume PO. Have the authors measured the selectivity as a function of propylene conversion, or co-fed PO to observe whether it is increasingly consumed with time on stream when co-fed during propylene epoxidation?
Response: 

Comment#7. In the "implication of reaction mechanism for DEP" section, if none of these peaks are observed when propylene is fed first, does that mean that if you first feed propylene, then feed propylene and oxygen, this catalyst does not make PO? Or is this observation just saying that if you feed propylene first, then O2 alone second, no PO is formed?
Response: We greatly thank you for carefully reviewing our manuscript. We conducted an analysis of the reaction mechanism by altering the feedstock introduction sequence. The results showed that the dispersed single atoms with low-coordinate Cu–N coordination structure avoided the co-adsorption of propylene and oxygen, and followed the sequential activation mechanism. This is obviously different from the traditional DEP reaction mechanism, which involves adsorbing oxygen first and activating propylene. Therefore, when propylene gas was first introduced into the test system, all the active sites were occupied by the propylene molecules. The non-oxidation of propylene was caused by the lack of active sites on the catalyst surface that could absorb the molecular oxygen that was introduced later. When the propylene gas was first fed, and then the propylene/oxygen mixed gas was fed, the results are as follows (Figure R1). In the initial stage, we could not observe the formation of any product through propylene oxidation, because the propylene molecule that was adsorbed on the surface had not been purged. However, a large number of products appeared shortly thereafter, indicating that the competitive adsorption of molecular oxygen in the mixed gas on low-coordinate Cu–N coordination sites was dominant. The activation of oxygen molecules resulted in the intermediate reactive oxygen species attacking the propylene molecule. 
These above results strongly support the conclusion that the excellent DEP performance via Cu1–N2 SAC primarily stems from the special reaction mechanism, that is, adsorption and activation of oxygen before the bridged adsorption of propylene. Finally, the results above have been appropriately included in the revised manuscript. 

Comment#8. The computational part was so thorough, it was great. Not every paper takes the time to determine pathways on the control/non-optimal samples (like the Cu(111) in this study). Nice work!
Response: We thank you for the highly positive comment. We hope this study will provide in-depth and meaningful insights for readers of Nature Catalysis.

Reply to Reviewer #3:


Fig. 2 | Structural characterizations. (a) Cu K-edge EXAFS of Cu foil, Cu2O, CuO, Cu1–N2, Cu1–N3 and Cu1–N4 SACs. (b) Wavelet transform of EXAFS of Cu foil, CuO, Cu1–N2, Cu1–N3 and Cu1–N4 SACs. (c) Cu K-edge XANES of Cu foil, Cu2O, CuO, Cu1–N2, Cu1–N3 and Cu1–N4 SACs. (d) Cu 2p XPS spectra of Cu1–N2, Cu1–N3 and Cu1–N4 SACs.


Supplementary Fig. 8 | XPS spectra of various catalysts. N 1s XPS spectra of C3N4, Cu1–N2, Cu1–N3 and Cu1–N4 SACs.


Supplementary Figure 13 | Schematic illustration of the plausible formation mechanism of low-coordinated Cu1–N2 centres.


Supplementary Figure 25 | Characterizations of spent catalysts. (a) Cu 2p XPS spectra and (b) Cu LMM spectra of spent Cu1–N2, Cu1–N3 and Cu1–N4 SACs. (c) Cu 2p XPS spectra of CuNP/C3N4–L and spent CuNP/C3N4–L.

Supplementary Table 4 | FWMH of C3N4 and C3N4–L for C 1s XPS.
Sample	FWMH (ev)
	C−C/C=C	N−C=N	C−NHX
C3N4	1.30	1.21	1.30
C3N4-L	1.30	1.26	1.30

Supplementary Table 4 | FWMH of C3N4, Cu1–N2 SAC, Cu1–N3 SAC and Cu1–N4 SAC for N 1s XPS.
Sample	FWMH (ev)
	C−N=C	C−NHx	N−(C)3	Cu−N
C3N4	1.35	1.30	1.31	/
Cu1–N2 SAC	1.35	1.35	1.25	1.28
Cu1–N3 SAC	1.40	1.25	1.40	1.40
Cu1–N4 SAC	1.40	1.40	1.20	1.37

Supplementary Table 4 | FWMH of Cu1–N2 SAC, Cu1–N3 SAC, Cu1–N4 SAC, CuNP/C3N4–L and their spent samples for Cu 2p XPS.
Sample	FWMH (ev)
	Cu+	Cu2+
Cu1–N2 SAC	1.84	1.83
Cu1–N3 SAC	1.85	1.85
Cu1–N4 SAC	1.85	1.85
Spent Cu1–N2 SAC	1.80	1.85
Spent Cu1–N3 SAC	1.80	1.85
Spent Cu1–N4 SAC	1.80	1.85
CuNP/C3N4–L	1.85	1.85
Spent CuNP/C3N4–L	1.85	1.85


Supplementary Figure 14 | Characterizations and physicochemical properties of carriers. (a) XRD pattern and (b) FT-IR spectra of melamine, L-cysteine and precursor. (c) FT-IR spectra of precursor in different steps. (d) C 1s XPS spectra of C3N4 and C3N4–L. (e) 13C/1H CP-MAS NMR spectra of C3N4 and C3N4–L. SEM images of (f) C3N4 and (g) C3N4–L. (h) N2 adsorption/desorption isotherms and (i) pore size analysis of pristine C3N4 and C3N4–L.


General Comment: The manuscript (Ref. NATCATAL-24020271) describes the synthesis of a series of Cu-based catalysts deposited on g-C3N4 and their subsequent use in the epoxidation of propylene in gas phase using molecular oxygen as sole oxidant. The manuscript reads very well in general and presents some very interesting insights of the interplay between synthetic parameters and catalytic performance. However, there are some fundamental flaws that significantly hinder the potential impact of the manuscript that must be addressed by the authors to deliver a more solid and robust work. I can only recommend publication of the manuscript after major revisions are made to the original text. The authors are kindly asked to revise their manuscript following these aspects, making the appropriate changes. 
Response: We thank you for the highly positive comment of “The manuscript reads very well in general and presents some very interesting insights of the interplay between synthetic parameters and catalytic performance”. We also greatly appreciate your constructive and meaningful suggestions on our manuscript. These suggestions are very important to us and we have made every effort to refine our study and carefully revised our manuscript based on your comments.

Comment#1. Concerning English usage, despite the manuscript very clearly and concisely written is not entirely devoid of flaws. Some grammatical and typographical errors may still be found which should be revised for a better reading. The authors are kindly asked to proof-read their manuscript in order to come up with a more robust and readable text. The following are just a few illustrative examples to support this point:
- In the Introduction section "For above discussion, the low-coordinate" should read "From the above discussion, the low-coordinate"
- In the section "Formation process of Cu1-N2 SAC" several Figure numbers are incorrect, please revise all Figure referencing and mention them accordingly
- "Understanding the migration of active metal species during catalysis is crucial for investigating the underlying mechanismss" should read "Understanding the migration of active metal species during catalysis is crucial for investigating the underlying mechanisms"
- "This was followed by the interaction of the remaining O- species (M2) with second propylene to create an oxametallopropylene intermediate" should read "This was followed by the interaction of the remaining O- species (M2) with a second propylene molecule to create an oxametallopropylene intermediate" 
Response: We thank the reviewer for carefully reviewing our manuscript. We have taken your comments seriously and have carefully proofread the manuscript to address these issues. We have completed the revisions and have highlighted the changes in yellow in the manuscript for your reference. We sincerely apologize for any inconvenience or confusion caused by these errors and appreciate your understanding and patience.

Comment#2. The authors mention that the figure shown in SF6 are Type I isotherms. This is a somewhat optimistic assumption. The isotherms are barely Type I, and they appear to be type II. Did the authors perform BET analyses to check the accessible surface area? The plots on PSD are simply not informative. The obtained surface areas do not correspond to microporous solids. Thus, the discussion following this identification is fundamentally not true.
Response: We thank this useful suggestion from the reviewer. In the isothermal adsorption process, the BET equation can be expressed in terms of adsorption volume (V) and relative pressure (P/P0) can be expressed as follows:

Where P0 represents the saturated vapor pressure of the adsorbed substance at the adsorption temperature, Vm represents the saturated adsorption amount of the adsorbed substance at the single molecular layer, C is the constant of BET theory.
According to the BET equation, the value of C has a significant influence on the characteristic of the curve. The value of C in the BET equation plays a crucial role in determining how P/P0 influences the gas adsorption when Vm reaches the saturation point. When V = Vm, the BET equation can be simplified to: 

Based on the BET analyses to check the accessible surface area, the experimental details are shown as follows: The Brunauer-Emmett-Teller surface area and Barrett-Joyner-Halenda (BJH) porosity of samples were analyzed by the N2 adsorption/desorption isotherms by using Micromeritics (ASAP 2460) at -196 °C, then dried at 100 °C for 6 h under vacuum conditions. According to the above equations and the obtained results, the C values of the Cu1–N2, Cu1–N3 and Cu1–N4 SACs have been calculated at 5.978, 5.977 and 5.985, respectively, which is fundamentally scientific. In addition, C > 2 is the characteristic of an adsorption isotherm conforming to type II or type IV. As shown in Supplementary Fig. 6, the type H3 hysteresis loop appears in the middle of the adsorption isotherms, which corresponds to the capillary condensation of the porous adsorbent. This is mainly due to the nonrigid aggregates of two-dimensional carbon nitride sheets. Based on the IUPAC classification, it appears that the as-prepared SACs have type IV adsorption isotherms.
We would like to express our sincere gratitude to you again for your valuable comments.

Comment#3. While the discussion is apt, the XPS analysis does not appear entirely correct. The FWHM values should be similar for all deconvoluted peaks, yet this does not appear to be the case. The authors should re-analyze the XPS spectra.
Response: Thank you for the careful and rigorous review. Based on the principles of XPS and the physical significance of FWHM, the same element should exhibit similar FWHM values under different chemical environment. Thanks to the reviewer's recommendations, we have performed a re-analysis and peak-fitting of the XPS spectra to ensure rigorousness in our revised manuscript. We sincerely appreciate your comment again. In case there are any other aspects you deem necessary for us to further consider, we are willing for the further revision.

Comment#4. Compared to pristine C3N4, the sp3- hybridized nitrogen peaks in all SACs shifted to a higher binding energy, suggesting an increased tendency for pyridinic N species to coordinate with Cu atoms." This observation is hardly visible from SF8. The shiftings should be marked for a better identification.
Response: Thank you for your thorough review of our manuscript and for bringing this important error to our attention. We deeply regret the typo mistake in our manuscript, where we incorrectly marked "sp3-hybridized nitrogen" instead of "sp2-hybridized nitrogen." This error has led to an inaccurate conclusion in our interpretation of the data presented in Supplementary Fig. 8. The N 1s spectrum showed a new peak at 397.9 eV which can be assigned to the characteristics of Cu–N coordination, further corroborating the interaction between Cu and N atoms. Furthermore, the peaks at 399.0 eV can also be observed, corresponding to sp2-hybridized nitrogen in the triazine ring (C–N=C). We acknowledge that the binding energy of the sp2-hybridized nitrogen peaks has shifted towards higher binding energies, as indicated in the corrected version, suggesting that the pyridinic N species tend to coordinate with Cu atoms. For better identification, the shifts have been marked in Supplementary Fig. 8 in the revised manuscript.

Comment#5. What do the arrows indicate in SF14a?
Response: We thank you for carefully reviewing our manuscript. In the original Supplementary Figure 14a, the arrows indicate the XRD diffraction peaks of melamine, demonstrating that the mixture obtained after Step I retained the original structure of the melamine precursor. To gain deeper insights into the products of Step I, we conducted a meticulous analysis of the XRD spectra. In the updated illustration, melamine structures were marked in black and the XRD diffraction peaks can be visible at 2θ of 17.6°, 21.6°, 22.1°, 26.2°, and 28.8°, which can be characteristic of (-111), (210), (-211), (-301), and (-311) of melamine (JCPDS 75-1288). Additionally, the XRD diffraction peaks at 2θ of 9.9°, 19.8°, 21.0°, 27.5°, 29.3°, 30.0°, and 46.3° can be assigned to (010), (020), (121), (200), (0-22), (014), and (304) of Cu(C2H7N5)2Cl2·2H2O (JCPDS 24-1654) , which came from the coordination complex between Cu2+ and melamine and was highlighted in red. The blue signs denote the copper oxychloride crystals arising from the hydrolysis of CuCl2 and the XRD diffraction peaks at 2θ of 16.2°, 32.3°, and 39.7° can be attributed to (-101), (210), and (220) of Cu2(OH)2Cl (JCPDS 86-1391). As shown in Supplementary Figure 14b, all the above-mentioned XRD diffraction peaks are well indexed, proving the reliability of the conclusion. We sincerely appreciate your thoughtful comments, as they have allowed us to provide a more detailed explanation. If there are additional points you would like us to address, we would be glad to incorporate further insights into the revised manuscript.

Comment#6. The decreased absorbance at 3564 cm-1, ascribed to the desorption of H2O, verifies the above point." This observation does not appear very realistic. The intensity of the broad band around 3500 cm-1 is very similar for the three samples.
Response: We thank you for carefully reviewing our manuscript and apologize for the oversight of not presenting the relevant information clearly and adequately. To better illustrate the decreased absorbance at 3564 cm-1, the spectral region between 3000 cm⁻¹ and 4000 cm⁻¹ range was appropriately magnified to facilitate detailed observation. As can be clearly seen in Supplementary Figure 14e, the decreased absorbance specifically at 3564 cm⁻¹ can be attributed to the desorption of H₂O. The evident result confirmed our aforementioned point, that is a significant dehydration process occurred due to the reaction between melamine and L-cysteine during the Step I in Supplementary Figure 14a. We apologize for our oversight and would like to express our sincere gratitude to you once again for your thorough review. 

Comment#7. NMR spectroscopy does not reveal, to the best of my perception, a clear signal for Carbons 1, 2, or 3. H NMR does show the differences in the NH2 signal, but other differences are practically invisible.
Response: We thank the reviewer for the insightful commen. As shown in the 13C CP-MAS NMR spectroscopy (Supplementary Figure 14e), both C3N4 and C3N4–L exhibit resonant signals at 156.2 ppm and 163.9 ppm, respectively, which can be attributed to the C4 atom in the N=C–N2 structure and the C5 atom in the N=C–N(NH2) structure of the heptazine unit (Refer to Angew. Chem. Int. Ed. 49, 441–444 (2010); Appl. Catal. B Environ. 254, 128–134 (2019); Chem. Eng. J. 383, 123132 (2020)). XPS C1s spectra indicates that the ratio of C5 to C4 has increased from 2.24 (C3N4) to 2.54 (C3N4–L), which aligns with the conclusions derived from the through further analysis (Refer to Adv. Sci. 11, 2408293 (2024)). Therefore, the successful modification by ACAS strategy was validated. The addition of the L-cysteine modifier during the synthesis of C3N4–L was limited to only 0.5% of the melamine content to ensure the feasibility of modification and that the periodicity of the carrier structure remains intact. This resulted in weak resonant signals for C1, 2 and 3 in the NMR spectra. Magnifying the spectral regions around 151.4, 158.7, and 169.4 ppm allows us to better illustrate this variation. We extend our heartfelt gratitude again to the reviewers for their incisive and profound comments.

Comment#8. SF13b is not a mechanism, merely the insertion of Cu (II) atoms in the polymerized structure deducted by the authors from their findings.
Response: We appreciate your thorough review of our manuscript and we also acknowledge the reviewer’s point that Supplementary Fig. 13b is not a mechanism. Thank to this suggestion, which has prompted us to refine the scheme to make the formation process of Cu1–N2 SAC through ACAS strategy clearer and more visualized. Specifically, the processes of the functionalization modification for C3N4 carrier and the Cu single-atom site anchoring were integrated, including the amidation reaction, the self-condensation reaction, the metal-heteroatom coordination, and the dehydration reaction, etc. The details are as follows.
Supplementary Fig. 13 specifically illustrates the synthetic steps for Cu1–N2 SAC utilizing the ACAS strategy. First of all, the hydrolysis of CuCl2 and the modifier L-cysteine creates an acidic medium that facilitates the hydrolysis of melamine. Compound I is formed by the reaction between L-cysteine and melamine in this step. The abundant terminal –NH2 of melamine would coordinate with free Cu2+ species to form Compound II, as evidenced by the XRD characterization (Supplementary Fig. 14a). Secondly, during the solvothermal reaction process, melamine underwent further hydrolysis to generate cyanuric acid units. The in-situ formed cyanuric acid and melamine spontaneously assembled into a supramolecular structure through hydrogen bonding (Refer to Chem. Eng. J. 405, 126685 (2021); Nano Res. 11, 3462–3468 (2018)). Due to the in-situ modification of nitrogen-containing units, the low-coordinate Cu–N structures can form (Compound III) during the assembly process, which further stabilizes the anchoring of Cu–N species. After that, the cyanuric acid in Compound III released the ammonia gas and left melamine to self-condense into heptazine units (Compound IV) (Refer to ACS Appl. Mater. Interfaces 11, 10651–10662 (2019); Small 20, 2309032 (2024); Langmuir 30, 447–451 (2014)). Subsequently, the modified moiety within Compound III underwent cyclization to form Compound V by releasing NH3 and H2S molecules. This was followed by dehydration to generate Compound VI during pyrolysis. The low-coordinate Cu1–N2 SAC was formed by repeating these steps iteratively and periodically. Throughout these above processes, the assembly of the supramolecular structure and the formation of asymmetric melon configurations created favorable conditions for the formation of high-loading and low-coordinate copper single-atom sites. 
We would like to express our sincere gratitude to you again for your valuable comments. The relevant content has been updated in the revised manuscript and the Supplementary information.

Comment#9. I apologize, but I find that the conclusions concerning electrophilic oxygen species is not entirely correct. Electrophilic species must inevitably possess a diminished electron density (delta+) for them to behave as electrophilic species, which would then target electron-rich domains (such as a double bond). Any species with a (delta minus) is usually considered Nucleophile and would thus never target a double bond. What electrophilic oxygen species have the authors identified?
Response: Thank you for your insightful comments regarding the electrophilic and nucleophilic nature of oxygen species. We value the opportunity to address this question in detail as it is a crucial aspect of understanding the DEP reaction mechanism.
Firstly, we appreciate your emphasis on the distinction between electrophilic and nucleophilic oxygen species. The classification of oxygen species based on their electron density and reactivity is supported by extensive research in the context of oxidation reactions. In particular, oxygen species adsorbed on metal sites typically undergo a series of electron transfers from the metal, progressing towards the lattice oxygen state. Among these species, singlet oxygen (~1O2) as well as molecular and atomic ion-oxygen species (O2-, O-, O0) are widely recognized for their strongly electrophilic properties. In contrast, lattice oxide ions (O2-) are typically nucleophilic reagents with the strongest nucleophilicity.
Reactions involving oxygen can therefore be divided into two groups based on the nature of the oxygen species: (1) electrophilic oxidation, which involves the activation of oxygen to its electrophilic forms and their attack on electron-rich domains of the reacting molecule, and (2) nucleophilic oxidation, where nucleophilic oxygen species add to a molecule that has been activated to make such addition favorable. For instance, in the case of hydrocarbons, electrophilic oxidation often leads to C–C/C=C bond cleavage, whereas nucleophilic oxidation typically tends to attack the allylic or partial oxidation. The alignment of this framework with Boreskov's early ideas on associative and stepwise oxidation mechanisms is reflected in contemporary studies (React. Kinet. Catal. L. 35, 369–379 (1987); Catal. Rev. 19(1), 1–41 (1979)). 
Furthermore, there is a plethora of literature that shows how electrophilic oxygen species are involved in olefin oxidation reactions(J. Am. Chem. Soc. 144, 7693–7708 (2022); ACS Catal. 9, 6262–6275 (2019)). In our previous work, we have similarly highlighted the critical role of electrophilic oxygen species in controlling the PO selectivity and catalytic performance of DEP reactions (J. Am. Chem. Soc. 142, 14134–14141 (2020)). 
In the present study, we have identified the electrophilic O2- species on Cu1–N2, Cu1–N3, and Cu1–N4 SACs through density functional theory calculations. The Bader charge analysis revealed that these oxygen species carry a charge of -0.08 |e| to -0.33 |e|, which is significantly less negative compared to oxygen species dissociated on Cu (111) surfaces or lattice oxygen in Cu2O (110) surface. Specifically, on Cu (111) surfaces, molecular oxygen is easily dissociated, and the resulting oxygen atoms have a significant negative charge of -0.90 |e|, which is typicalof nucleophilic oxygen species. For lattice oxygen in Cu₂O at elevated temperatures, the Bader charge reaches -0.97 |e|, and these nucleophilic oxygen species were found to require surmounting a significant barrier (0.87 eV) for oxygen insertion reactions. These results highlight the distinct reactivity of nucleophilic oxygen species, which is not conducive to the activation of C=C double bonds in propylene. In addition, the above results also emphasize the critical role of electrophilic oxygen species in the DEP reaction. Furthermore, the low-coordinate Cu–N single-atom sites effectively suppress the formation of strongly nucleophilic oxygen species, enabling precise control of selectivity in the DEP process. 
Your thoughtful comments have enabled us to provide a more detailed explanation and strengthen our conclusions, which we sincerely appreciate. If there are any additional points you want us to address, we are more than happy to incorporate them into the revised manuscript.

Comment#10. Concerning the catalytic data, while the authors have prepared a catalyst that performs quite well, the PO yield, propylene conversion and PO selectivity fall below the benchmark values which are considered the thresholds for their commercial implementation (as reviewed by Khatib and Oyama in S.J. Khatib, S.T. Oyama, Catal. Rev. 57 (2015) 306.) Furthermore, in terms of these parameters, the reported values fall below previously published papers (see for example J. García-Aguilar et al. J. Catal. 386 (2020) 94-105) which use significantly simpler and more cost-effective catalysts. The impact of this novel research needs to be further justified. While the PO production rate values are interesting, the other parameters are not impressive.
Response: Thank you for your thoughtful comments and the valuable references you have provided. They have helped us to better showcase the advantages of our study compared to the reported literature. We fully acknowledge that the propylene conversion, and PO selectivity reported in our study fall below the benchmark values required for commercial implementation, as highlighted in the review by Khatib and Oyama (Catal. Rev. 57, 306 (2015)). According to the reference of Catal. Rev. 57, 306 (2015), a possible target for commercialization of PO production is a PO selectivity of 70% at a propylene conversion of 10%. This was based on comparison to ethylene oxide (EO) processes, which currently operate with EO selectivity of 80-90% and ethylene conversion of 7-15%. It is worth noting that, our study was based on the direct epoxidation of propylene route with molecular oxygen alone, which is believed to be an ideal route for PO production and is significantly different from the traditional technology. Even though the DEP process struggles with the trade-off between activity and selectivity, the as-prepared Cu1–N2 SAC has achieved the optimal performance among the reported DEP catalysts and demonstrated highly competitive catalytic activity when compared with other catalysts using other PO production processes. The potential of this catalyst system is demonstrated in our study and provides a valuable foundation for future improvements and optimizations. To better illustrate the differences between García-Aguilar’s work (J. Catal. 386, 94–105 (2020)) and our work, a comparison was made in the following Table R1.
Table R1. The contrast between García-Aguilar’s work and our work.
	García-Aguilar’s work	This work
Goal	Study Ni/Ti-SiO2 catalysts for propylene epoxidation using H2/O2, with the aim of substituting noble metals with cost-effective alternatives.	Propose an ACAS strategy to synthesize low coordinate Cu1–N2 SAC for propylene epoxidation using O2 alone, significantly boosting the catalytic performance of copper-based catalysts.
Catalyst structure	Ni nanoparticles loaded on Ti-SiO2
(Ni/Ti-SiO2)	Cu1–Nx single-atom sites coordinated on C3N4 nanosheets (x=2, 3, or 4)
Research Strategy	Performing the catalytic reaction and characterizations over the Ni/Ti-SiO2 catalysts. The active site and mechanism were suggested based on the catalytic performance and structure characterization. The author demonstrated that Ni can serve as a substitute for Au in the epoxidation of propylene using H2/O2 processes.	Considering the particularity of the DEP reaction, which involves the use of molecular oxygen as the sole oxidant and the difficulty of the DEP reaction, we have developed an ACAS strategy to facilitate the stabilization of low-coordinate Cu1–N2 species. Cu1–N2 SAC has an excellent formation rate for PO with a high selectivity for PO and a high propylene conversion. Unveiling the intricate structure-activity relationship of Cu–N coordination via in-situ characterization and theoretical calculations.
Catalytic performance and reaction condition	Propylene epoxidation to PO with H2 and O2: 6 % propylene conversion, 85% selectivity to PO and 1.92 mmolPO gcat-1 h-1 PO formation rate. Reaction condition: WHSV=20000 h-1, C3H6:H2:O2=1:1:1 (vol.), 200 oC.	Propylene epoxidation to PO with only O2: 5.7 % propylene conversion, 78%  PO selectivity and 5.87 mmolPO gcat-1 h-1 PO formation rate. Reaction condition: GHSV=36000 h-1, C3H6:O2=2:1 (vol.), 275 oC.
Relationship between catalytic performance and catalyst structure	The metal species loaded on Ti-SiO2 determine its catalytic performance. The well-dispersed Ni particles and tetrahedral Ti species enhanced the catalytic performance, demonstrating high PO selectivity and stable catalytic activity, which can be comparable to some noble metal-based catalysts.	The low-coordinate Cu1–N2, with the dynamic regulation of valence state of Cuδ+ (where 1<δ<2) center,  facilitated the formation of electrophilic oxygen species, which targeted the electronrich C=C bond in propylene for efficient PO production. In contrast, Cu1–N4, characterized by higher Cu–N coordination numbers and Cu oxidation valence, delivered moderate catalytic performance. Cu1–N3 sites, on the other hand, allowed the co-adsorption of C3H6 and O2, resulting in poor performance for DEP reaction.
We are confident that through multi-dimensional comparisons between García-Aguilar’s work and our work, we can allay your concerns and gain your recognition for our study.

Comment#11. Concerning literature revision, I find it surprising that reference works (such as the papers mentioned in the previous point) have been seemingly overlooked by the authors. I strongly advise to check recent literature references on DEP to further strengthen the discussion and make the manuscript of a broader appeal.
Response: Thank you for your constructive feedback on our manuscript. We have thoroughly reviewed the latest literature on propylene epoxidation reaction in response to your advice, with a particular focus on advancements in DEP reaction. Our discussion has been enhanced by the additional literature search, which provides a broader context for our work.
Furthermore, to disperse your doubts directly, we have updated our manuscript to include a comprehensive table that systematically compares the catalysts and their performance, specifically highlighting only those studies that involve the direct epoxidation of propylene using molecular oxygen. The innovative nature and significant breakthrough of our work are clearly demonstrated in this table. According to the manuscript, the Cu1–N2 SAC (5.9 mmol gcat-1 h-1) has a PO formation rate that is 5 to 400 times higher than that of other catalysts. The PO formation rate over Cu1–N2 SAC has been increased by 1,073%~14,565%, based on the reported DEP catalysts with ≥60% PO selectivity. The catalytic performance of Cu1–N2 SAC has reached and even surpassed the performance of propylene epoxidation catalysts through H2/O2. Our catalyst's effectiveness and superiority in PO production rate are demonstrated by this unprecedented performance, which has great scientific and industrial value. 
We believe that these revisions have further strengthened our manuscript and will make it more appealing to a broader audience. Thank you once again for your valuable suggestions, which have undoubtedly improved the quality and comprehensiveness of our work.

Table R2. Comparison between Cu1–N2 SAC, Cu1–N3 SAC, Cu1–N4 SAC, CuNP/C3N4–L and other reported IB group metal catalysts for propylene epoxidation with O2.
No.	Catalyst	Conv. (%)	PO select. (%)	PO formation rate
(mmol gcat-1 h-1)	Temp. (oC)	Flow rate (mL min-1)	Pressure (MPa)	Ref.
1	Ag3/Al2O3	/	85	/	110	/	/	Science. 339, 1590-1593 (2013)
2	Ag-MoO3/ZrO2	0.6	58	0.236	350	62.5	0.1	J. Mol. Catal. A-Chem. 232, 165-172 (2005)
3	Ag-CaCO3-NaCl	3.7	45	1.24	260	30.0	0.3	Appl. Catal. Gen. 302, 283–295 (2006)
4	Ag/Y2O3-K2O/α-Al2O3	4	47	0.303	245	16.7	0.1	Catal. Lett. 119, 185–190 (2007)
5	Ni1Ag0.4/SBA-15	/	70.7	0.015	220	20.0	0.1	Appl. Catal. B-Environ. 243，304-312 （2019）
6	Au/TS-1-KOH	0.88	52	0.19	200	20.0	0.1	Angew. Chem. Int. Ed. 48, 7862-7866 (2009)
7	Cubic Cu2O	0.80	10	/	225	50.0	0.1	Angew. Chem. Int. Ed. 126, 4956–4961 (2014)
8	Octahedral Cu2O	0.67	3	/	200	50.0	0.1	
9	Rhombic dodecahedra Cu2O	0.2	50	0.057	250	50.0	0.1	
10	Rhombic dodecahedra Cl-Cu2O	0.05	95	0.02	150	50.0	0.1	J. Am. Chem. Soc. 142, 14134-14141 (2020)
		1.0	63	0.5	200	50.0	0.1	
11	Cu/SiO2	0.25	53	0.014	225	50.0	0.1	J. Catal. 236, 401-404 (2005)
12	VCe0.5Cu0.5-NaCl	0.26	33	0.165	250	60.0	0.1	J. Catal. 211, 552-555 (2002)
13	c-Cu2O-27	0.06	82	0.050	110	50.0	0.1	Nat. Commun. 12, 5921 (2021)
14	Cu7Ce	0.68	95	0.078	80	50.0	0.1	J. Phys. Chem. C 112, 7731–7734 (2008)
15	KAc-Cu/SiO2	1.3	61	/	275	100.0	0.1	J. Catal. 241, 225–228 (2006)
16	LaMn0.5Cu0.5O3	0.02	74	0.239	150	30.0	0.1	Catal. Sci. Technol. 12, 2426–2437 (2022)
17	LaCo0.8Cu0.2O3−δ-NaCl	10	12	1.04	250	30.0	0.1	ACS Sustain. Chem. Eng. 9, 794–808 (2021)
18	Cu/TiO2	0.3	24	0.22	250	50.0	0.1	ACS Catal. 14, 10172–10180 (2024)
19	Ag8Cu1/Cs2O-α-Al2O3	5.5	48.5	0.272	160	20.0	0.1	Rare Met. 34, 477–490 (2015)
20	AgCuCl/BaCO3-N2	0.45	85.29	0.5	275	30.0	0.1	ACS Appl. Nano Mater. 6, 9687–9696 (2023)
21	K+-1% CuOx/SBA-15	2.1	21	2.1	350	60.0	0.1	J. Catal. 241, 225–228 (2006)
22	CuNP/C3N4–L	1.6	8	0.163	325	30.0	0.1	This work
23	Cu1–N4 SAC	5.5	44	3.023	300	30.0	0.1	
24	Cu1–N3 SAC	0.4	/	/	350	30.0	0.1	
25	Cu1–N2 SAC	5.7	78	5.866	275	30.0	0.1	

Table R3. Comparison between Cu1–N2 SAC, Cu1–N3 SAC, Cu1–N4 SAC, CuNP/C3N4–L and other reported IB group metal catalysts for propylene epoxidation H2 and O2.

No.	Catalyst	Conv. (%)	PO select. (%)	PO formation rate
(mmol gcat-1 h-1)	Temp. (oC)	Flow rate (mL min-1)	Pressure (MPa)	Ref.
1	Au/TiO2	0.6	96	0.54	50	33.3	0.1	J. Catal. 178, 566–575 (1998)
2	Au/TS-1(48)–Na1	7.4	85	2.05	200	20.0	0.1	Appl. Catal. B Environ. 95, 430–438 (2010)
3	Au(mIWI)/TS-1	2.8	91.5	1.61	200	35.0	0.1	ChemCatChem 12, 5993–5999 (2020)
4	Au/TS-1PT	1.3	89	0.39	200	66.7	0.1	Angew. Chem. Int. Ed. 60, 18185–18193 (2021)
5	Au20Pt1/TS-1-B	11	88	6.14	200	35.0	0.1	AIChE J. 68, e17416 (2022)
6	Au/HTS-1(NIMG)	6	91.2	2.59	200	35.0	0.1	ACS Sustain. Chem. Eng. 10, 9515–9524 (2022)
7	Au/TS-1-B	3.99	91.7	2.34	200	35.0	0.1	Appl. Catal. B Environ. 319, 121837 (2022)
8	Au/TS-1-CTES	13.3	92.7	2.02	180	20.0	0.1	ACS Sustain. Chem. Eng. 11, 7042–7052 (2023)
9	Au1&Aun	0.29	75.6	0.2	200	35.0	0.1	Nat. Commun. 15, 3249 (2024)
10	Cu1–N4 SAC	5.5	44	3.023	300	30.0	0.1	This work
11	Cu1–N3 SAC	0.4	/	/	350	30.0	0.1	
12	Cu1–N2 SAC	5.7	78	5.866	275	30.0	0.1	



Reply to Reviewer #4:


Fig. 1 | Schematic illustration and characterizations. (a) Scheme for the synthesis of Cu1–N2 SAC. TEM images of (b) Cu1–N2, (c) Cu1–N3 and (d) Cu1–N4 SACs. Aberration-corrected HAADF-STEM images of (e) Cu1–N2, (f) Cu1–N3 and (g) Cu1–N4 SACs. Element mapping images of (h) Cu1–N2, (i) Cu1–N3 and (j) Cu1–N4 SACs.


Fig. 2 | Structural characterizations. (a) Cu K-edge EXAFS of Cu foil, Cu2O, CuO, Cu1–N2, Cu1–N3 and Cu1–N4 SACs. (b) Wavelet transform of EXAFS of Cu foil, CuO, Cu1–N2, Cu1–N3 and Cu1–N4 SACs. (c) Cu K-edge XANES of Cu foil, Cu2O, CuO, Cu1–N2, Cu1–N3 and Cu1–N4 SACs. (d) Cu 2p XPS spectra of Cu1–N2, Cu1–N3 and Cu1–N4 SACs.


Fig. 4 | Dynamic changes of single-atom sites during propylene epoxidation with O2. (a) The first derivatives of XANES spectra of Cu foil, Cu2O, CuO and Cu1–N2 SAC after the successive feeding of O2 and C3H6. (b) The dynamic changes in mean chemical valence of Cu1δ+–N2 sites according to the edge position and the first derivatives of XANES spectra. (c) Fitting of k3-weighted EXAFS data of Cu1–N2 SAC after the successive feeding of O2 and C3H6. In situ DRIFTS spectra were recorded in the mixed flow of C3H6 and O2, after the successive feeding of C3H6 and O2 and after the successive feeding of O2 and C3H6 over (d) Cu1–N2, (e) Cu1–N3 and (f) Cu1–N4 SACs. (g) In situ DRIFT spectroscopy of Cu1–N2 SAC for different wavenumber intervals.

Supplementary Figure 10 | The corresponding fitting results and atomic configurations. The corresponding EXAFS fitting results of Cu1–N2, Cu1–N3 and Cu1–N4 SACs of (a) R-space and (b) k-space. Computational models for (c) Cu1–N2 (d) Cu1–N3 and (e) Cu1–N4 SACs. Orange: Cu atoms, blue: N atoms, grey: C atoms and white: H atoms.


Supplementary Figure 12 | Structural analysis of various catalysts. (a) The first derivatives of XANES spectra of Cu foil, Cu2O, CuPc, Cu1–N2, Cu1–N3 and Cu1–N4 SACs. (b) The dynamic changes in mean chemical valence of Cu1δ+ sites according to the edge position and the first derivatives of XANES spectra.


Supplementary Figure 13 | Schematic illustration of the plausible formation mechanism of low-coordinated Cu1–N2 centres.


Supplementary Figure 26 | Structural characterizations. Cu K-edge XANES of Cu foil, Cu2O, CuO, Cu1–N2 SAC and Cu1–N2 SAC-O2-C3H6. 


Supplementary Figure 28 | Structural characterizations. Cu K-edge EXAFS of Cu foil, Cu2O, CuO, Cu1–N2 SAC and Cu1–N2 SAC-O2-C3H6.

Supplementary Table 3 | EXAFS fitting data of various catalysts.
Sample	Shell	N	R (Å)	σ2 (10-3Å2)	E (eV)	R-factor
Cu foil	Cu–Cu	12.0	2.55	-	-	-
CuO	Cu–O	4.0	1.95	-	-	-
Cu2O	Cu–O	2.0	1.85	-	-	-
Cu1–N2 SAC	Cu–N	2.2	1.95	6.7	9.0	0.67%
	Cu–C	2.4	3.20	3.0	9.0	
Cu1–N2 SAC	Cu–N	2.2	1.91	5.9	2.7	2.97%
	Cu–Cu	0.2	2.49	3.0	2.7	
Cu1–N3 SAC	Cu–N	2.8	1.90	6.7	2.6	1.73%
	Cu–C	1.1	3.21	3.0	2.6	
Cu1–N3 SAC	Cu–N	2.8	1.90	7.5	1.7	3.81%
	Cu–Cu	0.1	2.40	3.0	1.7	
Cu1–N4 SAC	Cu–N	3.7	2.04	12.3	13.1	1.98%
	Cu–C	1.2	3.24	3.0	13.1	
Cu1–N4 SAC	Cu–N	3.7	2.12	28.8	9.6	2.92%
	Cu–Cu	0.8	2.40	3.0	9.6	
Cu1–N2 SAC-O2-C3H6	Cu–N	2.1	1.92	7.1	3.8	1.29%
	Cu–C	0.7	2.97	3.0	3.8	
Cu1–N2 SAC-O2-C3H6	Cu–N	2.1	2.04	3.0	-49.0	4.30%
	Cu–Cu	0.1	2.25	9.0	-49.0	
N, coordination number; R, distance between absorber and backscatter atoms; σ2, DebyeWaller factor to account for thermal and structural disorders; ∆E0, threshold Energy Correction; R factor indicates the goodness of the fit.


General Comment: This manuscript is another attempt to produce propylene oxide (PO) by the direct epoxidation of propylene with molecular oxygen (a green process), where at least the catalytic results (high selectivity and stable conversion) suggest success. This manuscript suggests that for isolated low coordinated Cu sites the formation of electrophilic oxygen species could be formed that direct the insertion of electrophilic oxygen species in the C=C bond of propylene. As no nucleophilic oxygen species are present that lead to unwanted CO2 and acrolein formation, high selectivities were obtained. No experimental evidence however is provided on the presence or absence of either oxygen species.
Response: We thank you for the positive comment of “the catalytic results (high selectivity and stable conversion) suggest success”. We are sorry for the confusion caused by the incomplete description of oxygen species in the manuscript, and we would be grateful if we could address this question in detail.
Based on the in situ DRIFTS results presented in our study, the vibration peaks of the functional groups of the epoxidation intermediates can only be observed under the condition that oxygen was fed prior to propylene. In contrast, when propylene was introduced first followed by oxygen, no IR peaks corresponding to the reaction intermediates were detected. Moreover, comprehensive characterizations including the X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) consistently demonstrated that the low-coordinate Cu1–N2 SAC possessed an oxygen-free structure and maintained its stability at a reaction temperature of 275 oC. Nucleophilic lattice oxygen species can effectively be eliminated by the absence of Cu–O coordination structure and Cu2O/CuO crystal phases. Collectively, these above results clearly show that the chemisorbed molecular oxygen on the catalyst surface can be activated into the intermediate oxygen species, which play a crucial role in promoting the DEP reaction. 
As we all know, the electrophilic and nucleophilic nature of oxygen species is indeed a key aspect of understanding the DEP reaction mechanism. A substantial amount of research on this issue in oxidation reactions supports the classification of oxygen species based on their electron density and reactivity. When oxygen species are adsorbed on metal sites, they usually go through a series of electron transfers from the metal and move towards the lattice oxygen state. Among these species, molecular and atomic iono-oxygen species (O2-, O-, O0) are widely recognized for their strongly electrophilic properties. In contrast, lattice oxide ions (O2-) are typically nucleophilic reagents with the strongest nucleophilicity. In addition, numerous studies have shown that electrophilic oxygen species play a role in olefin epoxidation reactions. (J. Am. Chem. Soc. 144, 7693–7708 (2022); ACS Catal. 9, 6262–6275 (2019)). In our previous work, we have similarly highlighted the critical role of electrophilic oxygen species in controlling the PO selectivity and catalytic performance of DEP reactions (J. Am. Chem. Soc. 142, 14134–14141 (2020)). 
Based on the above, in our study, we used density functional theory calculations to identify the electrophilic O2- species on Cu1–N2, Cu1–N3, and Cu1–N4 SACs. The Bader charge analysis revealed that these oxygen species carry a charge of -0.08 |e| to -0.33 |e|, which is significantly less negative compared to oxygen species dissociated on Cu (111) surfaces or lattice oxygen in Cu2O (110) surface. This electrophilic oxygen species is suitable for activating C=C double bonds in propylene.  
Your thoughtful comments have enabled us to provide a more detailed explanation and strengthen our conclusions, and we are grateful for them. Incorporating additional insights into the revised manuscript is something we would happily do if there are any additional points you would like us to address.

Major comment#1. What is the catalytic active site?
In your results section you state: progression of Cu into NP with a size of 15 nm: This is something completely different as isolated Cu atoms. Please discuss the implication of these findings. Also, the XRD results suggest the presence of metallic Cu NPs larger than 5 nm: what role do these NP play in the catalysis? Why are these NPs not visible in the TEM (although Figure 1b suggest you have any)? EXAFS fitting results are missing (also missing the data, i.e. the k-space spectra), but already a look at the FT EXAFS suggest that there could be Cu-Cu contributions present (Furthermore, XAFS by itself is not a proof of the presence of isolated Cu atoms: https://pubs.acs.org/doi/full/10.1021/acscatal.3c01116). This has large implication on how data are presented: for example, the interpretation of the Cu-N coordination number obtained from EXAFS is based on your assumption that all Cu atoms are isolated. This is clearly not the case and with that the assignment of Cu-N2, Cu-N3 and Cu-N4 incorrect and thus assumptions that went into the DFT calculations do not hold up.
Response: Your insightful comments are greatly appreciated. We sincerely apologize for the inappropriate of expression in our manuscript, which has led to your misunderstanding of our experimental results.
In this study, the Cu-based catalysts with ~15 nm nanoparticles (NPs) loading are the controlled catalysts (namely CuNP/C3N4–L) and present the 0.20 nm lattice fringes that correspond to the (111) plane of crystalline Cu. While the Cu1–N2, Cu1–N3 and Cu1–N4 SACs are atomically dispersed without any Cu NPs. Fig. 1b presents the TEM image of Cu1–N2 SAC. As can be seen in Fig. 1b, no nanoparticles were observed in this TEM image and the white regions represent the pores on the surface of the two-dimensional sheets of the catalysts because this image was taken under bright field imaging mode. To better understand, we have added the clear signs in the revised manuscript to mark the porous structure. Besides, we cannot agree the reviewer’s point that the XRD results indicate the presence of metallic Cu NPs larger than 5 nm. The XRD patterns showed only high-angle-shifted (002) peaks at 2θ of 27.5° belonging to pi-pi packing with the interlayer distance around 3.35 Å in all SACs. The controlled sample CuNP/C3N4–L is clearly different from Cu-based SACs because of the uncontrollable proximity of Cu active sites, showing the XRD diffraction peaks belong to Cu (JCPDS 04-0836)
To dispel the reviewer's doubts, we have re-tested the XAFS characterizations of all Cu-based SACs samples and re-fitted the data, including both R-space and k-space. The revised manuscript and supporting information have been updated with the relevant results. First, only a prominent peak at ~1.43 Å in EXAFS (Fig. 2a) was observed in these Cu-based SACs, indicative of the first coordination sphere of Cu–N interactions instead of Cu–O interactions. Focusing on the second-shell, the absence of Cu–Cu scatterings confirmed that the copper species was atomically dispersed. Furthermore, the FT-IR, XPS and other characterizations indicated that the formation of the Cu–N coordination structure in the Cu-based SACs was reliable. According to the CO DRIFTS test and HAADF-STEM images, the copper species in the Cu-based SACs were highly dispersed and did not aggregate. In order to prove our viewpoint more scientifically and effectively, the corresponding fitting of the EXAFS data in both R-space and k-space have been carried out, and the fitting results are presented in Supplementary Table 3. The quantitative fitting results elucidated that the average N coordination numbers of the central Cu atom were 2.2, 2.8 and 3.7 for Cu1–N2, Cu1–N3 and Cu1–N4 SACs, respectively. This confirmed that one Cu atom was coordinated with approximately two (Cu1–N2 sites), three (Cu1–N3 sites) and four (Cu1–N4 sites) nitrogen atoms of different Cu based SACs. Next, we also conducted a systematic fitting of the second-shell. By comparing the fitting results using Cu–Cu path or Cu–C path, it was observed that there are no Cu–Cu interactions exist in the Cu-based SACs, while the weak Cu–C interactions can be proved. According to the computational models optimized by DFT calculations, the Cu–C interactions arising from the interaction between the Cu–N centre and the C atoms in the carbon nitride structure.
Based on the above results, we are certain that the copper species in the as-prepared Cu-based SACs are atomically dispersed. Additionally, other experimental results and DFT calculations are both scientifically and reliably. We have updated the manuscript accordingly, thanking the reviewer for this enhancement.

Major comment#2. What is the role of the support?
One recent example of direct propylene epoxidation, where no catalyst was needed, is the that proposed by Trunschke et al (Nature Communications 13, 2022) over boron nitride and silicon dioxide. Please also consider this manuscript and the influence of the support, which is not zero. Raman (and maybe XPS) suggests you have undercoordinated N sites, what could be the potential role of the support alone?
Response: We sincerely appreciate the time and effort you have dedicated to reviewing our manuscript. They have helped us to better showcase the advantages of our study compared to the reported literature. According to the reference of Catal. Rev. 57, 306 (2015), a possible target for commercialization of PO production could be achieved with a PO selectivity of 70% at a propylene conversion of 10%. Furthermore, the PO selectivity is a vital industrial evaluation metric, that significantly influences the utilization of raw material and the cost of product separation. As far as another important industrial indicator of the reaction temperature is concerned, the PO production reaction is exothermic and the high reaction temperatures are unfavorable for the chemical equilibrium movement, leading to an inevitable conflict between the reaction kinetics and thermodynamics. The implementation of the lowest attainable reaction temperature serves as a critical measure to suppress thermal runaway phenomena. It is worth noting that, the as-prepared Cu1–N2 SAC has achieved the optimal performance among the reported DEP catalysts. And highly competitive catalytic activity at a relatively low reaction temperature was achieved when compared to other catalysts using other PO production processes. Our study demonstrated the potential of this catalyst system, which provides a valuable foundation for future improvements and optimizations. 
Although the Trunschke’s work (Nat. Commun. 13:7504, (2022)) has scientifically and objectively demonstrated a new route for the direct oxidation of propane to produce PO, the issues such as high reaction temperatures and low PO selectivity still remain. We believe that the Trunschke’s study is highly influential and provides us with meaningful insights. To better illustrate the differences between Trunschke’s work and our work, a comparison was made in the following Table R1.
Response: Table R1. The contrast between Trunschke’s work and our work.
	Trunschke’s work	This work
Goal	Examine the process of directly oxidizing propane to propylene and propylene oxide using boron nitride and silicon dioxide, which can reduce CO2 emissions and energy consumption associated with traditional industrial processes.	Propose an ACAS strategy to synthesize low coordinate Cu1–N2 SAC for propylene epoxidation using O2 alone, significantly boosting the catalytic performance of copper-based catalysts.
Catalyst structure	Boron nitride, silica, and silicon carbide	Cu1–Nx single-atom sites coordinated on C3N4 nanosheets (x=2, 3, or 4)
Research Strategy	By optimizing process conditions, it was possible to synthesize propylene oxide from propane. The comparison was made between the performance of various inert fillers in propane oxidation. 
The techno-economic viability of producing propylene oxide directly from propane was evaluated through kinetic experiments, microkinetic simulations, and economic efficiency assessments.	Considering the particularity of the DEP reaction, which involves the use of molecular oxygen as the sole oxidant, we have developed an ACAS strategy to facilitate the stabilization of low-coordinate Cu1–N2 species. Cu1–N2 SAC has an excellent PO formation rate with a high PO selectivity and a high propylene conversion. Unveiling the intricate structure-activity relationship of Cu–N coordination via in situ characterization and theoretical calculations.
Catalytic performance and reaction condition	Propene oxidation to PO: about 40.0 % propene conversion, 11%  PO selectivity.
Reaction condition: C3H8:O2:He= 6:3:11, T=490oC, flow rate=3.3-20 mL min-1, mass was 0.666 g for SiO2.	Propylene epoxidation to PO with only O2: 5.7 % propylene conversion, 78% PO selectivity and 5.87 mmolPO gcat-1 h-1 PO formation rate.
Reaction condition: GHSV=36000 h-1, C3H6:O2:N2=2:1:17, T=275 oC, mass was 0.050 g for Cu1–Nx SAC.
Relationship between catalytic performance and catalyst structure	The fast reaction at non-specific interfaces facilitate the synthesis of products prone to overoxidation via  gas-phase reaction.
The gas-phase oxygen itself was essential for the initiation and propagation of the radical chain reactions by generating radical precursors like •OH, affecting reaction activity.	The low-coordinate Cu1–N2, with the dynamic regulation of valence state of Cuδ+ (where 1<δ<2) center,  facilitated the formation of electrophilic oxygen species, which targeted the electronrich C=C bond in propylene for efficient PO production. In contrast, Cu1–N4, characterized by higher Cu–N coordination numbers and Cu oxidation valence, delivered moderate catalytic performance. Cu1–N3 sites, on the other hand, allowed the co-adsorption of C3H6 and O2, resulting in poor performance for DEP reaction.
Downsizing the active center to the single-atom scale and decreasing the metal-heteroatomic coordination number can promote the special DEP reaction mechanism, that is, adsorption and activation of oxygen before the bridged adsorption of propylene, as well as the enhanced charge transfer dynamics. This finally improves the electrophilicity of intermediate oxygen species, which selectively activates the C=C double bond of propylene for PO formation. However, as the particle size and metal-heteroatomic coordination number decrease, the inherently high surface energy of SACs would rapidly increase and result in cluster formation. The underlying support plays a decisive role in affecting the metal-support interaction and structure-activity relationship (Refer to Nat. Chem. 3(8), 634–641 (2011), and Coordin. Chem. Rev. 505:215693 (2024)). Therefore, to anchor isolated and low-coordinate Cu sites, functionalized C3N4 with plenty of undercoordinated N sites and a strong enough bond is a crucial and ideal support. The detailed process of preparing low-coordinate Cu1–N2 SAC based on our proposed ACAS strategy can be found in the revised manuscript. As we can see in Supplementary Figure 17, the metal-free carbon nitride (support alone) demonstrated the negligible capacity to activate propylene, and predominantly formed side products, such as acrolein and CO2. And across all the investigated temperatures, no PO selectivity towards the DEP reaction was observed. Once the undercoordinated N sites interacted with Cu atoms to form the low-coordinate Cu1–N2 single-atom sites, the Cu1–N2 SAC outperformed all other catalysts in the DEP reaction, especially in achieving a state-of-the-art PO formation rate at relatively mild temperatures. In conclusion, there is a large number of undercoordinated N atoms in functionalized C3N4 with inherent tunability that can trap low-coordinate and high content single-atom metal centers, making it a more favorable support for precision catalysis. The sp2 hybridization in the C3N4 structure also produce a highly conjugated electronic structure, which improves the catalytic stability of SACs.
We hope this clarification addresses the concerns raised.

Minor comments:
The term single-atom catalyst is not correct, you have many atoms... and apart from isolated Cu sites also larger copper nanoparticles present.
Response: We appreciate the reviewer’s observation. The concept of "single-atom catalysis" was first introduced in 2011 by Tao Zhang, Jun Li, and Jingyue Liu. They reported the Pt single atoms loading on FeOx and defined the single atom catalysts (SACs) as heterogeneous catalysts in which all metal species are atomically dispersed (Refer to Nat. Chem. 3(8), 634–641 (2011)). SACs have attracted considerable interest because they offer 100% metal atom utilization and exhibit excellent catalytic performance compared to traditionally supported nanoparticles.
In our study, we have developed an innovative asymmetric cavity-anchoring single-atom (ACAS) strategy to address this above challenge. This approach involved introducing amino acids with carbon skeletons to generate a multitude of asymmetric anchoring sites on functionalized C3N4, thus facilitating the stabilization of low-coordinate Cu1–N2 species. The XRD patterns showed only high-angle-shifted (002) peaks at 2θ of 27.5° belonging to pi-pi packing with an interlayer distance of around 3.35 Å in all SACs. The appearance of distinct XRD peaks that represent metallic copper can be only observed in CuNP/C3N4–L. The CO DRIFTS showed an IR peak at 2054 cm-1 (Cu1–N2 SAC), 2057 cm-1 (Cu1–N3 SAC) and 2060 cm-1 (Cu1–N4 SAC) representing the CO linear adsorption (COL) on Cu atoms. It is worth noting that the absence of a bridge bonded CO band in the range of 1700-1900 cm-1 rules out the presence of multi-atom Cu species (Refer to Angew. Chem. Int. Ed. 62 (2023), e202217220, Angew. Chem. Int. Ed. 126, 4956–4961 (2014), Nat. Commun. 12, 6022 (2021), J. Catal. 405, 333–345 (2022), Appl. Catal. A Gen. 505, 334–343 (2015), J. Phys. Chem. C 113, 10689–10695 (2009) and J. Catal. 268, 367–375 (2009)). According to the TEM and HAADF-STEM images, the copper species were atomically dispersed and did not aggregate in as-prepared SACs. 
Most importantly, we have re-tested the XAFS data and re-fitted the EXAFS data of all Cu-based SACs samples. The relevant results have been updated in the revised materials. First, only a prominent peak at ~1.43 Å in EXAFS (Fig. 2a) was observed in these Cu-based SACs, which indicates the first coordination sphere of Cu–N interactions. Focusing on the second-shell, the absence of Cu–Cu scatterings confirmed that the copper species was atomically dispersed. The corresponding fitting of the EXAFS data in both R-space and k-space have been carried out, and the fitting results are presented in Supplementary Table 3. The quantitative fitting results revealed that the average N coordination numbers of the central Cu atom were 2.2, 2.8 and 3.7 for Cu1–N2, Cu1–N3 and Cu1–N4 SACs, respectively. This confirmed that one Cu atom was coordinated with approximately two (Cu1–N2 sites), three (Cu1–N3 sites) and four (Cu1–N4 sites) nitrogen atoms of different Cu based SACs. Next, we also conducted a systematic fitting of the second-shell. By comparing the fitting results using Cu–Cu path or Cu–C path, it was observed that no Cu–Cu interactions exist in the Cu-based SACs, while the weak Cu–C interactions can be proved. According to the computational models optimized by DFT calculations, the Cu–C interactions arise from the interaction between the Cu–N centre and the C atoms in the carbon nitride structure. As we can understand from the reaction mechanism for DEP in in situ DRIFTS spectra, O2 can be preferentially adsorbed and activated on the low-coordinate Cu1–N2 site and the propensity for C3H6 bridged adsorption happen later, favoring the oxygen insertion to producing PO. The single-atom center is responsible for promoting this specific reaction pathway.
The atomic dispersion of copper species in the as-prepared Cu-based SACs is confirmed by the above results. We hope this clarification satisfactorily addresses the reviewer’s concerns and further reinforces the conclusions drawn in our manuscript.

Minor comments:
Abstract, last sentence: you do not optimize metal oxide catalysts, you optimize the coordination environment of isolated metal centers.
Response: Thank the reviewer for pointing this out. As known to us all, the traditional DEP catalysts are generally copper-based oxides, such as Cu2O nanocrystals, which have Cu–O active sites and are prone to causing the deep oxidation of propylene. In contrast, in this study, the Cu–N active sites were introduced and effectively prevented the side reactions. Furthermore, the precise manipulation of metal-nitrogen (M–N) coordination environments shed light on the coordination sensitivity of low-coordinate active-sites. 
Our apologies for the misunderstandings caused by our negligence and inappropriate language. This part has been revised to be more reasonable based on this comment. “These insights pave the way for a new avenue in the optimization of the fine structure of isolated active sites through the precise manipulation of metal-nitrogen (M–N) coordination environments.”

Minor comments:
Introduction: why is the interatomic distance between copper atoms important when you claim that isolated Cu atoms do the job?
Response: We thank you for this comment and admire the reviewer's rigorous academic attitude. As known, the formation of surface oxygen species with eletrophilicity is crucial for the DEP reaction. Taking traditional DEP catalysts (Cu nanoparticals and Cu2O nanocrystals with continuous Cu–Cu or Cu–O active sites) as an example, at the reaction temperature, C3H6 and O2 molecules would both tend to co-adsorb on their surface due to the unseparated Cu sites. This promoted the Mvk mechanism and induced the formation of nucleophilic oxygen species. (Refer to Angew. Chem. Int. Ed. 53, 4856–4861 (2014), Nat. Commun. 12, 5921 (2021), and J. Am. Chem. Soc. 142, 14134–14141 (2020)). Specifically, the Bader charge analysis revealed that molecular oxygen is easily dissociated on Cu (111) surfaces, and the resulting oxygen atoms have a significant negative charge of -0.90 |e|, which is typical of nucleophilic oxygen species. For lattice oxygen in Cu₂O at elevated temperatures, the Bader charge reaches -0.97 |e|, and these nucleophilic oxygen species were found to require surmounting a significant barrier (0.87 eV) for oxygen insertion reactions. These results highlight the distinct reactivity of nucleophilic oxygen species, which is not conducive to activation of the C=C double bond in propylene. 
In our study, the single-atom Cu1–Nx center has been designed, boosting the special DEP reaction mechanism which is different from the Mvk mechanism on Cu nanoparticals and Cu2O nanocrystals. The reaction began with the adsorption of molecular oxygen at the single-atom center to yield O2- species with a charge of -0.08 |e| to -0.33 |e|, which subsequently underwent an oxygen insertion into the C=C bond of propylene, leading to the formation of PO. This finding highlights the crucial role played by the atomic dispersion state of copper in controlling the reaction mechanism and product selectivity for the DEP process. 

Minor comments:
Pyrolysis is typically an uncontrolled synthesis method; how do you manage to maintain isolated Cu sites (XRD/TEM suggests you do not)? Can you show the repeatability of this synthesis method? Are you sure that all S is out of the final structure, or could this influence the support chemistry as well? 
Response: We appreciate your insightful comment. The structural characteristics of SACs determine that the interaction between the atomic dispersed metal and the coordination atoms (such as N and others) on the support is crucial for the stabilization of metal single atoms. Various strategies based on coordination geometry engineering, confinement effects or chemical bonding interactions can be used to reduce the surface free energy and kinetically inhibit metal-atom aggregation (Refer to Nat. Synth. 3, 1427–1438 (2024), Angew. Chem. Int. Ed. 60, 15248–15253 (2021), and Science 353, 150–154 (2016)). Among the above strategies, the metal-support interaction (MSI) is often considered to be the key to stabilizing metal single-atom sites in synthesis and catalysis of SACs. Thus, the key to regulating the MSI lies in the rational design of the C3N4 support. Because of its polymer characteristics, easy synthesis, abundant N content and rich chemical properties, the C3N4 support can be designed at the molecular level to anchor isolated metal atoms with different coordination environments.
 In this study, we have developed an innovative asymmetric cavity-anchoring single-atom (ACAS) strategy. This approach involved introducing amino acids with carbon skeletons to generate a multitude of asymmetric anchoring sites on functionalized C3N4, thus facilitating the stabilization of low-coordinate Cu1–N2 species. Supplementary Fig. 13 specifically illustrates the synthetic steps for Cu1–N2 SAC utilizing the ACAS strategy. First of all, the hydrolysis of CuCl2 and the modifier L-cysteine creates an acidic medium that makes it easier to hydrolyze melamine. The reaction between L-cysteine and melamine results in the formation of Compound I. The abundant terminal –NH2 of melamine would coordinate with free Cu2+ species to form Compound II, as evidenced by the XRD characterization (Supplementary Fig. 14a). Secondly, during the solvothermal reaction process, melamine undergoes further hydrolysis to generate cyanuric acid units. A supramolecular structure is formed by the spontaneous assembling of in-situ formed cyanuric acid and melamine through hydrogen bonding (Refer to Chem. Eng. J. 405, 126685 (2021); Nano Res. 11, 3462–3468 (2018)). Due to the in-situ modification of nitrogen-containing units, Cu2+ forms low-coordinate structures (Compound III) during the assembly process, which further stabilizes the anchoring of Cu–N bnn species. Finally, the cyanuric acid in Compound III undergoes pyrolysis, releasing ammonia gas and leaving melamine to self-condense into heptazine units (Compound IV) (Refer to ACS Appl. Mater. Interfaces 11, 10651–10662 (2019); Small 20, 2309032 (2024); Langmuir 30, 447–451 (2014)). Meanwhile, the modified moiety within Compound III undergoes pyrolysis and cyclization to form Compound V, which is then dehydrated to yield Compound VI. These steps are iteratively and periodically repeated, ultimately forming the low-coordinate Cu1–N2 SAC. Throughout these above processes, the assembly of the supramolecular structure and the formation of asymmetric melon configurations create favorable conditions for the formation of high-loading and low-coordinate copper single-atom sites.
As the reviewer mentioned, pyrolytic strategies commonly used for synthesizing SACs generally suffer from aggregation especially at high metal loadings. In the extensive preliminary experiments conducted in this study, we identified similar issues and solved them. An improper ratio of metal precursor, nitrogen-containing precursor and L-cysteine, along with factors such as the temperature, and reaction time of the solvothermal process, as well as the temperature, time, atmosphere, and heating rate of the pyrolysis process, could all lead to aggregation or collapse of the catalyst structure. The following table lists some of the influence of the synthesis parameters. 

	Parameters	Catalysts state	Cu species
CuCl2:melamine:L-cysteine
(mass ratio)	0.78:2.00:0.01	Nanosheet	Single atom
	0.60:2.00:0.015	Nanosheet	Single atom
	0.95:2.00:0.00	Nanosheet	Single atom
	1.00:2.00:0.00	Nanosheet	Nanopartical
	0.78:1.60:0.05	Nanotube	Nanopartical
	1.20:2.00:0.01	Collapse	-
	0.78:2.00:0.30	Nanotube	Nanopartical
	More……
Temperature, time (solvothermal process)	220 oC, 4 h	Nanosheet	Single atom
	220 oC, 6 h	Collapse	-
	220 oC, 3 h	Nanosheet	Single atom
	220 oC, 2 h	Nanosheet	Nanopartical
	220 oC, 1 h	Nanosheet	Nanopartical
	180 oC, 4 h	Nanosheet	Nanopartical
	160 oC, 4 h	Collapse	-
	More……
Temperature, time, atmosphere, and heating rate (pyrolysis process)	550 oC, 3 h, 5%H2/Ar, 8 oC/min	Nanosheet	Single atom
	550 oC, 3 h, Ar, 8 oC/min	Nanosheet	Single atom
	520 oC, 3 h, Ar, 5 oC/min	Nanosheet	Nanopartical
	550 oC, 3 h, Air, 8 oC/min	Nanosheet	Nanopartical
	550 oC, 3 h, 5%H2/Ar, 3 oC/min	Nanosheet	Nanopartical
	550 oC, 3 h, Air, 10 oC/min	Collapse	-
	More……
Although the EDS, XPS, and XAS results indicate that the low-coordinate Cu1–N2 SAC contains no S element, the presence of extremely trace amounts of residual S cannot be ruled out. In light of the detailed mechanistic insights into ACAS strategy presented above, its universality was further explored. As shown in Supplementary Fig. 14, in addition to L-cysteine, glycine and L-arginine, which do not contain the S element, can also be used for the functionalization of C3N4 support. Moreover, the structural features including the crystal structures, infrared signatures, and coordination environments of the prepared copper-based single-atom catalysts shows no significant difference compared to that of Cu1–N2 SAC, resulting in the similarly excellent DEP performance (Supplementary Fig. 14). This result effectively rules out the interference of the S element and highlights the significance of the ACAS strategy proposed in our study.
We hope this clarification addresses the concerns raised and provides a deeper understanding of ACAS strategy for SACs preparation.



Minor comments:
The synthesis approach taken here is inspired by that published by Yang et al. (reference 33) where similarly low coordinated Cu sites were held responsible for high ORR activity. To obtain further stability, amino acids were used that should lead to stronger anchoring sites. Related to this, the catalyst synthesis procedure proposed here will be difficult to scale up, please comment.
Response: We sincerely thank you for your thoughtful review of our work. It is important to note that the fresh Cu-N-C SAC prepared by Yang et al. (reference 33) using a two-step approach generated only Cu–N4 coordination structure, which was anchored on a N-doped carbon support without displaying the characteristics of Cu–N2 and Cu–N3. Only under ORR working conditions, the dynamic evolution of Cu–N4 to Cu–N3 and further to HO–Cu–N2 can be driven by the applied potential. Comprehensively considering the structural characteristics of Cu-N-C SAC, there are obvious differences between Cu-N-C SAC and ours. In our study, we developed an innovative asymmetric cavity-anchoring single-atom (ACAS) approach for fabricating the low-coordinate Cu-based SAC. This approach involved introducing amino acids with carbon skeletons to generate a multitude of asymmetric anchoring sites on functionalized C3N4. This facilitated the stabilization of high content of low-coordinate Cu1–N2 species without the need for additional reaction conditions. The fitting of the EXAFS data elucidated that the average N coordination numbers of central Cu atom were 2.2 and 2.1 for on fresh and spent Cu1–N2 SAC, respectively. This confirmed the strong bonding between one Cu atom and two N atoms, which facilitates that the Cu1–N2 sites can be activated under the oxygen adsorption, simultaneously promoting the dynamically stability of Cu1–N2 sites can during the DEP reaction.
Overall, the preparation method of Cu1–N2 SAC was not inspired by that published by Yang et al, but referred to their descriptions of the low-coordinate active centers. We are sincerely grateful for your valuable insights once again, which have significantly propelled our research forward. Benefiting from the development of the ACAS strategy, the fine coordination structure of low-coordinate Cu1–N2 single-atom centers can be ultimately stabilized on functional carrier (C3N4–L). 
Considering the easy availability and low cost of raw materials such as the amino acids, melamine and metal precursors, and the theoretical synthesis mechanism underlying the ACAS method has been elucidated in our work, the scaled up synthesis of Cu1–N2 SAC can be achievable. We expanded the synthesis scale of Cu1–N2 SAC by 2 fold (100 mL), 4 fold (200 mL) and even 20 fold (1000 mL), respectively. It was found that the fill rates of the reaction materials in the solvothermal reactor significantly affected the pressure of solvothermal process, which was the key factor influencing the synthesis of Cu1–N2 SAC. Based on the variable regulation and optimization of the synthesis experiments, the fill rate of 55% was determined as the most excellent parameter. Eventually, the stable synthesis of ten-gram-scale Cu1–N2 SAC has be achieved. As shown in Supplementary Fig. 21, there are no conspicuous differences in the structure and the DEP performance of the catalysts prepared on different scales, demonstrating the practicality of the ACAS strategy.
 We declared that the main purpose of our work was to assess the structure-performance relationship between fine coordination structure of Cu1–Nx SACs and the DEP reaction systematically. We overlooked whether the catalysts could be synthesized on a large scale or not and we apologize for this. We have updated the manuscript accordingly, thanking the reviewer for this enhancement.


Minor comments:
Results: Not sure how a significant number of micropores is beneficial to electron transfer between active sites and oxygen species. Please elaborate. Also not clear how the Cu1-N2 leads to more micropores compared to Cu1-N3, what is done differently in the synthesis procedure?
Response: Thank the reviewer for pointing this out. Microporous structures in catalysts which can be beneficial for electron transfer, are is traceable and widely proven. 
As can be seen in Cao’s study (Refer to Nat. Commun. 14:2494, (2023)), they discussed in detail the beneficial effects of the microporous structure on electron transfer in single-atom catalysis for enhanced alkene epoxidation. The main functions of the microporous structure are as follows. (1) Modifying electronic structure: charge redistribution regulated by the confinement effect of micropores can alter the electron cloud of SACs. This creates an optimal electronic structure, facilitating electron transfer efficiency between olefins, intermediates and active sites, and promoting alkene epoxidation as well as selectivity. (2) Boosting charge transfer: the microporous micro-environment can lower the reaction activation energy and stabilize the reaction intermediates by interacting with reactants and active sites. This is crucial for accelerating electron transfer, and improving alkene epoxidation reaction rate.
Meanwhile, Zhu’s work (Refer to Nat. Catal. 6, 574–584 (2023)) showed that micropores can shorten the transport path of photo- generated excitons, enabling efficient separation of electron- hole pairs, ensuring high-efficiency electron transfer. The active sites within the micropores can utilize the energy of excitons to drive the reaction. Micropores structure can restrict the diffusion and adsorption behavior of reactant and product molecules within them. Additionally, Li’s study (Refer to Angew. Chem. Int. Ed. 64, e202415691 (2025)) discussed that the interconnected meso- and micropores structure in porous Fe-N/C materials provides higher specific surface area and efficient pathways for electron transfer during the oxygen reduction reaction. In addition, Luo’s study (Refer to Nat. Commun. 16:800, (2025)) proved that a microporous-dominated structure with a large specific surface area and pore volume can be conducive to the exposure of active sites and the diffusion of reactants. Besides, the microporous structure can also benefit electron transfer, resulting in the rapid conversion of PMS. This leads to an improvement in the catalytic performance..
To sum up, the microporous structure facilitates electron transfer during the catalytic process through multiple mechanisms, such as exposure of more active sites, generation of a confinement effect, influence of the electronic structure, enhancement of the contact between reactant and active sites, and optimization of charge transfer. This, in turn, improves the catalytic performance.
The synthesis methods of Cu1–N2 and Cu1–N3 SACs can be learned in detail from our manuscript and supplementary information. Cu1–N2 SAC was synthesized according to our proposed ACAS strategy. This approach involved introducing amino acids with carbon skeletons to generate a multitude of asymmetric anchoring sites on functionalized C3N4, thus facilitating the stabilization of low-coordinate Cu1–N2 species. As can be seen in Supplementary Figure 13, during the solvothermal reaction process, melamine underwent hydrolysis to generate cyanuric acid unit. The in-situ formed cyanuric acid and melamine spontaneously assembled into a supramolecular structure through hydrogen bonding. Subsequently, the introduction of L-cysteine resulted in the shear stress breaking the stacking interactions between layers and cutting the stacked structure into ultrathin layers. The modified moiety within Compound III was cyclized to form Compound V by releasing NH3 and H2S molecules, followed by dehydration to generate Compound VI during pyrolysis. Porous and ultra-thin nanosheets were formed due to the escape of these gas molecules. By contrast, Cu1–N3 SAC was synthesized according to the scalable two-step annealing method referring to Lu’s work (Refer to Nat. Nanotechnol. 17, 174–181 (2022)). Initially, bulk carbon nitride was synthesized. Subsequently, single-atom copper sites were anchored via impregnation and calcination procedures. Consequently, Cu1–N3 SAC has a relatively thick nanosheet morphology and is devoid of a porous architecture, leading to a limited specific surface area. 
We are sincerely grateful for your valuable insights once again. They have significantly propelled our research forward, enabling us to gain deeper in this study.

Minor comments:
Results XPS: Please explain what you mean with: 'It was important to recognize that the prevalence of Cu-N coordination and the relative quantities of other nitrogen species displayed an inverse relationship, underscoring a more unsaturated coordinative environment for the Cu1-N2 SAC' This is absolutely no proof for undersaturated sites.
Response: Thank you very much for your valuable comment. We agree with the reviewer’s suggestion that the prevalence of Cu–N coordination is not the evidence for undersaturated sites. The Cu–N peaks in the N 1s XPS spectrum can only corroborate the interaction between Cu and N atoms. We hope this clarification satisfactorily addresses the reviewer’s concerns and further reinforces the conclusions drawn in our manuscript.

Minor comments:
EXAFS, please show the full EXAFS spectra somewhere. Where are the fitting results of the EXAFS spectra? Also, I could not find any information anywhere on the fitting approach taken. This is not good practice.
Response: We appreciate the reviewer’s critical observation. First of all, we would like to express our apologies for any possible oversights. Secondly, the EXAFS data and the fitting results of the EXAFS spectra in the R-space have been both presented in our manuscript and Supplementary information (We apologize again for the misunderstanding caused by the inappropriate presentation format). To clear up any uncertainties, we have retested the XAFS characterizations of all SACs and re-fitted the data, covering both R-space and k-space. The relevant EXAFS spectra and the fitting results have been updated in the revised manuscript (Fig. 2a) and Supporting information (Supplementary Figure 10 and Supplementary Table 3). Fig. 2a showed only a prominent peak at ~1.43 Å in EXAFS in these Cu-based SACs, which was indicative of the first coordination sphere of Cu–N interactions. The quantitative fitting results elucidated that the average N coordination numbers of the central Cu atom were 2.2, 2.8 and 3.7 for Cu1–N2, Cu1–N3 and Cu1–N4 SACs, respectively. This confirms that one Cu atom is coordinated with approximately two (Cu1–N2 sites), three (Cu1–N3 sites) and four (Cu1–N4 sites) nitrogen atoms of different Cu based SACs. The reliability of our study can be demonstrated by the consistency of the conclusions drawn from the updated results with the initial ones. In addition, the EXAFS fitting approach has been supplemented in the Supplementary information and highlighted in yellow. Thank you again for your thoughtful comments, and if there are any additional points you would like us to address, we would be glad to incorporate further insights into the revised manuscript.

Minor comments:
As a matter of fact, most information on the analysis methods applied is missing in both the manuscript and SI.
Response: Thank the reviewer for pointing this out. As required, we have meticulously supplemented the information on all analysis methods item by item in the revised Supplementary information from page 3 to page 9.

Minor comments:
EXAFS: Not sure how you would get bond angle information out of EXAFS fitting.
Response: We sincerely apologize for any misunderstanding that may have occurred due to the inappropriate expressions in our manuscript. In fact, the bond angle information of Cu1–N2, Cu1–N3 and Cu1–N4 SACs was obtained from the optimized configurations through computational studies. The EXAFS fitting merely proves that the experiment and the theoretical models are in agreement. The corrected manuscript has been amended accordingly. We appreciate your feedback once more.

Minor comments:
XANES analysis, your spectra do not have the distinct feature from Cu(1) and also not from Cu(0), probably as linear combination fitting will show (The CuO and Cu2O reference spectra are not good references for Cu surrounded by N). And thus, I doubt the assignment to be that of Cu(1) (what is the fitting error of linear combination fits? Please have a look at the literature (XANES spectra of Cu in SSZ-13 for NOx oxidation for example) which is the closest to single site Cu XANES in environments closest to what you have here. The assignment of Cu(1) species further does not corroborate with the XPS/AES results and the observation of metallic Cu peaks in the XRD. 
Response: Thank you for your thoughtful comment. Your comment is truly insightful, and we are grateful for the chance to clarify this point. We have read J. Phys. Chem. Lett. 9, 3035-3042 (2018) and other references carefully. Furthermore, we also re-tested the XAFS characterizations of all SACs in order to ensure the reliability and reproducibility of the data. The updated Cu K-edge XANES analysis shows that the as-prepared SACs exhibit distinct features from Cu+. The obvious shoulder feature appearing in all Cu-based SACs around 8983 eV can be assigned to the 1s → 4pxy transition, indicative of a Cu+ cation in a linear configuration. There is no visible Cu2+ feature at about 8986 eV. Interestingly, compared to Cu1–N3 and Cu1–N4 SACs, the intensity of the peak around 8983 eV in Cu1–N2 SAC decreases and the 1s → 4pz transition around 9004 eV shifts the edge position to a lower-energy position. These results show that the linear configuration in Cu1–N2 SAC is broken, which is in agreement with the optimized configurations (Refer to Nature 622, 754-760 (2023) and J. Phys. Chem. Lett. 9, 3035-3042 (2018)). 
In view of the doubt from reviewer that the CuO and Cu2O reference spectra are not good references for Cu surrounded by N, we added the CuPc standard sample which is characterized by Cu–N coordination structure. As shown in Supplementary Fig. 12, based on the XANES spectra of Cu foil and CuPc, the valences of Cuδ+ in Cu1–N2, Cu1–N3 and Cu1–N4 SACs were estimated to be ~0.70, 0.93, and 0.83 according to our linear fitting results (R=0.99). This is consistent with the Cu 2p XPS/AES analysis results. Among them, the valence state of Cu1–N3 SAC is different from that of Cu 2p XPS. The reason is that the surface carbon atomic content in the Cu1–N3 SAC was less than that observed in the other two catalysts, suggesting a replacement of carbon atoms by copper in the Cu1–N3 structure, as confirmed by the distinct reduce of C–N group related vibration bands intensity, which affects the electron density of the copper centers (Refer to Nat. Catal. 5, 818-831 (2022) and Nat. Commun. 12:6022, (2021)). As we can see in Supplementary Figure 7, the metallic Cu XRD peaks can only be observed in CuNP/C3N4–L. The XRD patterns showed only high-angle-shifted (002) peaks at 2θ of 27.5° belonging to pi-pi packing with an interlayer distance of around 3.35 Å in all SACs. These indicate that the Cu species in Cu1–N2, Cu1–N3 and Cu1–N4 SACs are atomically dispersed.
We want to express our sincere appreciation to you once more for your valuable comments. The revised manuscript and Supplementary information now have the updated relevant content.

Minor comments:
Not clear at all how Figure 4d is obtained, i.e. oxidation state changes during reaction, is the feed changed? These results are based on linear combination fitting of XAS spectra using references which do not represent the species at hand (and thus lead to very approximate fits with large fit errors). How this is then direct proof for the formation of electrophilic oxygen species is really not clear. Also, the suggestion that a reduction in the FT EXAFS peak intensity at a certain bond distance suggests a reduction in electron density at the isolated Cu sites is not correct. When O species are formed, this might lead to a contribution at a different distance potentially cancelling out the Cu-N contributions. But, as stated before, as XAFS is a bulk averaging method and you have metallic Cu nanoparticles and Cu-Nx species, there might be other processes leading to a reduction of the intensity in the FT EXAFS spectra. 
Response: Figure 4d was obtained by the in situ DRIFT spectroscopy of Cu1–N2 SAC recorded in the mixed flow of C3H6 and O2, after the prior feeding of C3H6 and then feeding O2 and after the prior feeding of O2 and then feeding C3H6. The DRIFTS analysis highlighted that the molecular oxygen tends to be preferentially activated on Cu1–N2 sites and then the propylene molecule can be activated via bridged adsorption, favoring the oxygen insertion to produce PO. We apologize for the potential confusion caused by our imprecise description and think that you mentioned Figure 4b instead of Figure 4d.
In view of the doubt from reviewer that the references using for the linear combination fitting of XAFS spectra may be unbefitting, we have retested the XAFS data of all SACs and the in situ XAFS of Cu1–N2 SAC (Cu1–N2 SAC-O2-C3H6). Besides, we added the CuPc reference which is characterized by Cu–N coordination structure. As shown in Fig. 4b, the valences of Cuδ+–N2 in fresh and in situ Cu1–N2 SACs were estimated to be nearly 0.70 and 0.90 according to our linear fitting results based on the first derivatives of XANES spectra. The dynamic nature of Cu1–N2 site migration was also corroborated by Bader charge analysis, with charge states oscillating between +0.73e and +0.87e (Supplementary Fig. 27) during the DEP reaction progress. We agree with the reviewer’s suggestion that this cannot be direct proof for the formation of electrophilic oxygen species. Based on the special reaction mechanism that occurs at the low-coordinate Cu1–N2 center, that is, adsorption and activation of oxygen before the bridged adsorption of propylene, as well as the enhanced charge transfer dynamics, we have identified the electrophilic O2- species on Cu1–N2 SAC through density functional theory calculations. The Bader charge analysis revealed that these oxygen species carry a charge of -0.33 |e|, which is significantly less negative compared to oxygen species dissociated on Cu (111) surfaces or lattice oxygen in Cu2O (110) surface. Specifically, on Cu (111) surfaces, molecular oxygen readily dissociates, and the resulting oxygen atoms carry a substantial negative charge of -0.90 |e|, characteristic of nucleophilic oxygen species. For lattice oxygen in Cu2O at elevated temperatures, the Bader charge reaches -0.97 |e|, and these nucleophilic oxygen species were found to require surmounting a significant barrier (0.87 eV) for oxygen insertion reactions. Thus, evidence for electrophilic oxygen species originates from DFT calculations, and the theoretical reaction pathway strictly follows the evidence obtained from in situ spectroscopic characterizations. The misunderstanding caused by the inappropriate expression has been corrected in the revised manuscript.
Indeed, in conjunction with the EXAFS fitting data including the in situ results (Fig. 4c and Supplementary Table 3), it can be concluded that Cu1–N2 SAC remained atomically dispersed with stable low-coordinate Cu–N coordination structure (such Cu–N coordination numbers and Cu–N bond distances) without Cu–O centers. We also conducted a systematic fitting of the second-shell. By comparing the fitting results using Cu–Cu path or Cu–C path, it was observed that there are Cu–C interactions instead of Cu–Cu interactions existing in Cu1–N2 SAC during the DEP reaction, indicating that the Cu–N interactions are stable and cannot be oxidized easily to form the Cu–O coordination structure. Compared with the XAFS data of the spent Cu1–N2 SAC in our original manuscript, the more rigorous added in situ XAFS experimental results indicate that there is no contribution at different distances that could potentially interfere with the Cu–N coordination structure during the DEP reaction. This more objectively and scientifically demonstrates that the electron density at the isolated Cu sites did not decrease significantly during the DEP reaction. The above results also indicate that the DEP reaction does not disrupt the Cu–N coordination environment of the single-atom sites, but merely affects the electronic state of the Cu species. 
Moreover, the XRD patterns (Supplementary Fig. 7 and Supplementary Fig. 23) showed only high-angle-shifted (002) peaks at 2θ of 27.5° belonging to pi-pi packing with an interlayer distance of around 3.35 Å in all SACs. The diffraction XRD peaks of metallic copper are only present in the control sample CuNP/C3N4–L. Next, as can be seen in Fig. 1b, no nanoparticles were observed and the white regions in this TEM image represent the pores on the surface of the ultra-thin nanosheets of Cu1–N2 SAC because this image was taken under bright field imaging mode. To better understand, we have added the clear signs in the revised manuscript. Therefore, we believe that the above explanations can be sufficient to confirm the absence of Cu nanoparticles or other metal-heteroatom coordination structures in SACs. 
We sincerely appreciate your thoughtful comments, as they have allowed us to provide a more detailed explanation and strengthen our conclusions. If there are additional points you would like us to address, we would be happy to incorporate further insights into the revised manuscript.

Comments:
This brings me than to the last point: if all of your EXAFS data interpretation is correct, i.e. you have only isolated and undercoordinated Cu atoms, than the formation of a Cu-O bond should be very visible in the FT EXAFS spectra at larger bond distance as Cu-N. Please consider this. 
Response: Thank you for the insightful comments from the reviewer. In order to eliminate your doubts and ensure the authenticity of our experimental data, we have retested the XAFS data of all SACs and analyzed them in more detail. 
The relevant results, including figures and tables, have been updated in the revised materials. First, as shown in Fig. 2a, only a prominent peak at ~1.43 Å in EXAFS was observed in these Cu-based SACs, which indicates the first coordination sphere of Cu–N interactions. The absence of Cu–O scatterings at ~1.50 Å excluded the possibility of Cu–O coordination structure formation. Besides, the corresponding fitting of the EXAFS data in both R-space and k-space have been carried out, and the fitting results (Supplementary Table 3) also elucidated that one Cu atom was coordinated with approximately two (Cu1–N2 sites), three (Cu1–N3 sites) and four (Cu1–N4 sites) nitrogen atoms of different Cu based SACs. Next, we also conducted a systematic fitting of the second-shell. By comparing the fitting results using Cu–Cu path or Cu–C path, it was observed that there are Cu–C interactions instead of Cu–Cu interactions existing in the Cu-based SACs, which arise from the interaction between the Cu–N centre and the C atoms in the heptazine units on carbon nitride structure. Furthermore, the in situ XAFS (Cu1–N2 SAC-O2-C3H6) results also determine that the Cu–N interactions are stable and cannot be oxidized easily to form the Cu–O coordination structure, which strongly confirms our conclusion. 
We are truly grateful for your meticulous and insightful comments. In case there are any other aspects you deem necessary for us to further consider, we are willing for the further revisions.

Here, all of changes were highlighted in yellow in revised files. We hope the revised manuscript could meet the publication standards of Nature Catalysis.

Sincerely yours,
Gang Fu, Qingbiao Li, and Jiale Huang

College of Chemistry and Chemical Engineering, 
State Key Laboratory of Physical Chemistry of Solid Surfaces,
Xiamen University, Xiamen, 361005, China

This is a typical template for responding to reviewers, assuming you are a native English speaker and Chemist, please assist me in responding to reviewers' comments.
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 The language should not be wordy and the brief should be dry and polite.I may give you more than one question at a time, so please answer each one in turn.The overall response idea is [thanking the reviewer for pointing out the problem] [apologizing for the problem] [a revision or explanation of this problem] [thanking the reviewer again].At the same time I would like you as an expert to give me the changes I need to make to the text in response to this comment.I'll give you ideas for your answer, and you'll expand on them with modifications that take into account your status as an expert.

The original version of my manuscript is as follows:"Efficient harvest of electricity, aromatic aldehydes and H2 from lignin over nanoflower-like cobalt-based bifunctional electrocatalysts
Yichen Zhang 1,2 ‡, Daihong Gao1,2 ‡, Denghao Ouyang 1,2, Binhang Yan 2, Xuebing Zhao 1,2 *
1 Key Laboratory of Industrial Biocatalysis, Ministry of Education; Tsinghua University, Beijing 100084, China; 
2 Department of Chemical Engineering, Tsinghua University, Beijing 100084, China.
‡These authors contributed equally to this work.
* Corresponding author: Xuebing Zhao. Email: zhaoxb@mail.tsinghua.edu.cn
Abstract: Lignin is a promising feedstock for production of aromatic compounds and green energy. Here, based on cobalt-based bifunctional electrocatalysts, we have developed a new coupled system for efficient transformation of lignin to electricity, aromatic aldehydes and H2. A facile approach for preparing nanoflower-like catalysts has been developed by using high-current density electrodeposition of Co on Ni foam. The obtained electrode showed excellent performance for converting lignin to electricity in a direct lignin fuel cell (DLFC), and electro-oxidation of lignin to obtain aromatic aldehydes, as well as hydrogen evolution reaction in an electrolytic cell. The peak power density of the DLFC reached 196.1 mW‧cm–2. By using the DLFC as a power supply for lignin depolymerization and HER, endogenous electrons of lignin could be well transferred to water, resulting in formation of aromatic aldehydes and H2 without input of external electricity. The total aromatic aldehyde yields at the anodes of DLFC and the electrolytic cell were 2.90% and 3.34%, respectively, and the H2 production at the cathode of the electrolytic cell was 12.7 mL‧cm–2. This work thus can provide a novel system for lignin valorization and sustainable production of hydrogen.
Keywords: Lignin; electrolysis; oxidative depolymerization; aromatic aldehydes; hydrogen production 
1. Introduction
      Lignin, an amorphous aromatic polymer constructed by phenolpropane units, is a principal component of lignocellulosic biomass. It has attracted significant attention in recent years as a versatile feedstock for various industrial applications1. Traditionally lignin is regarded as a low-value by-product of the pulping industries. However, the rich aromatic structure of lignin renders it an attractive precursor for synthesis of high-value chemicals, such as aromatic aldehydes, and as a potential source for renewable energy production2. The valorization of lignin not only enhances the economic viability of biorefineries but also contributes to the sustainable utilization of biomass resources. In recent years, the direct conversion of lignin into electrical energy has emerged as a promising approach to harness its inherent redox-active properties. Direct lignin fuel cells (DLFCs) exploit the oxidative depolymerization of lignin at the anode to generate electricity3–5, simultaneously producing valuable aromatic compounds.6 This technology offers the dual advantage of energy generation and the production of high-value chemicals, positioning lignin as a pivotal component in the development of sustainable energy systems.
         On the other hand, hydrogen (H2) is recognized as the ultimate clean energy carrier, playing a crucial role in reducing carbon emissions and mitigating climate change.7 Among the various methods for hydrogen production, water electrolysis stands out as a green and straightforward technology. Currently, water electrolysis accounts for approximately 5% of the world's total H2 production.8 However, the high energy consumption associated with this process results in relatively expensive hydrogen production costs, generally exceeding $10 per kilogram. In contrast, the mature and large-scale methane reforming technology produces hydrogen at a cost of around $2 per kilogram.9 These economic challenges necessitate the development of efficient and cost-effective electrocatalysts or the construction of novel systems to reduce energy consumption and enhance energy efficiency in hydrogen production.
The traditional water electrolysis process involves the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. However, the kinetics of OER are inherently slow and complex, leading to significant energy losses. To mitigate these issues, more thermodynamically and kinetically favorable organic oxidation reactions are often employed in place of OER. This substitution reduces the anode potential and the overall energy consumption associated with hydrogen production. Biomass-derived substrates, such as glucose10, sorbitol8, cellulose 11, and lignin 11–13, can be utilized in these modified electrolysis processes due to their abundance, low cost, and lower oxidation potentials compared to OER. For instance, Liu et al.14 successfully coupled glucose oxidation with HER using nanostructured NiFeOx and NiFeNx catalysts, achieving a current density of 100 mA‧cm–2 at 1.39 V and a glucaric acid yield of 83%. Similarly, Ghatak et al.12 replaced OER with oxidation of black liquor in alkaline water electrolysis, significantly reducing the HER overpotential and enhancing reaction kinetics. Leveraging lignin as an electron donor in hydrogen production systems presents an innovative pathway to achieve sustainable hydrogen generation. By integrating lignin oxidation with HER, it is possible to develop bifunctional electrocatalysts that facilitate the simultaneous production of hydrogen and valuable aromatic aldehydes. This integrated approach not only enhances the overall energy efficiency of the system but also contributes to the valorization of lignin, aligning with the principles of green chemistry and the circular economy. Transition metals, such as cobalt (Co), have demonstrated excellent catalytic effects on lignin depolymerization and are also effective for HER15,16, making them ideal candidates for the development of such bifunctional catalysts.
Building on previous work, where an efficient DLFC being capable of generating electricity was developed 6, in the present work, we developed a novel nanoflower-structured bifunctional cobalt-based electrocatalyst (CoSx@NF) for converting lignin to electricity, and further achieving electrochemical depolymerization of lignin to aromatic aldehydes, as well as HER for hydrogen production. Using electrodeposition at a high-current density, we synthesized the CoSx@NF catalyst with nano-flower structure and outstanding electrochemical performance. The catalysts were then employed in a coupled DLFC and electrolytic cell system, which achieved transfer of the endogenous electrons of lignin to water for efficient and stable co-production of aromatic aldehydes and hydrogen without input of external electricity. This work thus can provide an easy-operated method for preparation of the bifunctional electrocatalysts, and a novel route for sustainable valorization of lignin and green hydrogen production.
2. Materials and methods
2.1 Materials
Potassium hydroxide (KOH) and cobalt chloride hexahydrate (CoCl2·6H2O) were purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China). Thiourea was purchased from Shanghai Meryer Biochemical Technology Co., Ltd (Shanghai, China). Vanadium pentoxide (V2O5) was purchased from Anhui Senrise Technology Co., Ltd (Anhui, China). Sodium hydroxide (NaOH), ammonium chloride (NH4Cl), p-hydroxybenzaldehyde, vanillin and syringaldehyde were analytic pure and purchased from Shanghai Aladdin Biochemical Technology Co., Ltd (Shanghai, China). Analytic pure concentrated hydrochloric acid and sulfuric acid were purchased from Beijing Tongguang Chemical Co., Ltd (Beijing, China). Anhydrous ethanol was purchased from Shanghai Titan (General-Reagent) Technology Co., Ltd (Shanghai, China). Pt/C powder (Johnson Matthey, 70%) was purchased from Shengernuo Energy Mall (Suzhou, China). Corn stover alkali lignin (CSAL) was purchased from Ji’nan Yanghai Environmental Protection Materials Co., Ltd (Ji’nan, China). Ni foam was purchased from Kunshan Shengshi Jingxin New Material Co., Ltd (Kunshan, China). The direct biomass fuel cell (DBFC) device was manufactured by Wuhan Zhisheng New Energy Co., Ltd. (Wuhan, China). Nafion®117 and 211 proton exchange membranes were produced by DuPont Company (DE, USA). The carbon felt was manufactured by Beijing Jinglong Special Carbon Graphite Factory (Beijing, China).
2.2 Preparation of the CoSx@NF electrode
The CoSx@Ni foam (CoSx@NF) electrode was prepared using electrodeposition method. The solution containing 0.1 mol‧L⁻1 CoCl2·6H2O, 2 mol‧L⁻1 NH4Cl and 0.2 mol‧L⁻1 thiourea was prepared as the electrolyte, and the cleaned 2.5×2.5 cm2 nickel foam was used as the working electrode, and the ruthenium-iridium-titanium mesh was used as the counter electrode. A voltage-stabilized and current-stabilized power supply was used for electrodeposition at the current density of 1 A‧cm⁻2 for 3 minutes. To ensure uniform deposition of the active layer on both sides of the electrode, the process was repeated on the other side for 3 minutes, followed by rinsing with water to obtain the CoSx@NF electrode.
The Pt/C@NF electrode was prepared by coating method. 20 mg of Pt/C powder, 10 mg of PVDF binder and the appropriate amount of NMP solvent were mixed, ground evenly with a mortar, and coated onto clean nickel foam. The electrode was then placed in an oven at 60 ℃ for 12 hours to remove the solvent. The loading of Pt/C on the electrode was 2.5 mg‧cm⁻2.
2.3 Depolymerization of lignin and hydrogen evolution reaction in electrolysis system
The device used for lignin electro-oxidative depolymerization and the hydrogen evolution reaction was a liquid flow electrolytic cell, and its structure was similar to the liquid flow fuel cell in the previous study 6. The anode electrolyte consisted of 2 g‧L⁻1 lignin solution, while the cathode electrolyte was 3 mol‧L⁻1 KOH solution. The temperature of the electrolyzer, anode reactor, and cathode reactor was maintained at 80 ℃. The electrolysis voltage was set to 1.2 V, and the reaction time was 2 h.
The electrolytic cell and DLFCs are connected to construct a coupled system. Two DLFCs with electrode area of 2×2 cm2 were connected in series to power the electrolytic cell. The DLFC anode, as well as the electrolyzer cathode and anode, were CoSx@NF electrodes, while the DLFC cathode was carbon-felt electrode. The anode reactor of the DLFC was filled with 50 mL of 2 g‧L⁻1 lignin solution, and the cathode reactor was filled with 1 L of (VO2)2SO4 solution. The anode tank of the electrolytic cell contained 50 mL of 2 g‧L⁻1 lignin solution, and the cathode electrolyte was 50 mL of 3 mol‧L⁻1 KOH solution. An electrochemical workstation was connected in parallel to both ends of the electrolytic cell to detect voltage, and the ammeter was connected in series in the circuit to monitor and record the electrolytic current. During the discharge and electrolysis reaction, the DLFC and the electrolytic cell were heated and maintained at 80 °C for 2 h.
2.4 Analytic methods
2.4.1 Physical characterization of the electrode
The anode surface morphology was characterized by scanning electron microscopy (SEM) (Hitachi S-3400 N II, Hitachi, Japan). The elements of electrodeposition modified nickel foam were detected by energy dispersive X-ray spectrometer (EDS) (Hitachi S-3400 N II, Hitachi, Japan). The ultrastructure of the electrode surface was further characterized by transmission electron microscopy (TEM) (Talos F200X, Thermo Fisher, America). Powder X-ray diffraction (XRD) data were acquired with a D/Max 2500H diffractometer (Rigaku, Japan). X-ray photoelectron spectroscopy (XPS) was conducted using a Thermo Fisher ESCALAB 250 Xi X-ray photoelectron spectrometer (MA, Thermo Fisher, America). The specific surface area of the electrode was measured using specific surface area and pore size distribution analyzer (IQ2, Quantachrome, America).
2.4.2 Characterization of lignin depolymerization products
The anolyte was sampled during the electrolysis and discharging process to detect the oxidative depolymerization products. The anolyte containing lignin was acidified with hydrochloric acid to pH 1~2. The aromatic compound monomers were quantified by ultra-performance liquid chromatography (UPLC) ACQUITY UPLC I-CLASS Plus system (Waters, USA) equipped with an Inertcore Plus C18 reversed-phase column (2.1 × 100 mm, 2.6 μm, GL Sciences, Japan). A range of concentrations of p-hydroxybenzaldehyde, vanillin, syringaldehyde, p-hydroxybenzoic acid, vanillic acid, syringic acid, p-hydroxyacetophenone, acetovanillone, acetosyringone, p-coumaric acid, ferulic acid and sinapinic acid standard solution were prepared to obtain standard curves.
2.4.3 Electrochemical measurements
The electrochemical performance tests were carried out in a typical three-electrode system by electrochemical workstation (CS1350, Wuhan Kostel Co., Ltd., China). The prepared electrode served as the working electrode, the Ag/AgCl electrode was used as the reference electrode, and the platinum mesh was used as the counter electrode. The electrolyte used 1 mol‧L⁻1 KOH solution, with 2 g‧L⁻1 alkaline lignin added as required by the experiment. The electrochemical active surface area (ECSA) was measured by cyclic voltammetry (CV) at various scan rates (2, 4, 6, 8, and 10 mV‧s⁻1) within the voltage range of –0.6~ –0.5 V vs. Ag/AgCl, and then the double layer capacitance (Cdl) was linearly fitted. Electrochemical impedance spectroscopy (EIS) testing was carried out at constant voltage of 0.45 V vs. Ag/AgCl during lignin oxidation and –1.145 V vs. Ag/AgCl during hydrogen evolution, with a voltage amplitude of 5 mV and a frequency range from 100 kHz to 0.01 Hz. The discharge performance of the DBFC was tested by linear sweep voltammetry (LSV) in a two-electrode system to obtain polarization curves. The cathode electrolyte was 0.37 mol‧L–1 (VO2)2SO4 solution with 2 mol‧L–1 H2SO4 as the supporting electrolyte. The anolyte was a 3 mol‧L–1 KOH solution, with different concentrations and types of lignin added according to experimental requirements. The test voltage ranged from zero to open circuit potential, and the scan rate was 10 mV‧s–1. The power density of the cell was calculated based on the obtained polarization curve using the formula P=UI. The hydrogen evolution polarization curve was determined using an electrochemical workstation (CHI660, Shanghai Chenhua Instrument Co., Ltd., China). Linear sweep voltammetry (LSV) was conducted at a scan rate of 5 mV‧s⁻1, with 95% internal resistance compensation applied.
2.4.4 Calculation method
Density functional theory (DFT) calculations were conducted using the Vienna ab initio simulation package (VASP 6.3.2) 17,18 with the Perdew-Burke-Ernzerhof (PBE) functional under the generalized gradient approximation (GGA).19,20 Based on the XRD and TEM results, slab models of the CoS2 (111) and (010) facets were constructed. The Ni (111) and Pt (111) facets were selected as references. The calculations utilized a 3 × 3 × 1 k-point mesh21 and a cutoff energy of 450 eV. The force tolerance for each atom and the SCF tolerance were set to 0.05 eV Å-1 and 1.0 × 10⁻5 eV, respectively. The Gibbs free energy (G) for each reaction step was calculated using the formula: G = E + ZPE - TS, where E, ZPE, T (298.15 K), and S represent the calculated free energy, zero-point energy, temperature, and entropy, respectively.
3. Results and discussion
3.1. Preparation of and characterization of CoSx@NF electrode 
3.1.1 Preparation of CoSx@NF electrode
A typical procedure for CoSx@NF is shown in Fig. 1a. Ni-based materials are well-known for their excellent good corrosion resistance and electrocatalytic activity. Ni foam, with its three-dimensional porous framework, large surface area, and high mechanical strength, is particularly suitable as a conductive base material. Additionally, studies have shown that the transition metal cobalt sulfide exhibits low hydrogen adsorption energy, good conductivity, and outstanding catalytic activity for HER16,22. Since high current-density electrodeposition can accelerate grain growth on the electrode surface, increase surface roughness, and form a porous structure that improves mass transfer and gas diffusion, resulting in improved catalytic performance9, the CoSx@NF electrode was thus prepared by using the high current density electrodeposition for application in the electrolysis system.
The performance of the electrodes prepared by electrodeposition for different time was investigated, as shown in Supporting Information Fig. S1. The current densities of all electrodes increased significantly at potential of –1.2~ –1.8V after addition of lignin, indicating that the electrodeposited active layer had strong catalytic effect on lignin oxidation. As observed in Fig. S1a, the oxidation current of lignin increased with deposition time, but after the deposition time exceeded 3 minutes, the oxidation current plateaued. SEM images of electrodes prepared under different deposition times are shown in Fig. S2. At a deposition current of 1 A·cm⁻2, the 3D microspherical nanoarray structure could be formed on the nickel foam skeleton. The microspheres obtained by deposition for 1 minute were smaller, and higher magnification image revealed that these microspheres were composed of nanosheet arrays. As the deposition time was extended to 3 minutes, the size of the microspheres grew larger, and high-magnification microscopic images showed nanoflower spherical structure formed by cross-stacked nanosheets. However, when the deposition time was extended to 6 minutes, the nanosheets within the microspheres became less distinct, and the microsphere diameter decreased. Thus, electrodeposition for 3 minutes could obtain optimal micromorphology of the cobalt active layer. Similarly, increasing the deposition current density could significantly increase the amount of deposited cobalt sulfide. As shown in Fig. S3, at a relatively low deposition current density of 0.2 A·cm⁻2, only a single layer of small microspheres was deposited on the electrode surface. However, as the deposition current increased to 1 A·cm⁻2, the microspheres on the electrode surface exhibited multi-layer morphology and increase in the size. Therefore, the appropriate deposition current determined to be 1 A·cm⁻2. Microspherical structure could significantly increase the specific surface area with a high loading capacity of the electrocatalyst. Moreover, the nanosheet structure in the nanoflower spheres further increased the specific surface area, exposing more catalytically active sites. As shown in Fig. S1b, the oxidation current of lignin indeed increased gradually with the increase in the electrodeposition current density. It indicated that by using electrodeposition at relatively high current density (1 A·cm⁻2), a large amount of active cobalt species could be deposited on the Ni foam matrix and the 3D structure of the catalysts could be well constructed. 
3.1.2. Characterization of the CoSx@NF electrode
A series of characterization techniques were employed to further analyze the structure of the CoSx@NF electrode prepared at a current density of 1 A‧cm⁻2 for 3 minutes. The microstructure of the surface active layer was observed by transmission electron microscopy (TEM), as shown in Fig. 1b-d. The surface of CoSx@NF was composed of nanosheets, which displayed locally ordered lattice fringes with interplanar spacings of 0.275 nm and 0.158 nm, corresponding to the diffraction points observed in the selected area electron diffraction (SAED) pattern (Fig. 1e). These diffraction points were attributed to the (020) and (222) crystal planes of CoS2, respectively. The energy-dispersive X-ray spectroscopy (EDS) spectrum (Fig. 1f) showed that the elements Ni, Co, O, and S were uniformly distributed on the electrode surface. The crystal phase of CoSx@NF was characterized by XRD spectrum (Fig. 1h). The diffraction peaks appeared at 44.47°, 51.82° and 76.33°, which corresponded to the standard spectrum positions of Ni (JCPDS No. 96-901-2971), and were attributed to the crystal phase of the nickel foam. XPS spectrum shown in Fig. 1i and Fig. S4a-b confirmed the presence of Ni, Co, O and S elements in the obtained electrocatalyst. After deconvolution, the XPS spectrum of Co 2p showed characteristic peaks at 780.9 eV and 796.6 eV, which was consistent with the binding energy of Co2+23, while the peaks at 779.5 eV and 794.8 eV were attributed to the chemical state of Co3+24. This indicated the presence of both Co2+ and Co3+ species on the CoSx@NF electrode. The characteristic peaks of Ni 2p at 855.7 eV and 873.5 eV corresponded to Ni(OH)225. However, due to the limited detection depth of XPS, the XPS spectrum of Ni 2p showed large noise with weak response signal (Fig. S4), indicating that Co element was the predominant metal element on the electrode surface while much less Ni element was present on the surface layer. The XPS spectrum of S 2p indicated the introduction of S and formation of CoSx during electrodeposition. These results confirmed that the CoSx catalyst layer was successfully formed on the nickel foam by electrodeposition.
In addition, the BET specific surface area of the CoSx@NF electrode and the unmodified nickel foam was carried out using nitrogen adsorption and desorption method. As shown in Fig. S5, the specific surface area of CoSx@NF was 11.10 m2‧g⁻1, being increased by 41.0% compared to the unmodified nickel foam (7.87 m2‧g⁻1). The nanoflower spheres on the surface of CoSx@NF featured hierarchical porous structure, which could significantly increase the specific surface area of the Ni foam.
 
Fig. 1. Preparation and structural characterization of CoSx@NF electrode: a Schematic diagram for preparation of CoSx@NF electrode by electrodeposition for lignin depolymerizaiton and hydrogen evolution; b-c TEM images; d HRTEM images; e SAED images; f EDS electron microscope energy spectrum images; g SEM images; h XRD spectra of CoSx@NF (upper line) and Ni (lower line); i XPS spectra of Co 2p of CoSx@NF
3.1.3 Electrochemical performance of the prepared electrocatalysts
To further investigate the catalytic performance of the prepared electrode, the electrochemical active surface area (ECSA) and electrochemical impedance spectroscopy (EIS) of the electrocatalyst were analyzed. The electrodes were subjected to cyclic voltammetry (CV) tests at different scan rates (2, 4, 6, 8, and 10 mV‧s⁻1as shown in Fig. S6a–b. The electric double layer capacitance (Cdl) was derived from the CV curve, and it was proportional to ECSA15. As shown in Fig. S6c, the Cdl of CoSx@NF was 587.38 mF‧cm⁻2, which was significantly higher than the Cdl value of unmodified nickel foam (0.56 mF‧cm⁻2). It indicates that the nanoflower spheres catalyst active layer supported on the nickel foam significantly increased the ECSA, and exposed more electrochemical active sites, thereby facilitating the electrocatalytic reaction.
Fig. S7 shows the Nyquist curves of different electrodes, where the impedance behavior reflects the reaction kinetics of the interface between the electrode and the electrolyte15. The equivalent circuit model is constructed by measuring the electrochemical impedance spectrum, and it is usually composed of solution resistance (Rs), constant phase angle element (CPE) and charge transfer resistance (Rct)14. The semicircle (capacitive reactance arc) corresponding to the high-frequency region of the Nyquist curve is associated with Rct. A smaller semicircle radius indicates a lower Rct and better conductivity of the electrode material. As shown in Fig. S7, compared with the unmodified nickel foam (Rct 480.33 Ω), CoSx@NF had a lower charge transfer resistance Rct (13.25 Ω), indicating easier charge transfer between the CoSx@NF electrode and the electrolyte, and better conductivity of the electrode material. After adding lignin, the Rct of CoSx@NF was further decreased to 9 Ω, indicating an accelerated electron transfer rate on the electrode surface and demonstrating good catalytic activity for lignin oxidation. The horizontal coordinate of the intersection of the Nyquist curve and the real axis (Z' axis) represents the solution resistance (Rs). After addition of lignin, Rs increased slightly, indicating a slight increase in the resistance of the electrolyte solution itself, suggesting that the resistance encountered by moving of ions in the lignin solution also increased slightly. However, since the Rs was relatively small, ranging from 0.98 to 1.9 Ω, it could be considered negligible compared with Rct. These results show that charge transfer at the electrode-electrolyte interface was the rate-controlling step, and thus the development of efficient electrode catalysts was particularly critical. The CoSx@NF electrode prepared by electrodeposition well reduced Rct from 480.33 Ω to 13.25 Ω, thereby demonstrating excellent electrochemical performance.
3.1.4 CoSx@NF as the anode for efficient conversion of lignin to electricity
Since the prepared CoSx@NF electrocatalyst showed good catalytic performance to catalyze the oxidative depolymerization of lignin, it was firstly used as the anode in a direct lignin fuel cell (DLFC) for direct conversion of lignin to electricity.  DLFC is a fuel cell that can directly convert lignin to electricity under mild conditions.  However, direct conversion of lignin to electricity at low temperatures is challenging, because of the recalcitrant structure of lignin to degradation by oxygen under mild conditions. To promote the kinetics of electrode reactions, in the previous works, bioinspired electron transport chains (ETCs) were constructed with redox couples as the electron mediators to increase the rate of electron transfer from lignin to oxygen thus achieving great improvement of the power density26. The anode electron mediators could be soluble redox couples such as polyoxometalates (POMs)27, methylene blue28 and ferric salts such as FeCl3 29 and ferricyanides30, while efficient solid mediators (catalysts) are metals sulfides.3 For the cathode electron mediators, various metal-based redox couples such as (VO2)2SO4, POMs, Fe(NO3)3  have been used to facilitate the electron transfer to oxygen, among which (VO2)2SO4 showed the best performance for electricity generation.30 Consequently, the CoSx@NF electrode was employed as the anode of the fuel cell system with (VO2)2SO4 as the electron mediator at the cathode for discharging testing. As shown in Fig. 2, when Nafion 117 was used as the proton exchange membrane and sodium lignosulfonate or alkali lignin as substrates, the peak power density of DLFC using CoSx@NF anode was 149.4 mW‧cm⁻2 and 113.2 mW‧cm⁻2, respectively, significantly higher than the peak power density of the cell without lignin (69.0 mW‧cm⁻2). When Nafion 211 was used as the proton exchange membrane, the discharge performance of the DLFC improved significantly, with a peak power density of 196.1 mW‧cm⁻2 when sodium lignosulfonate was used as the substrate. These discharging results indicated that the CoSx@NF anode exhibits excellent performance in DLFCs for electricity generation. 
 
Fig. 2. Direct conversion of lignin to electricity with CoSx@NF as the anode. a The working principle of the direct lignin fuel cell; b Power density of the fuel cell equipped with Nafion 117 membrane; c Power density of the fuel cell equipped with Nafion 211 membrane
3.1.5. Electro-oxidative depolymerization of lignin for producing aromatic aldehydes
The effect of CoSx@NF on electrocatalytic oxidative depolymerization of lignin was further investigated. A liquid flow electrolysis cell was used as an electrolysis device for lignin oxidative depolymerization and hydrogen evolution reaction, and CoSx@NF electrode served as both the anode and cathode. The influence of reaction conditions such as electrolysis voltage, alkali concentration, lignin concentration, and temperature on the yield of aromatic aldehydes produced by lignin oxidative depolymerization on the anode was explored.
The change in the yield of aromatic aldehydes with varying electrolysis voltage is shown in Fig. 3. As the electrolysis voltage increased from 1 V to 1.2 V, the yields of p-hydroxybenzaldehyde, vanillin and syringaldehyde exhibited an increasing trend. This increase was attributed to the higher electrolysis voltage elevating the current density and accelerating the rate of lignin oxidation on anode. However, when the voltage exceeded 1.2 V, the yield of aromatic aldehydes initially increased and then decreased, indicating that the aromatic aldehydes were over-oxidized. Notably, the yield of syringaldehyde was the highest at an electrolysis voltage of 1.1 V, reaching 1.20%. Among the three aromatic aldehydes, syringaldehyde was the most sensitive to the reaction conditions, followed by vanillin and then p-hydroxybenzaldehyde. This order of reaction activity is consistent with the reported literature.31 Overall, the optimal electrolysis voltage for the electrolysis system was determined to be 1.2 V, at which the total yield of aromatic aldehydes reached 5.53%, with yields of 0.99% for p-hydroxybenzaldehyde, 3.57% for vanillin, and 0.97% for syringaldehyde, respectively.
 
Fig. 3. Effect of electrolysis voltage on the yield of aromatic aldehydes prepared by electro-oxidative depolymerization of lignin: a p-hydroxybenzaldehyde; b vanillin; c syringaldehyde; d total aromatic aldehydes; reaction conditions: Clignin, 2 g‧L⁻1; CKOH, 3 mol‧L⁻1; T, 80 ℃; A, 2×2 cm2 electrode area; e possible reaction mechanism of lignin oxidation on the CoSx@NF anode.
The effect of alkali concentration on the yield of aromatic aldehydes is shown in Fig. S8. The yield of aromatic aldehydes increased with increasing alkali concentration. On the one hand, a higher alkali concentration increased the concentration of supporting electrolytes, reduced the internal resistance of the electrolytic cell, and increased the current density in the circuit, thereby accelerating the lignin oxidation reaction at the anode. On the other hand, OH- participated in the oxidation and cleavage of ether bonds and carbon-carbon bonds in the lignin side chain, and their increased concentration promoted the formation of aromatic aldehydes. However, when the alkali concentration exceeded 3 mol‧L⁻1, the yield of aromatic aldehydes decreased slightly, possibly due to the strong alkaline conditions causing lignin intermediates to undergo polycondensation or further oxidation of the aromatic aldehydes. Therefore, the optimal alkali concentration was determined to be 3 mol‧L⁻1.
The effect of lignin concentration on the yield of aromatic aldehydes is shown in Fig. S9. The yields of aromatic aldehydes increased as the lignin concentration decreased. When the lignin concentration was 4 g‧L⁻1, the total yield of aromatic aldehydes was only 4.18%. At this concentration, the high lignin content increased the viscosity of the electrolyte solution, leading to higher internal resistance in the electrolytic cell, and the higher lignin concentration might have blocked the catalyst pores, resulting in electrocatalyst deactivation. When the lignin concentration was reduced to 0.5 g‧L−1, the total yield of aromatic aldehydes increased to 8.10%, with yields of 1.55% for p-hydroxybenzaldehyde, 5.00% for vanillin, and 1.54% for syringaldehyde.
Reaction temperature is an important factor affecting lignin oxidative depolymerization in the electrolytic cell. As shown in Fig. S10, the yield of aromatic aldehydes increased with rising temperature. When the temperature of the electrolytic cell was 25 °C, the total yield of aromatic aldehydes was only 1.61%. However, when the reaction temperature increased to 90 °C, the total yield of aromatic aldehydes rose to 6.01%, with yields of 1.15% for p-hydroxybenzaldehyde, 3.82% for vanillin, and 1.04% for syringaldehyde. This was primarily due to the increase in temperature, which reduced the internal resistance of the electrolytic cell and enhanced the rate of lignin oxidative depolymerization. Considering that further increasing the temperature could cause damage to the ion exchange membrane of the electrolytic cell, the optimal reaction temperature was determined to be 90 °C.
3.1.6. Stability of the catalysts for lignin depolymerization
The CoSx@NF electrode was reused for five times, and the yield of the aromatic aldehydes produced by electrolysis on the anode is shown in Table S1. The yield of aromatic aldehydes did not significantly decrease after multiple uses, indicating that the catalyst has good stability in catalyzing the oxidative depolymerization of lignin. The structure of the CoSx@NF anode after the electrolytic oxidation of lignin was characterized, as shown in Fig. S11. The SEM image revealed that the electrode morphology after use was still three-dimensional microsphere structure, which was not significantly different from the newly prepared electrode, but the thickness of the nanosheets in the nanoflower sphere increased after catalyzing the lignin oxidation reaction. The XPS energy spectrum showed that its elemental composition and valence states were consistent with the newly prepared catalyst. From the deconvoluted Co 2p energy spectrum, the relative content of Co3+ increased, indicating a transformation from Co3+ to Co2+ on the electrode surface during the lignin oxidation depolymerization process. The XRD spectrum showed that the CoSx@NF anode after use still exhibited the Ni crystal phase of the nickel foam. Notably, a weak diffraction peak attributed to CoOOH was also observed. This result confirmed that the Co2+/Co3+ redox couple played an important catalytic role in the lignin oxidation depolymerization reaction.
3.2. Hydrogen evolution performance of CoSx@NF electrode
3.2.1. Comparison of hydrogen evolution performance
The hydrogen evolution performance of the electrode, influenced by electrodeposition time and current density, is shown in Fig. S12. According to the hydrogen evolution polarization curve results (Fig. S12a), the hydrogen evolution current initially increased and then decreased as electrodeposition time extended. The optimal hydrogen evolution performance was achieved at a deposition time of 3 minutes. The effect of different deposition current densities on hydrogen evolution performance during electrode preparation was also explored. As shown in Fig. S12b,  the hydrogen evolution current of the electrode gradually increased as the deposition current density rose from 0.2 A‧cm−2 to 1 A‧cm−2. This improvement was attributed to the increased catalyst loading on the electrode surface, which provided more catalytic active sites, thereby enhancing the hydrogen evolution current. Additionally, the interspace between the nanoflower spheres and the nanosheets within them facilitated the rapid escape of H2, preventing material collapse due to gas pressure during the hydrogen evolution process and contributing to the stability of the electrode material. Therefore, the optimal electrode preparation conditions were a deposition time of 3 min and a deposition current density of 1 A‧cm−2.
The hydrogen evolution polarization curves of different types of electrodes were further compared. As shown in Fig. 4, the hydrogen evolution activity of the modified NF electrode was significantly improved, and the performance of the CoSx@NF electrode was comparable to that of commercial Pt/C@NF. At lower current density (less than 250 mA‧cm-2), the hydrogen evolution overpotential of Pt/C@NF electrode was smaller than that of CoSx@NF electrode, while at higher current density (more than 250 mA‧cm-2), the hydrogen evolution overpotential of the CoSx@NF electrode was similar to or even smaller than that of the Pt/C@NF electrode. According to the results in Fig. 4a, the overpotential for the hydrogen evolution reaction of different electrodes is shown in Fig. 4b. At a current density of 10 mA·cm−2, the hydrogen evolution overpotential of the CoSx@NF electrode was only 23 mV, similar to that of the Pt/C@NF electrode (30 mV), while the hydrogen evolution overpotential of unmodified nickel foam was as high as 202 mV. At a current density of 100 mA·cm−2, the hydrogen evolution overpotential of the CoSx@NF electrode was 153 mV, much lower than that of nickel foam (369 mV), while the performance of the Pt/C@NF electrode (111 mV) was slightly better than that of the CoSx@NF electrode. These results can be explained by considering two factors: the electrochemical activity of the electrode material and its morphology. The binding energy between the catalyst and adsorbed hydrogen is a crucial factor affecting the rate of HER. According to the volcano plot curve32, Pt is located at the top of the “volcano curve” indicating that its binding energy with hydrogen is moderate, and it has the highest exchange current density, making it the best HER catalytic material. Metals located to the left side of Pt on the curve have progressively stronger hydrogen binding energy, causing the adsorbed hydrogen to bind tightly to the active sites and preventing desorption, which is detrimental to the reaction. Conversely, metals to the right of Pt have weaker binding energies, leading to less adsorbed hydrogen on the catalyst surface and slower reaction rates. Co is located on the left side of Pt, and has a binding energy similar to Pt, resulting in good catalytic activity. Additionally, the surface morphology of the electrode plays a crucial role in determining the hydrogen evolution overpotential. The CoSx@NF electrode catalyst has a large specific surface area, and its nanoporous structure allows effective gas diffusion, enhancing mass transfer performance16, which contributes to reducing the overpotential of the hydrogen evolution reaction. Consequently, the CoSx@NF electrode demonstrated excellent hydrogen evolution catalytic activity.
 
Fig. 4. Hydrogen evolution performance test of different electrodes and Gibbs free energy and bader charge analysis for HER: a polarization curve; b comparison of hydrogen evolution overpotential; c Tafel slope; d exchange current density; e Nyquist curves of CoSx@NF and Pt/C@NF cathodes in the high-frequency region at 113 mV overpotential; f Long-term stability test showing potential-time curves of hydrogen evolution electrolysis at different electrodes. g Gibbs free energy of H adsorption (ΔGH*) on CoS2 (111), CoS2 (010), Pt (111), and Ni (111); h Bader charge of adsorbed H2O on CoS2 (111), CoS2 (010), Pt (111), and Ni (111).
3.2.2. Electrochemical performance test
Fig. 4c shows the electrochemical parameters of the electrode obtained by fitting the HER polarization curve. The Tafel slope reflects the charge transfer resistance kinetics of the catalyst, and its value is inversely proportional to the charge transfer rate, which is related to the electrode reaction mechanism.14 The Tafel slope of the Pt/C@NF electrode was the lowest at only 35.4 mV‧dec−1, closely matching the value reported in the literature16, indicating that the precious metal has a faster hydrogen evolution kinetic rate. Compared with the Tafel slope of nickel foam (154.3 mV‧dec−1), the Tafel slope of the CoSx@NF electrode was reduced to 136.9 mV‧dec−1, indicating that the modified electrode promoted the hydrogen evolution rate. Based on the Tafel slope value obtained from the experimental fitting (more than 120 mV‧dec−1), it was preliminarily speculated that the Volmer step in the hydrogen evolution process might be the rate-determining step33. The exchange current density (j0), which is related to the electrode material, reflects the inherent catalytic activity of the catalyst. The j0 value of the CoSx@NF electrode was the highest at 6.71 mA‧cm−2, significantly higher than that of the unmodified nickel foam (0.43 mA‧cm−2) and the Pt/C@NF electrode (1.22 mA‧cm−2), indicating that the electrode had strong depolarization ability (Fig. 4d).
The electrochemical impedance spectrum was fitted using the Randles equivalent circuit model34, and the results are shown in Fig. S13a and Fig. 4e. Compared with the unmodified nickel foam (Rct was 96.44 Ω), the CoSx@NF electrode exhibited a much lower charge transfer resistance (Rct was 4.08 Ω), indicating enhanced conductivity and accelerated charge transfer. This improvement might be attributed to its large active area and abundant catalytic active sites, leading to better hydrogen evolution performance. Additionally, the Pt/C@NF electrode had an even lower Rct value (2.02 Ω), confirming that the precious metal was indeed highly active in catalyzing the hydrogen evolution reaction. The impedance of the CoSx@NF electrode was measured at different overpotentials. As shown in Fig. S13b, as the overpotential increased, the capacitive arc radius gradually decreased, indicating a reduction in charge transfer resistance. Thus, increasing the polarization potential was conducive to accelerating the hydrogen evolution reaction rate. This phenomenon was consistent with the conclusion reported by Liao et al.35 Compared with the catalytic performance of related hydrogen evolution electrodes reported in recent years, as shown in Table S2, the CoSx@NF electrode developed in this study had excellent catalytic performance, especially low overpotential.
3.2.3. Electrode stability assessment 
The structure of the CoSx@NF cathode after the electrolytic hydrogen evolution reaction was characterized, as shown in Fig. S14. The CoSx@NF cathode retained its three-dimensional microsphere stacking structure after use, similar to the morphology of the newly prepared electrode. However, the nanosheets in the microstructure became slightly thicker after catalyzing the hydrogen evolution reaction. The XPS spectrum of the CoSx@NF cathode after use was similar to that of the newly prepared catalyst, and the XRD spectrum was still consistent with the Ni crystal phase of the nickel foam, indicating that the crystal structure was relatively stable. These results demonstrate that the CoSx@NF electrode had excellent mechanical stability.
To test the stability of the electrode, long-term electrolysis experiment was conducted, with the results shown in Fig. 4f. At the constant current density of 10 mA‧cm−2, the hydrogen evolution catalytic activity of the CoSx@NF electrode did not decay significantly over an 80-hour operation period. The hydrogen evolution overpotential remained between 96 and 120 mV, and no significant shedding of active layer on the electrode surface was observed after electrolysis, indicating that the electrode has excellent electrocatalytic stability. Additionally, the hydrogen evolution overpotentials of several other electrodes were compared. The Pt/C@NF electrode exhibited superior hydrogen evolution performance with an overpotential of only 62 mV. The overpotential of nickel foam was as high as 361 mV, while the CoSx@NF electrode reduced the overpotential by approximately 253 mV, demonstrating that the energy consumption of the hydrogen evolution half reaction could be reduced by about 70%.
3.2.4. Density functional theory study
To further investigate the mechanism behind the superior HER catalytic performance of the CoSx@NF catalyst, we conducted density functional theory (DFT) calculations. The theoretical models were constructed based on TEM and XRD experimental results, with detailed structural models of the CoS2 (111) facet, CoS2 (010) facet, Ni (111) facet, and Pt (111) facet shown in Fig. S15. Adsorption of H atoms and H2O was added to the respective models, and structural relaxation was performed to obtain the corresponding energy configurations.
The Gibbs free energy of H adsorption (ΔGH*) is considered a critical indicator for evaluating HER catalysts. For an effective HER catalyst, the adsorption and desorption of H should be thermodynamically balanced. The closer the ΔGH* is to zero, the higher the expected HER activity. As shown in Fig. 4g, the Gibbs free energies for H adsorption on the CoS2 (111) and (010) facets are 0.26 eV and 0.24 eV, respectively, which are closer to the −0.15 eV value for the Pt (111) facet and significantly lower than the −0.61 eV for the Ni (111) facet. The small positive ΔG values indicate weak H adsorption on these CoS2 facets, which, compared to the strong H adsorption on Ni (111), is more favorable for the HER process.
The dissociation of water is another crucial step in the HER process. Fig. 4h shows the Bader charges of H2O on these four different surfaces, where the Bader charges on the CoS2 (111) and (010) facets are +0.013 e⁻ and + 0.048 e⁻, respectively, while the Ni (111) and Pt (111) facets exhibit Bader charges of +0.039 e⁻ and + 0.101 e⁻, respectively. This indicates that during adsorption, electrons are transferred from H2O to the catalyst. The higher Bader charge of H2O on the CoS2 (010) facet compared to that on Ni (111) suggests a water activation ability closer to that of Pt, which favors the dissociation process of water.
In conclusion, both experimental and theoretical results confirm that the CoSx@NF catalyst optimizes the H adsorption and H2O dissociation processes, thereby exhibiting excellent HER activity.
3.3 Coupling direct lignin fuel cell with electrolysis system 
Because the CoSx@NF showed excellent performance on lignin oxidation and HER, a coupled system was further constructed by coupling DLFC with electrolysis system (Fig. 5a) to achieve co-production of aromatic aldehydes and H₂ through transfer of the endogenous electrons from lignin to water.  In this coupled system, DLFC provides electricity to the electrolysis cell, resulting in the formation of aromatic aldehydes at the anodes of both the fuel cell and the electrolysis cell, while H2 was generated at the cathode of the electrolysis cell. The primary driving force of the system is the Gibbs energy change of lignin oxidation by VO2+. Such a coupled system can achieve production of high added-value products (aromatic compounds) and green hydrogen without using external electricity. 
Before running the coupling system, we investigated the effects of lignin oxidation on the electrolytic current of hydrogen evolution in a flow electrolysis cell. As shown in Fig. 5a, at room temperature and voltage of 1.2 V, the current was nearly zero within the first hour in 1 mol‧L−1 KOH electrolyte, indicating that no reaction took place in the electrolysis system. Then, 2 g‧L−1 lignin was added to the anode solution, and the current density increased rapidly and significantly. When the reaction temperature was increased to 80 °C, the increase in the current density was even more pronounced. Additionally, the current exhibited a wavy pattern. This was due to the integration of a negative feedback temperature controller with the heating device of the electrolytic cell, resulting in heat exchange between the surrounding environment and the electrolytic cell. Consequently, the temperature fluctuated around the set target temperature, causing the fluctuation trend of the current density. Initially, without lignin, the current was large within the first hour. This was because the active species of the electrocatalyst underwent rapid oxidation at this electrolysis voltage. Once the active species reached equilibrium, the hydrogen evolution current remained stable at a current density of 2.85 mA‧cm⁻2, indicating that oxygen evolution reaction occurred at the anode at this voltage. When lignin was added after 1 h, the initial current was significantly enhanced due to the oxidation of the functional groups of lignin that were easy to oxidize. As the reaction proceeded, the concentration of highly reactive substances in lignin decreased, and the current gradually declined.
Fig. 5(b) shows the change in current density with and without adding lignin at the beginning of the reaction during 2h continuous electrolysis. The system of lignin oxidation coupled with HER exhibited higher current density than water electrolysis at the same potential. At room temperature, the average current density increased from 0.35 mA‧cm−2 to 2.03 mA‧cm−2 after adding lignin, while the average current density increased from 3.63 mA‧cm−2 to 5.83 mA‧cm−2 at 80 ℃. Since the cathode electrolyte was KOH solution, hydrogen production followed Faraday's law36. Therefore, when the electrolysis voltage applied to the electrolytic cell was 1.2 V at room temperature, the H2 production capacity reached 0.845 mL‧h−1‧cm−2 (electrode area) when lignin was added, representing a 479% increase in hydrogen production efficiency compared to the control system without addition of lignin (0.146 mL‧h−1‧cm−2). At 80 ℃, the H2 production capacity was 2.435 mL‧h−1‧cm−2, which was 61% higher than that of control system (1.515 mL‧h−1‧cm−2). 
 
Fig. 5 Schematic and performance evaluation of the system for lignin oxidation coupled with hydrogen evolution: a Schematic diagram of the coupling system (DLFC and electrolytic cell); b Effect of lignin addition after 1 hour on hydrogen evolution electrolysis current; c ontinuous electrolysis for 2 hours with and without lignin addition at room temperature and 80°C. Yields of aromatic aldehydes prepared by depolymerization of lignin at the DLFC anode and the electrolytic cell anode in the coupling system: d p-hydroxybenzaldehyde; e vanillin; f syringaldehyde; g total aromatic aldehydes; the reaction conditions of DLFC and electrolytic cell: Clignin, 2 g‧L−1; CKOH, 3 mol‧L−1; A, 2×2 cm2 electrode area
Two DLFCs each with an electrode area of 2×2 cm2 were connected in series to power the electrolysis cell device. As shown in Fig. S16, the electrolysis voltage was maintained between 1.3 and 1.4 V, and the electrolysis current density ranged from 10 to 25 mA‧cm−2. The current density was relatively high at the early stage due to the high initial lignin concentration in the DLFC and the electrolysis cell. As the reaction proceeded, the lignin concentration decreased, resulting in a corresponding decrease in current density. For comparison, the current density during constant voltage electrolysis with an electrochemical workstation as a power source was recorded. The current density of the coupled system was within the range of that obtained in constant-voltage electrolysis at 1.3 V and 1.4 V, indicating that the coupled system could achieve a similar performance to the standalone electrolysis system.
Fig. 5d-g shows the yield of aromatic aldehydes in the coupled system. It is clear that the coupled system successfully achieved oxidative depolymerizaiton of lignin to produce aromatic aldehydes while simultaneously generating H2 at the cathode of the electrolytic cell. For the anodic reaction of DLFC, the highest total aromatic aldehyde yield was observed at 20 minutes, reaching 2.90%, with yields of 0.82% for p-hydroxybenzaldehyde, 1.60% for vanillin, and 0.48% for syringaldehyde, respectively. For the anodic reaction in the electrolysis cell, the highest total aromatic aldehyde yield was observed at 30 minutes, reaching 3.34%, with yields of 0.75% for p-hydroxybenzaldehyde, 1.95% for vanillin, and 0.64% for syringaldehyde, respectively. At this time, H2 production at the cathode reached 12.7 mL‧cm−2. These results confirm the feasibility of the coupled system to produce aromatic aldehydes and H2. Based on the optimization results for the production of aromatic aldehydes, an electrolysis voltage of 1.2 V was found to be preferable. Therefore, the electrolysis voltage could be further adjusted by connecting a variable resistor box in series in the circuit to achieve a higher yield of aromatic aldehydes. 
4. Conclusions
An efficient bifunctional electrocatalyst, CoSx@NF with 3D nanoflower spheres morphology composed of nanosheets was developed through a high-current density electrodeposition process. The catalyst showed good performance for anodic oxidation of lignin to obtain aromatic aldehydes as well as cathodic HER to produce H2. The unique 3D structure endowed the catalysts a high specific surface area, and the hierarchical porous nature facilitated the rapid escape of gas, thereby enhancing the material's stability. The CoSx@NF could well work as an anode in a DLFC to effectively convert lignin to electricity with maximal power density of196.1 mW‧cm⁻2 at 90 °C. The CoSx@NF electrode also showed good performance in an electrolytic cell to catalyze the electro-oxidative depolymerization of lignin, yielding aromatic aldehydes. At an electrolysis voltage of 1.2 V and  lignin concentration of 0.5 g‧L⁻1, the total yield of aromatic aldehydes reached 8.10%. Additionally, the CoSx@NF cathode demonstrated excellent hydrogen evolution catalytic activity. At current densities of 10 mA‧cm⁻2 and 100 mA‧cm⁻2, the hydrogen evolution overpotentials were only 23 mV and 153 mV, respectively. Pairing lignin oxidation with the hydrogen evolution process resulted in higher current density at the same electrolysis potential. A novel coupled system was further developed by coupling the DLFC with the paired electrolytic cell to achieve co-production of aromatic aldehydes and H2 without input of external electricity. The highest yield of total aromatic aldehydes at the anodes of the DLFC and electrolytic cell were 2.90% and 3.34%, respectively, with H2 productivity of 12.7 mL‧cm⁻2 at the cathode of the electrolytic cell, demonstrating the success of the coupled system. This work thus may provide novel idea to develop green system for conversion of lignin to high added-value chemicals and renewable fuels. 

Author contributions
Yichen Zhang: methodology, formal analysis, investigation, resources, data curation, writing-original draft, and visualization. Daihong Gao: methodology, formal analysis, investigation, data curation, visualization and writing-original draft. Denghao Ouyang: formal analysis, investigation, and visualization. Binhang Yan: resources, software, formal analysis. Xuebing Zhao: conceptualization, resources, funding acquisition, project administration, supervision, writing—review and editing.
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Comment#2. References cited in the SI pay attention to formatting issues and it is recommended that they remain consistent with those in the main text. For example, journal names are italicized, etc.

Comment#3. In the main text, there is an inaccuracy in the figure number when describing Figure 5. For example: “As shown in Fig.
5a, at room temperature and voltage of 1.2 V, the current was nearly zero within the first hour in 1 mol‧L−1 KOH electrolyte, indicating that no reaction took place in the electrolysis system.
Then, 2 g‧L−1 lignin was added to the anode solution, and the current density increased rapidly and significantly”. This description should actually correspond to Fig. 5b. It is recommended to conduct a careful check.

Comment#4. There are also some formatting issues in the main text. For instance, there should be a space between the number and the unit. In line 548 of the main text, there is no space between "2" and "h" in "2 h".
In line 552, the symbol for degrees Celsius is not in the Time New Roman font. In line 559, there is a lack of a space in the expression "of196.1…". There are other similar formatting issues in the main text. We won't list them one by one. Please check carefully.