Advancements in Cobalt-Based Catalysts for Enhanced CO₂ Hydrogenation: Mechanisms, Applications, and Future Directions: A Short Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Hydrogenation of CO₂ is one of the most important issues related to mitigation of environmental hazard caused by the increasing cocentration of this greenhouse gas in the atmosphere. Cobalt offers unique possibilities in heterogeneous catalysis with concern to both its effectiveness, availability and price. Nevertheless, despite the extreme numbers of studies there is still no clear conception of the ways one may capitalize on these properties of cobalt catalysts. This is the first and perhaps most important conclusion of the present review. Accordingly, much research has to be devoted to developing active, selective and durable cobalt-based catalysts. All this supports that the topic of the review is timely and important. In accord, the references (totalling at 111) are mostly recent, with 56 citations from the 2020-2024 period, with 13 from 2024 alone.

As sometimes said, heterogeneous catalysis is more an art than science. This opinion is supported by the fact, that properties of a catalyst depend an being mono- or multimetallic, supported or unsupported, where metal-metal and metal-support intercations may have decisive role in the catalysis. And then the role of additives is not even mentioned. One of the best features of the present review is in that it takes the reader form the simplest (bit still not at all simple) catalysts to the much more complicated bi-, or multimetallic ones supprted on various supports from the well known silica or alumunium oxide to the more sophisticated MOF-s. As much as possible, this tour is guided by the available kinetic and mechanistic informations. The figures and especially the tables are much informative.

Comments:

1) Although from practical (industrial) use heterogeneous catalysts are much preferred over homogeneous (soluble) ones, a short paragraph with comparison of the role of cobalt-based catalysts in heterogeneous and homogeneous catalysis could give valuable insight to readers to the mechanism of cobalt-based CO₂ hydrogenations.

2) Surpisingly, results of theoretical calculations are missing from the manuscript. True, in the „Future perspecives” section the authors themselves suggest more use of the results of calculations, as an already established way to get more information on very complex and experimentally highly demanding systems.

3) One of the key points in catalysis is the long-time durability of the catalysts. This review does not turn much attention on this feature, and gives only one example of long-time stability of a specific catalyst (line 612).

4) While the paper is written in good and readable English, the sentence in lines 581-582 is incorrect.

5) In line 580 „CO₂ hydroformylation” is mentioned. In priciple, such a reaction could supply glyoxylic acid as product, If it is really so, please treat it in more detail since this rection is not discussed elsewhere in the text.

6) There is a language error in line 456, and Pd0 is used instead of Pd⁰ in line 464.

7) I certainly cannot recommend the use of the units of catalytic efficiency as, e.g. „0.096 gMeOH kg cat⁻¹ h⁻¹” (l. 551) or „the yield was … 0.54 mmol g cat⁻¹ h⁻¹”. This may be accepted as laboratory slang but does not agree with the mass specific activity of the catalyst.

Comments on the Quality of English Language

Comments in the overall report

Author Response

Question 1:

Although from practical (industrial) use heterogeneous catalysts are much preferred over homogeneous (soluble) ones, a short paragraph with comparison of the role of cobalt-based catalysts in heterogeneous and homogeneous catalysis could give valuable insight to readers to the mechanism of cobalt-based CO₂ hydrogenations.

I would like to express my sincerest gratitude for your insightful comments. Homogeneous catalysts represent a crucial component of CO₂ hydrogenation catalysts, and we extend our apologies for having previously failed to adequately address this aspect. In response to your suggestion, we have included a brief discussion of homogeneous catalysts in the introductory section and have compared them with heterogeneous catalysts, as well as made the necessary adjustments to the paragraphs to ensure that the content is presented in an appropriate manner. The aforementioned discussions will provide a more rigorous logical approach to the rationale behind the selection of non-homogeneous thermal catalysts for review in this paper. Secondly, in the section entitled 'Mechanisms', we provide a brief overview of the mechanism of Co catalysts for homogeneous CO₂ hydrogenation in comparison with non-homogeneous catalysts. Given the dearth of literature on first-row transition metals employed in homogeneous reactions (a point explicitly highlighted in some of the literature，e.g.‘Jeletic, M. S., Helm, M. L., Hulley, E. B., Mock, M. T., Appel, A. M., & Linehan, J. C. (2014). A cobalt hydride catalyst for the hydrogenation of CO₂: pathways for catalysis and deactivation. ACS Catalysis, 4(10), 3755-3762.’ and ‘Onishi, N., & Himeda, Y. (2022). Homogeneous catalysts for CO₂ hydrogenation to methanol and methanol dehydrogenation to hydrogen generation. Coordination Chemistry Reviews, 472, 214767.’), we have striven to synthesise the prevailing perspectives from the limited literature we have collated (see yellow highlights). We hope that the contributions we have made to this section will provide a comprehensive overview. We trust that our contributions to this section will meet with your approval.

 

Question 2:

Surpisingly, results of theoretical calculations are missing from the manuscript. True, in the “Future perspecives” section the authors themselves suggest more use of the results of calculations, as an already established way to get more information on very complex and experimentally highly demanding systems.

We would like to express our sincerest gratitude for your invaluable feedback. Simulation calculations are an invaluable tool for studying reaction mechanisms, and they are frequently employed in conjunction with characterisation techniques such as in-situ DRIFTS and in-situ FTIR spectroscopy to elucidate reaction pathways. It should be noted that some of the references included in the mechanism section have been derived from calculations, although they have not been explicitly labelled as such. In Chapter 3 of the revised article, we have explicitly stated that model calculations are an invaluable tool for mechanistic studies. Furthermore, we have incorporated a detailed discussion of the original references, along with new calculations, into the revised article (see yellow highlights). We trust that you will be satisfied with our revised article.

 

Question 3:

One of the key points in catalysis is the long-time durability of the catalysts. This review does not turn much attention on this feature, and gives only one example of long-time stability of a specific catalyst (line 612).

I would like to express my gratitude for your insightful comments. We would like to begin by apologising for the lack of rigour in our presentation. In the paper, we put forward the view that the Co-based catalysts are relatively better in terms of stability. This is based on a comparison of the catalysts with the Ni, as well as other catalysts, as cited in [63], [64], [66] and [107]. The Cu catalysts revealed that the Co catalysts, in comparison with Ni and Cu catalysts, exhibited a reduced formation of coke on the surface during the CO₂ hydrogenation reaction. Additionally, they demonstrated a diminished propensity for carbon and sintering issues, along with a comparatively enhanced stability when compared to Ni and Cu catalysts. In light of the comments provided, we have revised the relevant sections of the article with the aim of enhancing its rigour. We would like to thank you once more for your comments, which have helped us to improve the quality of our article.

 

Question 4:

While the paper is written in good and readable English, the sentence in lines 581-582 is incorrect.

We sincerely thank you for your careful reading, and we have corrected the sentences that appeared to be incorrect (see yellow highlights).

 

Question 5:

In line 580 “CO₂ hydroformylation” is mentioned. In priciple, such a reaction could supply glyoxylic acid as product, If it is really so, please treat it in more detail since this rection is not discussed elsewhere in the text.

We apologise for the error we made due to carelessness, it was not ‘CO₂ formylation’ but ‘CO₂ methanation’ that was intended and we have corrected it. Thank you for your careful reading and for pointing out our mistake.

 

Question 6:

There is a language error in line 456, and Pd⁰ is used instead of Pd0 in line 464.

Thank you very much for your comments. However, there is no place in our text where we use the expression ‘Pd0’, and we found the article near where you located it that incorrectly wrote ‘Fe⁰’ as ‘Fe0’, which we think you are referring to this error, which we have corrected (see yellow highlights).

 

Question7:

 I certainly cannot recommend the use of the units of catalytic efficiency as, e.g. „0.096 g_MeOH kg _cat^-1 h^-¹” (l. 551) or the yield was … 0.54 mmol g _cat^-¹ h^-¹”. This may be accepted as laboratory slang but does not agree with the mass specific activity of the catalyst.

I am grateful for your meticulous examination of this matter. It is acknowledged that the units employed in these sections of the text are not sufficiently rigorous; they have been revised accordingly.

Reviewer 2 Report

Comments and Suggestions for Authors

Major Comments

1) Motivation to study Co over other metals is not very well defined in the main body of the paper. The authors mention it in their abstract (Line 16), but since the primary focus of this review is Co based catalyst, they should include a short paragraph in the introduction high lighting the benefit of Co catalysts over others. There is some discussion in lines 121-125 but references need to be included.

2) Authors only examine heterogeneous Co catalysts (Line 73). There are clear advantages to heterogenous catalysts, but this reviewer believes that summarizing the current developments in homogeneous catalysts will be beneficial to the purpose of the review. Some good reference could include (but not limited to) "Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations, Weiping Liu, Basudev Sahoo, Kathrin Junge, and Matthias Beller, Accounts of Chemical Research 2018 51 (8), 1858-1869"

3) In section 4, the authors discuss various mechanisms and relative performances for Co-based catalysts. There is a lot of good but scattered discussion (spread out between lines 345-417) around the catalyst/support interactions that dictate selectivities and yields. This reviewer believes that clubbing all the active site/support interactions discussion under one sub-section in section 4 would be beneficial to researchers looking to select the optimal substrate for their catalyst. Some good references to include here would be "Parastaev, A., Muravev, V., Huertas Osta, E. et al. Boosting CO2 hydrogenation via size-dependent metal–support interactions in cobalt/ceria-based catalysts. Nat Catal 3, 526–533 (2020)." And " J. Díez-Ramírez, P. Sánchez, V. Kyriakou, S. Zafeiratos, G.E. Marnellos, M. Konsolakis, F. Dorado, Effect of support nature on the cobalt-catalyzed CO2 hydrogenation, Journal of CO2 Utilization, Volume 21,2017,Pages 562-571"

4) Section 5.1 "Conclusion" should be renamed as "limitations" since that is most of the discussion in that section. Labelling it as "Conclusion" ends up negating the necessity of the extensive review presented in the manuscript. One more limitation to examine in this section is the availability of sustainably obtained hydrogen and how that impacts the endeavors of this valorization.

Minor Comments

1) In the abstract, the authors allude to China's goal of "achieving carbon peak and carbon neutrality by 2020". Since its 2024, introducing the paper with this goal makes it less convincing. The authors should consider rewording their motivation for this very important climate concern.

2) There seems to be a typo in lines 581-582, where the authors have quoted "The conversion of LaCo₀.₇Ga₀.₃O₃ to CO₂ was 9.8%, with 74.7% selectivity to alcohols and 88.1% ethanol content in the alcohol mixture". I believe they meant " The conversion over LaCo₀.₇Ga₀.₃O₃ of CO₂ was 9.8%, with 74.7% selectivity to alcohols and 88.1% ethanol content in the alcohol mixture". The reviewer requests the authors to review their manuscript carefully for more typos that might cause confusion.

Comments on the Quality of English Language

English language is fine but will need a review to look for typos that might lead to confusing or misleading conclusions. I have included this feedback in Minor comment #2.

Author Response

Response:

We greatly appreciate your time and effort in reviewing our manuscript! We have conducted another thorough check to ensure that any remaining expressions are appropriately revised. Once again, we express our sincere gratitude for your diligent review and guidance.

 

Question 1:

Motivation to study Co over other metals is not very well defined in the main body of the paper. The authors mention it in their abstract (Line 16), but since the primary focus of this review is Co based catalyst, they should include a short paragraph in the introduction high lighting the benefit of Co catalysts over others. There is some discussion in lines 121-125 but references need to be included.

Thank you for your valuable suggestion. In response to your suggestion, we have added a short paragraph in the Introduction, lines 92-102, to describe the advantages of Co catalysts and the breakthroughs made in recent years, as well as supplementing the references in the original lines 121-125(see yellow highlights). In addition, it is added that Co catalyst is a catalyst with more average advantages in all aspects, strong CO₂ activation ability at low temperature, strong hydrogenation ability, better stability and lower cost. The reason why it has not received as much attention as Cu, Ni, In, Pd, etc. in CO₂ hydrogenation may be that it is less expensive compared to the precious metals, but more expensive and less abundant for Ni, Fe, In, which are also non-precious metals. Therefore, rather than using Co catalysts for the synthesis of its dominant product methane, we would expect it to be able to exploit the ability to grow carbon chains that it has demonstrated in Fischer-Tropsch reactions, catalysing the CO₂ hydrogenation of products such as hydrocarbon-olefins, long-chain alkanes, and higher-carbon alcohols, as discussed in Sections 4.2 and 4.3 of this paper. This is the reason why we mention in this supplement the breakthrough of Co-based catalysts in obtaining long-chain products.

 

Question 2:

Authors only examine heterogeneous Co catalysts (Line 73). There are clear advantages to heterogenous catalysts, but this reviewer believes that summarizing the current developments in homogeneous catalysts will be beneficial to the purpose of the review. Some good reference could include (but not limited to) "Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations, Weiping Liu, Basudev Sahoo, Kathrin Junge, and Matthias Beller, Accounts of Chemical Research 2018 51 (8), 1858-1869"

Thank you very much for your suggestion and the references you have given, which is a very good and rigorous introduction to the new cobalt complex homogeneous hydrogenation catalysts being used in various types of hydrogenation reactions, and following the guidance of this paper we have found many more related papers that provide an in-depth understanding of the application of homogeneous catalysts in CO2 hydrogenation reactions. Homogeneous catalysts are a very important part of CO2 hydrogenation catalysts, and we apologise for neglecting this before. In response to your suggestion we have briefly discussed homogeneous catalysts and compared them with non-homogeneous catalysts prior to the original 73 lines, as well as adjusting the paragraphs to suit the content(see yellow highlights). These discussions will provide a more rigorous and logical thought process as to why non-homogeneous thermal catalysts were chosen for review in this paper.

 

Question 3:

In section 4, the authors discuss various mechanisms and relative performances for Co-based catalysts. There is a lot of good but scattered discussion (spread out between lines 345-417) around the catalyst/support interactions that dictate selectivities and yields. This reviewer believes that clubbing all the active site/support interactions discussion under one sub-section in section 4 would be beneficial to researchers looking to select the optimal substrate for their catalyst. Some good references to include here would be "Parastaev, A., Muravev, V., Huertas Osta, E. et al. Boosting CO₂ hydrogenation via size-dependent metal-support interactions in cobalt/ceria-based catalysts. Nat Catal 3, 526-533 (2020)." And " J. Díez-Ramírez, P. Sánchez, V. Kyriakou, S. Zafeiratos, G.E. Marnellos, M. Konsolakis, F. Dorado, Effect of support nature on the cobalt-catalyzed CO₂ hydrogenation, Journal of CO₂ Utilization, Volume 21,2017,Pages 562-571"

Thank you very much for your comments, we have reorganised chapter 4.1 according to your comments, and concentrated all the discussion of active site/carrier interactions into one subsection(see yellow highlights). We are also grateful for the references you gave, which we have cited, and which have enriched our discussion of active site/carrier interactions and fleshed out our ideas. The structural changes you gave helped us to make the article flow more logically and at the same time make it easier for researchers to get the information they need from the article.

 

Question 4:

Section 5.1 "Conclusion" should be renamed as "limitations" since that is most of the discussion in that section. Labelling it as "Conclusion" ends up negating the necessity of the extensive review presented in the manuscript. One more limitation to examine in this section is the availability of sustainably obtained hydrogen and how that impacts the endeavors of this valorization.

①Thank you very much for your suggested change to the title of chapter 5.1, he does fit better with what we are exploring in this chapter, and I have revised the title of this chapter based on your suggestion(see yellow highlights).②Thank you very much for your comments, which have added a very important piece to our discussion of the limiting factors of CO₂ hydrogenation reactions. The ideal source of sustainable hydrogen from feedstock gas was envisioned in the introduction(see green highlights), where ideally we could sustainably obtain hydrogen by electrolysis of water, with renewable energy sources including hydro, geothermal, wind, and solar power generating the energy to sustain the process, and the by-product water from the CO₂ hydrogenation reaction being separated out to be used in the water electrolysis process. However, there are many loopholes in the implementation of this scenario, so we have discussed it as a limiting factor in this subsection based on your suggestion(see yellow highlights).

 

Question 5:

 In the abstract, the authors allude to China's goal of "achieving carbon peak and carbon neutrality by 2020". Since its 2024, introducing the paper with this goal makes it less convincing. The authors should consider rewording their motivation for this very important climate concern.

First of all, thank you very much for your tolerance, and I am humbled by the fact that we made an incorrect statement in the first sentence of the abstract. What we want to say here is not ‘achieving carbon peak and carbon neutrality by 2020’, but ‘introducing a goal of carbon peak and carbon neutrality in 2020’, which we have amended. Secondly, according to the information we have gathered, on 22 September 2020, at the 75th session of the United Nations General Assembly, China proposed for the first time a ‘dual-carbon’ goal, that is, to achieve carbon peak by 2030 and strive to achieve carbon neutrality by 2060. At the same time, according to the Fourteenth Five-Year Plan and the Vision 2035 programme, in the future, China will restructure the energy pattern of society and gradually replace traditional fossil fuels with green energy. CO₂ hydrogenation can achieve the recycling of carbon sources, mitigate the greenhouse effect and synthesise fuels and chemicals, which is conducive to the realisation of the ‘dual-carbon’ goal. CO₂ hydrogenation reaction is an important way to realise the ‘dual-carbon’ goal, and the proposed goal has also promoted the rapid development of CO₂ hydrogenation reaction research.

 

Question 6:

There seems to be a typo in lines 581-582, where the authors have quoted "The conversion of LaCo_0.7Ga_0.3O₃ to CO₂ was 9.8%, with 74.7% selectivity to alcohols and 88.1% ethanol content in the alcohol mixture". I believe they meant " The conversion over LaCo_0.7Ga_0.3O₃ of CO₂ was 9.8%, with 74.7% selectivity to alcohols and 88.1% ethanol content in the alcohol mixture". The reviewer requests the authors to review their manuscript carefully for more typos that might cause confusion.

We were really sorry for our careless mistakes. Thank you for your reminder. We have amended the misrepresentation here.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript under consideration is a review paper devoted to cobalt catalysts for reactions of carbon dioxide with hydrogen.

The Reviewer believes that this paper should not be accepted in the present form because of the following:

1. Almost identical review was recently published in Catalysts, namely "Catalytic Hydrogenation of CO2 to Methanol: A Review" by M.Ren et al., Catalysts, 2022. The current manuscript is 90% devoted just to methanol synthesis.

2. Thermodynamic considerations, which are discussed in Chapter 2, are not relevant and in some cases mislead drastically. In particular, the statement in lines 149-150 gives an incorrect idea of why the methanol synthesis is an exothermal process while the RWGS is endothermal. In fact, it has nothing to do with temperature range.

3. The problem of mechanism is not addressed properly. Indeed, a common approach teaches that RWGS is the first step of any H2-CO2 interaction, so then everything is a "syngas chemistry" complicated by RWGS side effects. The authors do not argue, just cite a couple of works where alternative formiatic mechanism is considered for very specific cases. No discussion introduced.

4. The Reviewer is generally concerned with terminology, especially with application of the term "hydrogenation" to C1 chemistry reactions of formation of hydrocarbons, methanol and higher alcohols. The Reviewer believes that for industrially important processes it is advisable to be keen on terminology, which is common among industry chemists. It would be scholastic to call "hydrogenation" all the reactions where hydrogen is involved. CO2 hydrogenation is a process of methane formation from CO2 and H2, just that. The Reviewer insists on that despite numerous publications where almost any reaction with hydrogen participation is called "hydrogenation". The authors have to cite numerous sources and show illustrations where "hydrogenation to CO" is called in a common way "reverse water gas shift reaction" or RWGS.

Author Response

Response:

We greatly appreciate your time and effort in reviewing our manuscript! We have conducted another thorough check to ensure that any remaining expressions are appropriately revised. Once again, we express our sincere gratitude for your diligent review and guidance.

 

Question 1:

Almost identical review was recently published in Catalysts, namely "Catalytic Hydrogenation of CO₂ to Methanol: A Review" by M.Ren et al., Catalysts, 2022. The current manuscript is 90% devoted just to methanol synthesis.

First of all, thank you very much for this excellent article and we are very sorry to have missed such a shining pearl in our collection of literature. The content of this article has given us a deeper understanding of the development of CO₂ hydrogenation to methanol and a clearer understanding of the reaction mechanism, which has helped us to revise this article and we have included references to this article in the revised article.

Secondly, we feel that you may have some misunderstanding about the similarity between the two articles; we do not believe that this article is very similar to the existing articles, so please allow us to explain our reasons here.

1)The articles as a whole have different research targets. This article focuses on the study of cobalt-based catalysts in the application of CO₂ hydrogenation for the synthesis of methane, long-chain hydrocarbons, and alcohol chemicals, whereas the existing literature mainly discusses the progress of research on the synthesis of methanol products by CO₂ hydrogenation.

2) Catalyst Differences. The types of catalysts discussed in this paper are limited to cobalt-based catalysts, which are classified by the differences in the products generated, and then the effects of metal additives, carriers, morphological structures and other factors on the reaction performance are stated in paragraphs within the chapters. The types of catalysts discussed in previous papers have focused on the study of Cu-based, In-based and alloy catalysts for methanol synthesis, which is a much wider range of catalyst types.

3) Differences in reaction mechanism: Firstly, this paper combines the statement of reaction mechanisms such as metal-carrier interaction, interfacial effect, size effect, and oxygen vacancy with the catalyst design ideas, and does not summarise them in a separate chapter. In contrast, metal-carrier interaction, interfacial effect, size effect, and oxygen vacancy are summarised as separate subsections in the chapter on reaction mechanisms. Secondly, it is argued that there are three reaction paths of Co-based catalysts involved in CO₂ hydrogenation, the formic acid path, the reverse water gas shift (RWGS)+CO hydrogenation(FTS) path , and the direct hydrogenation path, and that these paths have a significant effect on the selectivity of the different products. The established literature, on the other hand, focuses more on the methanol synthesis pathway, suggesting that the reaction proceeds via the formate/carbonate intermediate pathway and the RWGS pathway. Finally, the paper summarises the different states of Co active sites, classified and discussed according to chemical valence. The existing literature, on the other hand, provides a review of the different chemical states of Cu, discussing them differentiated according to the different metal crystal planes.

4) Conclusion. The conclusion of this paper summarises the current limitations of catalytic reaction research and addresses these limitations with a vision of future breakthroughs in catalyst design for synergistic interactions at multiple active sites, the development of advanced characterisation techniques, the introduction of theoretical calculations, and reactor design. The existing literature concludes with a summary of design ideas for methanol synthesis catalysts and a vision for future in-depth kinetic studies, optimisation of the laboratory, and scaling up of reaction experiments.

5) Others. Firstly, a comparison of the table of contents of the two articles shows that the overall structure of the articles is different; this article tends to give an overview of the catalysts after stating the mechanism, whereas the existing literature tends to give an overview of the catalysts before discussing the mechanism. Secondly, in addition to the discussion of catalysts for the synthesis of methanol, this paper also deals with catalysts for the synthesis of higher carbon alcohols. The literature has discussed the catalysts for the synthesis of methanol.

Thank you very much for taking the time out of your busy schedule to read our article and reply, we hope that our response will change your mind.

 

Question 2:

Thermodynamic considerations, which are discussed in Chapter 2, are not relevant and in some cases mislead drastically. In particular, the statement in lines 149-150 gives an incorrect idea of why the methanol synthesis is an exothermal process while the RWGS is endothermal. In fact, it has nothing to do with temperature range.

We sincerely thank you for reading our article carefully and thoughtfully and for your valuable comments. We apologise for the errors in the thermodynamic discussion section, which could indeed be misleading to the reader, and are very grateful that you pointed them out to us. Since there is a thermodynamic limit (a chemical reaction eventually reaches its maximum thermodynamically permissible conversion rate), our discussion of temperature factors in the second section was not rigorous, and we have revised the relevant part of the article(see yellow highlights). Your serious and rigorous approach to science is an example for us to follow.

 

Question 3:

The problem of mechanism is not addressed properly. Indeed, a common approach teaches that RWGS is the first step of any H₂-CO₂ interaction, so then everything is a "syngas chemistry" complicated by RWGS side effects. The authors do not argue, just cite a couple of works where alternative formiatic mechanism is considered for very specific cases. No discussion introduced.

Thank you for your valuable comments. Based on your comments we have modified the mechanism section. Firstly, because of the large number of reactions involved, we have divided the discussion of the reaction pathways for CO₂ hydrogenation over Co-based catalysts into two parts: the C1 product and the C₂+ product, for which there are three main reaction pathways, and for the C₂+ product for which RWGS is the prerequisite step. Secondly, most of the literature, e.g.‘Niu, J., Liu, H., Jin, Y., Fan, B., Qi, W., & Ran, J. (2022). Comprehensive review of Cu-based CO2 hydrogenation to CH3OH: Insights from experimental work and theoretical analysis. International Journal of Hydrogen Energy, 47(15), 9183-9200’and ‘Han, W. A. N. G., Sheng, F. A. N., Sen, W. A. N. G., Mei, D. O. N. G., Qin, Z. F., Fan, W. B., & WANG, J. G. (2021). Research progresses in the hydrogenation of carbon dioxide to certain hydrocarbon products. Journal of Fuel Chemistry and Technology, 49(11), 1609-1619’, the formic acid reaction pathway is discussed as a separate pathway from the RWGS + CO-hydro pathway, so this categorisation has been adopted in this paper as well, which I hope you will understand.Every comment you make helps us improve our articles, and it's greatly appreciated.

 

Question 4:

The Reviewer is generally concerned with terminology, especially with application of the term "hydrogenation" to C1 chemistry reactions of formation of hydrocarbons, methanol and higher alcohols. The Reviewer believes that for industrially important processes it is advisable to be keen on terminology, which is common among industry chemists. It would be scholastic to call "hydrogenation" all the reactions where hydrogen is involved. CO₂ hydrogenation is a process of methane formation from CO₂ and H₂, just that. The Reviewer insists on that despite numerous publications where almost any reaction with hydrogen participation is called "hydrogenation". The authors have to cite numerous sources and show illustrations where "hydrogenation to CO" is called in a common way "reverse water gas shift reaction" or RWGS.

We would like to express our sincerest gratitude for your invaluable contribution to this discussion. We extend our sincerest apologies for the imprecise terminology employed in our article. As you rightly observed, it is appropriate to employ specialised chemical terminology in the context of industrially significant processes. For example, the reaction involving the hydrogenation of CO is referred to as a syngas reaction, while the reaction involving the hydrogenation of CO and CO₂ to methane is known as a methanation reaction. Similarly, the reaction involving the hydrogenation of CO₂ to CO is designated a reverse water gas reaction. It would be prudent to avoid an overly academic approach when using the term 'hydrogenation' for all reactions involving hydrogen, as this may lack academic rigour. As a result, we have conducted a review and amended the terminology used in the article for all industrially significant processes. Furthermore, a substantial corpus of literature has been consulted in order to ascertain whether CO₂ hydrogenation to CO should be designated a reverse water gas reaction. This is presented herewith. It is evident that both academic research and industrial R&D are of equal importance for the advancement of society as a whole. The findings of the research are interdisciplinary and mutually reinforcing. The comments you have made have provided us with a profound insight into this matter, and it is our hope that the revised content will meet your expectations.The literature is listed below：

[1]Yu, J., Muhetaer, A., Li, Q., & Xu, D. (2024). Solar Energy-Driven Reverse Water Gas Shift Reaction: Photothermal Effect, Photoelectric Activation and Selectivity Regulation. Small, 2402952.

[2]Triviño, M. L. T., Arriola Jr, N. C., Kang, Y. S., & Seo, J. G. (2024). Transforming CO₂ to valuable feedstocks: Emerging catalytic and technological advances for the reverse water gas shift reaction. Chemical Engineering Journal, 150369.

[3]Chen, X., Chen, Y., Song, C., Ji, P., Wang, N., Wang, W., & Cui, L. (2020). Recent advances in supported metal catalysts and oxide catalysts for the reverse water-gas shift reaction. Frontiers in Chemistry, 8, 709.

[4]Villora-Picó, J. J., González-Arias, J., Pastor-Pérez, L., Odriozola, J. A., & Reina, T. R. (2024). A review on high-pressure heterogeneous catalytic processes for gas-phase CO₂ valorization. Environmental Research, 240, 117520.

[5]Ren, M., Zhang, Y., Wang, X., & Qiu, H. Catalytic hydrogenation of CO₂ to methanol: a review. Catalysts. 2022.

[6]Zhou, W., Cheng, K., Kang, J., Zhou, C., Subramanian, V., Zhang, Q., & Wang, Y. (2019). New horizon in C1 chemistry: breaking the selectivity limitation in transformation of syngas and hydrogenation of CO₂ into hydrocarbon chemicals and fuels. Chemical Society Reviews, 48(12), 3193-3228.

[7]Scarfiello, C., Pham Minh, D., Soulantica, K., & Serp, P. (2023). Oxide supported cobalt catalysts for CO₂ hydrogenation to hydrocarbons: Recent progress. Advanced Materials Interfaces, 10(15), 2202516.

[8]Niu, J., Liu, H., Jin, Y., Fan, B., Qi, W., & Ran, J. (2022). Comprehensive review of Cu-based CO₂ hydrogenation to CH3OH: Insights from experimental work and theoretical analysis. International Journal of Hydrogen Energy, 47(15), 9183-9200.

Zeng, F., Mebrahtu, C., Xi, X., Liao, L., Ren, J., Xie, J., ... & Palkovits, R. (2021). Catalysts design for higher alcohols synthesis by CO₂ hydrogenation: Trends and future perspectives. Applied Catalysis B: Environmental, 291, 120073.

Reviewer 4 Report

Comments and Suggestions for Authors

Hydrogenation of carbon dioxide to obtain valuable chemical products is currently one of the topical areas of catalysis. Rapid development of modern materials science allows to create new catalytic systems showing improved properties. New methods of analysis, including computational ones, contribute to a deeper understanding of the processes occurring on the surface of catalytic systems. The authors' article is a review of modern catalysts and ideas about the processes occurring on the surface of cobalt catalysts and the possibility of influencing the routes of flow in order to affect the performance of the catalytic process. The article is written in a good language, the article contains extensive factual material. It is possible to formulate a minor number of remarks and comments:

1.         The choice of Co-based catalysts as the object of study should be explained in the Introduction section.

2.         How can a catalyst solve the ‘thermodynamic’ problem of CO2 conversion (line 105)?

3. How does equation (3) differ from equation (4)?

4.         In Fig 3, what is the difference between the route *NSOO  CO2 and CO2 *NSOO?

5.         It should be deciphered what is CoAlLDH? (line 179)

6.         The thesis described in lines 331-333 should be explained because the dG of equations (5) and (6) is greater than 0.

7.         It is necessary to decipher what Co/KIT-6 is (line 389).

8.         In Table 1,2, correct the column signatures (for H2/CO2, the coefficient 2 for Co2 has moved away).

9.         Typos. For example lines 458, 480, 498, 575.

Comments on the Quality of English Language

The article is written in a good language.

Author Response

Response:

We are grateful for the comprehensive and constructive review, which has provided valuable insights and feedback. We concur with the recommendations set forth and have integrated these proposed alterations into the manuscript. In the following, the responses to all the comments are provided one by one.

Question 1:

The choice of Co-based catalysts as the object of study should be explained in the Introduction section.

Thank you for your valuable suggestions. In response to your suggestion, we have included a small paragraph in the Introduction (see yellow highlights), to explain why we chose Co-based catalysts as the subject of our review by adding a short paragraph on the advantages of Co-based catalysts and the breakthroughs made in recent years. I believe that this addition will make it easier for the reader to understand the reasons for our review of Co-based catalysts for CO₂ hydrogenation.

 

Question 2:

How can a catalyst solve the ‘thermodynamic’ problem of CO₂ conversion.

Thank you for your question. We have added to this question in lines 127-131 of the text(see yellow highlights), where we argue that ‘The researchers used catalyst design ideas such as optimising metal-carrier interaction (MSI), constructing symmetry-breaking active centres, and regulating the number of oxygen vacancies on the surface, which lowered the thermodynamic energy barriers to carbon dioxide activation, facilitated the carbon dioxide hydrogenation reaction, and regulated the distribution of reaction products.’ I hope the additions meet your expectations. Thanks again for your patience!

 

Question 3:

How does equation (3) differ from equation (4)?

We are very sorry that we did not find such a low-level error when checking our article, we mistakenly wrote ‘C₂H₅OH’ as ‘CH₃OH’ in Equation 4, and we have corrected Eq. 4 (see yellow highlights). Thank you very much for reading our article carefully and finding such a detailed error, guarding the rigour of our article, thank you again!

 

Question 4:

In Fig 3, what is the difference between the route *NSOO CO₂ and CO₂ *NSOO?

Thank you very much for your query. There is no difference in the intended meaning of the two line indications, this was an error in our drawing, probably forgetting to remove the indicative lines from the sketch when adjusting the shape and position of the lines for aesthetics. We have already modified and replaced the picture, keeping only one line indication. Thank you for your patience in pointing out such a detailed problem for us.

 

Question 5:

It should be deciphered what is CoAlLDH? (line 179)

Thank you for your valuable suggestions.The CoAlLDH catalyst is a layered double hydroxide Co-Al catalyst, to which we add in the paper(see yellow highlights).

 

Question 6:

The thesis described in lines 331-333 should be explained because the dG of equations (5) and (6) is greater than 0.

Thank you very much for your valuable suggestions. Firstly, we would like to apologise to you here, we have misspelt the value of the ΔG in the reaction equation and the minus sign before the value appeared to be missing. Therefore, we have modified Eq. 5, and we feel that the existence of Eq. 6 is a bit redundant, so we have deleted Eq. 6. Second, according to your suggestion, we have added the discussion of the ΔG in this paragraph(see yellow highlights), because the Gibbs function value is less than zero, the CO₂ methanation reaction can be carried out spontaneously at room temperature, which indicates that methane is stable at room temperature, which provides strong support for the use of methane as a hydrogen storage medium. This provides strong support for the use of methane as a hydrogen storage medium. We hope that our modification can make you feel satisfied.

 

Question 7:

It is necessary to decipher what Co/KIT-6 is (line 389).

We sincerely appreciate your valuable comments. Based on your comments we add an explanation of the KIT-6 material(see yellow highlights), which is a molecular sieve material with ordered mesopores.

 

Question 8:

In Table 1,2, correct the column signatures (for H₂/CO₂, the coefficient 2 for Co₂ has moved away).

We are very sorry for our careless mistakes and thank you for the heads up. We have checked all the tables in the article and corrected all formatting and content errors.

 

Question 9:

Typos. For example lines 458, 480, 498, 575.

We sincerely thank you for your careful reading. The typos have been corrected in our resubmitted manuscript. Thank you for correcting them.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors did rather deep re-editing job after the first revision and succeeded in meeting all the Reviewer's concerns. However they obviously did it in great hurry, so allowed unfortunate misprints.

1. In particular, in Lines 164-165 one can see a duplicate of a "For example" expression.

 

After correction of this and other minor issues, the manuscript is recommended for publication.

Comments on the Quality of English Language

The English is generally good enough. However, there are misprints in the manuscript, in particular in Lines 164-165.