Research Progress of Co-Catalysts in Photocatalytic CO₂ Reduction: A Review of Developments, Opportunities, and Directions

Round 1

Reviewer 1 Report

This review describes the achievements in the field of heterogenous photocatalysis effected by titanium dioxide and related systems. Co-catalysts may have significant influence on the catalyst performance. In the manuscript, various aspects of additional metals (co-catalysts) are discussed in detail with the emphasis on recent literature. I have a nice impression of the reading, as it provides valuable information on the state of the research in this field. When I checked the authors’ self-citation, I was surprised that there are NO citations of papers of the corresponding authors. Overall, I would suggest acceptance in the present form.

Author Response

Dear Editors and Reviewers,

Thanks for your comments concerning our manuscript entitled “Advances in Semiconductor-Based Nanocomposite Photocatalysts and Electrocatalysts for Nitrogen Reduction to Ammonia” (manuscript ID processes-2249178). Those comments are all valuable and helpful for revising and improving our paper. We have carefully studied the comments and made corrections based on your comments. Modified portions are highlighted in red in the revised version. As suggested, we improved the manuscripts by improving the writing and adding more data and analyses. The revised manuscript is responded to, point by point.

The primary corrections in the paper and the responses to the reviewer’s comments are as follows.

Thank you and best regards.

Yours sincerely,

Corresponding author: Qian Su [email protected]

                   Xueyuan Yan  [email protected]

Reviewer #1: 1. This review describes the achievements in the field of heterogenous photocatalysis effected by titanium dioxide and related systems. Co-catalysts may have significant influence on the catalyst performance. In the manuscript, various aspects of additional metals (co-catalysts) are discussed in detail with the emphasis on recent literature. I have a nice impression of the reading, as it provides valuable information on the state of the research in this field. When I checked the authors’ self-citation, I was surprised that there are NO citations of papers of the corresponding authors. Overall, I would suggest acceptance in the present form.

Response: Thanks for the comment. 

Author Response File: Author Response.doc

Reviewer 2 Report

This is review article which covers research progress of co-catalysts in photocatalytic reduction of CO2.

The comments about this manuscript are given below:
1. English of this manuscript is quite worst. Please polish the English language of your revised manuscript carefully before submission; for that, the authors are suggested to find a professional English language service to do so
2. This review article should include a separate section titled “mechanic and kinetic achievements”. Although there is one small section “2.1 Mechanistic role of co-catalysts in CO2 photocatalytic reduction”, but it doesn’t cover maximum articles.
3. At present, future research perspectives are briefly lumped in the last section "5-. Conclusions and Outlook". To make the review article more forward-looking, please separate this section into two sections, i.e. a section with title like "Challenges and Perspectives" and a section with title like "Conclusions". The new section "Challenges and Perspectives" should be expanded to systematically outline the authors' detailed views on the key challenges, existing research gaps and future research directions etc., including also potential future developments in research methodologies, that are important to future development of the related field.
4. This review article should cover the latest articles. However, there is one article of 1972 and six articles before 2000. I think it should cover recent articles maybe from 2000 onward
5. It is surprising to note that there is no article mentioned in this review for several years i.e. 2000, 2001, 2003, 2018, 2023. Only one article is mentioned for year 2022. It seems that this review is not thorough. Probably authors have not reviewed complete literature published in this area.
6. Probably figures have been copied from various articles. However proper reference of that article in the figure caption is missing.
7. Page 1, line 23, it is written “….of China economy, …”. This article is not for China only. Therefore, word China should be replaced with world.
8. A separate table should be included for all abbreviations
9. In several places it is mentioned as [author name] et. al. Authors name should be avoided in the article.
10. Section 3: “Precious metal-based catalysts” should be modified to “nobel metalbased co-catalysts”.
11. A separate table should be provided with summary of all results mentioned in this article
12. Section 4.3.2 is titled “Wrapping method”. However, in this section term “cladding method” is used.

Comments for author File: Comments.docx

Author Response

Dear Editors and Reviewers,

Thanks for your comments concerning our manuscript entitled “Advances in Semiconductor-Based Nanocomposite Photocatalysts and Electrocatalysts for Nitrogen Reduction to Ammonia” (manuscript ID processes-2249178). Those comments are all valuable and helpful for revising and improving our paper. We have carefully studied the comments and made corrections based on your comments. Modified portions are highlighted in red in the revised version. As suggested, we improved the manuscripts by improving the writing and adding more data and analyses. The revised manuscript is responded to, point by point.

The primary corrections in the paper and the responses to the reviewer’s comments are as follows.

Thank you and best regards.

Yours sincerely,

Corresponding author: Qian Su [email protected]

                   Xueyuan Yan  [email protected]

Reviewer #2: 1. English of this manuscript is quite worst. Please polish the English language of your revised manuscript carefully before submission; for that, the authors are suggested to find a professional English language service to do so.

Response: Thanks for the comment. The language has been touched up and revised and marked in red in the manuscript.
2. This review article should include a separate section titled “mechanic and kinetic achievements”. Although there is one small section “2.1 Mechanistic role of co-catalysts in CO₂ photocatalytic reduction”, but it doesn’t cover maximum articles.

Response: Thanks for the comment. 

2.1. Mechanic and kinetic achievements

      Due to the linear structure of CO₂, the energy required to cleave the C-O bond was much higher than that required to cleave the C-C, C-O and C-H bonds[95]. In addition, the relatively wide energy gap and electron affinity of CO₂ lead to a negative single electron transfer redox potential. Therefore, different reaction pathways have been developed to reduce CO₂ at lower energies. In general, the photocatalytic CO₂ reduction process must follow the following four steps [96-98]: (1) the electrons on the photocatalyst VB was stimulated and transferred to its CB to generate a photogenerated charge The photogenerated charge could either be transferred separately to the photocatalyst surface for photocatalytic reaction or recombined to release photons or heat; (2) the catalyst surface for CO₂ absorption; (3) photogenerated electrons on the photocatalyst surface to convert CO₂ into fuel; (4) desorption of photocatalyst products. A conductive potential more negative than the required standard potential, and effective electron transfer to the CO₂ adsorbed semiconductor surface, determine the efficiency of CO₂ reduction.

[95].G. Gao, Y. Jiao, E. R. Waclawik, A. Du. Single atom (Pd/Pt) supported on graphitic carbon nitride as efficient photocatalyst for visible-light reduction of carbon dioxide. J. Am. Chem. Soc. 2016, 138, 6292.

[96]. N. Nie, L. Zhang, J. Fu, B. Cheng, J. Yu, Self-assembled hierarchical direct Zscheme g-C₃N₄/ZnO microspheres with enhanced photocatalytic CO₂ reduction performance, Appl. Surf. Sci. 441 (2018) 12–22.

[97]. L. Zhou, H. Kamyab, A. Surendar, A. Maseleno, A.Z. Ibatova, S. Chelliapan, N. Karachi, Z. Parsaee, Novel Z-scheme composite Ag₂CrO₄/NG/polyimide as high performance nano catalyst for photoreduction of CO₂: design, fabrication,

[98]. characterization and mechanism, J. Photochem. Photobiol., A 368 (2019) 30–40.

[99]. P. Raizada, A. Kumar, V. Hasija, P. Singh, V.K. Thakur, A.A.P. Khan, An overview of converting reductive photocatalyst into all solid-state and direct Z-scheme system for water splitting and CO₂ reduction, J. Ind. Eng. Chem. 93 (2021) 1–27.

3. At present, future research perspectives are briefly lumped in the last section "5-. Conclusions and Outlook". To make the review article more forward-looking, please separate this section into two sections, i.e. a section with title like "Challenges and Perspectives" and a section with title like "Conclusions". The new section "Challenges and Perspectives" should be expanded to systematically outline the authors' detailed views on the key challenges, existing research gaps and future research directions etc., including also potential future developments in research methodologies, that are important to future development of the related field.

Response: Thanks for the comment. 

5. Challenges and Perspectives

In conclusion, the selection of high activity, high selectivity and high stability is necessary to improve the CO₂ reduction activity. The co-catalysts have several properties: 1) promoting the separation of photoexcited electron-hole pairs, 2) inhibiting side reactions, 3) improving the selectivity of target products.

Currently, about half of the literature reports the use of noble metal-based co-catalysts (e.g., Pt-, Au-, and Ag-), alloys (i.e., Au/Cu and Pt/Cu) for facilitating photocatalytic CO2 reduction reactions. While, until recently, researchers have also developed many noble metal-free co-catalysts to aid in the photocatalytic reduction of CO2 such as Cu-, Ni- and graphene. Among them, Cu-based co-catalysts are the most commonly used catalysts for photocatalytic reduction of CO₂. Loading co-catalysts on semiconductor surfaces is a technique for CO₂ reduction synergy enhancement. Although many results on high activity, stability and selectivity of catalysts for CO₂ reduction have been reported in the literature. However, it is still in the initial stage of exploration and there is still much room for development in the future. Several issues are cited below. Although the activity, stability, and selectivity of co-catalyst-loaded semiconductor catalysts have been studied more in the literature. However, the physicochemical properties, catalytic activity, and the coupling of multi-electron and multi-metal reaction mechanisms of co-catalysts have been less studied. Because, this direction is the focus of future research. In addition, theoretical calculations (e.g. Density functional theory) are needed to study the process of CO₂ reduction to provide theoretical knowledge for photocatalytic CO₂ reduction. Unlike other literature, this paper is optimistic about the application prospect of graphene photocatalytic CO₂ reduction for the following two reasons: 1. Graphene has excellent electrical conductivity and huge specific surface area, so it could improve the general semiconductor materials with low visible light utilization and high excitation electron-hole complex probability. 2. Graphene was a well co-catalyst for the environment and no pollution to the environment before and after the reaction. In the future research, we should focus on the direction of graphene photocatalytic CO2 reduction.

6.Conclusions

(1)Co-catalyst preparation and dispersion. The activity, selectivity and stability of the co-catalyst/photocatalyst system depend largely on the co-catalyst preparation and dispersion methods, as these methods directly affect the physicochemical properties (e.g. chemical composition, phase structure, size, morphology, structure, size distribution, valence and surface area) of the co-catalyst. Therefore, the activity of CO₂ reduction could be improved by controlling the preparation of the catalyst. Currently, monometallic and bimetallic co-catalysts are the most studied in the literature, but the lack of studies on three or more polymetallic co-catalysts is less. In the future research, we should focus on developing multi-metal and multi-functional co-catalysts. Finally, it is important to note that to reach the scale of industrial applications of semiconductor photocatalysts, co-catalysts that require environmental friendliness, energy efficiency and other advantages are essential.

(2)To explore high selectivity, high activity and low cost co-catalyst. According to previous studies, catalysts like Cu/Pt, Cu/Au, etc. are more effective for CO₂ reduction. In view of this, it could be tried to find new and more efficient photocatalysts such as three metals or metal oxides of more than three metals, metal nitride, metal phosphide, bentonite, spinel and chalcocite etc. Multi-component active ingredients with synergistic effects could enhance the photocatalytic CO₂ activity. In addition, the stability of semiconductor photocatalysts is also a great challenge. Finding effective techniques to stop chemical or photocorrosion of co-catalysts will be a key direction for future research.
4. This review article should cover the latest articles. However, there is one article of 1972 and six articles before 2000. I think it should cover recent articles maybe from 2000 onward.

Response: Thanks for the comment. 

• H. Nguyen, H.Y. Wu, H. Bai, Photocatalytic reduction of NO₂and CO₂ using molybdenum-doped titania nanotubes. Chem. Eng. J. 269 (2015) 60–66.

[17]. H. Maimaitizi, A. Abulizi, K. Kadeer, D. Talifu, Y. Tursun, In situ synthesis of Pt and N co-doped hollow hierarchical BiOCl microsphere as an efficient photocatalyst for organic pollutant degradation and photocatalytic CO₂ reduction, Appl. Surf. Sci. 2020, 502, 144083.

[38]. Zhang, R.; Wang, H.; Tang, S.; Liu, C.; Dong, F.; Yue, H.; Liang, B. Photocatalytic Oxidative Dehydrogenation of Ethane Using.

CO₂ as a Soft Oxidant over Pd/TiO₂ Catalysts to C₂H₄ and Syngas. ACS Catal. 2018, 8, 9280−9286.

[40]. S. Yoshino, K. Sato, Y. Yamaguchi, A. Iwase, A. Kudo, Z-schematic CO₂ reduction to CO through interparticle electron transfer between SrTiO₃: Rh of a reducing photocatalyst and BiVO₄ of a water oxidation photocatalyst under visible light, ACS Appl. Energy Mater. 2020, 3, 10001–10007.

• Huang, Z. Duan, Y. Song, Q. Li, L. Chen, BiVO₄microplates with oxygen vacancies decorated with metallic Cu and Bi nanoparticles for CO₂ photoreduction, ACS Appl. Nano Mater. 2021, 4, 3576-3585.

[79]. H. Zhao, J. Duan, Z. Zhang, W. Wang, Bi-Ti-In trimetallic sites in the Indiumdoped Bi₄Ti₃O₁₂-CuIn₅S₈ S-scheme heterojunction for controlling the selectivity of CO₂ photoreduction, Fuel. 2022, 325, 124993.

5. It is surprising to note that there is no article mentioned in this review for several years i.e. 2000, 2001, 2003, 2018, 2023. Only one article is mentioned for year 2022. It seems that this review is not thorough. Probably authors have not reviewed complete literature published in this area.

[11]. N.H. Nguyen, H.Y. Wu, H. Bai, Photocatalytic reduction of NO₂ and CO₂ using molybdenum-doped titania nanotubes. Chem. Eng. J. 269 (2015) 60–66.

[17]. H. Maimaitizi, A. Abulizi, K. Kadeer, D. Talifu, Y. Tursun, In situ synthesis of Pt and N co-doped hollow hierarchical BiOCl microsphere as an efficient photocatalyst for organic pollutant degradation and photocatalytic CO₂ reduction, Appl. Surf. Sci. 2020, 502, 144083.

[38]. Zhang, R.; Wang, H.; Tang, S.; Liu, C.; Dong, F.; Yue, H.; Liang, B. Photocatalytic Oxidative Dehydrogenation of Ethane Using.

CO₂ as a Soft Oxidant over Pd/TiO₂ Catalysts to C₂H₄ and Syngas. ACS Catal. 2018, 8, 9280−9286.

[40]. S. Yoshino, K. Sato, Y. Yamaguchi, A. Iwase, A. Kudo, Z-schematic CO₂ reduction to CO through interparticle electron transfer between SrTiO₃: Rh of a reducing photocatalyst and BiVO₄ of a water oxidation photocatalyst under visible light, ACS Appl. Energy Mater. 2020, 3, 10001–10007.

[58]. L. Huang, Z. Duan, Y. Song, Q. Li, L. Chen, BiVO₄ microplates with oxygen vacancies decorated with metallic Cu and Bi nanoparticles for CO₂ photoreduction, ACS Appl. Nano Mater. 2021, 4, 3576-3585.

[79]. H. Zhao, J. Duan, Z. Zhang, W. Wang, Bi-Ti-In trimetallic sites in the Indiumdoped Bi₄Ti₃O₁₂-CuIn₅S₈ S-scheme heterojunction for controlling the selectivity of CO₂ photoreduction, Fuel. 2022, 325, 124993.

[95].G. Gao, Y. Jiao, E. R. Waclawik, A. Du. Single atom (Pd/Pt) supported on graphitic carbon nitride as efficient photocatalyst for visible-light reduction of carbon dioxide. J. Am. Chem. Soc. 2016, 138, 6292.

[96]. N. Nie, L. Zhang, J. Fu, B. Cheng, J. Yu, Self-assembled hierarchical direct Zscheme g-C₃N₄/ZnO microspheres with enhanced photocatalytic CO₂ reduction performance, Appl. Surf. Sci. 441 (2018) 12–22.

[97]. L. Zhou, H. Kamyab, A. Surendar, A. Maseleno, A.Z. Ibatova, S. Chelliapan, N. Karachi, Z. Parsaee, Novel Z-scheme composite Ag₂CrO₄/NG/polyimide as high performance nano catalyst for photoreduction of CO₂: design, fabrication,

[98]. characterization and mechanism, J. Photochem. Photobiol., A 368 (2019) 30–40.

[99]. P. Raizada, A. Kumar, V. Hasija, P. Singh, V.K. Thakur, A.A.P. Khan, An overview of converting reductive photocatalyst into all solid-state and direct Z-scheme system for water splitting and CO₂ reduction, J. Ind. Eng. Chem. 93 (2021) 1–27.
6. Probably figures have been copied from various articles. However proper reference of that article in the figure caption is missing.

Response: Thanks for the comment. Figure 3. Mechanism diagram of CO₂ reduction by Pt-TiO₂ photocatalyst. [90]; Figure 4. Mechanism of photocatalytic reduction of CO₂ by Ag-loaded BaLa₄Ti₄O₁₅. [71]; Figure 5. Diagram of the mechanism of photocatalytic CO₂ reduction occurring on the Pd-TiO₂ photocatalyst covered with Nafion layer. [33]; Figure 6. a) Ultraviolet photoelectron spectroscopy (UPS) determination of the work function of graphene oxide and Cu/graphene oxide composites. b) Energy band edge positions of pristine graphene oxide and Cu/GO composites compared to CO₂/CH₃OH and CO₂/CH₃CHO formation potentials. c) Schematic diagram of the photocatalytic reaction. [85] Figure 7. TEM image of Cu-TiO₂ photocatalyst. [89]; Figure 8. Mechanism of CO₂ reduction by Cu dispersed on TiO₂ nanoflakes. [89].
7. Page 1, line 23, it is written “….of China economy, …”. This article is not for China only. Therefore, word China should be replaced with world.

Response: Thanks for the comment. With the rapid economic development of the world.
8. A separate table should be included for all abbreviations

Response: Thanks for the comment.

Table 2. Abbreviations of professional terms

9. 
   In several places it is mentioned as [author name] et. al. Authors name should be avoided in the article.

Response: Thanks for the comment. Researchers [71] added a series of co-catalysts (NiO, Ru, Au, Cu, and Ag) to the synthesized BaLa₄Ti₄O₁₅, respectively. Researchers [72] investigated the crystallographic selectivity of Pd co-catalysts using 2D g-C₃N₄ with low layer thickness.
10. Section 3: “Precious metal-based catalysts” should be modified to “nobel metalbased co-catalysts”.

Response: Thanks for the comment. Noble metal based co-catalysts

11. 
    A separate table should be provided with summary of all results mentioned in this article

Response: Thanks for the comment.

Table 1. Co-catalysts for photocatalytic CO₂ reduction of metals and non-metals.

12. 
    Section 4.3.2 is titled “Wrapping method”. However, in this section term “cladding method” is used.

Response: Thanks for the comment. The wrapping method basically solves the problem of increasing defect concentration in the crystal itself due to additives.

Author Response File: Author Response.doc

Reviewer 3 Report

This manuscript tried to summarize the  research progress of co-catalysts in photocatalytic CO2 reduction, but it was poor organized and the discussion is superficial and confusing. At the present stage the reviewer must rejects this manuscript, but recommends the authors to resumbit after thoroughly re-organization, and also the English should be polished by a native speaker. 

Author Response

Dear Editors and Reviewers,

Thanks for your comments concerning our manuscript entitled “Advances in Semiconductor-Based Nanocomposite Photocatalysts and Electrocatalysts for Nitrogen Reduction to Ammonia” (manuscript ID processes-2249178). Those comments are all valuable and helpful for revising and improving our paper. We have carefully studied the comments and made corrections based on your comments. Modified portions are highlighted in red in the revised version. As suggested, we improved the manuscripts by improving the writing and adding more data and analyses. The revised manuscript is responded to, point by point.

The primary corrections in the paper and the responses to the reviewer’s comments are as follows.

Thank you and best regards.

Yours sincerely,

Corresponding author: Qian Su [email protected]

                   Xueyuan Yan  [email protected]

Reviewer #3: 1. This manuscript tried to summarize the research progress of co-catalysts in photocatalytic CO₂ reduction, but it was poor organized and the discussion is superficial and confusing. At the present stage the reviewer must rejects this manuscript, but recommends the authors to resumbit after thoroughly re-organization, and also the English should be polished by a native speaker. Response: Thanks for the comment. First of all, I would like to thank the reviewers for their reasonable, scientific and patient comments on the revision. This has benefited me to be more rigorous in my future research. We express our deepest gratitude for this. The manuscript has been carefully revised repeatedly for its defects. At the same time, we enriched the content in the manuscript. We hope to meet the submission standards of the journal. The valuable comments made by the reviewers have served as a excellent guide in writing papers in the future. We could found my own shortcomings from this manuscript. We will work diligently to write a better article.

Author Response File: Author Response.doc

Round 2

Reviewer 2 Report

1.     New section 2.1 Mechanic and kinetic achievements has been added.

a.      However there is no kinetic data given in this section.

b.     Section 2.2 should be a subsection of 2.1 as 2.1.1. It should not be a separate section

Author Response

Dear Editors and Reviewers,

Thanks for your comments concerning our manuscript entitled “Advances in Semiconductor-Based Nanocomposite Photocatalysts and Electrocatalysts for Nitrogen Reduction to Ammonia” (manuscript ID processes-2249178). Those comments are all valuable and helpful for revising and improving our paper. We have carefully studied the comments and made corrections based on your comments. Modified portions are highlighted in red in the revised version. As suggested, we improved the manuscripts by improving the writing and adding more data and analyses. The revised manuscript is responded to, point by point.

The primary corrections in the paper and the responses to the reviewer’s comments are as follows.

Thank you and best regards.

Yours sincerely,

Corresponding author: Qian Su [email protected]

                   Xueyuan Yan  [email protected]

Reviewer #2: 1. New section 2.1 Mechanic and kinetic achievements has been added.

1. However there is no kinetic data given in this section.
2. Section 2.2 should be a subsection of 2.1 as 2.1.1. It should not be a separate section

Response: Thanks for the comment. a. During the reduction process, the products obtained change with the change of reaction conditions and catalytic materials during the reduction process. The photon energy required for photoexcitation depends on the band gap of photocatalyst. The edge position of the energy band of the photocatalyst should match the redox potential of the relevant reaction. The different reduction products and corresponding electrode potentials obtained from the photocatalytic reduction reaction of CO₂ in aqueous solution are shown in Table 1. The ideal CO₂ photocatalytic reduction reaction must meet the requirement that the CB potential of photogenerated electrons is more negative than the potential of the reduction products/CO₂ (CH₄/CO₂, CH₃OH/CO₂, HCHO/CO₂, HCOOH/CO₂ or CO/CO₂), and the VB that generates holes is corrected than the potential of the oxidation reaction (O₂/H₂O) of H₂O. To sum up, photocatalytic reduction of CO₂ must meet two conditions: photon energy is greater than or equal to the size of band gap; The CB potential is more negative than the surface electron acceptor potential, and the VB potential is corrected than the surface electron donor potential. In this way, the reaction process of photocatalytic reduction of CO₂ can be realized [98].

Table 1. Different reduction products obtained from photocatalytic reduction of CO₂ in aqueous solution and the corresponding electrode potentials (vs. standard hydrogen electrode, 25°C, pH=7)

b.2.1.1 Mechanistic role of co-catalysts in CO₂ photocatalytic reduction

Author Response File: Author Response.doc

Reviewer 3 Report

Thanks for careful revision and a publication is recommended.

Author Response

Dear Editors and Reviewers,

Thanks for your comments concerning our manuscript entitled “Advances in Semiconductor-Based Nanocomposite Photocatalysts and Electrocatalysts for Nitrogen Reduction to Ammonia” (manuscript ID processes-2249178). Those comments are all valuable and helpful for revising and improving our paper. We have carefully studied the comments and made corrections based on your comments. Modified portions are highlighted in red in the revised version. As suggested, we improved the manuscripts by improving the writing and adding more data and analyses. The revised manuscript is responded to, point by point.

The primary corrections in the paper and the responses to the reviewer’s comments are as follows.

Thank you and best regards.

Yours sincerely,

Corresponding author: Qian Su [email protected]

                   Xueyuan Yan  [email protected]

Reviewer #3: Thanks for careful revision and a publication is recommended.

Response: Thanks for the comment. 

Author Response File: Author Response.doc