CO₂ Hydrogenation to Methanol on CuO-ZnO/SiO₂ and CuO-ZnO/CeO₂-SiO₂ Catalysts Synthesized with β-Cyclodextrin Template

Round 1

Reviewer 1 Report

 - It would be appreciated if the authors include some data in the abstract, it is very general.

    - The authors could improve the explanation of the selection of the synthesis method. Lines 90-91 should be revised.

    - What is the relevance of the TGA of the precursors? It would be advisable to include some comment on the signals that are observed in the distribution of products.

    - On line 240, do the authors mean that the Si:Zn ratio is lower than the Si:Cu ratio or is there an error?

I hope that these comments serve to improve the quality of the manuscript and I remain at your disposal for any additional review. 

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Author Response

We sincerely thank our referees for their valuable comments and suggestions on improving our manuscript. All the comments are listed below with our responses.

Reviewer 1

1) It would be appreciated if the authors include some data in the abstract, it is very general.

Response:

Some data about the samples with the best catalytic activity results was added to the abstract.

2) The authors could improve the explanation of the selection of the synthesis method. Lines 90-91 should be revised.

Response:

This method was previously used to obtain silica materials. However, in this work it was proposed to use the modified method to obtain more complex systems, such as those described in the text. It is explained in the novelty of the work.

3)  What is the relevance of the TGA of the precursors? It would be advisable to include some comment on the signals that are observed in the distribution of products.

Response:

This method was used to choose the calcination temperature, since the synthesis method is relatively new. Besides, it is known, that the decomposition temperature for complex systems can differ due to a variety of the factors.

4) On line 240, do the authors mean that the Si:Zn ratio is lower than the Si:Cu ratio or is there an error?

Response:

It is a typo, it was fixed.

Reviewer 2 Report

Catalysts

Manuscript ID:           catalysts-2505562

Title:                            CO₂ hydrogenation to methanol on CuO-ZnO/SiO₂ and CuO-ZnO/CeO₂-SiO₂ catalysts synthesized with β-cyclodextrin template

In this manuscript, the authors investigated the reaction of CO₂ hydrogenation to methanol over CuO-ZnO/SiO₂ and CuO-ZnO/CeO₂-SiO₂ catalysts. The catalysts were characterized by XRD, SEM-EDX, low-temperature nitrogen physisorption, XPS, EPR and TGA, while the magnetic properties were also measured. The authors showed that the catalysts with cerium added at the gelation stage demonstrated the best performance.

I think that is an interesting and well-presented research article. However, some issues should be improved. I recommend a publication of this manuscript to the Catalysts journal after the authors consider the following major revisions.

Comment #1

The authors need to improve on their bibliography. I think that they should add more references in order to enrich the introduction section. Please add the most important findings of the following studies in terms of the CO₂ hydrogenation reaction and the physicochemical properties of the cerium oxide (basicity, increased oxygen mobility, oxygen vacancy sites etc.). Furthermore, the addition of aliovalent ions (Cu²⁺, Mg²⁺, La³⁺, Sm³⁺, Pr³⁺) into the crystal structure of CeO₂ improve the catalytic activity and stability.

[1].    A.I. Latsiou, N.D. Charisiou, Z. Frontistis, A. Bansode, M.A. Goula, CO₂ hydrogenation for the production of higher alcohols: Trends in catalyst developments, challenges and opportunities. Catalysis Today 420 (2023) 114179.

[2].    Y. Zhou, F. Liu, S. Geng, M. Yao, J. Ma, J. Cao, Tuning the content of S vacancies in MoS2 by Cu doping for enhancing catalytic hydrogenation of CO₂ to methanol. Molecular Catalysis 547 (2023) 113288.

[3].    G.I. Siakavelas, N.D. Charisiou, A. AlKhoori, S. AlKhoori, V. Sebastian, S.J. Hinder, M.A. Baker, I.V. Yentekakis, K. Polychronopoulou, M.A. Goula, Highly selective and stable Ni/La-M (M=Sm, Pr, and Mg)-CeO₂ catalysts for CO₂ methanation. Journal of CO2 Utilization 51 (2021) 101618.

Comment #2

In the manuscript there are several typos, please check and correct carefully.

Comment #3

Please improve the quality of the Figures 2, 3, 4, 5, 7, and 8. Please correct the x-axis name position.

Comment #4

Is it possible for the authors to calculate the population of the basic sites of the reduced samples?

Comment #5

Which is the lifetime of the catalytic material presented herein? Could the authors provide more data in terms of the catalytic stability? Is there any structural change of the active sites on the catalyst surface after the experiment?

Author Response

We sincerely thank our referees for their valuable comments and suggestions on improving our manuscript. All the comments are listed below with our responses.

Reviewer 2

Comment #1

The authors need to improve on their bibliography. I think that they should add more references in order to enrich the introduction section. Please add the most important findings of the following studies in terms of the CO₂ hydrogenation reaction and the physicochemical properties of the cerium oxide (basicity, increased oxygen mobility, oxygen vacancy sites etc.). Furthermore, the addition of aliovalent ions (Cu²⁺, Mg²⁺, La³⁺, Sm³⁺, Pr³⁺) into the crystal structure of CeO₂ improve the catalytic activity and stability.

 [1].    A.I. Latsiou, N.D. Charisiou, Z. Frontistis, A. Bansode, M.A. Goula, CO₂ hydrogenation for the production of higher alcohols: Trends in catalyst developments, challenges and opportunities. Catalysis Today 420 (2023) 114179.

[2].    Y. Zhou, F. Liu, S. Geng, M. Yao, J. Ma, J. Cao, Tuning the content of S vacancies in MoS2 by Cu doping for enhancing catalytic hydrogenation of CO₂ to methanol. Molecular Catalysis 547 (2023) 113288.

[3].    G.I. Siakavelas, N.D. Charisiou, A. AlKhoori, S. AlKhoori, V. Sebastian, S.J. Hinder, M.A. Baker, I.V. Yentekakis, K. Polychronopoulou, M.A. Goula, Highly selective and stable Ni/La-M (M=Sm, Pr, and Mg)-CeO₂ catalysts for CO₂ methanation. Journal of CO2 Utilization 51 (2021) 101618.

Response:

New references were added ([14-19]).

Comment #2

In the manuscript there are several typos, please check and correct carefully.

Response:

Fixed.

Comment #3

Please improve the quality of the Figures 2, 3, 4, 5, 7, and 8. Please correct the x-axis name position.

Response:

Fixed.

Comment #4

Is it possible for the authors to calculate the population of the basic sites of the reduced samples?

Response:

It is not obvious how the basic or acidic sites can affect the catalytic parameters in these systems, textural or phase characteristics are more strongly influenced. Therefore, such task was not posed.

Comment #5

Which is the lifetime of the catalytic material presented herein? Could the authors provide more data in terms of the catalytic stability? Is there any structural change of the active sites on the catalyst surface after the experiment?

Response:

One of the samples was tested for catalytic stability for more than 50 hours, as you can see, there is a decrease in sample productivity by about 30% in the first 6 hours of the experiment, then the sample remains quite stable (data are shown in the figure below).

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript describes the synthesis and characterisation of a series of mixed copper (II) – zinc oxide catalysts supported on unmodified and ceria-modified silica supports using β-cyclodextrin as a template.

The topic of CO2 utilisation is of significance. The scope is adequate and the methods and results are comprehensive.

However, the work lacks reproducibility in that the key operating parameters are missing and the full product information (serial No., brand, manufacturer, city, country) of the instruments are also missing.

Although due to the page limit (is this right?) the majority of the methods other than the catalyst synthesis methods have been moved to SI. Perhaps the test on catalyst performance in CO2 hydrogenation reaction, which is of importance, should be moved back to the main text.

The plots are not standardised and some axis labels are missing and some have different styles, 1) parameter, unit 2) parameter (unit) 3) parameter / unit.  

The result discussion is not adequate. Please discuss the underlying mechanism. The computational characterization of the role of nano-structuring and alloying in Cu-based catalysts on the activity and selectivity of CO₂ reduction to generate formic acid (HCOOH), formaldehyde (H₂C=O), methanol (CH₃OH) and methane (CH₄) should be cited doi.org/10.3390/nano13010087

Author Response

We sincerely thank our referees for their valuable comments and suggestions on improving our manuscript. All the comments are listed below with our responses.

Reviewer 3

The manuscript describes the synthesis and characterisation of a series of mixed copper (II) – zinc oxide catalysts supported on unmodified and ceria-modified silica supports using β-cyclodextrin as a template.

The topic of CO2 utilisation is of significance. The scope is adequate and the methods and results are comprehensive.

However, the work lacks reproducibility in that the key operating parameters are missing and the full product information (serial No., brand, manufacturer, city, country) of the instruments are also missing.

Response:

Additional information of the instruments was added to Supplementary Information.

Although due to the page limit (is this right?) the majority of the methods other than the catalyst synthesis methods have been moved to SI. Perhaps the test on catalyst performance in CO2 hydrogenation reaction, which is of importance, should be moved back to the main text.

Response:

The part about catalysis was added to the main text (paragraph 2.3).

The plots are not standardised and some axis labels are missing and some have different styles, 1) parameter, unit 2) parameter (unit) 3) parameter / unit.  

Response:

The figures and the axis were fixed.

The result discussion is not adequate. Please discuss the underlying mechanism. The computational characterization of the role of nano-structuring and alloying in Cu-based catalysts on the activity and selectivity of CO₂ reduction to generate formic acid (HCOOH), formaldehyde (H₂C=O), methanol (CH₃OH) and methane (CH₄) should be cited doi.org/10.3390/nano13010087

Response:

The mechanism of the reaction was discussed more carefully, the reference was added [14].

Reviewer 4 Report

Comments

This study reports the rational synthesis of CuO-ZnO/SiO2 and CuO-ZnO/CeO2-SiO2 using β-cyclodextrin template for the hydrogenation of CO₂ to methanol. The article is well designed, organized, and supported with some pieces of evidence and, therefore, could be accepted after addressing the following comments.

1.     The authors should provide SEM or TEM images for the catalysts because the catalyst's shape is one of the main factors determining the catalytic activity.

2.     The XRD in Figure 2 showed only Cu-oxide peaks, and the authors should rewrite and add more explanation. Also, the XRD references or card numbers should be added.

3.     The authors should add the XPS spectra for the elements and their related fitting because they can reflect the presence of mixed oxide phases.

4.     The authors should unify the format of the  references

5.     The authors should cite more recent references related to CO₂ reforming or hydrogenation or conversion like https://doi.org/10.3390/nano12142379 

https://doi.org/10.1016/j.ijhydene.2021.11.239

6.     The utilization of mixed metal oxides in CO₂ conversion is extensively studied, so the authors should emphasize the novelty of this study in the abstract, introduction

Moderate editing of the English language is required in terms of coherency and flow

Author Response

We sincerely thank our referees for their valuable comments and suggestions on improving our manuscript. All the comments are listed below with our responses.

Reviewer 4

This study reports the rational synthesis of CuO-ZnO/SiO2 and CuO-ZnO/CeO2-SiO2 using β-cyclodextrin template for the hydrogenation of CO₂ to methanol. The article is well designed, organized, and supported with some pieces of evidence and, therefore, could be accepted after addressing the following comments.

1. The authors should provide SEM or TEM images for the catalysts because the catalyst's shape is one of the main factors determining the catalytic activity.

Response:

An example of SEM image was added to the Supplementary Materials.

2. The XRD in Figure 2 showed only Cu-oxide peaks, and the authors should rewrite and add more explanation. Also, the XRD references or card numbers should be added.

Response:

There is explanation in the text: zinc oxide peaks are invisible due to the small amount. The card numbers are also represented in the figures.

3. The authors should add the XPS spectra for the elements and their related fitting because they can reflect the presence of mixed oxide phases.

Response:

The XPS spectra were added to the Supplementary Information.

4. The authors should unify the format of the  references

Response:

Fixed.

5. The authors should cite more recent references related to CO₂reforming or hydrogenation or conversion like https://doi.org/10.3390/nano12142379 

https://doi.org/10.1016/j.ijhydene.2021.11.239

Response:

New references are added ([14-19]).

6. The utilization of mixed metal oxides in CO₂conversion is extensively studied, so the authors should emphasize the novelty of this study in the abstract, introduction

Response:

The novelty of this work lies in the use of cyclosextrins for the template synthesis. This explanation is added in the abstract.

Round 2

Reviewer 1 Report

The authors have answered to the comments of the other reviewers and have modified the text to make it publishable.

In general, there is a high level of English.

Author Response

We sincerely thank our referees for their valuable comments and suggestions on improving our manuscript. 

Reviewer 2 Report

Accept in the presence form

Author Response

We sincerely thank our referees for their valuable comments and suggestions on improving our manuscript. 

Reviewer 3 Report

The authors have adressed the majority of the issues raised.

Although the plots are not of publishable scientific quality, e.g. Fig. 1, x-axis label should be Mass (%), y-axis label Temperature (°C); Fig. 2, 3 do not have y-axis label and x-axis label should be centered. The same applies to the rest. Fig. 5, a and c y-axis scale needs to be adjusted to eliminate the large blank space.

Author Response

We sincerely thank our referees for their valuable comments and suggestions on improving our manuscript.

Reviewer 4 Report

The authors have addressed all the comments and the article could be accepted as is 

Author Response

We sincerely thank our referees for their valuable comments and suggestions on improving our manuscript.