Unraveling FeO_x Nanoparticles Confined on Fibrous Mesoporous Silica Catalyst Construction and CO Catalytic Oxidation Performance

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors synthesized and characterized KCC-1-supported FeOx catalysts for CO oxidation. The manuscript combined different characterization methods to evaluate the catalytic performance. After a major revision, this manuscript might be taken into consideration for acceptance.

Comment 1: In page 3, the authors reported that TEM images did not show distinct lattice structures for KCC-1 and Fe2O3 over 7% Fe@KCC-1 catalyst. The status of this testing catalyst needs to be clarified (e.g., pristine or spent/reduced). TEM/SEM images for spent/reduced catalysts (after reducing Fe2O3 to Fe) need to be provided if the authors wanted to claim the anti-agglomeration (namely, anti-sintering) properties of metal particles over mesoporous silica supports. Calcination at inert gas is not a harsh condition to prove anti-agglomeration property of metal oxide particles.

Comment 2: In page 5, the authors mentioned as Fe was introduced, the temperatures corresponding to all the reduction processes moved towards lower temperatures, showing that the reduction properties of the material gradually increased. Here authors may use the peak temperature to make the comparison. Pure Fe2O3 itself (the top line in Figure 2) contains more sites for reduction, hence exhibiting higher peak temperature (because more oxygen ions from bulky phase needs to be reduced so the peak will appear late in the ramping temperuate profile). Authors need to rephrase this statement.

Comment 3: For Section 2.2, since more Fe2O3 sites would participate to the reaction of CO oxidation, catalyst with higher Fe2O3 loading is expected to exhibit lower temperature to reach the 100% of CO conversion. The authors found this general trend, but 10%Fe@FCC-1 showed lower activity compared to its counterpart with 7% Fe loading, reason needs to be given to explain this phenomenon.

Comment 4: In order to have a better comparison for CO oxidation activity, the authors can normalize converted CO based on Fe loading amount (per gram), then compare normalized rates at different reaction temperatures. By virtue of the Arrhenius equation, activation energies among different catalysts can be calculated and then compared.

Comment 5: For Figure 5, a zoomed-in figure especially for adsorbed CO should be provided since now this part of IR spectra (marked as pink) is nearly flat and few information can be observed.

Comment 6: As a common sense, CO is a reducing gas and it is more active than CO2, normally CO or syngas (CO + H2) has more industrial application. Hence more words of introduction / background should be given in Section 1 to introduce the importance of CO oxidation to CO2.

Comments on the Quality of English Language

The authors drafted the manuscript with good Engligh expression. 

Author Response

Dear Editor and Reviewers,

Thank you for giving the opportunity to modify the paper again, and we also thank the reviewers for the insightful comments regarding the manuscript # Manuscript ID: catalysts-2771079 entitled “Unraveling the CO Catalytic Oxidation Mechanism of FeO_x Nanoparticles Confined on Fibrous Mesoporous Silica”. Those comments are valuable and very helpful in improving the quality of the work. We have studied the comments carefully and have made corrections (Modified in red). We expect that the revised manuscript can meet the criteria of Catalysts A point-to-point response to reviewers’ comments is shown below.

 

Reviewer 1

Comment 1: In page 3, the authors reported that TEM images did not show distinct lattice structures for KCC-1 and Fe2O3 over 7% Fe@KCC-1 catalyst. The status of this testing catalyst needs to be clarified (e.g., pristine or spent/reduced). TEM/SEM images for spent/reduced catalysts (after reducing Fe2O3 to Fe) need to be provided if the authors wanted to claim the anti-agglomeration (namely, anti-sintering) properties of metal particles over mesoporous silica supports. Calcination at inert gas is not a harsh condition to prove anti-agglomeration property of metal oxide particles.

Response to Comment 1: We thank the reviewer for your valuable feedback. We apologize for the lack of clarity regarding the testing status of the catalyst in the TEM images. The TEM images for reduced 7%Fe@KCC-1 catalyst have been provided in the revised Supplementary to reinforce the evidence for our findings (Figure S2). Once again, we appreciate the reviewer's insightful comments, and we will address each concern thoroughly in the revised manuscript.

Comment 2: In page 5, the authors mentioned as Fe was introduced, the temperatures corresponding to all the reduction processes moved towards lower temperatures, showing that the reduction properties of the material gradually increased. Here authors may use the peak temperature to make the comparison. Pure Fe2O3 itself (the top line in Figure 2) contains more sites for reduction, hence exhibiting higher peak temperature (because more oxygen ions from bulky phase needs to be reduced so the peak will appear late in the ramping temperate profile). Authors need to rephrase this statement.

Response to Comment 2: Thank you for your valuable feedback on the manuscript. We appreciate your attention to detail and your insights regarding our statement on page 5. We apologize for any confusion or misinformation that may have arisen from our original statement, and we thank you for pointing out this important clarification. We have revised the statement in the manuscript to make it more scientifically sound.

Comment 3: For Section 2.2, since more Fe2O3 sites would participate to the reaction of CO oxidation, catalyst with higher Fe2O3 loading is expected to exhibit lower temperature to reach the 100% of CO conversion. The authors found this general trend, but 10%Fe@FCC-1 showed lower activity compared to its counterpart with 7% Fe loading, reason needs to be given to explain this phenomenon.

Response to Comment 3: We appreciate the reviewer's comment regarding the observed lower activity of the 10%Fe@FCC-1 catalyst compared to its counterpart with 7% Fe loading in Section 2.2. We have explained this phenomenon based on the previous references. The CO conversion of 10%Fe@KCC-1 is slightly lower than that of 7%Fe@KCC-1, which can be attributed to the higher content of the active component leading to its aggregation on the surface of KCC-1, resulting in a reduction of exposed Fe active sites, which is also observed in other reported catalysts. ^{23, 24}

References

23. Feng, B.; Shi, M.; Liu, J.; Han, X.; Lan, Z.; Gu, H.; Wang, X.; Sun, H.; Zhang, Q.; Li, H.; Wang, Y.; Li, H. An efficient defect engineering strategy to enhance catalytic performances of Co₃O₄ nanorods for CO oxidation. J. Hazard. Mater. 2020, 394, 122540.
24. Wang, Y.; Liu, C.; Liao, X.; Liu, Y.; Hou, J.; Pham-Huu, C. Enhancing oxygen activation on high surface area Pd-SnO₂ solid solution with isolated metal site catalysts for catalytic CH₄ combustion. Appl. Surf. Sci. 2021, 564, 150368.

Comment 4: In order to have a better comparison for CO oxidation activity, the authors can normalize converted CO based on Fe loading amount (per gram), then compare normalized rates at different reaction temperatures. By virtue of the Arrhenius equation, activation energies among different catalysts can be calculated and then compared.

Response to Comment 4: Thank you for the valuable suggestion regarding a better comparison of CO oxidation activity in the manuscript. Normalizing the converted CO based on the Fe loading amount per gram and comparing the normalized rates at different reaction temperatures would indeed provide a more insightful analysis. By applying the Arrhenius equation, we have calculated the activation energies for the CO oxidation reaction across different catalysts and subsequently compare them. This will enable a quantitative evaluation of the catalytic performance and allow for a more comprehensive understanding of the differences observed among the catalysts. This addition will enhance the scientific rigor and contribute to a more thorough comparison of the catalytic activity.

Comment 5: For Figure 5, a zoomed-in figure especially for adsorbed CO should be provided since now this part of IR spectra (marked as pink) is nearly flat and few information can be observed.

Response to Comment 5: Thank you for your valuable feedback on Figure 5 of the manuscript. We have modified it to make the meaning clearer. As shown in Figure 5, the in situ DRIFTs versus time curves for transient reactions of 5% O₂ (30 mL·min^-1) and pre-adsorbed CO/N₂ species over 7% Fe@KCC-1 catalyst at 120 °C for 60 min. The detected peaks near 2119 cm^-1 and 2174 cm^-1 are attributed to CO species linearly adsorbed on the surface Fe³⁺ sites (Fe³⁺-CO) ²⁶. The CO peak for 60 min of adsorption faded with the passage of 5% O₂.

Reference

28. Duan, H.; Zeng, W.; Tang, X.; Yu, B.; Chen, S.; Lian, X. Theoretical investigation for the reaction of CO oxidation by N₂O on Fe₆M (M = Fe, Co, Ni, Mn) clusters. React. Kinet. Mech. Cat. 2022, 135, 3021–3030.

Comment 6: As a common sense, CO is a reducing gas and it is more active than CO2, normally CO or syngas (CO + H2) has more industrial application. Hence more words of introduction / background should be given in Section 1 to introduce the importance of CO oxidation to CO2.

Response to Comment 6: Thank you for bringing this to our attention, the revised manuscript included a more detailed introduction explaining the importance of CO oxidation to CO₂. By elaborating on these points in the introduction, you can provide a broader context for the research and underscore the practical applications and significance of studying CO oxidation catalysts.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This thesis is devoted to research in the field of catalytic oxidation of CO. This is a very actively developing field of research where a large number of different catalysts are being tested. This paper is clearly written and can be published after the following comments have been answered.

1 According to the title of the paper, the authors should disclose the mechanism of oxidation, but descriptions of such mechanism already exist in the literature https://doi.org/10.1016/j.msea.2006.05.119 . Perhaps it is necessary to emphasize the novelty of the results obtained regarding the mechanism itself, or to change the title of the paper.

2 Page 3, line 106. Figure 2c-f is displayed incorrectly. There must be a different drawing here.

3 Page 5, line 160. It is necessary to make a link to the description of the active centers of active oxygen.

4 Page 5, line 165. You should write Figure 3 instead of Figure S3.

5 The term Fe2O3 nanoparticles is sometimes used (e.g., line 108), but there is no data on the structure of this particular composition. The formula FeOx is also used in the title.

6 Page 8 line 238 Figure 10. There is no such drawing.

Author Response

Dear Editor and Reviewers,

Thank you for giving the opportunity to modify the paper again, and we also thank the reviewers for the insightful comments regarding the manuscript # Manuscript ID: catalysts-2771079 entitled “Unraveling the CO Catalytic Oxidation Mechanism of FeO_x Nanoparticles Confined on Fibrous Mesoporous Silica”. Those comments are valuable and very helpful in improving the quality of the work. We have studied the comments carefully and have made corrections (Modified in red). We expect that the revised manuscript can meet the criteria of Catalysts A point-to-point response to reviewers’ comments is shown below.

 

Reviewer 2

This thesis is devoted to research in the field of catalytic oxidation of CO. This is a very actively developing field of research where a large number of different catalysts are being tested. This paper is clearly written and can be published after the following comments have been answered.

1. According to the title of the paper, the authors should disclose the mechanism of oxidation, but descriptions of such mechanism already exist in the literature https://doi.org/10.1016/j.msea.2006.05.119 . Perhaps it is necessary to emphasize the novelty of the results obtained regarding the mechanism itself, or to change the title of the paper.

Response 1: We thank the reviewer for your valuable feedback. We have changed the title to make it more relevant to the main idea of this manuscript.

2. Page 3, line 106. Figure 2c-f is displayed incorrectly. There must be a different drawing here.

Response 2: Thank you for bringing this issue to our attention. We apologize for the error in Figure 2c-f on page 3, line 106. Upon review, we have identified the mistake and will correct it in the revised version of manuscript. We will ensure that the correct drawings are included for Figure 2c-f to accurately represent the data and findings of our study.

3. Page 5, line 160. It is necessary to make a link to the description of the active centers of active oxygen.

Response 3: Thank you for bringing this issue to our attention. We have added references to this analytical statement to make it more scientifically sound.

4. Page 5, line 165. You should write Figure 3 instead of Figure S3.

Response 4: Thank you for bringing this issue to our attention. We apologize for the error in Figure 3 on page 5, line 165. Upon review, we have identified the mistake and will correct it in the revised version of manuscript.

5 The term Fe2O3 nanoparticles is sometimes used (e.g., line 108), but there is no data on the structure of this particular composition. The formula FeOx is also used in the title.

Response 5: Thank you very much for pointing out the problem, we have made a distinction between Fe₂O₃ and FeO_x in the manuscript. In general, Fe₂O₃ specialises in ferric oxide species, while FeO_x refers to the iron oxide species loaded in KCC-1.

6. 6. Page 8 line 238 Figure 10. There is no such drawing.

Response 6: Thank you for bringing this issue to our attention. We apologize for the error in Figure 10 on page 8, line 238. Upon review, we have identified the mistake and will correct it in the revised version of manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper “Unraveling the CO Catalytic Oxidation Mechanism of FeOx Nanoparticles Confined on Fibrous Mesoporous Silica” discusses an interesting topic, on new catalytic systems. The manuscript is well written and rather well argumented, but with numerous small errors.

After reviewing the paper I recommend that it should be accepted after minor revision. Here are some recommendations:

- rows 16-17 and row 190: the authors write “Catalytic oxidation is used to control carbon monoxide (CO) emissions from automobile exhaust”. While this is absolutely true, the control of automobile exhaust involves many other oxidation and reduction reactions for a complex mixture of pollutants. If the catalysts tested in this paper cannot fulfill the same tasks the authors should not imply this. There are other important applications for the CO oxidation reactions, such as the fuel cells, where CO concentrations should be very low, therefore I recommend the authors do not mention automobile exhaust as an application for their catalysts.

- rows 65-66: “the catalyst showed a mineralization rate of about 40% over a wide pH range.” – what is the relevance of this phrase in the context of the manuscript?

- Figure 1: the SEM images in figures a-f should be atributted to the specific catalysts instead of letting the reader guess to which catalyst is each one referring;

- rows 110 and 260 (Conclusions): “some of which are even smaller than 10 nm” – please explain how you reached this conclusion;

- Figure S3: please modify the marker sizes in order for the curves to be seen;

- rows 132-133: “Pure Fe₂O₃ was found to have a specific surface area of only 4 m²·g^-1 and a pore volume of only 1.68 cm³·g^-1” – the pore volume value in Table 1 is different, please chek and correct;

- rows 133-134: “Fe₂O₃ is similarly porous, but has an almost nil pore diameter” – what difference there is between “pure Fe₂O₃” and “Fe₂O₃”? Where are the values for the latter is Table 1?

- row 137: “both x%Fe@KCC-1 and 7%Fe@KCC-1” – the 7%Fe sample is included in the x%Fe samples, so why mentioning both?

- rows 139-140: “the specific surface area of x%Fe@KCC-1 (x = 0, 1, 3, 5, 7 and 10) catalysts decreased from 576 m²·g^-1 to 397 m²·g^-1” if the sample with 0% Fe is included, then the specific surface area is 645 m²·g^-1, not 576 m²·g^-1;

- rows 144-145: “Pore size distribution analysis of x%Fe@KCC-1 showed that mesopores of 2.7 nm size were embedded in the floral fibre microspheres of KCC-1 molecular sieves” – this is not visible in Figure S3, so please explain;

- rows 155-156: “As Fe was introduced, the temperatures corresponding to all the reduction processes moved towards lower temperatures” – this is not visible in Figure 2 for the peak at lower temperature, so please rephrase.

- row 165: the catalytic activities are presented in Figure 3, not S3;

- rows 231-232: “Figure 6 shows the transient in situ DRIFTs curves” – Figure 6 is not showing DRIFT curves;

- row 238: “Figure 10” does not exist;

- row 240: “to form species” – what species?

- rows 242 and 245: “formed a perovskite structure (FeO_x+1).” – please explain why do you consider this a perovskite phase;

- rows 247-248: “Thus, the FeO_x-1 structure is recovered by oxidation of O₂ gas molecule” – this is confusing. The sequence explained in the pragraph above is this: CO and O₂ are adsorbed and react to form CO₂ in adsorbed state. Then CO₂ is desorbed and the oxygen remaining on the catalyst surface is removed by a new CO molecule that is adsorbed on Fe sites, forming a new CO₂ molecule. What oxidation of O₂ gas molecule is discussed here?

- rows 281- 282: preparation “in compliance with previously reported methods” – please give the references;

- row 297: “homogeneous precipitation” – precipitating Fe(OH)₃ from iron nitrate and ammonia is a simple precipitation, why did the authors name it homogeneous? The precipitating agent is not generated in the solution, is just added to the solution;

Grammar and Spelling:

- rows 25-26: “The CO catalytic removal mechanism was favored by a combination of in situ DRIFTS and DFT calculations” – should be changed, maybe to investigated;

 

 

Author Response

Dear Editor and Reviewers,

Thank you for giving the opportunity to modify the paper again, and we also thank the reviewers for the insightful comments regarding the manuscript # Manuscript ID: catalysts-2771079 entitled “Unraveling the CO Catalytic Oxidation Mechanism of FeO_x Nanoparticles Confined on Fibrous Mesoporous Silica”. Those comments are valuable and very helpful in improving the quality of the work. We have studied the comments carefully and have made corrections (Modified in red). We expect that the revised manuscript can meet the criteria of Catalysts A point-to-point response to reviewers’ comments is shown below.

 

Reviewer 3

The paper “Unraveling the CO Catalytic Oxidation Mechanism of FeOx Nanoparticles Confined on Fibrous Mesoporous Silica” discusses an interesting topic, on new catalytic systems. The manuscript is well written and rather well argumented, but with numerous small errors.

After reviewing the paper I recommend that it should be accepted after minor revision. Here are some recommendations:

- rows 16-17 and row 190: the authors write “Catalytic oxidation is used to control carbon monoxide (CO) emissions from automobile exhaust”. While this is absolutely true, the control of automobile exhaust involves many other oxidation and reduction reactions for a complex mixture of pollutants. If the catalysts tested in this paper cannot fulfill the same tasks the authors should not imply this. There are other important applications for the CO oxidation reactions, such as the fuel cells, where CO concentrations should be very low, therefore I recommend the authors do not mention automobile exhaust as an application for their catalysts.

Response: Thank you for the suggested changes, we have removed the inappropriate industry background description!

- rows 65-66: “the catalyst showed a mineralization rate of about 40% over a wide pH range.” – what is the relevance of this phrase in the context of the manuscript?

Response: Thank you for pointing this out, we have removed the expression that is not relevant to this manuscript.

- Figure 1: the SEM images in figures a-f should be atributted to the specific catalysts instead of letting the reader guess to which catalyst is each one referring;

Response: Apologies for our illogical nomenclature, we have changed the figure caption of Figure 1 to ensure a better understanding for the reader.

- rows 110 and 260 (Conclusions): “some of which are even smaller than 10 nm” – please explain how you reached this conclusion;

Response: Thank you for pointing this out, we have removed unjustified statements from the manuscript to ensure its scientific validity.

- Figure S3: please modify the marker sizes in order for the curves to be seen;

Response: Thank you for your careful review, we have modified the marker sizes in order for the curves to be seen.

- rows 132-133: “Pure Fe₂O₃ was found to have a specific surface area of only 4 m²·g^-1 and a pore volume of only 1.68 cm³·g^-1” – the pore volume value in Table 1 is different, please chek and correct;

Response: Thank you for your careful review and for pointing out this error, which greatly enhances the rigour of this manuscript, which we have revised.

- rows 133-134: “Fe₂O₃ is similarly porous, but has an almost nil pore diameter” – what difference there is between “pure Fe₂O₃” and “Fe₂O₃”? Where are the values for the latter is Table 1?

Response: Thank you very much for pointing out the problem, we have made a distinction among Fe₂O₃, pure Fe₂O₃ and FeO_x in the manuscript. In general, pure Fe₂O₃ refers to pure Fe₂O₃ catalysts, Fe₂O₃ specialises in ferric oxide species, while FeO_x refers to the iron oxide species loaded in KCC-1.

- row 137: “both x%Fe@KCC-1 and 7%Fe@KCC-1” – the 7%Fe sample is included in the x%Fe samples, so why mentioning both?

Response: Thank you very much for pointing out the error. This was a clerical error on our part and has now been corrected.

- rows 139-140: “the specific surface area of x%Fe@KCC-1 (x = 0, 1, 3, 5, 7 and 10) catalysts decreased from 576 m²·g^-1 to 397 m²·g^-1” if the sample with 0% Fe is included, then the specific surface area is 645 m²·g^-1, not 576 m²·g^-1;

Response: Thank you for the suggested changes, we have revised the values (change 576 m²·g^-1 to 645 m²·g^-1) in the manuscript.

- rows 144-145: “Pore size distribution analysis of x%Fe@KCC-1 showed that mesopores of 2.7 nm size were embedded in the floral fibre microspheres of KCC-1 molecular sieves” – this is not visible in Figure S3, so please explain;

Response: I apologized for the confusion caused by the incorrect value in the manuscript, where 2.7 nm should be 3.1 nm, and we have corrected it.

- rows 155-156: “As Fe was introduced, the temperatures corresponding to all the reduction processes moved towards lower temperatures” – this is not visible in Figure 2 for the peak at lower temperature, so please rephrase.

Response: Thank you for your valuable feedback on the manuscript. We appreciate your attention to detail and your insights regarding our statement in Figure 2. We apologize for any confusion or misinformation that may have arisen from our original statement, and we thank you for pointing out this important clarification. We have revised the statement in the manuscript to make it more scientifically sound.

- row 165: the catalytic activities are presented in Figure 3, not S3;

Response: Thank you very much for pointing out the error. This was a clerical error on our part and has now been corrected.

- rows 231-232: “Figure 6 shows the transient in situ DRIFTs curves” – Figure 6 is not showing DRIFT curves;

Response: Thank you very much for pointing out the error. This was a clerical error on our part and has now been corrected.

- row 238: “Figure 10” does not exist;

Response: Thank you very much for pointing out the error. This was a clerical error on our part and has now been corrected.

- row 240: “to form species” – what species?

Response: Thank you for your detailed review, we have added to the analysis of this paragraph to make it scientifically sound.

- rows 242 and 245: “formed a perovskite structure (FeO_x+1).” – please explain why do you consider this a perovskite phase;

Response: Thank you for pointing out the implausible statement, we have revised “perovskite” to “peroxide”.

- rows 247-248: “Thus, the FeO_x-1 structure is recovered by oxidation of O₂ gas molecule” – this is confusing. The sequence explained in the pragraph above is this: CO and O₂ are adsorbed and react to form CO₂ in adsorbed state. Then CO₂ is desorbed and the oxygen remaining on the catalyst surface is removed by a new CO molecule that is adsorbed on Fe sites, forming a new CO₂ molecule. What oxidation of O₂ gas molecule is discussed here?

Response: We apologized for this unclear description. The transfer of oxygen species from oxygen molecules in the adsorbed state to oxygen-containing vacancy structures promotes the restoration of the Fe₂O₃ structure, that is FeO_x-1 + O_2ads → FeO_x+O_ads.

- rows 281- 282: preparation “in compliance with previously reported methods” – please give the references;

Response: Thank you for your careful review, we have provided previous literature to support the statement.

- row 297: “homogeneous precipitation” – precipitating Fe(OH)₃ from iron nitrate and ammonia is a simple precipitation, why did the authors name it homogeneous? The precipitating agent is not generated in the solution, is just added to the solution;

Response: Thank you for your careful review, We have revised the manuscript for statements that do not make sense.

Grammar and Spelling:

- rows 25-26: “The CO catalytic removal mechanism was favored by a combination of in situ DRIFTS and DFT calculations” – should be changed, maybe to investigated;

Response: Thank you for the suggested changes, we have revised the values (change favored to investigated) in the manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors offer their new insights for the application of nano-catalysts in important oxidation reactions and the development of efficient CO oxidation catalysts in this paper. They have thoroughly addressed my comments, and now the manuscript has been carefully revised and exhibited a better demonstration. The manuscript can be accepted in this present form.