H₂ Production from Catalytic Methane Decomposition Using Fe/x-ZrO₂ and Fe-Ni/(x-ZrO₂) (x = 0, La₂O₃, WO₃) Catalysts

Round 1

Reviewer 1 Report

The authors present the preparation and characterization of different Fe/x-ZrO2 and Fe-Ni/(x-ZrO2) (x = 0, La2O3, WO3) catalysts for the catalytic decomposition of methane to produce hydrogen and nanostructured carbon. Although this work falls within the topics of the Catalysts, the English of the manuscript should be corrected in depth. Likewise, the authors should undertake the following changes.              

1. Abstract: what “TPR/O” is? It must be changed to TPO. Additionally, H2-TPR is not necessary since TPR is enough.
2. Introduction: the authors do not mention works on Ni-Fe or Fe-Ni catalysts for this reaction.
3. The catalyst preparation must be clarified since some aspects are missing. On the other hand, the authors said “Fe and Fe-Ni contents were maintained at 20 wt. %.” but this is not clear enough. What is the Fe/Ni ratio used? Is this percentage refereed to the amount of metal in the fresh or reduced catalyst? If this is the content on the fresh catalysts, Fe and Ni are oxidized. Also, the amount of Fe and Ni in the reduced catalyst must be calculated and included in the manuscript. Please, clarify this.
4. The comparison of catalysts in terms of H₂ or carbon yield is limited or none. Table 2 only shows Fe catalyst references. The carbon formation must be also tabulated. No Fe-Ni articles are referenced for comparison. References for this and for the Introduction section:

• Torres, D.; Pinilla, J.L.; Suelves, I. Catalysts. 2018, 8, 300
• Wang, W.; Wang, H.; Yang, Y.; Jiang, S. J. Hydrogen Energy. 2012, 37, 9058.
• Bayat, N.; Rezaei, M.; Meshkani, F. J. Hydrogen Energy. 2016, 41, 1574.

5. Format: The size of the figures is too big. Also their titles. Please correct this.
6. Figure 1. The phases could be introduced to know the identity of each peak.
7. Figure 2 is not clear. The authors should split this into two figures. Additionally, spent catalysts do not show microporosity? What the carbon ratio in these catalysts? Carbon present microporosity if it is in a large amount in the catalyst after use.
8. Figure 4. Data would be easier to follow if they are put as lines instead of columns.
9. Figure 6. Two catalysts pointed to the red line.
10. Results must be separated in 3 big sections: “characterization of catalysts”, “catalytic tests” or “catalytic decomposition of methane” and “characterization of spent catalyst and carbon produced”
11. Reference 2 must be checked.

Author Response

Comments and Suggestions of Reviewers

Reviewer #1:

The authors are grateful to the reviewer for the comments which we hope will make our research article worthy of publication. Thus, we have responded to them to the best of our ability.

Comment #1:

The English of the manuscript should be corrected in depth

Response #1:

The English of the revised manuscript is thoroughly revised.

Comment #2:

Abstract: what “TPR/O” is? It must be changed to TPO. Additionally, H2-TPR is not necessary since TPR is enough.

Response #2:

The authors appreciate the comment of the reviewer. Our intention was to represent TPR and TPO as TPR/O since both has TP (Temperature Programmed) in common. But it has been changed to TPO as suggested.

Comment #3:

Introduction: the authors do not mention works on Ni-Fe or Fe-Ni catalysts for this reaction.

Response #3:

The authors are much obliged to the reviewer for calling our attention to acknowledging related research that involves catalysts with composition like Ni-Fe. Some important research works as regard this have been added.

Comment #4:

The catalyst preparation must be clarified since some aspects are missing. On the other hand, the authors said “Fe and Fe-Ni contents were maintained at 20 wt. %.” but this is not clear enough. What is the Fe/Ni ratio used? Is this percentage refereed to the amount of metal in the fresh or reduced catalyst? If this is the content on the fresh catalysts, Fe and Ni are oxidized. Also, the amount of Fe and Ni in the reduced catalyst must be calculated and included in the manuscript. Please, clarify this.

Response #4:

We have added some explanation to clarify the catalyst preparation section. The percentage loading quoted is for the fresh catalyst samples and Fe/Ni ratio in the bimetallic is 1. We couldn’t agree more with the reviewer in determining the amount of Fe and Ni in the reduced catalysts, however, the research center that does Elemental Dispersive X-ray (EDX) or ICP-MS analysis for us is yet to open due to the global coronavirus outbreak.

Comment #5:

The comparison of catalysts in terms of H2 or carbon yield is limited or none. Table 2 only shows Fe catalyst references. The carbon formation must be also tabulated. No Fe-Ni articles are referenced for comparison. References for this and for the Introduction section:

Torres, D.; Pinilla, J.L.; Suelves, I. Catalysts. 2018, 8, 300

Wang, W.; Wang, H.; Yang, Y.; Jiang, S. J. Hydrogen Energy. 2012, 37, 9058.

Bayat, N.; Rezaei, M.; Meshkani, F. J. Hydrogen Energy. 2016, 41, 1574.

Response #5:

We are grateful to the reviewer for this comment. Additional table i.e. Table 3, that compares the carbon yield from the past studies with the present work has been added. Also, Fe-Ni articles have been referenced in Table 2 and the introductory section in lines 80 – 86.

Comment #6:

Format: The size of the figures is too big. Also their titles. Please correct this. 

Response #6:

We have adjusted the size of the figures as well as that of the titles.

Comment #7:

Figure 1. The phases could be introduced to know the identity of each peak.

Response #7:

The authors agree with the reviewer; hence symbols have been added to the figure for the identification of the peaks.

Comment #8:

Figure 2 is not clear. The authors should split this into two figures. Additionally, spent catalysts do not show microporosity? What the carbon ratio in these catalysts? Carbon present microporosity if it is in a large amount in the catalyst after use.

Response #8:

We tender our unreserved apologies for the ambiguity in Figure 2. The figures are actually for fresh catalyst samples. They represent the N₂ adsorption-desorption isotherms and pore size distribution of the fresh catalysts. They have been separated for clarity.

Comment #9:

Figure 4. Data would be easier to follow if they are put as lines instead of columns.

Response #9:

Figure 4 has been changed to lines for the ease of readability.

Comment #9:

Figure 6. Two catalysts pointed to the red line.

Response #10:

We agreed with the reviewer. The mistake has been corrected.

 

 

Comment #11:

Results must be separated in 3 big sections: “characterization of catalysts”, “catalytic tests” or “catalytic decomposition of methane” and “characterization of spent catalyst and carbon produced”

Response #11:

The results section has been separated into 3 big sections as shown in lines 111, 167 and 212.

Comment #12:

Reference 2 must be checked.

Response #12:

Reference 2 has been replaced with a better one.

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

This short paper deals with the methane decomposition over Fe-Ni catalysts. However, some points need a further explanation/discussion. Following are some comments/questions to be taken into consideration by the authors.

Page 2. Introduction. The authors should more clearly specify the so-called acidity of WO3.

Apparently the composition of the investigated samples is not experimentally measured. These data are of great importance for understanding the results derived from the used characterisation techniques.

Figure 1. The characteristic diffraction lines should be marked in the figure.

Table 1B. The accuracy of the data from textural characterisation is very high. Unless at least triplicate measurements have been made, the precision of these data should be revised.

Even with a metallic loading of 20%wt, the dramatic decrease in surface area of the catalysts supported on bare ZrO2 is really dramatic, from 325 to 7-13 m2/g. Do the authors have a convincing explanation?

H2-TPR profiles have not been quantitatively analysed. Is the H2 uptake coherent with the metallic loading of the samples?

Suggestion for figure 4. For a better, quick understanding, instead using bars, the evolution of H2 yield with time could be best depicted with symbols.

The role of the support and its composition is not deeply discussed.

Apparently Figure 5 and 6 provide the same information and conclusions for the characterisation of coke deposits.

Author Response

Comments and Suggestions of Reviewers

 

Reviewer #2:

The authors are much obliged to the reviewer for their comments which we believe will make our research article worthy of being published. Therefore, we have responded to the comments to the best of our ability.

Comment #1:

Page 2. Introduction. The authors should more clearly specify the so-called acidity of WO₃.

Response #1:

Brief discussion about the special acidity of WO₃ has been added to the introductory section at line 94.

Comment#2:

Apparently the composition of the investigated samples is not experimentally measured. These data are of great importance for understanding the results derived from the used characterisation techniques.

Response #2:

The authors appreciate the comment of the reviewer. We have made efforts towards doing this, but the research Center that could do the Elemental Dispersion X-ray (EDX) analysis or ICP-MS is closed due to the covid-19 outbreak.

Comment #3:

Figure 1. The characteristic diffraction lines should be marked in the figure.

Response # 3:

The characteristic diffraction peaks showing the identified phases have been marked.

Comment #4:

Table 1B. The accuracy of the data from textural characterization is very high. Unless at least triplicate measurements have been made, the precision of these data should be revised.

 

Response #4:

The authors are grateful for this and the precision of the data in table 1B has been revised.

Comment #5:

Even with a metallic loading of 20%wt, the dramatic decrease in surface area of the catalysts supported on bare ZrO2 is really dramatic, from 325 to 7-13 m²/g. Do the authors have a convincing explanation?

Response #5:

We are grateful to the reviewer for calling our attention to this. The BET surface areas of the support in table 1 are as-prepared from the manufacturer, without calcination. We have calcined them at the same condition as our catalyst samples and carried out BET analysis on the calcined supports. The results are tabulated below.

Catalyst supports	Specific surface area (m2/g)
	From manufacturer	After calcination
ZrO2	325	58.7
WO3+ZrO2	112	97.7
La2O3+ZrO2	67.3	62.8

 

 

Comment #6:

H₂-TPR profiles have not been quantitatively analysed. Is the H₂ uptake coherent with the metallic loading of the samples?

Response #6:

H₂-TPR profiles have been quantitatively analyzed using 0.07g of catalyst. The values obtained correlate with the specified loading of Fe and Fe-Ni.

 

Comment #7:

Suggestion for figure 4. For a better, quick understanding, instead using bars, the evolution of H₂ yield with time could be best depicted with symbols.

Response #7:

The figure has been adjusted for better understanding of the H₂ yield.

Comment #8:

The role of the support and its composition is not deeply discussed.

Response #8:

Additional information about the support has been added to the manuscript in lines 94-97 and 179-182.

Comment #9:

Apparently Figure 5 and 6 provide the same information and conclusions for the characterisation of coke deposits.

Response #9:

The authors are much obliged to the reviewer and would like to point out that Figure 5 only gave a quantification of the amount of carbon deposits on the used catalysts while Figure 6 gave a classification of the carbon deposits as a function of the temperature at which the oxidation takes place.

 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

This work presents experimental  results  of  methane decomposition using Fe and Fe-Ni catalysts which were dispersed over different supports by the wet-impregnation method. The authors observed the effect of modifying ZrO2 with La2O3 and WO3 in terms of H2 yield and carbon deposits. The modification led to higher H2 yield in all cases and WO3-modified support gave the highest yield of about 90% and was stable throughout the reaction period. Adding Ni to Fe/x-ZrO2 gave higher H2 yield and stability for ZrO2 and La2O3+ZrO2 supported catalysts whose prior performance and stability were very poor. In these studies extensive diagnostic techniques (XRD, BET, TPR/O, TGA and Raman spectroscopy) were used.

The research area is of interest to both scientists and engineers.

Generally, the quality of this paper is quite good, including quality of the figures and tables. I can recommend it for publication after minor editorial corrections.

I have minor comments:

1. Lines 221 and 278: "-1" in the expression "cm-1" should appear as superscript;
2. Line 19: "cat" in the expression "mL(hgcat)-1" should appear as subscript;
3. Lines 185 and 282: "-1" in the expression "mL(hgcat)-1" should appear as superscript, but “cat” as subscript;
4. In some places, spaces between the number and the sign "℃" should be removed;
5. Table 2 requires a commentary regarding comparison with references [23], [24] and [25].

Author Response

Comments and Suggestions of Reviewers

Reviewer #3:

The authors are grateful to the reviewer for the comments which we hope will make the research article worthy of being published. Owing to that, we have responded to the comments leaving no stones unturned.

Comment #1:

Lines 221 and 278: "-1" in the expression "cm-1" should appear as superscript.

Response #1:

The “-1” and "cm-1" have been changed to superscript.

Comment #2:

Line 19: "cat" in the expression "mL(hgcat)-1" should appear as subscript.

Response #2:

We have changed the "cat" in the expression "mL(hgcat)-1" to subscript.

Comment #3:

Lines 185 and 282: "-1" in the expression "mL(hgcat)-1" should appear as superscript, but “cat” as subscript;

Response #3:

We have adjusted this and ensured subscript and superscript at the appropriate place as suggested.

Comment #4:

In some places, spaces between the number and the sign "℃" should be removed;

Response #4:

We have removed the space between number and the sign "℃".

Comment #5:

Table 2 requires a commentary regarding comparison with references [23], [24] and [25].

Response #5:

The authors have added a commentary as regard comparing present work with the references like [23], [24], [25] and others.

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Authors addressed the suggestions and the revised manuscript is suitable to be accepted in Catalysts 

Author Response

Thanks

Reviewer 2 Report

Most of comments/suggestions have been taken into consideration. The paper is now ready for publication.

Author Response

Thanks