Hydrogen Production by Partial Oxidation Reforming of Methane over Ni Catalysts Supported on High and Low Surface Area Alumina and Zirconia
, Samsudeen O. Kasim 1, Sofiu Mahmud 1 and Ahmed S. Al-Fatesh 1,*Round 1
Reviewer 1 Report
A critical point is regarding the best sample catalyst. The authors say that the “The Ni-Al-H-600 catalyst illustrates better methane conversion than the other catalysts; the conversion reached 90% at 650°C.” (row 265).
From TPR-H2 (figure 4) it is possible to see that this sample catalysts isn’t fully reduced. The reduction condition declared by authors are “ catalyst was activated inside the reactor at 600 °C by passing hydrogen at a rate 104 of 40 mL/min for 2 h followed by 20 min of N2 at a rate of 30 mL/min” then the question is : How the authors explain the better catalytic performance of Ni-Al-H-600 with respect to other fully activated sample catalysts?
What was the H2 consumption of the samples? Was the total amount compatible with Ni2+ reduction to Ni° and NO3- reduction processes?
The nominal content of Nickel correspond to the real content on the sample catalysts?
Another critical point : why the increasing of temperature promote the coke formation? In the scientific literature it is reported opposite trend. The authors can explain the reaction that occurs? Then the sentence “As the reaction temperature was increased, the production of carbon grew up.”(row 349) should be explained.
What is the particle size used for the experiments? This can have a strong influence on internal mass transfer limitations.
A deep characterization of spent catalysts should be added. In particular the TEM characterization could to allows to identify the different carbon species customarily assessed by Raman analysis.
The discussion part in the manuscript is very limited. The author should provide more discussion about the role of support and each active sites in this case in order to prove the originality of the prepared catalysts.
Due to the fact that the reaction has a change in the number of molecules between reactants and products, the answer to the following question should be addressed in the manuscript: How was the molar change taken into account to calculate the composition in the reactor outlet? How calculated the conversion of CH4 and yield?
In catalytic results the H2/CO molar ratio should be reported in order to understand what secondary reactions occur.
Some comparisons with other Ni supported catalysts to POM reaction should be reported.
The resolution of the figures is rather low . Fig.1, fig.2, fig. 3 are not aligned and have different dimension , the tick number in the Fig.1 are too little
The caption of figure 4 is wrong, also the legend in figure 6 (twice Ni-Zr-600).
The quality of figure 7 is very poor.
Overall, the paper needs significant editing, but the technical portion is interesting and worthy of publication.
Author Response
Comments and Suggestions of Reviewer
Reviewer #1:
The authors are very appreciative to comments and suggestions of the reviewer, which enhanced the standard of the present manuscript.
Comment #1:
From TPR-H2 (figure 4) it is possible to see that this sample catalyst isn’t fully reduced. The reduction condition declared by authors are “catalyst was activated inside the reactor at 600 °C by passing hydrogen at a rate 104 of 40 mL/min for 2 h followed by 20 min of N2 at a rate of 30 mL/min” then the question is: How the authors explain the better catalytic performance of Ni-Al-H-600 with respect to other fully activated sample catalysts?
Response #1:
The authors are grateful for the keen observation of the reviewer. In fact the activation was performed at 800 °C not 600 °C. The statement is corrected in the revised manuscript.
Comment #2:
What was the H2 consumption of the samples? Was the total amount compatible with Ni2+ reduction to Ni° and NO3- reduction processes?
Response #2:
Total H2 consumption of the catalyst reduction was inserted in table 1 (please, see the last column), and the nickel reduction was found harmonious with the total H2 consumption
Comment #3:
The nominal content of Nickel corresponds to the real content on the sample catalysts?
Response #3:
The ICP metal oxide microanalysis of the fresh nickel supported alumina and zirconia catalysts was performed. It gave values of Ni close to nominal value as shown in the table below.
Table 1. ICP metal oxide microanalysis of the calcined catalysts
|
Ni-Al-H-600 |
||
|
Catalyst component |
NiO |
Al2O3 |
|
Theoretical wt/wt% |
10.00 |
90.00 |
|
Experimental Wt/wt%
|
10.23 |
89.77 |
|
Ni-Zr-H-600 |
||
|
Catalyst component |
NiO |
ZrO2 |
|
Theoretical wt/wt% |
10.00 |
90.00 |
|
Experimental Wt/wt%
|
10.31 |
89.69 |
Comment #4:
Another critical point: why the increasing of temperature promotes the coke formation? In the scientific literature it is reported opposite trend. The authors can explain the reaction that occurs? Then the sentence “As the reaction temperature was increased, the production of carbon grew up.”(row349) should be explained.
Response #4:
We found slight increase of carbon formation from 4% at 500 °C to 8% at 650 °C, as result of increased reaction temperature. We agree with the reviewer, that the rate of gasification increases with the increase of temperature. However, in this case, the rate of carbon formation surpasses its rate of removal and the apparent slight increase is observed.
Comment #5:
What is the particle size used for the experiments? This can have a strong influence on internal mass transfer limitations.
Response #5:
The particle size analysis was done and it is as shown in the table below. Indeed, particle size can have a great influence on the mass transfer limitations. Based on the particle size obtained and other parameters, the Weisz-Prater equation (equation 1) was used to determine the effect of internal mass transfer limitations.
… (1)
Where -rA(obs) is the observed rate of reaction, density of the bulk catalyst, R average radius of the catalyst particle, De effective diffusivity and CAs concentration at the surface. According to the criterion, if <<1 the effect of mass transfer limitation is negligible. Inserting the parameters of this present study in the Weisz-Prater equation, the criterion is satisfied. Thus, the mass transfer limitation has no significant effect.
Table 2. Particle size analysis of the prepared catalysts
|
|
Particle size in nm |
||||
|
SAMPLE |
D10 |
D50 |
D90 |
||
|
10Ni-Al-L-800 |
177.1 |
257.6 |
466.7 |
||
|
10Ni-Al-H-600 |
407 |
708.1 |
1051 |
||
|
10Ni-Al-H-800 |
420.5 |
604.2 |
1169.3 |
||
|
10Ni-Zr-L-600 |
507.6 |
709.1 |
1151 |
||
|
10Ni-Zr-L-800 |
2358.6 |
2813.6 |
3467.4 |
||
|
10Ni-Zr-H-600 |
1893.9 |
2314.1 |
3282.2 |
||
|
|
|||||
|
|
|||||
Comment #6:
A deep characterization of spent catalysts should be added. In particular the TEM characterization could to allow to identify the different carbon species customarily assessed by Raman analysis.
Response #6:
The authors agree with the reviewer, Characterizations with TEM will boost further the manuscript. However due to COVID -19 outbreak, the TEM and many other analyses techniques are not available for now.
Comment #7:
The discussion part in the manuscript is very limited. The author should provide more discussion about the role of support and each active sites in this case in order to prove the originality of the prepared catalysts.
Response #7:
The influence of catalyst support has been discussed in the introduction section and extra references were added to strengthen the effect of support type.
Comment #8:
Due to the fact that the reaction has a change in the number of molecules between reactants and products, the answer to the following question should be addressed in the manuscript: How was the molar change taken into account to calculate the composition in the reactor outlet? How calculated the conversion of CH4 and yield?
Response #8:
The authors are highly grateful to the reviewer for the invaluable questions raised and thus feel obliged to answer them to the best of our ability. According to the experimental set up, the composition at the reactor outlet goes to the gas chromatograph (GC) that gives the amount of the individual gas that makes up the composition. It is well known that the concentration of any gas in the composition that enters the GC is solely a function of the partial pressure of the gas.
So the higher the vapor pressure of a gas, the higher will be its concentration in the composition. By proper calibration, the GC takes care of the molar change to give the amount of each gas that makes up the composition at the reactor outlet.
The CH4 conversion and H2 yield are calculated according to the following relation:
Where “in” and “out” refer to the amount of the gas in the feed and product stream, respectively given by the GC. The relations have been added to the manuscript.
Despite the above elaborated discussion, it is very important to mention that the volume of the gas to be analyzed is controlled by the size of the sample loop which is fixed in our analysis.
Comment #9:
In catalytic results the H2/CO molar ratio should be reported in order to understand what secondary reactions occur.
Response #9:
The H2/CO ratio is computed as shown in the table below. The results indicate that side reactions offset the expected values, however, as the temperature of the system increases, the ratio progressively approaches the theoretical values.
|
Temperature ( °C ) |
H2/CO Average value |
|
500 |
3.73 |
|
550 |
2.71 |
|
600 |
2.36 |
|
650 |
2.1 |
Comment #10:
Some comparisons with other Ni supported catalysts to POM reaction should be reported.
Response #10:
The authors are thankful to reviewer for the constructive suggestions. A comparison was carried out among the works accomplished before and this work as shown in table below. The table is also inserted in the revised manuscript.
Table 2 comparison of catalytic performance of partial oxidation of methane
|
Catalyst |
Weight (g) |
CH4:O2 |
Space Velocity (mL/g/h) |
Reaction Temperature (°C) |
% CH4 conversion |
Reference |
|
25%Ni/Al2O3+TiO2+CaO |
0.05 |
1.78:1.00 |
6x104 |
650 |
86 |
[24] |
|
10%Ni/Ce0.7Zr0.3O2-Al2O3 |
0.5 |
2.00:1.00 |
4x104 |
650 |
67.8 |
[25] |
|
La2NiZrO6 |
0.01 |
2.00:1.00 |
300x104 |
750 |
40 |
[26] |
|
8%Ni/CeO2 -ZrO2 -Al2O3 |
0.15 |
2.00:1.00 |
20x104 |
650 |
88.5 |
[37] |
|
6%Ni/SiO2 |
0.1 |
2.00:1.00 |
6x104 |
600 |
85 |
[38] |
|
Ni-Al-H-600 |
0.1 |
2.00:1.00 |
1.95 x104 |
650 |
90 |
This work |
Comment #11:
The resolution of the figures is rather low. Fig.1, fig.2, fig. 3 are not aligned and have different dimension, the tick number in the Fig.1 are too little
Response #11:
The resolutions of the figures are improved and the figure formats are reviewed
Comment #12:
The caption of figure 4 is wrong, also the legend in figure 6 (twice Ni-Zr-600).
Response #12:
The caption of figure 4 is corrected and the legend of figure 6 is correctly adjusted.
Comment #13:
The quality of figure 7 is very poor.
Response #13:
Figure is redrawn to improve its quality.
Comment #14:
Overall, the paper needs significant editing, but the technical portion is interesting and worthy of publication.
Response #14:
The authors are grateful for the reviewer for constructive suggestions and corrections and the revised manuscript is thoroughly edited.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscripted presented by Al-Fatesh et al. reports on the H2 production from CH4 over supported Ni catalysts. Interestingly, the researchers achieved up to 90% methane conversion together with a good H2 yield. In addition, they found that the new catalysts feature low undesirable carbon deposition.
Overall, the manuscript is very well written and contains interestng new data, which will useful for other researchers in the field. I would recomend to publish it as it is.
Author Response
Comments and Suggestions of Reviewe
Comments and Suggestions of Reviewer #2:
Comment
Overall, the manuscript is very well written and contains interesting new data, which will useful for other researchers in the field. I would recommend to publish it as it is.
Response
The authors are very grateful to the reviewer for his encouraging comments.
Author Response File: Author Response.docx
Reviewer 3 Report
The paper by the Al-Fatesh group studies the transformation of an oxygen/methane mixture to CO/hydrogen, catalyzed by a Ni/alumina composite. This reviewer's background is in solid materials characterization, not in petroleum or energy research. However, it seems obvious that the topic is of great commercial interest. On the other hand, I do not believe that the authors did a careful job of explaining the distinction between their work and other reports of Ni/Al2O3 and possibly Zr catalysts in the literature. In particular, there is a paper (Lunsford et al, J. Catalysis, 132, 117-127, 1991) with over 700 citations that essentially studies the same system, even arriving at a similar temperature for their optimized yield (700°C vs. 650 for the present study) Dozens of other papers deal with a Ni/Al2O3 catalyst, but the authors only cite a few and do so in a distant way (see for instance line 69, which needs a nice lineup of citations).
Some improvement in experimental methods and data analysis description would be useful. For instance, N2 sorption experiments are described, but no fitting parameters, histograms, etc. are provided. This reviewer, a chemist, would also like more details about the catalyst preparation (2.1). Was the reaction stirred? Was it sealed in a particular atmosphere? What were the sources for the starting materials?
The figure quality is not good in many cases. Figures 2, 3, 14 are pixelated and poor in resolution, and 11 is distorted. Figures 11 and 13 are poorly designed to convey information. The authors should redesign 9, 10, 11, and 13 to emphasize any differences in the data. Consider providing raw data, full-scale spectra, or Origin/Excel files to reviewers or in Supporting Info. In particular, I'd say that fixing the low-resolution, pixelated spectra figures is essential to supporting the authors' conclusions.
Finally, I note that no parameters for a control experiment using unmodified alumina/zirconia were reported. In general, catalyst studies should report a control without the catalyst. This is especially important since the authors report no information on the source, nature, particle size, etc. of their alumina support (which, as I said, I recommend they do) If the authors wish to argue that this is unnecessary due to previous published work in the field, they should explicitly do so in a rebuttal or section of the manuscript.
A few minor typos, of no great concern, are in the manuscript. For instance, 362 (ZO2) and 367-368 "catalyst for range" and "suitability performance."
Author Response
Comments and Suggestions of Reviewer
Comments and Suggestions of Reviewer #3:
The authors are very thankful to comments and suggestions of the reviewer, which upgraded the standard of the present manuscript.
Comment #1:
The paper by the Al-Fatesh group studies the transformation of an oxygen/methane mixture to CO/hydrogen, catalyzed by a Ni/alumina composite. This reviewer's background is in solid materials characterization, not in petroleum or energy research. However, it seems obvious that the topic is of great commercial interest. On the other hand, I do not believe that the authors did a careful job of explaining the distinction between their work and other reports of Ni/Al2O3 and possibly Zr catalysts in the literature. In particular, there is a paper (Lunsford et al, J. Catalysis, 132, 117-127, 1991) with over 700 citations that essentially studies the same system, even arriving at a similar temperature for their optimized yield (700°C vs. 650 for the present study) Dozens of other papers deal with a Ni/Al2O3 catalyst, but the authors only cite a few and do so in a distant way (see for instance line 69, which needs a nice lineup of citations).
Response #1:
We appreciate this comment of the reviewer. We agree that Lunsford et al; performed nice work on Ni/Al2O3. It deserves to be included in the introduction. However, this work comprises the use of both alumina and zirconia supports. More useful citations are included in the revised manuscript.
Dissanayake et al. investigated the partial oxidation of methane using 25%Ni supported on alumina [a].The result showed that complete conversion could obtained if the reaction temperature was above 700° C was operated. The result also revealed that the catalyst bed may be subdivided into three zones with different catalytic activity aspects. Qingwei et al; studied the partial oxidation of methane using textural promoted alumina supported on Ni (Ni/CeO2-ZrO2/γ-Al2O3) [b]. The result indicate the effect of different preparation processes. On the other hand, Sajjadi and Haghighi examined Ni/Al2O3 by means of different preparation techniques namely sol-gel, impregnation and hybrid sol-gel plasma [M]. They found that each preparation technique possessed advantageous properties over the others such as good dispersion, stability and coke resistance etc.
Dong et al; performed the comparative investigation of partial oxidation of CH4 over Ni/ZrO2, Ni/CeO2 and Ni/Ce–ZrO2 catalysts [N]. They found that over Ni/ZrO2, CH4 and O2 are activated on the surface of metallic Ni, and then adsorbed carbon reacts with adsorbed O2 to generate CO, which formed the main path for the partial oxidation of CH4.
Comment#2:
Some improvement in experimental methods and data analysis description would be useful. For instance, N2 sorption experiments are described, but no fitting parameters, histograms, etc. are provided. This reviewer, a chemist, would also like more details about the catalyst preparation (2.1). Was the reaction stirred? Was it sealed in a particular atmosphere? What were the sources for the starting materials?
Response #2:
The authors are much obliged to the reviewer, however, we believe that the adsorption-desorption isotherms with the hysteresis loops are sufficient for the N2 physisorption analysis. On catalyst preparation, the reaction was stirred over a heated hotplate and the processes were carried out in the atmosphere. The starting materials were purchased from the manufacturers e.g. Nickel(II) nitrate hexahydrate from Alfa Aesar. More details about the synthesis can be found in one of our previous publications.
Comment #3:
The figure quality is not good in many cases. Figures 2, 3, 14 are pixelated and poor in resolution, and 11 is distorted. Figures 11 and 13 are poorly designed to convey information. The authors should redesign 9, 10, 11, and 13 to emphasize any differences in the data. Consider providing raw data, full-scale spectra, or Origin/Excel files to reviewers or in Supporting Info. In particular, I'd say that fixing the low-resolution, pixelated spectra figures is essential to supporting the authors' conclusions
Response # 3:
The authors are thankful to reviewer for the suggestions which definitely raises the standard of the paper. All suggested figures are revised and redrawn to offer better resolutions.
Comment #4:
Finally, I note that no parameters for a control experiment using unmodified alumina/zirconia were reported. In general, catalyst studies should report a control without the catalyst. This is especially important since the authors report no information on the source, nature, particle size, etc. of their alumina support (which, as I said, I recommend they do) If the authors wish to argue that this is unnecessary due to previous published work in the field, they should explicitly do so in a rebuttal or section of the manuscript.
Response #4:
A section describing the output of a control experiment is added in the result section. The result of the experimental reaction using only the supports is reported.
As a control experiment, reactions using empty reactor without catalysts under the same feed ratio and various temperatures (500–650 °C) were performed. The CH4 conversions registered during the test were 0.00% in the 500– 650 °C range. Therefore, the control experiment denoted negligible interaction of the reactor tube to the catalytic activity.
Comment #5:
A few minor typos, of no great concern, are in the manuscript. For instance, 362 (ZO2) and 367-368 "catalyst for range" and "suitability performance."
Response #5
The typo errors in lines 362 and 367-368 are fixed in the revised manuscript.
Author Response File: Author Response.docx
Reviewer 4 Report
In this manuscript, the authors described the performance of partial oxidation reforming of methane through Ni catalysts supported on high and low surface area Al2O3 and ZrO2 to produce hydrogen. Catalysts were prepared by using wet impregnation method, calcined at 600 and 800 °C and characterized by diverse techniques. The catalytic activity, stability, and carbon formation were determined to measure the catalytic performance as the reaction temperature at 450-650 °C. And the Ni-Al-H-600 turned out to be the best catalyst with highest methane conversion, hydrogen yield and stability when reaction temperature was set to 650 °C. Although the result is of great interest in POM catalyst design, there are still few experiments could be conducted to improve the completeness.
According to the authors’ previous report (Processes 2019, 7, 141. DOI: 10.3390/pr7030141), the binary oxide of Al2O3 and ZrO2 can enhance the surface area. Since the best catalyst in current manuscript is Ni-Al-H-600, I am curious if the Ni catalyst supported on high surface area of the binary oxide of Al2O3 and ZrO2 could have the better performance.
The Figures in manuscript are extremely unprofessional for publication, eg. not centered, different thickness for four axes, different font of text and non-uniform picture size even in the same Figure, also, the unit for degree (°C) is symbol 0, not superscript zero or O.
Page 17 line 362, ZO2 should be ZrO2. Also, a space should be input between the number and unit (°C) in the whole article.
Author Response
Comments and Suggestions of Reviewer
Comments and Suggestions of Reviewer #4:
The authors are very grateful to valuable comments and suggestions of the reviewer, which positively modified the standard of the present manuscript.
Comment #1:
According to the authors’ previous report (Processes 2019, 7, 141. DOI: 10.3390/pr7030141), the binary oxide of Al2O3 and ZrO2 can enhance the surface area. Since the best catalyst in current manuscript is Ni-Al-H-600, I am curious if the Ni catalyst supported on high surface area of the binary oxide of Al2O3 and ZrO2 could have the better performance
Response #1:
We are thankful to the reviewer for the excellent observation. In that previous work we tested different reaction temperatures and different active metals. It was also established adding ZrO2 to Al2O3 enhanced Ni metal-support interaction, which was beneficial to the catalytic activity and sintering resistance. Therefore your comment identifies the right speculation. We do share with you that if mixtures of alumina and zirconia are used it might produce better performance.
Comment #2:
The Figures in manuscript are extremely unprofessional for publication, eg. not centered, different thickness for four axes, different font of text and non-uniform picture size even in the same Figure, also, the unit for degree (°C) is symbol 0, not superscript zero or O.
Response #2:
In the revised manuscript all the figures are revised to improve the quality in term of thickness, sizes, proper formatting and using correct symbol for the degree (°C).
Comment #3:
Page 17 line 362, ZO2 should be ZrO2. Also, a space should be input between the number and unit (°C) in the whole article.
Response #3:
In line 362 ZrO2 is replaced with ZO2. Also space is placed before the unit (°C) in the whole article.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Dear authors the manuscript has been significantly improved.
However, in the revised version of the manuscript, the mistakes remain in the pictures probably due to the file conversion from doc to pdf. In the references 24 and 25, check the format. In various figures the labels of numbers are not homogeneous, and the box figures are not aligned. Again, in figure 6, two curves refer to the same sample (NiZr_H-600).
Reviewer 3 Report
The authors have addressed several of my concerns with this revision. However, I still have a few more concerns which affect the reproducibility, and therefore the validity, of the work.
The authors have added details on their catalyst preparation. The authors say in their response letter "More details about the synthesis can be found in one of our previous publications." Please cite this preparation detail explicitly here.
As I stated in my previous review, figures 10, 13, and 14 are poorly designed to the point that it is difficult to get information from them. Surely the y-axis should be changed to show more differences between the curves in 10 and 14. The scale of Figure 13 reveals no differences between the samples, despite interpretation of TGA traces as showing differences in carbon production. If the point of the Raman spectra is simply that there is graphitic carbon present, that point could be made in the text and perhaps a representative full spectrum (or broader, at least) provided, possibly in an ESI document. Simply showing a wider x-axis range would improve this figure.
This reviewer has a further question about this spectrum - what sort of Raman spectrometer gives a spectrum containing zero visible noise? The author's previous work (Catalysts 2020, 10, 242) does not look like this, and the instrument listed is the same NMR-4500. The authors should provide raw data or a full spectrum for each of these spectra.
The authors have added information for an empty-tube control experiment.
Overall, I believe that this study is suitable for publication but that the quality of the figures still should be upgraded in order to support the authors' claims. Finally, the authors should review the document for the typos indicated by me and other reviewers in the first round of review.
Reviewer 4 Report
The authors addressed my concerns properly. So I recommend the publication in Processes.
