Highly Selective Syngas/H2 Production via Partial Oxidation of CH4 Using (Ni, Co and Ni–Co)/ZrO2–Al2O3 Catalysts: Influence of Calcination Temperature
, Yasir Arafat 1, Ahmed Aidid Ibrahim 1, Hamid Shaikh 2, Hanan Atia 3,*, Ahmed Elhag Abasaeed 1, Udo Armbruster 3 and Ahmed Sadeq Al-Fatesh 1,*Round 1
Reviewer 1 Report
The paper shows a detailed study of selective syngas/H2 production via partial 3 oxidation of CH4 using different catalysts with main focus on calcination temperature. I would like to ask about the following points:
They mentioned they get rid of the residual H2 by purge (line119), can they quantify this residual H2? Is it much?
From XRD, for Ni-800 it looks like there is the same peak as for NiCo-800. How can they be sure it is not noise? Could they check with reproducible tests in XRD? Have they done multiple XRD tests?
English should be checked as some mistakes were found among others:
101-102: A homogeneous solution was obtainED with slightly pink in color in case of Co while Ni based solution
102 appears to be slightly green in color...
106: Another part of the solid also calcined at 800°C under similar conditionS...
Author Response
Comments and Suggestions of Reviewer
Comments and Suggestions of Reviewer 1
We really appreciate the reviewer’s comments and his deep awareness of the presented topic. His remarks are highly appreciable and contributed to the improvement of this manuscript.
Comment 1
They mentioned they get rid of the residual H2 by purge (line119), can they quantify this residual H2? Is it much?
We use GC with a TCD detector to monitor the H2 and other reactant or product gases. The feed is not introduced to the reactor unless H2 is completely removed from the system.
Comment 2
From XRD, for Ni-800 it looks like there is the same peak as for NiCo-800. How can they be sure it is not noise? Could they check with reproducible tests in XRD? Have they done multiple XRD tests?
We have reviewed the samples for a second analysis
Comment 3
English should be checked as some mistakes were found among others:
101-102: A homogeneous solution was obtainED with slightly pink in color in case of Co while Ni based solution 102 appears to be slightly green in color...
We apologize for the poor use of English language in general. This sentence has been edited for better clarity. Apart from this, we thoroughly revised the orthography and grammar in the complete manuscript and all corrections are highlighted red.
Comment 4
106: Another part of the solid also calcined at 800°C under similar conditionS...
This was corrected as well.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors provide characterization of Ni and Co based auto-thermal reforming catalyst. The work is well described and thorough, albite somewhat conventional. Some additional literature review and description of the novelty in the text would make the work more impactful.
1. Aside from any cases listed below, the English is generally understandable and grammatically correct. However, it is somewhat stilted, with several awkward word or grammatical choices. Thorough review of the text by someone highly proficient in English is strongly recommended.
2. The motivation of the work is fairly well conveyed. However, it would befit from some additional text in the introductions and conclusions to better distinguish the novelty of the current work from the large body of similar catalyst systems. As Co is a less common reforming catalyst some literature comparison of Co and Ni/Co performance relative to conventional Ni would also be appropriate.
3. In section 3.3 the authors do not make clear what they mean by “interactions” and “metal-support-interactions”. Specifically In the authors write-up “It has weaker interaction with the support at less than 400°C, while exhibits a stronger interaction with the support at 400-600°C.” there usage of “interaction” does not appear to be the correct word. It is likewise unclear what they mean by “It can be inferred that the calcination of the bimetallic catalyst at 550°C has not only improved metal-support interaction... “ This is not helped by the fact that the literature definitions and interoperations of metal-support-interactions are inconsistent and contested. The authors must revise this section of the write-up to make their interpretations clear.
4. “Consequently, it can be deduced that higher calcination treatment and reaction temperature pose no adverse effect to our catalysts because they were the less susceptible to carbon deposition.” Given the lower surface area and metal dispersions of these catalyst, lower amounts of coke would be expected. Furthermore, the authors have not established by that point in the
manuscripts whether or not the calcination affects to performance of the catalyst for reforming, making the construction of the statement confusing.
5. The length of time the samples were exposed to coking conditions before TPO is unclear. A calculation on the estimated selectivity of the system to coke would be valuable. The separation of data from sections 3.9 and 3.4 is also somewhat confusing.
6. The catalyst preparation procedure should be edited for clarity. It is unclear if the % for CO and Ni are weight or molar ratios. As written, the “molar-unit ratio” presumably means 1:1 Zr to Al atoms, but the phasing is not clear.
7. “Total programmed oxidation” should be “Temperature Programmed Oxidation”
8. In Table 1. “m3/g” is assuredly the incorrect unit. cm3/g is likely what the authors intended.
Author Response
Comments and Suggestions of Reviewer
Comments and Suggestions of Reviewer 2
We are very thankful to this reviewer for his valuable comments, which are highly appreciable and contributed to improve the presentation of this manuscript.
The authors provide characterization of Ni and Co based auto-thermal reforming catalyst. The work is well described and thorough, albite somewhat conventional. Some additional literature review and description of the novelty in the text would make the work more impactful.
Comment 1
Aside from any cases listed below, the English is generally understandable and grammatically correct. However, it is somewhat stilted, with several awkward word or grammatical choices. Thorough review of the text by someone highly proficient in English is strongly recommended.
We appreciate the reviewer’s very gracious comment about the language quality. In accordance with recommendation from reviewer 1, we intensively revised the English language use.
Comment 2
The motivation of the work is fairly well conveyed. However, it would befit from some additional text in the introductions and conclusions to better distinguish the novelty of the current work from the large body of similar catalyst systems. As Co is a less common reforming catalyst some literature comparison of Co and Ni/Co performance relative to conventional Ni would also be appropriate.
Thank you for this suggestion. We now have included some more information in the introduction section of the revised manuscript with latest references and concluded. We added the following text to the introduction:
“On the other hand, other studies have demonstrated comparable performance at high temperature or by using precious metals. For instance, Dedov and co-workers utilized neodymium-calcium cobaltate based catalysts for syngas production via partial oxidation of methane [17]. They reported to attain 85% methane conversion and selectivity of CO and H2 close to 100% at very high temperature (925 °C). Likewise, another study used Ni(Co)-Gd0.1Ti0.1Zr0.1Ce0.7O2 catalyst and obtained comparable H2 selectivity at higher temperature (900 °C) for the production of syngas via partial oxidation of methane [14]. The present work is driven by our previous work [18] where it is has been shown that by using a single catalysis system of cobalt over CeO2 and ZrO2 supports; the hydrogen yield only up to 60% and 75 respectively was achieved for this system. Moreover, CeO2 support yield low hydrogen and cobalt alone is considered less reforming catalysis. Therefore in this work, the effect of binary metal system and support has been studied. It was observed that this system performs much better than single catalyst where hydrogen production was achieved up to 100 %. Several studies have employed Co-based catalysts for reforming reactions [19, 20, 14]. For instance, Zagaynov et al. [14] examined (Ni, Co and Co-Ni)/–Gd0.1Ti0.1Zr0.1Ce0.7O2 mesoporous catalysts obtained by co-precipitation for partial oxidation and dry reforming of methane. Interestingly, the results showed that the Co- and Ni-Co- containing catalysts exhibited excellent catalytic performance in partial oxidation of methane than the Ni sample, while the Ni-catalysts demonstrated tremendous catalytic performance in dry reforming of methane. “
Accordingly, the significance of this research contribution was to obtain a high catalytic performance at relatively low temperature using mono and bimetallic Co and Ni supported on (ZrO2+Al2O3) which are capable of producing syngas via partial oxidation of methane. In addition, they must be stable to overcome the deactivation processes like carbon accumulation, metal agglomeration and thermal sintering. The study of catalyst design started with a systematic investigation of the desired reaction together with potential side reactions. The sol-gel method of preparation was proposed to generate strong metal-support interaction (MSI) and to produce smaller metal particles, which is expected to be active in the catalytic reaction
Comment 3
In section 3.3 the authors do not make clear what they mean by “interactions” and “metal-support-interactions”. Specifically In the authors write-up “It has weaker interaction with the support at less than 400°C, while exhibits a stronger interaction with the support at 400-600°C.” there usage of “interaction” does not appear to be the correct word. It is likewise unclear what they mean by “It can be inferred that the calcination of the bimetallic catalyst at 550°C has not only improved metal-support interaction... “ This is not helped by the fact that the literature definitions and interoperations of metal-support-interactions are inconsistent and contested. The authors must revise this section of the write-up to make their interpretations clear.
According to Iriondo et al. [Top Catal (2008) 49:46–58]: “Stronger interactions of nickel oxide entities with supports provoke higher difficulty in their reducibility. The origin of the interaction between NiO particles and support remains a matter of controversy, and not unambiguous theoretical explanation of this phenomenon has yet been offered. Two different explanations about the origin of this interaction appear in the studies published in literature: (i), nickel occupying different sites in the alumina, octahedral or tetrahedral, this latter more difficult to reduce […]; or (ii), the incorporation of Al3+ […], Ce3+ […] or La3+ […] ions on the surface of NiO particles during impregnation.”
We add the following text in the manuscript (discussion section 3.3)
“In our system ZrO2 obviously doesn’t interact with Al2O3 strongly and the interaction between ZrO2 and Ni is weak as was found by J. Asencios et al. , [Fuel 97 (2012) 630–637] so Ni and/or Co is able to interact with Al2O3 to form Ni/CoAl2O4 and by this the Ni and/or Co are highly dispersed and as they are small crystals it is not possible to observe by XRD. The extent of this transformation increased with calcination temperature and is evidenced by the shift in reduction temperature for samples calcined at different temperatures. Also G. P. Berrocal et al., [Catalysis Today 149 (2010) 240–247] found that Ni strongly interacts with aluminum forming small NiAl2O4 particles that have the highest reduction temperature. At the same time, this sample showed the highest catalytic activity for the partial oxidation of methane. We observed similar dependency in our results.”
Comment 4
Consequently, it can be deduced that higher calcination treatment and reaction temperature pose no adverse effect to our catalysts because they were the less susceptible to carbon deposition.” Given the lower surface area and metal dispersions of these catalyst, lower amounts of coke would be expected. Furthermore, the authors have not established by that point in the manuscripts whether or not the calcination affects to performance of the catalyst for reforming, making the construction of the statement confusing.
We have updated the manuscript with the following text and the effect of calcination temperature on the reduction behavior and performance has been addressed in our reply to comment 3 from this reviewer.
“We assume that increasing the calcination temperature from 550 to 800 °C may form new surface sites due to the strong metal-support interaction. This might stabilize the high Ni dispersion against metal agglomeration and deactivation. Apart from this, ZrO2 might activate the oxidation of coke at high temperature and prevent the catalysts from coking. Also Co-800 catalyst operated at 800 °C had excellent stability for 24 h on stream without deactivation (as it wil be discussed latter). Henceforth, monometallic catalysts presented better performance with higher calcination temperature than bimetallic ones. ”
Comment 5
The length of time the samples were exposed to coking conditions before TPO is unclear. A calculation on the estimated selectivity of the system to coke would be valuable. The separation of data from sections 3.9 and 3.4 is also somewhat confusing.
The selectivity of carbon was calculated for Co-800 after long term test (24 h) at 800 °C. The selectivity of carbon is 0.02% based on CH4 conversion and carbon content of the catalyst (from TGA data).
Comment 6
The catalyst preparation procedure should be edited for clarity. It is unclear if the % for CO and Ni are weight or molar ratios. As written, the “molar-unit ratio” presumably means 1:1 Zr to Al atoms, but the phasing is not clear.
Weight percentages were quoted for Ni, Co and Ni-Co in the catalyst preparation. The total metal loading is 5 wt% of Co and/or Ni in the monometallic catalyst, while for the bimetallic the total metal loading was 5wt.% with 1:1 mole ratio. The Zr to Al atoms are also 1:1 mole ratio.
Comment 7
Total programmed oxidation” should be “Temperature Programmed Oxidation”
This has been corrected accordingly.
Comment 8
In Table 1. “m3/g” is assuredly the incorrect unit. cm3/g is likely what the authors intended.
We are grateful to the reviewer for his observation. The unit of pore volume was corrected in Table 1.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The paper has substantially improved and everything has been amended. All comments were tackled and the quality has been enhanced. It is worth publishing now.
