Carbon Formation in the Reforming of Simulated Biomass Gasification Gas on Nickel and Rhodium Catalysts

Round 1

Reviewer 1 Report

The manuscript describes the study of biomass gasification with the use of nickel and rhodium catalysts. The reaction conditions lead to the formation of solid carbon on the catalyst which inhibits the gasification reaction. The study discusses the effects of pressure, time-on-stream, and  ethylene content on the carbon formation on nickel and rhodium catalysts. The authors concluded that the most likely explanation for different behaviors of the catalysts was that in atmospheric conditions the nickel catalyst promoted carbon formation whereas rhodium catalyst promoted carbon oxidation, leading to lower carbon formation on rhodium catalyst.  Authors state that the use of noble metal catalysts could be  considered at least as they can offer significant advantage in long-term operation.

The work is supported by extensive research and the results are clearly presented. The conclusions are important from the point of view of biomass gasification. This is an important issue related to energy production.

In my opinion, the work is suitable for publication in its current form.

Author Response

In response to Reviewer 1,

Thank you for the positive feedback. We agree that this topic is important for the reforming of biomass gasification gas. We are happy that after studying this phenomenon in several different projects during the past decade, we had an opportunity to combine and present these results, which hopefully will be useful for the fellow scientists and process developers.

Sincerely,

Authors

Reviewer 2 Report

The manuscript entitled ‘Carbon formation in autothermal reforming of simulated biomass gasification gas on nickel and rhodium catalysts’ successfully presents a systematic study of carbon formation on nickel and rhodium catalysts. After reading, the reviewer found that the manuscript is well written and well presented. The introductory part is clear and concise with the emphasis of research objectives. The methodology described in the Materials and Methods part is clear with sufficient details. The Results and Discussion is well written as the effect of different operating parameters are discussed separately in great details with proper references. The figures (both figures and images) are of good quality. The reviewer therefore comes to the conclusion that the manuscript will be recommended for publication in Catalyst in its current form.

Author Response

In response to Reviewer 2,

Thank you for the positive feedback. As the paper discusses of several different aspects of the process that might affect the carbon formation, we made several major revisions before submitting the draft to Catalysts, in order to find the right balance between the Introduction and the Discussion. It is good to hear that it is clear and concise also for a reader.

Sincerely,

Authors

Reviewer 3 Report

This study analyzes the steam reforming of a simulated tar using Ni and Rh based catalyst. The authors have conducted a wide range of experiments in order to analyze the influence of pressure, TOS, ethylene content on the coke formation. The results are of great interest for the potential readers of catalysts. However, on the one hand, how the authors attained autothermal conditions in all the runs is not adequately explained (this study seems to be a conventional steam reforming process), and on the other hand, this study focuses on the carbon deposition, and therefore further characterization techniques of the deactivated samples should be included to support the results.

Accordingly, prior to the manuscript publication, the following points must be addressed:

• What is the metal loading of the commercial Rh based catalyst? What chemical components are present on it (type of support…)? Please, include it.
• In Table 3, the decimals should be expressed with dots.
• Did the authors reduce the catalyst prior the reforming experiments? The active phase in reforming reactions is the metal in its reduced form.
• Sintering at 900 ºC?
• How did the authors determine the conditions required to attain autothermal regime? The reforming reaction is a highly endothermic reaction, and in order to attain ATR, O2 is usually included in the feed. As observed in Table 2, Gas2 has a small concentration of O2. However, the amount of O2 required to attain ATR conditions will vary depending on the reforming conditions (T, P…). Did the authors corroborate that autothermal reforming was attained in all the runs? Moreover, Table 3 should include the ER used in all the runs.
• Why in Figure 2a and 3a (runs using Rh catalyst), the conversion of ethylene is not shown? Likewise, the C2H4 is not shown in the 144 hour runs.
• The carbon amount presented in Figure 5, at what deactivation stage (time on stream) corresponds? Please, include it in the text. Moreover, how was it determined? By oxidation? Please, include it in the text.
• The authors ascribed the catalyst deactivation to coke deposition and possible sulphur poisoning. What about metal sintering? If the reforming temperature is conducted in the 900-950 ºC range, is it likely that metal particles will be sintered. The authors are required to analyze it (by Scherrer Eq. in the XRD profiles…) and provide further explanations.
• The authors are suggested to clearly reference where the results are presented when commented it on the text. Sometimes is misleading where the results are displayed (Figures or Table).
• In figure 5b, a decrease of C content is predicted with temperature due to the promotion of C gasification in the reforming step. However, the results obtained in this study reveals that “when the reaction temperature was increased with nickel catalyst to 960 °C, the obtained carbon more than doubled again (test run 14)”. What about with Rh catalysts? There was any difference in the trends? Please, provide further explanations.
• The authors stated that different type of cokes can be distinguished in the deactivated catalyst (encapsulating and graphitic). However, these results are only based on the TPO and FTIR results of one run. The authors are suggested to include additional characterization techniques of the samples to complement the information provided in this study (SEM, TEM images, Raman….)

Author Response

In response to Reviewer 3,

Thank you for the positive feedback and valuable comments. We have made several changes and clarifications to the paper based on your suggestions, which are detailed in the attached document. This study has been carried out in several projects during past decade, and therefore we unfortunately don’t have possibility to do further analysis of samples. Also the chosen experimental methodology and the use of commercial catalysts both prevent certain types of analysis, which otherwise would have brought interesting further information about the carbon formation.

Sincerely,

Authors

Author Response File: Author Response.pdf

Reviewer 4 Report

This manuscript deals with the deposition of carbon on Nickel or Rhodium-based catalysts from simulated biogas. The authors have compared the tendency of nickel-based catalysts to coke under simulated biogas conversion conditions and compared them to a commercial rhodium-based catalyst. The authors present compelling evidence for the rhodium-based catalyst to be preferable when solid carbon is an unwanted side-product in the reactor. Overall, the research is well thought out and systematical, the manuscript is well written and relevant to the reader of Catalysts. There are some minor improvements that could be made, however:

• The authors have described three catalytic routes from hydrocarbons to solid carbon, but electrochemical synthesis routes from hydrocarbons (Z. Fan, W. Xiao, Electrochemical Splitting of Methane in Molten Salts To Produce Hydrogen, Angew. Chemie Int. Ed. 60 (2021) 7664–7668. https://0-doi-org.brum.beds.ac.uk/10.1002/ANIE.202017243) and CO2 (S. Ratso, P.R. Walke, V. Mikli, J. Ločs, K. Šmits, V. Vītola, A. Šutka, I. Kruusenberg, CO2turned into a nitrogen doped carbon catalyst for fuel cells and metal-air battery applications, Green Chem. 23 (2021) 4435–4445. https://0-doi-org.brum.beds.ac.uk/10.1039/d1gc00659b and C. Palmer, D.C. Upham, S. Smart, M.J. Gordon, H. Metiu, E.W. McFarland, Dry reforming of methane catalysed by molten metal alloys, Nat. Catal. 3 (2020) 83–89. https://0-www-nature-com.brum.beds.ac.uk/articles/s41929-019-0416-2) have also gained popularity recently. Some discussion on these competing methods should be added.
• Scanning electron microscopy analysis of the catalyst particles prior to and after the synthesis should also be undertaken. It is known from the electrochemical deposition of carbon, that nickel-based electrodes can erode and be incorporated into the carbon while it is forming, which leads to surface roughening (this is the reason why the carbon can be much harder to remove from such materials). SEM could reveal if that is also the case here.

• The authors have presented the carbon only as an unwanted side-product, but it can also bring added value if it can be removed from the catalyst without too much trouble. In the manuscript, the authors have only removed the carbon via oxidation, but a simple experiment of comparing this method to sonication or other methods and their efficiency could bring a lot of added value to the research.

Author Response

In response to Reviewer 4,

Thank you for the positive feedback and valuable comments, especially about the interesting topic of electrochemical methods. We have made additions and clarifications to the paper based on your suggestions, which are detailed in the attached document. This study has been carried out in several projects during past decade, and therefore we unfortunately don’t have possibility to do further analysis of samples. Also the chosen experimental methodology and the use of commercial catalysts both prevent certain types of analysis, which otherwise would have brought interesting further information about the carbon formation.

Sincerely,

Authors

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Taking into account the advice of the reviewers, the authors have thoroughly revised the paper and soundly responded and clarified their comments and doubts. Undoubtedly, the authors have done a great effort and the paper has been greatly improved and clarified, and in the reviewer humble opinion it is now publishable.