Methane Production from H2 + CO2 Reaction: An Open Molecular Science Case for Computational and Experimental Studies
, Andrea Capriccioli 2, Marzio Rosi 1,3, Carles Martì 4, Marco Parriani 1 and Antonio Laganà 3Round 1
Reviewer 1 Report
The P2G strategy is unequivocally a hot topic nowadays and therefore all the possible informations are relevant to solve the main problem human beings are facing today, global warming.
However, it is not clear at all what is the objective of the paper but to present a pre-commercial prototype that is roughly described. The reported results are scarce and not well defined and the synergy between theory and experiment are not supported by data but to stated by the authors. Moreover, the two applications, interesting by themselves are just enunciated without any results.
In th present form this cannot be onsidered a scientific paper worth of being published in a physicochemical periodic journal. Perhaps, economic or sosial sciuences journals may be appropriate for this piece of work.
Author Response
Responses to the Reviewer #1 comments:
Reviewer #1 – general comments: “The P2G strategy is unequivocally a hot topic nowadays and therefore all the possible informations are relevant to solve the main problem human beings are facing today, global warming. However, it is not clear at all what is the objective of the paper but to present a pre-commercial prototype that is roughly described. The reported results are scarce and not well defined and the synergy between theory and experiment are not supported by data but to stated by the authors. Moreover, the two applications, interesting by themselves are just enunciated without any results. In th present form this cannot be onsidered a scientific paper worth of being published in a physicochemical periodic journal. Perhaps, economic or sosial sciuences journals may be appropriate for this piece of work.”.
Author reply and made modifications: We thank the reviewer #1 for his criticism but we think that his final conclusion is quite ungenerous and do not correspond to the real scientific quality of our research work described and discussed in the manuscript. In fact, our paper reports only a general and synthetic description of the prototype methanation reactor, since it has been fully described in a previous published article (see ref. 3 of the submitted manuscript). Furthermore, experimental data are reported and discussed concerning the CH4 yield (see Fig. 1), the plasma assisted methanation reaction (see Fig. 5) reporting on the CO2 % dissociation measured as a function of the applied microwave discharge power and the mass spectrum of the final products so obtained. In order to highlight the importance of the presented data and the synergy between theory and experiment, as requested by the reviewer we have added the following sentences at lines 112-130 of the revised version of the manuscript: “Heterogeneous catalysis employing solid catalysts for gas reactions involves a large list of several elementary surface processes in which different mechanisms can compete in both main and side reactions. As detailed in ref. [16] our simulations were based on a Kinetic Monte Carlo (KMC) (see ref. [16] and references therein) treatment of the kinetics of the surface catalyzed gas phase processes expected to be involved in the PROGEO apparatus by making use of a rejection-free algorithm.
The goal of the simulation was to reproduce the measured temperature dependence of the CH4 yield (see Fig. 3 below) and single out the role played by the different elementary steps in the overall mechanism once that a satisfactory agreement with the experiment was obtained. To this end, the set of differential equations relating the consumption and the formation of the species intervening in the included elementary processes to their con-centration of the species of interest depending on the relevant direct and inverse rate coefficients are integrated. Particular care was therefore devoted to include an accu-rate description of adsorption, desorption of reactants and reaction intermediates as well as surface diffusion and surface reactions [16]. When appropriate, in order to accelerate the calculations steady-state (assuming that the net rate of formation for intermediates is zero which does not imply that the coverage by the intermediates is small) and quasi-equilibrium approximations (using the corresponding equilibrium equations instead of the kinetic equations for the fast steps) can also be introduced (see ref. [16] and references therein).”.
Sincerely yours,
Stefano Falcinelli.
Reviewer 2 Report
This manuscript by Falcinelli et al surveys recent developments in the production of methane from Paul Sabatier (PS) reaction using a consortium of computational and experimental facilities at various institutions in Europe. The subject of study obviously has great importance for industry and a more sustainable future with circular economy. As such the efforts reported in the survey are laudable and interesting. However for an original research article, several details are missing. The authors should provide more details on the methods while cutting short section 3.2. For example, details about the kinetic Monte Carlo method used are not provided. Also, Figures 2, 4 and 5 will be more easier to read and interpret if the different components were labeled (e.g. labeling the different peaks in Figure 5 (b).)
Author Response
Responses to the Reviewer #2 comments:
Reviewer #2 – general comments: “This manuscript by Falcinelli et al surveys recent developments in the production of methane from Paul Sabatier (PS) reaction using a consortium of computational and experimental facilities at various institutions in Europe. The subject of study obviously has great importance for industry and a more sustainable future with circular economy. As such the efforts reported in the survey are laudable and interesting.”.
Author reply and made modifications: We thank the reviewer #1 for his positive and encouraging comments.
Reviewer #2 - addressed point 1: “However for an original research article, several details are missing. The authors should provide more details on the methods while cutting short section 3.2. For example, details about the kinetic Monte Carlo method used are not provided.”.
Author reply and made modifications: We thank the reviewer #2 for his suggestion and we agree with him. To address his comment and suggestion, we added the following sentences at lines 112-130 of the revised version of the manuscript: “Heterogeneous catalysis employing solid catalysts for gas reactions involves a large list of several elementary surface processes in which different mechanisms can compete in both main and side reactions. As detailed in ref. [16] our simulations were based on a Kinetic Monte Carlo (KMC) (see ref. [16] and references therein) treatment of the kinetics of the surface catalyzed gas phase processes expected to be involved in the PROGEO apparatus by making use of a rejection-free algorithm.
The goal of the simulation was to reproduce the measured temperature dependence of the CH4 yield (see Fig. 3 below) and single out the role played by the different elementary steps in the overall mechanism once that a satisfactory agreement with the experiment was obtained. To this end, the set of differential equations relating the consumption and the formation of the species intervening in the included elementary processes to their con-centration of the species of interest depending on the relevant direct and inverse rate coefficients are integrated. Particular care was therefore devoted to include an accu-rate description of adsorption, desorption of reactants and reaction intermediates as well as surface diffusion and surface reactions [16]. When appropriate, in order to accelerate the calculations steady-state (assuming that the net rate of formation for intermediates is zero which does not imply that the coverage by the intermediates is small) and quasi-equilibrium approximations (using the corresponding equilibrium equations instead of the kinetic equations for the fast steps) can also be introduced (see ref. [16] and references therein).”.
Reviewer #2 - addressed point 2: “Also, Figures 2, 4 and 5 will be more easier to read and interpret if the different components were labeled (e.g. labeling the different peaks in Figure 5 (b).).”.
Author reply and made modifications: We agree and thank the reviewer for his suggestion. For such a reason we modified the Figures 2, 4 and 5 adding the labels, allowing a more clear reading of them (see the new figures in the revised version of the manuscript).
Sincerely yours,
Stefano Falcinelli.
Reviewer 3 Report
This paper reports an experimental and computational study of the CO2 reduction by hydrogen gas to give methane (Sabatier process). This is important seeing the demand for a sustainable economy. The results are promising, showing a high conversion ratio at the Ni catalyst at elevated temperatures.
In addition, CO2 dissociation in CO2/H2 plasma mixtures are also investigated. Intruigingly, CO2 dissociation appears to be catalyzed by H2, which is also a reactant in the reducing hydrogenation.
Publication can be recommended after the following points have been addressed:
- The alternative of fully reducing carbon dioxide to methane is the formation of methanol from CO2 through reaction with hydrogen. This is not even mentioned in the introduction.
Please cite here the following work: Chem. Soc. Rev. 2020, 49, 1385.
1. Discuss in the introduction the "pros and cons" for both alternative reducing CO2 fixation processes. CH3OH could be even more useful than methane, which can also be burned but not used as fuel.
- This brings me to the point why methanol does not even appear to be a side product in the process (Table 1). Do the authors have an explanation for this and could their reactor also be modified to produce methanol as well ?
- Formation of methane clathrate hydrates (Fig. 1) to store and transport methane is not further elaborated on in the paper. This idea is somewhat strange, because methane (i.e. natural fossile gas) is transported commonly in pipelines or in tanks liquefied under pressure - why should someone want to convert it into a clathrate, seeing that it has to be recovered from there too to make use of it? Using the clathrate itself as fuel is not very effective.
- The observation that higher ratios of H2 favor CO2 dissociation in CO2/H2 mixtures is interesting. There are indeed already a couple of reactions known in which a reactant acts also as a catalyst in a complex mechanism. The recognition of such processes is still in an infant state in chemistry, whereas chemists are well accustomed to products acting as catalysts (i.e. autocatalysis).
Please cite the following paper here, which provides another example from a different area (some others are cited therein): ChemPhysChem 2020, 21, 1775.
Author Response
Responses to the Reviewer #3 comments:
Reviewer #3 – general comments: “This paper reports an experimental and computational study of the CO2 reduction by hydrogen gas to give methane (Sabatier process). This is important seeing the demand for a sustainable economy. The results are promising, showing a high conversion ratio at the Ni catalyst at elevated temperatures. In addition, CO2 dissociation in CO2/H2 plasma mixtures are also investigated. Intriguingly, CO2 dissociation appears to be catalyzed by H2, which is also a reactant in the reducing hydrogenation. Publication can be recommended after the following points have been addressed:…”.
Author reply and made modifications: We thank the reviewer #3 for his positive and encouraging comments.
Reviewer #3 - addressed point 1: “The alternative of fully reducing carbon dioxide to methane is the formation of methanol from CO2 through reaction with hydrogen. This is not even mentioned in the introduction. Please cite here the following work: Chem. Soc. Rev. 2020, 49, 1385.”.
Author reply and made modifications: We thank the reviewer #3 for his suggestion and we agree in order to mention in the “Introduction” section the alternative reduction process of CO2 into methanol citing the interesting review paper suggested. To address his comment and suggestion, we added the following sentence at lines 72-75 of the revised version of the manuscript: “It has to noted that a very interesting and promising alternative way to convert carbon dioxide into a useful chemical compound is the formation of methanol from CO2 through the reaction with hydrogen. The interested reader can be found the state of art in this topic in a recent review paper by Zhong et al. [9].”.
The new added ref. [9] is the following:
- Zhong, J.; Yang, X.; Wu, Z.; Liang, B.; Huang, Y.; Zhang, T. State of the art and perspectives in heterogeneous catalysis of CO2 hydrogenation to methanol. Chem. Soc. Rev. 2020, 49, 1385-1413.
Reviewer #3 - addressed point 2: “Discuss in the introduction the "pros and cons" for both alternative reducing CO2 fixation processes. CH3OH could be even more useful than methane, which can also be burned but not used as fuel. This brings me to the point why methanol does not even appear to be a side product in the process (Table 1). Do the authors have an explanation for this and could their reactor also be modified to produce methanol as well ?”.
Author reply and made modifications: We thank the reviewer for his suggestion but since our paper is focused on the rationalization (and possible improvement) of the efficiency of methane production from CO2 in reactor performing the Sabatier reaction, we think that the added sentence and related new ref. [9] are sufficient in order to address also this reviewer comment (we are not experts in methanol production).
Reviewer #3 - addressed point 3: “Formation of methane clathrate hydrates (Fig. 1) to store and transport methane is not further elaborated on in the paper. This idea is somewhat strange, because methane (i.e. natural fossile gas) is transported commonly in pipelines or in tanks liquefied under pressure - why should someone want to convert it into a clathrate, seeing that it has to be recovered from there too to make use of it? Using the clathrate itself as fuel is not very effective.”.
Author reply and made modifications: We agree with the reviewer #3 on droppping the mention to clathration, since it is superfluous and completely marginal respect to the real target of work. For this reason, the Fig. 1 and its caption as well as the part
of the text where clathrate hydrates were mentioned have been modified and/or deleted (see line 54 of the revised manuscript).
Reviewer #3 - addressed point 4: “The observation that higher ratios of H2 favour CO2 dissociation in CO2/H2 mixtures is interesting. There are indeed already a couple of reactions known in which a reactant acts also as a catalyst in a complex mechanism. The recognition of such processes is still in an infant state in chemistry, whereas chemists are well accustomed to products acting as catalysts (i.e. autocatalysis). Please cite the following paper here, which provides another example from a different area (some others are cited therein): ChemPhysChem 2020, 21, 1775.”.
Author reply and made modifications: We thank the reviewer #3 for his suggestion and in order to address this comment, we added the following sentence at lines 252-256 of the revised version of the manuscript: “The observation that a higher concentration of H2 favors the dissociation of CO2 in different CO2/H2 mixtures is an interesting case of reactions in which a reactant also acts as a catalyst in a complex mechanism. It should be noted that the recognition of such processes, despite being still in an immature state, is of great relevance in chemistry [26].”.
The new added ref. [26] is the following:
- Tortora, C.; Mai, C.; Cascella, F.; Mauksch, M.; Seidel-Morgenstern, A.; Lorenz, H.; Tsogoeva, S. B. Speeding up Viedma Deracemization through Watercatalyzed and Reactant Self-catalyzed Racemization. ChemPhysChem 2020, 21, 1775– 1787.
Round 2
Reviewer 1 Report
The considerations presented by the authors are soundly and answer most of the concerns raised in the previous report
Author Response
We thank again the reviewer #1 for his comments and suggestions from his report-round1 aimed to improve the quality of the manuscript.
Reviewer 2 Report
The manuscript can now be accepted for publication.
Author Response
We thank again the reviewer #2 for his comments and suggestions from his report-round1 aimed to improve the quality and readability of the manuscript.
