Mechanistic Insights into the Effect of Sulfur on the Selectivity of Cobalt-Catalyzed Fischer–Tropsch Synthesis: A DFT Study

Round 1

Reviewer 1 Report

In this article, the influence of sulfur on the structure and performance of Co based Fischer-Tropsch catalyst were studied by theoretical calculation. The literature review is comprehensive and sufficient, the calculation results are well discussed. The paper is also well organized.  From these aspects, the paper is acceptable, but some minor revisions are needed to be made.

1. According to the published article Phys. Chem. C, 2016, 120, 9132-9147, CHO insertion is more plausible for C-C chain formation compared with CO insertion and CH-CH coupling. So, the investigation for the carbon chain growth derived from CH coupling is not well studied in this paper. The authors should refine their research work.
2. The content in Fig.3 and Fig.4 is the same, the authors should review the content in Fig.3 carefully.
3. The vdW-DF functional was adopted, the cutoff energy is as high as 600 eV, which is to high compared with the reported data that used the PBE functional. Are the authors teste all the parameters by using the new functional?

Author Response

Reviewer1:

In this article, the influence of sulfur on the structure and performance of Co based Fischer-Tropsch catalyst were studied by theoretical calculation. The literature review is comprehensive and sufficient, the calculation results are well discussed. The paper is also well organized.  From these aspects, the paper is acceptable, but some minor revisions are needed to be made.

 

1. According to the published article Phys. Chem. C, 2016, 120, 9132-9147, CHO insertion is more plausible for C-C chain formation compared with CO insertion and CH-CH coupling. So, the investigation for the carbon chain growth derived from CH coupling is not well studied in this paper. The authors should refine their research work.

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Author’s reply:

The following part is added to Page 12 of the manuscript to address the reviewer’s comment:

“In a previous DFT investigation, it was reported that CH + HCO reaction has a slightly lower barrier compared to CH+CH coupling[25] , while in another DFT study, both CH+HCO and CH+CH coupling reactions were proposed to be among the most plausible 3 coupling pathways[43]. However, recent experimental studies on Co(0001) single crystals, performed under CO pressure of 1x10^-7 mbar showed that not HCO, but CH is the coupling species, based on IR spectroscopy[46] and XPS[47] measurements. This result is also backed by observations on cobalt catalysts under applied FTS conditions[44,45]⁠. Furthermore, our calculations indicate that sulfur decreases the adsorption energy of HCO significantly, while for CH species, the adsorption energy is not much affected. Also, presence of sulfur increases the activation barrier for HCO formation, and promotes the decomposition of HCO back to H and CO species. These results indicate that for sulfur covered Co(111) surfaces, HCO coverage would be significantly low compared to CH coverage.”

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2. The content in Fig.3 and Fig.4 is the same, the authors should review the content in Fig.3 carefully.

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Author’s reply:

Figure 3 is replaced with the correct figure.

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3. The vdW-DF functional was adopted, the cutoff energy is as high as 600 eV, which is to high compared with the reported data that used the PBE functional. Are the authors teste all the parameters by using the new functional?

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Author’s reply:

All parameters used in the calculations were tested and the results of the tests are reported in Section 2.2., Computational details, including the effects of number of layers, number of k-points, vacuum height, lateral interactions and ZPE corrections. The accuracy of the calculations, in particular the adsorption energies, increase with increasing the cut-off energy. Therefore, we have used 600 eV cut-off energy, to calculate adsorption energies with high accuracy. The following comment was added to Page 5 of the manuscript to comment on the effect of the cut-off energy: “Further increase of the cut-off energy results in less than 1 kJ/mol difference in the CO adsorption energy.”

Reviewer 2 Report

The results are interesting and can be published.
My suggestion is to indicate and discuss the rate-limiting
steps in more detail. It makes also sense to discuss
the experimentally observed kinetics of the reaction and 
to mention how high are the reactant coverages at practically 
important conditions. The reason why the reaction kinetics 
have not been calculated theoretically should also be mentioned.

Author Response

The results are interesting and can be published. My suggestion is to indicate and discuss the rate-limiting steps in more detail.

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Author’s reply:

For the determination of the rate limiting steps, specific mechanisms have to be assumed and the elementary reactions in these mechanisms have to be calculated (E.g., as in Ref. 37 of the manuscript). Considering the complexity of the mechanism of FTS, our approach in this article is not to investigate a specific mechanism, but to qualitatively discuss the effects of sulfur on the selectivity, based on reactions that are proposed to be selectivity descriptors in the literature, as also mentioned in Page 4 of the manuscript as “As the exact mechanism of FTS is under debate, assuming a specific mechanism may lead to results that are restricted to only the studied mechanism. However, fundamentally, it can be concluded that a cobalt FTS catalyst has to be active in 4 types of main reactions, which are CO dissociation, carbon hydrogenation, oxygen hydrogenation and carbon coupling[15].” This approach is used successfully in previous publications (As in Ref. 25 of the manuscript and Su et al, J. Phys. Chem. C 124, 11040–11049).

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It makes also sense to discuss the experimentally observed kinetics of the reaction and to mention how high are the reactant coverages at practically important conditions.

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Author’s reply:

The experimentally observed kinetics are discussed in Pages 1-2 of the manuscript in Introduction part. Furthermore, a comparison between the insights obtained in the current study with the experimental kinetics is performed in Pages 16 of the manuscript as: “As sulfur decreases the adsorption energies (and as a result coverages) of CO and OH more strongly compared to H and CHx, this effect would also result in lower selectivity to heavier hydrocarbons and olefins, compared to methane and saturated hydrocarbons. These findings compliment and provide molecular understanding for the recent experimental observations[6,7]⁠ related to the effect of sulfur on selectivity of cobalt catalyzed FTS.”

 

Under experimental conditions, FTS occurs on cobalt nanoparticles which, in addition to Co(111), include other Miller index surfaces such as Co(311), Co(100), together with defect sites. As different surface structures result in different reactant coverages, this complicates the measurement of surface coverages for cobalt nanoparticles under reaction conditions. Furthermore, the application of high pressure and temperature conditions during experiment, hinders the use of surface science techniques that can measure surface concentrations, as these techniques typically need vacuum conditions. Therefore, up to our knowledge, no measurements are present in the literature that report surface coverages for sulfur poisoned cobalt nanoparticles under experimental conditions.

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The reason why the reaction kinetics have not been calculated theoretically should also be mentioned.

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Author’s reply:

For the calculation of reaction kinetics, the following comment is added on Page 16 of the manuscript:

 

“Although it is of interest to calculate the reaction kinetics based on DFT modelling, the calculation of kinetics necessitates assumptions about the surface coverage and the exact reaction mechanism that is responsible for the formation of various products. As the calculated reaction kinetics would depend on the reliability of these assumptions, the kinetics are not calculated in this manuscript. The comparison of the energetics of the investigated elementary reactions allow us to qualitatively discuss the effects of sulfur on the selectivity of cobalt catalyzed FTS, without the need of making assumptions on the surface coverages and exact reaction mechanism.”

Reviewer 3 Report

The authors made a valuable contribution towards effect of sulfur on the selectivity of cobalt catalyzed Fischer-Tropsch Synthesis. I recommended that the manuscript is accepted in its present form.

Author Response

Author’s reply:

We thank the reviewer for their time and consideration.