Preparation of Mn-Doped Co₃O₄ Catalysts by an Eco-Friendly Solid-State Method for Catalytic Combustion of Low-Concentration Methane

Round 1

Reviewer 1 Report

1) The manuscript contains a number of mistakes and should be spell-checked.

2) Why the "Materials and methods" section is following the "Results and discussion" and not vice versa?

3) Line 69 says "Synthesizing composite oxides by the solid-state 69 method using carbonates as precursors is a green route without wastewater and exhaust 70 gas". However, the initial cobalt precursor is a fully synthetic substance, the production of which is accompanied by the formation of wastewater and the release of CO2. Why, in this case, it was the main cobalt carbonate that was taken, and not, for example, the main copper carbonate, which is a natural mineral. In addition, it is known that the activity of copper in oxidative reactions is even higher.

4) How do the authors propose to prepare an industrial catalyst? Do the authors think that the obtained specific surface area is sufficient for the CMC reaction to proceed under industrial conditions?

5) The resulting system is positioned as an efficient catalyst for the oxidation of methane in a lean mixture. In turn, the catalytic experiment is given for pure methane.

6) The obtained results of catalytic experiments seem quite logical, since the effect of introducing manganese into the structure of cobalt oxide is fairly well described in the literature. I would like the authors to pay more attention to the issues of catalyst deactivation and regeneration.

 

Author Response

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Reviewer 2 Report

The manuscript deals with preparing a Mn-doped Co₃O₄ catalyst for lean methane combustion by a solid-state method. The experimental techniques used throughout the work have been adequately described. The characterisation of the synthesised catalyst is thorough and the conclusions are supported by the results presented. It could be argued that the scientific novelty of the manuscript is not significantly high, since doping of Co₃O₄ catalyst for methane oxidation, especially with Mn, Cu, Ni or Ce, is an extensively studied technique. The novelty presented in the manuscript is the synthesis method's eco-friendliness; thus, that aspect should be more extensively described and studied in the manuscript. Otherwise, the manuscript lacks scientific novelty. Apart from that, some issues should be addressed:

1. In lines 117-119, the authors ascribe the growth of the spinel lattice constant to the substitution of Co³⁺ ions with Mn³⁺ and the larger crystal radius of the latter. However, Mn²⁺ ions also present a crystal radius generally larger than that of Co²⁺. Therefore, could it be that the lattice constant growth is a consequence of the substitution of Co²⁺ with Mn²⁺?

2. In XPS characterisation, the authors should try to observe also the Mn2p spectrum in order to determine the +3/+2 ratio and check if it's compatible with the charge equilibrium described in line 176.

3. Did the authors calculate the H₂ uptake of the samples in the H₂-TPR experiments? That could also give an indication of the shift in the Co³⁺/Co²⁺ equilibrium of the catalysts. Also, what could be the reason behind the shift of the reduction profiles towards lower temperatures?

4. In lines 234-237 the authors state that the catalyst synthesised by the solid-state method is more active than other relevant catalysts reported in the literature. However, the parameters they use for this comparison (Table S1) are the T₁₀, T₅₀ and T₉₀ taken from other works where the reaction conditions (CH₄/O₂ ratio and GHSV) were completely different. Thus, that comparison cannot be taken as valid and should be removed.

Author Response

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Reviewer 3 Report

The manuscript deals with the synthesis of a series of MnCo catalysts suitable for the oxidation of methane.

 

Minor revision

 

Have ICP analysis been performed to verify the actual content of the samples? Please comment on this point.

 

In the article it is said that no characteristic diffraction peaks associated with different phase of Co3O4 have been found. However, especially in the case of Mn0.1Co1 sample, other low peaks can be appreciated in the range between 30-35º and around 55º mainly. Please comment on this point.

 

The amount of Co2+ and oxygen species has been estimated by means of the deconvolution of XPS curves. It can be apparently observed that the profiles are very similar but the corresponding molar ratios obtained varied noticeably. Please comment on this point.

 

Regarding the redox ability, an attempt to assign the peaks to the reducible species has been made. Regarding the area of the peaks, a smaller area in the case of the sample with higher manganese content is attributed to a lower presence of oxygen species. However, the area of these peaks would have to be related to the amount of reducible Co3+, Co2+, Mn4+ and Mn3+ species. Please comment on this point.   In the activity results (Figure 6) the performance of a sample with a 20% of manganese has been included. What about the characterization results of this sample? Please comment on this point.

 

Has the nature of the methane combustion products been evaluated? Is CO produced at some extent in the combustion? It would be convenient to compare the activity of the catalysts based on the conversion to CO2 as it is the desired combustion product. Please comment on this point.

 

Can the acidity of the catalysts have an influence on the adsorption capacity of methane in the catalysts? Please comment on this point.

 

A study of the physico-chemical properties of the catalysts after the stability reaction could be adequate. Please comment on this point.

Author Response

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Reviewer 4 Report

Journal: Catalysts (ISSN 2073-4344)

Manuscript ID: catalysts-2216275

Type: Article

Title: Preparation of Mn-doping Co3O4 catalysts by an eco-friendly solid-state method for catalytic combustion of low-concentration methane.

Authors: Linshuang Xue , Chenyi Yuan , Shipeng Wu , Zhen Huang * , Zhen Yan , Stéphane Streiff , Hualong Xu , Wei Shen *.

 

[1]         Introduction: write the perspective of the present work carefully.

[2]   Why the author didn’t measure the mechanical properties and Eg of the material prepared?

[3]   Why the author didn’t measure the dielectric constants for the materials?

 

References: Please, cite the following recent references

DOI: https://doi.org/10.1088/1742-6596/1795/1/012059

DOI: https://doi.org/10.1016/j.mseb.2021.115191

Best Regards

Author Response

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Round 2

Reviewer 1 Report

The manuscript can be accepted.