Influence of Ni on Fe and Co-Fe Based Catalysts for High-Calorific Synthetic Natural Gas

Round 1

Reviewer 1 Report

Figure 1 it is necessary to add what the gamma symbol stands for.

What is the error of determining a) the elements content by the ICP-OES method and b) products selectivity?

The choice of the pre-reduced temperature (500 °C) is not clear, is not explained by the authors.

Table 2 It would be great to see the heating value of the products presented in the literature to evaluate the effectiveness of the proposed catalysts.

Figure 4 Which phase does the blue filled triangle represent (CoO?). It is not worth talking about the presence of any phase in only one reflex (for example, Co₃O₄ for calcined Ni-containing catalysts, Fe₂O₃ for calcined Co-Fe catalyst, Ni-Fe alloy for reduced 5Co-15Fe-2Ni and CoO? for reduced 5Co-15Fe-5Ni), this requires additional evidence.

Have catalysts been investigated after reaction for coke content? Have you studied the structural characteristics of the catalysts after the reaction (in particular, the question of increasing the average particle size of the active component is of interest)?

Author Response

Thanks for your reviews

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

It is a well-structured paper dealing with an important field of energy transition and sustainability. However, there are a few points that need to be improved before considered for publication.

Introduction

• The authors correctly point out that the fossil resources (coal, petroleum and NG) are main sources of GHG (and in particular CO2) and thus alternatives are needed. Methanation thus could be a solution but not from coal as authors state in line 37.this would cancel out the sustainability point raised above, so coal has to be removed from this sentence. Also adding LPG(fossil-based) is not an option for SNG because again it cancels out the green character of such a fuel. Comma for thousands separator is not used internationally, so it has to be avoided (e.g. 9,520 kcal/Nm3 in line 40).
• Metric /SI unit system should be used (applicable to all the sections of the manuscript)

2. Results and discussion

• Figure 1 what does γ indicate ? is it the γ-alumina
• the double peaks between 65-70 degrees could be an indication of α-alumina formation given also the high temperature that the catalysts exposed to during reduction. BET analysis can also help to identify this transition.
• XRD analysis indicated presence of spinel structures. Interestingly, no NiAl2O4 was identified even if the catalysts exposed to high temperatures. How is this related to higher dispersion.
• Ni is known to being a very mobile metal on alumina.   In the absence of nickel aluminate formation, the authors should describe and explain  the stabilization effect of iron presence.
• The 60.6 nm of Ni crystallite are indicative of sintering either during synthesis and calculation of during reduction. 
• Figures 3 and 5 should look similar  and is better if the merge in 1 graph, so the reader can compare the obtained results.
• figures 1 and 4 can be merged together.
• the amount of catalyst used, and the reactor size indicate very small bed height(5 mm was estimated based on gamma alumina bulk density of 0.7 g/cc) given the particle size report there is a high probability of channelling of the gas and therefore, duplicates of experiments should be performed.
• Given the high conversion achieved in all the runs how did the authors ensure isothermal bed and that the actual conversion took place at the reported temperatures?was the temperature measured inside the reactor?

3. Materials and methods

•  the wavelength of the CuKα radiation should be mentioned.
• The Scherrer equation should be reported or a reference to the equation should be provided.
• What is the value of the constant ain the equation, and how were the instrument broadening effect taken into account?
• Line 264: does the volumetric flow refer to 200oC? If it is standard conditions it has to be specified.
• The textural properties of the catalysts at different stages should be reported (i.e. specific surface area ,pore size distribution pore volume etc.)
• Chemisorption experiments are suggested to be carried out to indicate dispersion of different metals.
• For the same reason TEM investigation can indicate possible sintering and be used as a crystal size and dispersion evaluation tool.

Conclusions

The conclusion section needs considerable improvement. It should not summarize the results only.

Author Response

Thanks for your reviews

Please see the attachment.

Author Response File: Author Response.docx