Developing Multifunctional Fe-Based Catalysts for the Direct Hydrogenation of CO₂ in Power Plant Flue Gas to Light Olefins

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper reported a multifunctional Fe-based catalysts for CO2 hydrogenation to light olefins. The authors shows good results with detailed characterization. I suggested to accept the paper after major revision. Here are some suggestions:

1.  Line 85. The wet-impregnation method has to be introduced with detailed steps. 

2. For Figure 1b, the study of temperature effect is not enough. At least three data points are needed. 

3. For study of addition water effect, the author needs to indicate the percent of added water. Similarly, the author needs to indicate the sample they tested for effect of water dosage.

3. Line 202-203, why does uneven dispersion illustrate the increase of specific surface area and the formation of mesoporous structure? Any reference? Generally, small particle size leads to higher surface area. However, from SEM, sample in c and d looks larger.

4. Please label the name of peaks in XRD (Figure 4).

5. Please relabel the temperature in Figure 7a. It's hard to determine the temperature with the peak.

6. There is a small peak at around 200C for KFMM0.15 in Figure 7a, what's that?

7. It would be better if author can compare their performance results with current data points.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The work is devoted to the development of new catalysts for direct hydrogenation of CO2 in power plant flue gas with low CO2 concentration. The authors need to further show how significant this particular formulation of the task is. The problem is that, from an energy point of view, it is much more efficient to concentrate CO2 from exhaust gases and then convert it into valuable products (methanol, olefins, etc.). What the authors propose leads to the formation of olefins in the exhaust gases with a very low concentration (less than 1%). The separation of these olefins from a common mixture containing N2, CO, and paraffins is a very energy-intensive process.

Additional notes are provided below.

- Figure 1a shows the effect of Mg dosage on the catalytic performance of various catalysts in CO2 hydrogenation. When Mg dosage changes from 0 to 0.2%, CO2 conversion varies from 21 to 25%. These changes in CO2 are within the experimental error, but the authors conclude that magnesium plays a decisive role in this reaction.

- in Fig. 1b there is no data on CO2 conversion at 340 C and 2000 ml/gh.

- data on specific surface area and pore measurements must be provided in a separate table for all samples.

- The RWGS reaction is reversible, and the maximum CO2 conversion under the selected reaction conditions (340 C and 3 MPa) is about 30%, which was observed by the authors in their work. The introduction of water into the initial mixture leads to a shift in thermodynamic equilibrium and a decrease in CO2 conversion, which was observed by the authors. The authors do not consider thermodynamics in this system at all, and the role of water is determined by its contribution to the reaction H2O + Fe5C2.

 

- the Fe5C2 phase is proposed as active in the FTS reaction. The main argument here is that the introduction of water leads to its decrease and a decrease in CO2 conversion. Taking into account the previous remark, this cannot be an argument for considering Fe5C2 as the active phase for the FTS reaction. More evidence is needed for this claim.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript deals with novel multifunctional Fe-based catalyst for direct hydrogenation of CO2 to light olefins.

Global energy-related CO₂ emission has been estimated to grow at a rate of ca 1%/year through 2050 and thus climate changes have been received widespread attention due to a large amount of CO₂ continuously emitted by extensive human activities. At the same time, CO2 can be used as an abundant, non-toxic, and renewable feedstock for producing various value-added chemicals such as CH₃OH, DME, aronatics, light olefins (C₂⁼-C₄⁼), etc. Particularly, light olefins are widely used in industry of rubber, resins, fibers, pharmaceuticals and fine chemicals. Light olefins can be obtained via thermal cracking of naphtha, therefore, the direct hydrogenation of CO₂ into light olefins would be highly desirable. The sustainable production route will reduce CO2 concentrations in the atmosphere, reduce the greenhouse effect and help to replace non-renewable fossil fuels, contributing to the goal of carbon neutrality. From this point of view, the article is of high importance.

The manuscript is well structured and will be of interest to a wide range of readers since provided valuable information and the content is very significant in this field.

 I recommended a minor revision of the article before it can be published in Catalysts.

 Some specific comments are as follows:

 1) line 78: KFMMx abbreviation should be deciphered.

2) line 143: “the CO selectivity decreases to a certain extent”

As I can see, changes of CO selectivity follow a sinusoidal trend. The authors should discuss this effect.

3) line 145: “olefins/paraffins (o/p) ratio both increase after doping Mg”

The same as in line 143. The O/p ratio follows an even more sinusoidal trend. There is no tendency to its increase.

4) line 151: “The effects of reaction temperature and weight hour space volume (WHSV) on light olefin synthesis were further studied”.

Here the authors should indicate for which catalyst these effects will be studied, since this is not clear from the caption to Fig. 1b.

5) line 155: “due to the accelerated conversion of CO₂ to 33.1%”

This value is not shown in the figure.

6) line 184: Due to the complexity of Figure 1, its captions should be more clear and informative. Thus it is not clear which catalysts are described in Figures 1с and 1d.

The caption for fig. 1c is absolutely uninformative. According to the text of the article, this figure describes the effect of water steam on light olefins synthesis. However, the caption to Fig. 1c says that the figure compares catalysts with different stacking manners. What does it mean?

line 191: “Specifically, the KFMM0.15 catalyst 191 shows a BET specific surface area of 82 m2/g, which is 1.7 times that of KFMM0 (48 m2/g)”

It looks like the phrase has not been completed.

line 198: “(b) pose size distributions of series KFMMx”

May be pore size?

line 204: “The SEM images of spent KFMM0.15 catalysts are shown in Figure 204 3e and 3f. It can be found that the morphology of KFMM0.15 is almost unchanged after the 205 CO2 hydrogenation reaction, indicating good stability”

For greater objectivity, I would advise authors to compare images with the same resolution. If possible, of course.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The research is related to a very exciting field but there are relevant issues that must be addressed:

1. The authors did not present any information related to stability tests but in some paragraphs do mention to ¨higher stability¨.
2. The main conclusion related the hydrophobic component is attributed to the constraining the oxidation of iron carbides by water. But it is very speculative. The results points out for a modification of the acid-basic properties of the catalysts.

SEM images indicated the morphology os spent catalysts did not changed in comparison with the fresh samples. Thus, the authors concluded that this behavior is an evidence of good stability, which is erroneous. Although the morphology was kept the catalytic reactions take place at the nanoscale, which is not reflected by SEM images. Moreover, any stability tests was not presented by the authors.

4. XRD results should be more explored. A good strategy should be the presentation of the patterns in a normalized scale. Moreover, the identification of relevant diffraction lines in the diffractograms can make easier the understanding of these results.

Comments on the Quality of English Language

From what I read I believe some improvements can be made. For example, the verb ¨conduce¨ appears repeteadly in the text.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Good now.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors took into account the comments of the reviewers. The revised version may be published in the journal.