Influences of Co-Content on the Physico-Chemical and Catalytic Properties of Perovskite GdCo_xFe_1−xO₃ in CO Hydrogenation

Round 1

Reviewer 1 Report

In this manuscript, the authors present a series of GdCo_xFe_1-xO₃ catalysts to reveal the effect of the state of iron and cobalt atoms in Fischer-Tropsch Synthesis reaction. The physicochemical properties of the synthesized catalysts have been characterized in detail and kinetic characteristics was studied to clarify the results clearly. The results are significant to study the influence of the catalytic performance of the complex oxides, thus I recommend its publication in Catalysts. Prior to its publication, the following issues should be considered.

1. In Figure 2, please unify the magnification of the SEM images.

2. In Figure 3, GdCo_0.2Fe_0.8O₃ catalyst exhibits the weakest peak of Gd 3d in XPS spectra, please explain the reasons.

3. In Table 2, GdCo_0.8Fe_0.2O₃ catalyst exhibits the largest crystallite size, please explain the reasons.

4. Whether there is synergy between cobalt and iron to affect the adsorption of CO？

5. In page 15, the years of some references are not bold.

Author Response

Dear reviewer!

Thank you for reviewing our manuscript.
We have carefully reviewed the comments and have revised the manuscript accordingly. Our responses are given in a point-by-point manner below. Changes to the manuscript are shown in red.

We hope the revised version is now suitable for publication and look forward to hearing from you in due course.

Sincerely, authors.

1. In Figure 2, please unify the magnification of the SEM images.

Response: Figure 2 and others have been modified for better clarity.

2. In Figure 3, GdCo₂Fe_0.8O₃catalyst exhibits the weakest peak of Gd 3d in XPS spectra, please explain the reasons.

Response: In Figure 3 in XPS spectra for GdCo_0.2Fe_0.8O₃ catalyst all peaks including Gd 3d are weaker than for other samples. The explanation is in the smallest amount of sample for XPS analysis in this case. Most important that this difference does not affect on the relation of elements.

3. In Table 2, GdCo₈Fe_0.2O₃catalyst exhibits the largest crystallite size, please explain the reasons.

Response: Thank you kindly for your question. Due to comment we correct values in the text not to have difference with the data in Table 1.

“The crystallite size of the obtained samples estimated using the Scherrer formula is in the range from ~ 47 to 65 nm (Table 1)” (Lines 93-94).

In any case we think and wrote that there is not dependence of crystallite size on composition. Fluctuation in crystallite size is to the particularities of the citrate-nitrate sol-gel technique. It is important that these values do not change significantly during cationic substitution.

4. Whether there is synergy between cobalt and iron to affect the adsorption of CO?

Response: Thank you for your deep question. We didn't observe a clear synergistic effect between iron and cobalt with respect to CO adsorption. But we can speak of synergism since the formation rates of methane and ethylene on GdCo_0.8Fe_0.2O₃ are significantly higher than the others. We attribute this effect on GdCo_0.8Fe_0.2O₃ to the different structural characteristics of this sample from others. At the same time, the ethylene selectivity on this sample is not high.

We rewrite the part of text; you can see it in the lines 220-224.

5. In page 15, the years of some references are not bold.

Response: We correct all references in the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors report on the effect of the substitution of cobalt into the GdFeO3 perovskite structure on the selective hydrogenation of CO was investigated. The purpose of this work was to study the catalytic activity of systems based on perovskite-type iron- and cobalt-containing complex oxides in the process of carbon monoxide hydrogenation and to establish the relationship between the physico-chemical properties of complex oxides and their catalytic characteristics. Good introduction, the details of the experiments are provided, also an adequate list of references were used. This work is acceptable for publication in catalysts after following revisions.

1.    The novelty of the work must be clearly addressed and discussed, compare your research with existing research findings and highlight novelty.

2.    Authors are advised to add purity of all the precursors used in synthesis process (methods and materials section)

3.    Use the same font size in Figure 1. Better to write doping percentage on horizontal axis.  The quality of the Figures could be improved for better understanding.  

4.    In Figure 5. Why there is decrease in CO conversions up to X=0.5 and then we observed sharp increase. Elaborate precisely?

5.    Similarly for CO conversions the explanation regarding dependence on the temperature is missing.

6.     In Table: 1. BET values are fluctuating, could you provide plausible reasons for that, and can you also state the % error of the quoted values?

Comments for author File: Comments.pdf

Author Response

Dear reviewer!

Thank you for reviewing our manuscript.

We have carefully reviewed the comments and have revised the manuscript accordingly. Our responses are given in a point-by-point manner below. Changes to the manuscript are shown in red.

We hope the revised version is now suitable for publication and look forward to hearing from you in due course.

Sincerely, authors.

1. The novelty of the work must be clearly addressed and discussed, compare your research with existing research findings and highlight novelty.

Response:       According to your comment, we added some information about the oxidation reactions and application of Co- and Fe-containing perovskites in these processes. You can see it in the lines 60-68.

2. Authors are advised to add purity of all the precursors used in synthesis process (methods and materials section)

Response:       Thank you for your comment. We added the purity of all precursors used in the synthesis process. Changes are shown in the section «Methods and Materials», the lines 306-308.  

3. Use the same font size in Figure 1. Better to write doping percentage on horizontal axis.  The quality of the Figures could be improved for better understanding.  

Response:       Due to comment we correct the quality of figures.

4. In Figure 5. Why there is decrease in CO conversions up to X=0.5 and then we observed sharp increase. Elaborate precisely?

Response:       Thank you for your comment. We decided to add more detailed information of this fact. These data “Substitution of iron for cobalt (x=0.2; 0.5) leads to improvement of oxygen mobility and to decrease in the amount of surface oxygen O_s. This is reflected in the decrease of CO conversion as CO is adsorbed through the surface oxygen.” are shown in the lines 172-175.

5. Similarly for CO conversions the explanation regarding dependence on the temperature is missing.

Response:       We added some information about dependence on the temperature in the lines 183-185.

Also, the effect of composition and temperature on CO conversion and CO₂ production are discussed in our manuscript. And this explanation you can see in the lines 200-211.

6. In Table: 1. BET values are fluctuating, could you provide plausible reasons for that, and can you also state the % error of the quoted values?

Response:       The error is 10%. Really, fluctuations of crystallite size are a little more than error and are the result of the particularities of citrate-nitrate sol-gel technique to reproduce fully nuances of the multistage reaction. It is important that these values do not change significantly (2.7 – 3.3) and they are intermediate between 2.3 for GdCoO₃ and 9.9 for GdFeO₃. Due to this, we mentioned in text that “the values of the specific surface area do not show dependence on the composition”.

We added in text «the value for the samples synthesized by high temperature ceramic method – “less than 1.0 m²/g”, the lines 100-102.

Author Response File: Author Response.pdf