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Open AccessArticle
Peer-Review Record

Carbon Spheres as CO2 Sorbents

Appl. Sci. 2019, 9(16), 3349; https://doi.org/10.3390/app9163349
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Appl. Sci. 2019, 9(16), 3349; https://doi.org/10.3390/app9163349
Received: 27 June 2019 / Revised: 5 August 2019 / Accepted: 12 August 2019 / Published: 15 August 2019

Round 1

Reviewer 1 Report

Very nice article.

For better reproducibility of your sorbents preparation, the chapter "Sample preparation" should be more specified in terms of molar quantities of reagents and concentration of ammonium hydroxide.

Author Response

Dear Reviewer,

We are very grateful for your pertinent comments, enabling us to improve our paper.

All changes made in the paper are marked in blue.

The detailed answers are below.

Very nice article.

For better reproducibility of your sorbents preparation, the chapter "Sample preparation" should be more specified in terms of molar quantities of reagents and concentration of ammonium hydroxide.

The concentration of ammonium hydroxide (25 wt.%) was indicated.

The quantities and concentrations of other reactants were given.

Reviewer 2 Report

The present manuscript reports on the carbon spheres as CO2 sorbents. The author's made efforts to explain their work by using various techniques. In my opinion, the work of the manuscript is good but there are many grammatical problems and with no clear structure. Therefore, I strongly recommend a major revision of the manuscript before it can be considered for publication and the detailed comments are as follows:

1) Grammatical errors and the sentence structures make difficult to understand.

2) What is the role of microwave here?

3) Briefly elaborate the mechanism between ethylene diamine and potassium oxalate 

4) Show the size distribution curves of all samples

6) At what stage, the TGA was measured? It would be ideal if the authors show a tentative graph

7) Discussion and summary section is small and it could be elaborated further. Ideally, the author can remove summary from the title.

8) Microporous plays a key role in the sorption of molecules. The author could dig literature further in order to validate and compare the results 

9) Reproducibility of the results is missing. 

Author Response

Dear Reviewer,

We are very grateful for your pertinent comments, enabling us to improve our paper.

All changes made in the paper are marked in blue.

The detailed answers are below.

The present manuscript reports on the carbon spheres as CO2 sorbents. The author's made efforts to explain their work by using various techniques. In my opinion, the work of the manuscript is good but there are many grammatical problems and with no clear structure. Therefore, I strongly recommend a major revision of the manuscript before it can be considered for publication and the detailed comments are as follows:

Grammatical errors and the sentence structures make difficult to understand.

A native speaker – Mr. Robert D. Cormia from USA is among the authors, he proofread the paper once more and asked another expert to repeat the verification.

What is the role of microwave here?

The role of microwave is very important, enabling to shorten the reaction time – from several hours in the case of an autoclave to several minutes in the case of microwave assisted solvothermal reactor. The application of microwaves enabled to perform a very homogenous heating of the reactor volume. In the result, there was no temperature profile in the reactor volume, no local overheating and finally – very uniform shape and size of the obtained particles.

In the literature there are many examples of production of nanoparticles using microwave assisted solvothermal reactors, reporting a good quality of the obtained products – small size, narrow size distribution and homogeneous shape.

The comments were added in the manuscript.

Briefly elaborate the mechanism between ethylene diamine and potassium oxalate 

Ethylene diamine introduced in the reaction system can react with resorcinol and formalin, but also with potassium oxalate, forming a potassium chelate. The ability of EDA to form chelates with metals (“amino acid metals”) is well known. A half of EDA produced by Dow Chemical Company is used as a chelating agent, forming complexes with certain metal ions to prevent the ions from interfering with processing or to promote buffering, concentration, separation, or transport. At elevated temperatures EDA decomposes with release of ammonia, carbon dioxide, carbon monoxide, nitrogen oxides, or volatile amines.

Potassium oxalate decomposes firstly to release carbon dioxide and form potassium carbonate. The latter decomposes above 700oC only, then the samples carbonised at this temperatures had better properties.

The potassium chelate decomposed in the result of carbonization, however some potassium remained in the samples, improving the basicity. It is possible that some amine groups remained as well, but the quality of the XPS spectra of nitrogen was to poor to judge about it.

It is possible that potassium formed the carbides. According to the literature, there is a possibility of formation of the following potassium carbides: KC8, KC16, KC24, KC32, KC48, KC60 (Carbides, Properties, Production, and Applications, T. Ya. Kosolapova, Plenum Press, New York – London, 1971. These carbides have graphite-like lattices in which the metal atoms are situated between the layers of carbon atoms. The metal atoms are located at the centers of the carbon hexagons.

Show the size distribution curves of all samples

A diagram showing the size distribution of the samples was added (Fig.2).

At what stage, the TGA was measured? It would be ideal if the authors show a tentative graph

TGA was measured for the final samples, after carbonization. A graph showing the TGA curves was added as Fig.5.    

Discussion and summary section is small and it could be elaborated further. Ideally, the author can remove summary from the title.

We removed “summary” from the title of the chapter. The paper was restructured, some text was added, some moved from “results” to “discussion” section.

Microporous plays a key role in the sorption of molecules. The author could dig literature further in order to validate and compare the results 

A text about microporosity was added.

Reproducibility of the results is missing. 

The measurements were repeated three times and an average value was taken into account.

Reviewer 3 Report

 

The authors report on the synthesis of carbon-based adsorbents from a typical combination of resorcinol and HCHO and reported their CO2 capture behaviour at 0 °C and 25 °C in their manuscript titled “Carbon spheres as CO2 sorbents”. They adopted potassium oxalate monohydrate as a chemical activating agent and EDA as the source of nitrogen and employed a solvothermal approach for the synthesis. I believe the use of resorcinol and HCHO to synthesize carbon spheres has been demonstrated in the previous literature on several occasions. Similarly, the use of potassium salts and EDA type amines to incorporate high micropore surface area and N group is very common. From CO2 capture performance point of view, these materials do not show any appreciable improvement as several other adsorbents with higher values for CO2 adsorption exists in the literature. The material characterization is also not very elaborate. Therefore, I believe the work is merely a repetition of what has already been established and the findings do not add much for the readers of this journal. Furthermore, I think the paper won’t appeal to the broad readership of the journal and as such won't fetch enough citations. Therefore, I do not recommend the publication of this research paper in this journal. Authors can consider the following points for publication in another journal.

1. A tabulated comparison of the obtained CO2 capture values with the existing materials, especially the ones that have been prepared using chemical activation would add more value to the paper.

2. In figure 4, why the sample RF 9/1 700 is not plotted within the relative pressure value 0-0.1. Also, it seems as if the desorption curve has gone below the adsorption curve. Authors may want to increase the quality of this figure.

3. I believe the addition of XRD, CHN and FTIR data is necessary to explain the structure, compare the bulk elemental composition with surface elemental composition and the nature of functional groups.

4. The use of potassium salts will leave many basic residues including elemental potassium in the carbonized material which needs to be eliminated using ~2 M HCl and water. However, the authors mentioned washing only with water. I think if washed with acid prior to water, the materials would have a higher surface area and pore volume which would help in higher CO2 adsorption.

5. Authors need to include the XPS survey spectra of the materials.

6. Section TGA provides only superficial information. It needs to be elaborated as to why and how the stability of materials is varied with pot oxalate or EDA.

7. In table 3, several numerical values have commas in place of decimal. It needs to be corrected.

8. Section 4 discussion and summary need to be elaborated more. Again, only superficial information about fig.8-11 is provided but no detailed discussion.

9. Where are refs 33 and 41 in the reference section?


Author Response

Dear Reviewer,

We are very grateful for your pertinent comments, enabling us to improve our paper.

All changes made in the paper are marked in blue.

The detailed answers are below.

The authors report on the synthesis of carbon-based adsorbents from a typical combination of resorcinol and HCHO and reported their CO2 capture behaviour at 0 °C and 25 °C in their manuscript titled “Carbon spheres as CO2 sorbents”. They adopted potassium oxalate monohydrate as a chemical activating agent and EDA as the source of nitrogen and employed a solvothermal approach for the synthesis. I believe the use of resorcinol and HCHO to synthesize carbon spheres has been demonstrated in the previous literature on several occasions. Similarly, the use of potassium salts and EDA type amines to incorporate high micropore surface area and N group is very common. From CO2 capture performance point of view, these materials do not show any appreciable improvement as several other adsorbents with higher values for CO2adsorption exists in the literature. The material characterization is also not very elaborate. Therefore, I believe the work is merely a repetition of what has already been established and the findings do not add much for the readers of this journal. Furthermore, I think the paper won’t appeal to the broad readership of the journal and as such won't fetch enough citations. Therefore, I do not recommend the publication of this research paper in this journal. Authors can consider the following points for publication in another journal.

It is true that potassium salts and EDA were described as activators to improve sorption capacity of carbon materials, but nobody used the two modificators simultaneously. We also tried to explain the interaction between them. Another novelty of the paper was the use of the microwave assisted solvothermal reactor, enabling to significantly shorten the reaction time and to obtain good quality of the produced material (uniform shape, narrow size distribution).

A tabulated comparison of the obtained CO2capture values with the existing materials, especially the ones that have been prepared using chemical activation would add more value to the paper.

A tabulated comparison of the results achieved on other materials was added.

There are no better results on CO2 adsorption on solid sorbents than 5-7 mmol CO2/g

and we achieved this level.

In figure 4, why the sample RF 9/1 700 is not plotted within the relative pressure value 0-0.1. Also, it seems as if the desorption curve has gone below the adsorption curve. Authors may want to increase the quality of this figure.

It was corrected.

I believe the addition of XRD, CHN and FTIR data is necessary to explain the structure, compare the bulk elemental composition with surface elemental composition and the nature of functional groups.

It is a very pertinent remark, thank you very much, we would like to do it. Nevertheless it can be done for the next paper only, due to the shortage of time to prepare the revised version of this paper. The paper in the recent form contains already a lot of experimental results and a lot of figures.

The use of potassium salts will leave many basic residues including elemental potassium in the carbonized material which needs to be eliminated using ~2 M HCl and water. However, the authors mentioned washing only with water. I think if washed with acid prior to water, the materials would have a higher surface area and pore volume which would help in higher CO2

Yes, we washed the material with water only. We do not think that washing with acid would improve the porosity, however we will try, thank you for the suggestion. Anyway, our goal would be not to remove potassium, which improves sorption ability of CO2.

Authors need to include the XPS survey spectra of the materials.

A figure with XPS survey spectra was added (Fig.3).

Section TGA provides only superficial information. It needs to be elaborated as to why and how the stability of materials is varied with pot oxalate or EDA.

It is true, we improved the content of TGA section.

In table 3, several numerical values have commas in place of decimal. It needs to be corrected.

It is true, we improved the content of TGA section.

Section 4 discussion and summary need to be elaborated more. Again, only superficial information about fig.8-11 is provided but no detailed discussion.

The section “discussion” was revised and completed.

Where are refs 33 and 41 in the reference section?

The references list was verified, completed and corrected.

Round 2

Reviewer 2 Report

The authors have addressed the comments carefully and would recommend for publication after fixing few things.

Line 152, replace Figure 1b) to Figure 2b

Figure 2: mention the total number of particles counted in the figure caption

Line 356-358: Reference is missing

 

 

Author Response

Dear Reviewer,

Thank you very much for your remarks.

Line 152, replace Figure 1b) to Figure 2b - I cannot find the error.

Figure 2: mention the total number of particles counted in the figure caption.

The total number of particles taken into account was 50 in every case. To avoid repetition, I added an appropriate  sentence in the text, not in every figure caption.

Line 356-358: Reference is missing

The corresponding reference was added.

Faithfully yours,

Urszula Narkiewicz

Reviewer 3 Report

The authors have addressed the comments carefully and hence I recommend the acceptance in its present form.

Author Response

Dear Reviewer,

Thank you very much for your positive assessment.

Concerning the research design, we will apply your comments to do it better in the next paper.

In the present paper, data analysis looks to try to understand SA and porosity (and pore volume) as a function of additives and carbonization, and secondly, to comment on observed carbon dioxide uptake as a function of additive and treatment, and especially the correlation with SA and pore volume.
Our future research will look more at carbon bonding using Raman spectroscopy, more detailed XPS and TEM studies.


Faithfully yours,

Urszula Narkiewicz

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