Cold Nitrogen Plasma: A Groundbreaking Eco-Friendly Technique for the Surface Modification of Activated Carbon Aimed at Elevating Its Carbon Dioxide Adsorption Capacity

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper shows the advantage of N2 plasma treatments for the activated carbon to increase the CO2 adsorption capacity of it. I understood that a little adsorption capacity was observed but it was not so large improvement. The author did not explain the detail why the adsorption capacity was increased and why the increasing was so small. I think that the author needs more exact advantages of N2 plasma.

Author Response

Answer to the Reviewer 1

This paper shows the advantage of N2 plasma treatments for the activated carbon to increase the CO2 adsorption capacity of it. I understood that a little adsorption capacity was observed but it was not so large improvement. The author did not explain the detail why the adsorption capacity was increased and why the increasing was so small. I think that the author needs more exact advantages of N2 plasma.

 

Answer:

Many authors introduce nitrogen into activated carbons in various ways. However, such actions always lead to changes in other parameters, especially textural parameters. Consequently, although these authors attempt to demonstrate the positive impact of increased nitrogen content on CO2 adsorption, the conclusions drawn from these studies are not reliable because the introduction of nitrogen involves changes in other properties. It is well known that the textural parameters are crucial for CO2 adsorption. Simultaneous changes in nitrogen content and textural parameter values do not justify conclusions indicating that nitrogen content is the cause of changes in CO2 adsorption.

 

The undeniable advantage of nitrogen plasma treatment is the possibility of enriching activated carbon with nitrogen without changing the values of textural parameters. In this case, the conclusion about the positive impact of increased nitrogen content is fully justified. The article presents a method of introducing nitrogen into activated carbons using nitrogen plasma, which allowed for a slight increase in nitrogen content, and consequently, a slight increase in CO2 adsorption. These are preliminary but pioneering studies, and that is why we decided to publish them. We are currently working on modifying cold plasma treating to increase the amount of nitrogen on the surface of activated carbon.

 

These information has been included in the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

n this work, it has been unequivocally demonstrated that the increased presence of nitrogen is responsible for the enhanced adsorption of CO2. Low-temperature nitrogen plasma treatment of activated carbon is a promising method for enhancing CO2 capture. Overall, the results are useful. However, several limitations need to be overcome.

1. They should improve introduction and at the last paragraph, superiority of the paper should be defined.

2. The BET data of all the samples is necessary.

3. It is the chemical adsorption?

4. The language should be improved to avoid some mistakes.

Comments on the Quality of English Language

n this work, it has been unequivocally demonstrated that the increased presence of nitrogen is responsible for the enhanced adsorption of CO2. Low-temperature nitrogen plasma treatment of activated carbon is a promising method for enhancing CO2 capture. Overall, the results are useful. However, several limitations need to be overcome.

1. They should improve introduction and at the last paragraph, superiority of the paper should be defined.

2. The BET data of all the samples is necessary.

3. It is the chemical adsorption?

4. The language should be improved to avoid some mistakes.

Author Response

Answer to the Reviewer 2

In this work, it has been unequivocally demonstrated that the increased presence of nitrogen is responsible for the enhanced adsorption of CO2. Low-temperature nitrogen plasma treatment of activated carbon is a promising method for enhancing CO2 capture. Overall, the results are useful. However, several limitations need to be overcome.

1. They should improve introduction and at the last paragraph, superiority of the paper should be defined.
2. The BET data of all the samples is necessary.
3. It is the chemical adsorption?
4. The language should be improved to avoid some mistakes.

 

Answers:

1. Two paragraphs were added on this subject (highlighted in yellow)

Many authors introduce nitrogen into activated carbons in various ways. However, such actions always lead to changes in other parameters, especially textural parameters. Consequently, although these authors attempt to demonstrate the positive impact of increased nitrogen content on CO₂ adsorption, the conclusions drawn from these studies are not reliable because the introduction of nitrogen involves changes in other properties. It is well known that the textural parameters are crucial for CO₂ adsorption. Simultaneous changes in nitrogen content and textural parameter values do not justify conclusions indicating that nitrogen content is the cause of changes in CO₂ adsorption.

According to our knowledge, there is a lack of studies presenting materials with the same textural properties but different nitrogen contents. In our study, we present commercially available activated carbon modified using barrier and spark discharge low-temperature nitrogen plasma treatment. As a result of this treatment, no properties of the activated carbon changed except for the nitrogen concentration on the surface. This allowed us to demonstrate that the increase in surface nitrogen is the reason for the increased CO₂ adsorption. The preliminary studies presented here concern a slight increase in nitrogen concentration and a slight increase in CO₂ adsorption. However, it is important that we have demonstrated for the first time an increase in CO₂ adsorption specifically due to the increase in nitrogen content while keeping other properties unchanged. The research will continue towards increasing nitrogen content using this method.

The undeniable advantage of nitrogen plasma treatment is the possibility of enriching activated carbon with nitrogen without changing the values of textural parameters. In this case, the conclusion about the positive impact of increased nitrogen content is fully justified. The article presents a method of introducing nitrogen into activated carbons using nitrogen plasma, which allowed for a slight increase in nitrogen content, and consequently, a slight increase in CO₂ adsorption. These are preliminary but pioneering studies, and that is why we decided to publish them. We are currently working on modifying cold plasma treating to increase the amount of nitrogen on the surface of activated carbon.

 

2. We described three samples: DT0, DT0_1, DT0_2 and all the BET data are listed in Table 1
3. It is physical adsorption. See: “The values of the isosteric heat of adsorption are lower than 50 kJ/mol, indicating the physical nature of CO₂ adsorption over DT0 samples”
4. The langue was improved. The changes were highlighted in yellow.

 

 

 

Reviewer 3 Report

Comments and Suggestions for Authors

The authors treated an activated carbon sample with nitrogen plasma to introduce nitrogen-containing functionalities onto the surface without altering other structural aspects of the sample, such as surface area, micropore volume, and pore size distribution. The resulting samples were analyzed and compared based on their CO2 adsorption capacities and adsorption enthalpies.

 

CO2 adsorption on carbon materials has been extensively researched, with many experimental and theoretical studies providing insights into CO2 capture and separation. For instance, Monte Carlo simulations of CO2 adsorption have examined how both pore size and the texture of the carbon surface affect CO2-N2 adsorption. The authors might consider citing the following reference: 10.1016/j.micromeso.2014.08.052

 

The authors list various fit-model equations linked to the shapes of adsorption isotherm curves, but only a select few are used in the manuscript. Specifically, the Sips equation is used to model the isotherm curves for CO2 adsorption, as illustrated in figure 11. Additionally, the Toth model is employed to fit these same isotherms for enthalpy calculations, specifically to input the pressures related to specific loadings into the Clausius-Clapeyron equation. How do the isosteric curves vary when a different model is used to describe the CO2 adsorption isotherms?

 

 

As illustrated in table 3, the fitted parameters for the CO2 isotherms of the three samples are nearly identical at a given temperature. This is also true for the curves representing the isosteric heats of CO2 adsorption shown in figure 13. Does this suggest that the nitrogen content on the surface has less impact on CO2 adsorption than porosity or BET surface area? Or does it indicate that the degree of nitrogen functionalization is similar across the DTO_1 and DTO_2 cases and the pristine DTO sample?

Author Response

Answer to the Reviewer 3

The authors treated an activated carbon sample with nitrogen plasma to introduce nitrogen-containing functionalities onto the surface without altering other structural aspects of the sample, such as surface area, micropore volume, and pore size distribution. The resulting samples were analyzed and compared based on their CO2 adsorption capacities and adsorption enthalpies.

 

CO2 adsorption on carbon materials has been extensively researched, with many experimental and theoretical studies providing insights into CO2 capture and separation. For instance, Monte Carlo simulations of CO2 adsorption have examined how both pore size and the texture of the carbon surface affect CO2-N2 adsorption. The authors might consider citing the following reference: 10.1016/j.micromeso.2014.08.052

 

Thank you for pointing out such an interesting literature reference. We have reviewed it with interest and cited it in our work.

 

The authors list various fit-model equations linked to the shapes of adsorption isotherm curves, but only a select few are used in the manuscript. Specifically, the Sips equation is used to model the isotherm curves for CO2 adsorption, as illustrated in figure 11. Additionally, the Toth model is employed to fit these same isotherms for enthalpy calculations, specifically to input the pressures related to specific loadings into the Clausius-Clapeyron equation. How do the isosteric curves vary when a different model is used to describe the CO2 adsorption isotherms?

 

The Toth model was the best fitted and used to modelling the curves. In the article we write that "The lowest HYBRID values were obtained for the Toth model." We also provide the parameters of the Toth equation in Table 3. Toth model was also used to determine the isosteric heat of adsorption. Only the caption under Figure 11 is incorrect. Instead of Sips, it should be the Toth equation. The error has been corrected.

 

As illustrated in table 3, the fitted parameters for the CO2 isotherms of the three samples are nearly identical at a given temperature. This is also true for the curves representing the isosteric heats of CO2 adsorption shown in figure 13. Does this suggest that the nitrogen content on the surface has less impact on CO2 adsorption than porosity or BET surface area? Or does it indicate that the degree of nitrogen functionalization is similar across the DTO_1 and DTO_2 cases and the pristine DTO sample?

 

 

In Table 3, the fitted parameters for the CO2 isotherms of the DT0 and DT0_1 samples were identical at a given temperature because we mistakenly copied and pasted the DT0 values twice. This error has been corrected. You can now see that the values of the parameters are different.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you very much for revising the manuscript. I have read it, but I cannot feel the advantage of nitrogen plasma treatment into the activated carbon. CO2 adsorption capacity of AC with plasma treatment increased only 2~3 %. That was too small to evaluate the structure change or surface property changes. I recommend you to apply more effective condition of plasma if you want to modify the surface of AC. This plasma treatment has a significant improvement in adsorption process than that of  liquid treatment or thermal treatment AC? I think this paper needs more big effect of N2 plasma treatment if you want to submit again.

 

Comments on the Quality of English Language

I think no problem.

Author Response

Reviever 1

Comments:

Thank you very much for revising the manuscript. I have read it, but I cannot feel the advantage of nitrogen plasma treatment into the activated carbon. CO2 adsorption capacity of AC with plasma treatment increased only 2~3 %. That was too small to evaluate the structure change or surface property changes. I recommend you to apply more effective condition of plasma if you want to modify the surface of AC. This plasma treatment has a significant improvement in adsorption process than that of  liquid treatment or thermal treatment AC? I think this paper needs more big effect of N2 plasma treatment if you want to submit again.

 

Response

The aim of this article was to highlight the potential of plasma treatment of activated carbon to enhance CO2 adsorption. It is true that we have achieved a slight increase in adsorption, but these are pioneering studies, and we are only at the beginning of this research path. Currently, we are exploring the possibilities of increasing the nitrogen content on the surface of activated carbon as a result of nitrogen plasma treatment. We would like to publish this article to give others the opportunity to familiarize themselves with the potential of using nitrogen plasma to enhance CO2 adsorption on activated carbon. We hope that in the next manuscript, we will be able to present a more significant effect of N2 plasma treatment.

Reviewer 3 Report

Comments and Suggestions for Authors

all my concerns have been addressed in the revised version of the manuscript.

the manuscript is ready for publication.

Author Response

Reviewer 3

Comments:

all my concerns have been addressed in the revised version of the manuscript.

the manuscript is ready for publication.

 

Response

Thank you for your comments that our manuscript is ready for publication.