Plasma Catalysis: Distinguishing between Thermal and Chemical Effects

Round 1

Reviewer 1 Report

The changes introduced in the new version of the paper contributes to clarify some of the points that were unclear in the previous one. Particularly, they make clear that CO2 formation is a temperature dependent effect (i.e. decomposition of CaCO3), while CO formation is due to the plasma decomposition of CO2. There would be a very simple test to reinforce this conclusion: follow the thermal decomposition of the CaCO3 at e.g. 680ºC 8 (i.e, assuming an increase in 50º C in temperature due to plasma thermal effects). The observed increase in CO+CO2 formation at, let say, 680º in the absence of plasma should be equivalent in the presence of plasma to an increase in CO2 with respect to the results in Figure 2 at 630 ºC. Also interesting would be to know the time needed for this local increase in temperature to occur. According to figure 3 the stabilization of the CO2(O2,CO) evolution as a function of time when applying  higher powers is about one minute. Is this the time required to stabilize the local temperature?

Otherwise, I think that the paper would gain in clarity if the results are presented in a more concise and comprehensive way trying to quantitatively compare the differences (for example calculating CO/CO2, O2/CO2 or O2/CO2 ratios to determine the incidence of the plasma reaction in the different samples) and reducing the number of figures.

Author Response

Dear reviewer,

Thanks for the suggestions, all the comments have been answered. Please find the answers in the attached file.

Kind regards,

Guido Giammaria

Author Response File: Author Response.docx

Reviewer 2 Report

- the chemical formulas in the header of Figure 10 are not indexed.

All comments were taken into consideration. Thus, the manuscript is ready for publication.

Author Response

Dear reviewer,

Thanks for your approval, the chemical formulas on the header of figure 10 have been indexed.

Kind regards,

Guido Giammaria

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments to the manuscript no. catalysts-413393: Plasma Catalysis; Thermal or Chemical Effect?

The authors of the manuscript tried to differentiate between thermal and chemical effects in DBD systems for decomposition of CaCO3 in Ar environment. Using Ar plasma as inert gas, the effects of specific surface and presence of CO2 in the formation of CO were tested and assumptions were derived. These basic scientific approaches helped to differentiate between thermal effects and plasma-chemical aspects in degradation of CaCO3 and most probably these results may be transferable to other industrial approaches. Hence, the content of this mansuscript will reveal a high relevance in applications of NTP and also fits the scopes of ‘catalysts’. However, there are a few comments which should be taken into consideration before publication:

·         Page 2 Line 53:’ …rather low rotational and translational temperatures, typically in the order or smaller than 100 K [14, 15].’ Both references point out that gas temperature at the outlet is still at room temperature or less than 26 °C. So the temperature increase by 100 K seems to be too high. The low relevance of heating up exhaust gas by NTP discharge was also shown in Dobslaw et al. (2017; https://dx.doi.org/10.1016/j.jece.2017.10.015), where temperature was in the range of -3,3 – 34 °C due to variations of ambient air. Beside additional informations about the plasma temperature, this reference also gives additional information about methane removal, which is also mentioned in the introduction of the manuscript. Hence, it its strongly recommend to add this reference to the existing reference list.

·         Page 2 Line 64: Please clarify how this efficiency of 10% was calculated. What efficiency is meant – electrical one? transformation efficiency?

·         Page 3 Line 106: …study that penetration of plasma in pores is possible to some extent…

·         Figure 5b: Please add an explanation for the sharp change in concentrations of CO and that way the sum of CO + CO2 after 9 minutes.

·         Chapter Diskussion: The indexes of chemical formulas in the text are not indexed.

·         Line 302: …gases vary, it can be concluded…

·         Conclusion: The primary conclusion of this manuscript is, that degradation effects caused by  the increase in gas temperature by DBD plasma are not negligible and a main factor in transformation of educts. Some references are given, where an increase of temperature between 70 °C to more than 200 °C were observed. However, how could this assumption be fixed, when other manuscripts (like the one mentioned in the first comment) reveal no significant temperature changes in gas outlet in comparison to gas inlet. Please add a critical paragraph dealing with this problem.

·         Appendices: Postulation that the appendices are published as supporting materials in a separate file, the figures of the appendices should be better linked in the main text.

Author Response

We thank the reviewer for his/her suggestions. We answered all the comments, improving the manuscript. The answers are in the file attached.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper reports about the effect of a DBD plasma on the CO2 decomposition reaction to O2 and CO. CO2 is produced in the reactor by thermal decomposition of CaCO3. Based on numerous speculations and many experimental tests authors concluded that CO  and O2 (this delayed with respect to CO) is only produced by the plasma processes. A brief literature analysis the thermal decomposition of CaCO3 should have sufficed for these conclusions: after more than 40 years of research there are no claims that CaCO3 may decompose into CO plus O2 (e.g., Thermochimica acta 255 (1995) 383; ibim 6 (1973) 67; Eur. J. Mineral processing and Environm. Protec. 1 (2001) 89). Then, based on casual results, authors try to find a correlation between decomposition rate and surface area of the CaCO3 samples (five samples, three of them with similar size and two other with a smaller size). The purpose is to determine to which extent thermal or plasma effects are interconnected, (i.e., to determine how plasma may heat the CaCO3). After a cumbersome set of experimental results, authors conclude that CaCO3 “decomposition is thermally controlled, whereas CO2 dissociation is “obviously” plasma controlled. I fear that this conclusion would have not need so many experiments based on hypothesis that need to be proved. For example, authors claim that surface area of samples and their internal porosity are key parameters to control the decomposition reaction of CaCO3 and the posterior decomposition of CO2 by plasma (i.e., discussion around the R1, R2, R3 scheme in the discussion). A simple survey of literature reveals that factors such as existence of particle agglomerates, geometry of sample pile and size, partial pressure of CO2, degree of crystallinity, shape of the particles, etc. may have a tremendous effect in controlling the kinetics of CaCO3 decomposition. None of these factors have been examined and any conclusion based merely on BET surface vales and porosity determined from mercury porosimetry analysis has many probabilities to be circumstantial.

On the other hand, authors base some of their conclusions on the shape of MS profiles over time. This is a rather delicate exercise and authors should be sure that the actual shape of these profiles is not affected by experimental factors (for example the reported delayed evolution of oxygen might have other reasons that the non-proven proposal that it interacts with the CaO surface).

In summary, although the strategy to follow the evolution of a thermally induced solid state decomposition reaction and how it is affected by DBD plasmas might be justified, the present paper is too speculative to grant a definitive credit to the conclusions about the surface area effects. Therefore, my suggestion is that authors proceed to a radical revision of the reported results and try to summarize them in a much shorter version where they just concentrate in those points which are clear and not affected by solid state kinetics which are not the core subject of the paper.

Author Response

We thank the reviewer for his/her suggestions. We answered all the comments, improving the manuscript and we hope we have clarified the scope of this work. The answers are in the file attached.

Author Response File: Author Response.docx

Reviewer 3 Report

In the manuscript titled, “Plasma catalysis; Thermal or Chemical Effect?” the authors studied the effect of Ar plasma on the decomposition of calcium carbonate by comparing the thermal effects and plasma chemistry based on the reaction rates and dynamics. The authors did a detailed study on the application of DBD Ar plasma by decomposing calcium carbonate as a model reaction. This study provides a systematic method for distinguishing thermal effects and plasma effect in DBD plasma applications. This work is novel enough to be published in the journal catalysts. However, there are few concerns and issues that were listed below and need to be addressed before getting accepted for publication.

1. This is the study about distinguishing whether a particular reaction is catalyzed by thermal effect or chemical effect in plasma catalysis. In the present study, the authors used decomposition of calcium carbonate as a model reaction and here thermal effect is responsible for catalysis. What happens if a different reaction is considered? What are the factors responsible for a thermal effect or chemical effect? If the authors could study another reaction and establish the general factors responsible for thermal effect or chemical effect for any reaction that will be great.

2. Page1, line 9, change the sentence “develop a method to distinct between” to develop a method to distinguish between”. The same correction need to be done in line 28 of page 1 (change distinct to distinguish).

3. There are few typos and grammatical mistakes throughout the manuscript. Please go through the entire manuscript and address these issues.

Author Response

We thank the reviewer for his/her suggestions. We answered all the comments, improving the manuscript. The answers are in the file attached.

Author Response File: Author Response.docx