Microwave-Assisted Oxidation of N2 into NOx over a La-Ce-Mn-O Perovskite Yielding Plasmas in a Quartz Flow Reactor at Atmospheric Pressure
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsMeunier et al. used a microwave-assisted method to oxidize N2 into NOx on a La0.8Ce0.2MnO3 perovskite at atmospheric pressure. Up to 2.5 vol.% of NOx concentrations in one pass were obtained at 600 W. However, more characterizations and analysis should be conducted to study the perovskite material and underlying catalysis active sites and mechanism. Necessary improvement should be made following below comments.
1. The English should be further improved and the mistakes about superscripts and subscripts should be corrected through the manuscript.
2. In the introduction and experiment parts, descriptions about the structure of La0.8Ce0.2MnO3 perovskite are missing. At least, the elements at A- and B-sites and space group should be analyzed. Please refer to the paper with DOI of 10.1039/D1TA10652J.
3. Additionally, necessary electronic characterizations should be measured and analyzed for La0.8Ce0.2MnO3 perovskite, e.g., XPS spectra for Ce, Mn and O elements. Especially, the detailed oxygen species are important for N2 oxidation, which should be fitted and analyzed. The authors are suggested to refer to the paper with DOI of 10.1002/cey2.465.
4. The active sites and underlying catalysis mechanism on La0.8Ce0.2MnO3 should be discussed in the revised version, which will help improve the quality of this manuscript.
Comments on the Quality of English Language
Moderate editing of English language required.
Author Response
Meunier et al. used a microwave-assisted method to oxidize N2 into NOx on a La0.8Ce0.2MnO3 perovskite at atmospheric pressure. Up to 2.5 vol.% of NOx concentrations in one pass were obtained at 600 W. However, more characterizations and analysis should be conducted to study the perovskite material and underlying catalysis active sites and mechanism. Necessary improvement should be made following below comments.
Comment 1: The English should be further improved and the mistakes about superscripts and subscripts should be corrected through the manuscript.
Reply 1: we have improved the written English and corrected typos. Note that the font from the MDPI template makes subscripts appear at the same level as normal case characters.
Comment 2: In the introduction and experiment parts, descriptions about the structure of La0.8Ce0.2MnO3 perovskite are missing. At least, the elements at A- and B-sites and space group should be analyzed. Please refer to the paper with DOI of 10.1039/D1TA10652J.
Reply 2: we have added the sentence “The large La and Ce cations occupy of the A-type sites of the perovskite structure, while Mn cations occupy B-sites [two new references].” In the experimental section and two new references that are generic reviews on the structure of perovskite catalysts ( Bhalla, A.S. ; Guo, R. ; Roy, R. The perovskite structure—a review of its role in ceramic science and technology. Materials Research Innovations 2000, 4, 3-26, https://doi.org/10.1007/s100190000062 and Keav, S.; Matam, S.K.; Ferri, D.; Weidenkaff, A. Structured Perovskite-Based Catalysts and Their Application as Three-Way Catalytic Converters—A Review. Catalysts 2014, 4, 226-255. https://doi.org/10.3390/catal4030226 ). This review details the basic characteristic of ABO3 perovskites, in particular for A = La or Ce and B = Mn that are relevant to our paper.
Comment 3: Additionally, necessary electronic characterizations should be measured and analyzed for La0.8Ce0.2MnO3 perovskite, e.g., XPS spectra for Ce, Mn and O elements. Especially, the detailed oxygen species are important for N2 oxidation, which should be fitted and analyzed. The authors are suggested to refer to the paper with DOI of 10.1002/cey2.465.
Reply 3: The reviewer is correct that the oxidation states of Ce and Mn and crucial to understand the role of these cations in the oxidation of NO. Yet, the actual reaction is taking place at such high temperatures (above 1600 °C) that the structure of the catalyst is likely changing and would not be captured by typical XPS sampling methods. We are planning to carry out XAS analyses, but need to apply for beam time and determine a microwave-compatible line.
Comment 4: The active sites and underlying catalysis mechanism on La0.8Ce0.2MnO3 should be discussed in the revised version, which will help improve the quality of this manuscript.
Reply 4: This question is indeed important and relates to the previous comment. As mentioned in the paper, the first step will be to determine the fraction of homogeneous and heterogeneous reaction. We are currently working on this aspect and hope to report future data in a subsequent paper.
Reviewer 2 Report
Comments and Suggestions for AuthorsThis article focuses on the use of a La-Ce-Mn-O perovskite catalyst, for the microwave-assisted oxidation of N2 to NOx.
Although the topic of the article is interesting, the writing of the article is rough, the discussion lacks depth and too many times the authors emphasize that further investigation and work is needed to clarify some aspects. In general, more than a scientific article, it seems like a technical report. Whoever writes a scientific article must highlight the novelties and extol the results, only in rare cases is it appropriate to refer to subsequent studies.
1. The English must be improved. For example, the title of the article must be written in correct English.
2. The authors state, at the end of the introduction, that energy balance is out of the scope of the article, however in the introduction, line 64-66, the microwave-excited hot plasma is proposed as a technology for decarbonization and energy-efficiency of the process. The actual “usability” of processes of this type cannot ignore an economic analysis.
3. Experimental section. Although the catalyst preparation method has been described previously in another article, the procedure used for its preparation needs to be described in this article.
4. The description of the catalytic experiment must be completed by adding the mass and volume of the catalyst, the size of the catalytic bed, the gases supplied and their percentages, and whether any pretreatments were done on the catalyst. The reader of an article should not look for information in the captions, this information should be written first in the experimental section and then in the captions.
5. The discussion lacks depth, it needs to be improved
The measure of the temperature in the MW cavities is not problematic, but it is necessary to use the right devices and requires a study on how to position the probes. the authors should explain why an orange colour corresponds to a temperature above 700°C.
6. Lines 181-183. Th authors should explain why it is difficult to determine whether the reaction Is heterogeneous and/or homogeneous.
7. A number of interesting characterizations are listed on the spent catalyst; however the related comments are sketchy, e.g. lines 234-235 the authors stated: “The proportion of minor perovskite phases such as La2Ce2O6.72 and LaMnO3 somewhat evolved”. This needs to be discussed and clarified.
8. The process described by the authors is on a laboratory scale, I ask if the authors hypothesized a scale up of the plant described in this article.
Comments on the Quality of English LanguageThe English must be improved
Author Response
Comment 1: This article focuses on the use of a La-Ce-Mn-O perovskite catalyst, for the microwave-assisted oxidation of N2 to NOx. Although the topic of the article is interesting, the writing of the article is rough, the discussion lacks depth and too many times the authors emphasize that further investigation and work is needed to clarify some aspects. In general, more than a scientific article, it seems like a technical report. Whoever writes a scientific article must highlight the novelties and extol the results, only in rare cases is it appropriate to refer to subsequent studies. The English must be improved. For example, the title of the article must be written in correct English.
Reply 1: We agree with the reviewer that the present work is rather a short communication aimed at reporting exciting new results. We indeed stressed that more work is needed to understand the reaction mechanism, which will be highly complex and demanding because of the unusually harsh reaction conditions. This surely will require years of work by many teams, but we feel that it is important to quickly report that it is possible to fix nitrogen in such a simple reactor lay-out. We are actually hopeful that this paper should become highly cited.
Comment 2: The authors state, at the end of the introduction, that energy balance is out of the scope of the article, however in the introduction, line 64-66, the microwave-excited hot plasma is proposed as a technology for decarbonization and energy-efficiency of the process. The actual “usability” of processes of this type cannot ignore an economic analysis.
Reply 2: The development of viable commercial processes will indeed require careful energy balance, but the work presented here does not pretend to have achieved this. In addition to viable energy balance, catalyst and reactor (quartz here) durability will have to be investigated. Economic analyses can be done by other teams, once optimum materials, reactor scale and irradiating power have been determined. As a matter of fact, most of the catalyst development studies do not include economic analyses.
Comment 3: Although the catalyst preparation method has been described previously in another article, the procedure used for its preparation needs to be described in this article.
Reply 3: We have added the sample preparation in the Supplementary Materials section, which reads:
“The synthesis was carried out under microwave heating using an advanced Labstation microwave oven (from Pyro Touch system- Milestones, Italy) which operated at 2.45 GHz with a power supply of 1000 W. The instrument was a multimode cavity equipped with temperature and pressure monitoring device. The reagents were supplied by Aldrich Chemical: La(NO3)3 · 9H2O (98%), Mn(NO3)2 · 4H2O (98%) and (NH4) 2Ce(NO3)6 (99.9%) and were used as received. A homogeneous aqueous solution was prepared containing the mixture of soluble salts added in adequate proportions. The mixture was transferred into a 250 ml Pyrex flask and then subjected to microwave irradiation to concentrate the solution by gradual removal of the solvent. The solution on exposure to microwave irradiation (P = 300 W) boiled with evolution of nitrogen oxide gases, leading to a viscous colored gel, which was rapidly transformed into black foam when the output power was increased to 600 W. Finally, by increasing the heating power (1000 W microwave irradiation for a short time) the powder, obtained after grinding the foam, ignites with appearance of flame and becomes red hot resulting in the product formation.”
Comment 4: The description of the catalytic experiment must be completed by adding the mass and volume of the catalyst, the size of the catalytic bed, the gases supplied and their percentages, and whether any pretreatments were done on the catalyst. The reader of an article should not look for information in the captions, this information should be written first in the experimental section and then in the captions.
Reply 4: We have added the mass and feed compositions under each figure because those were varied and so we believe that it is easier for the reader to have those directly in the corresponding figure legend. No pre-treatments were done, which has now been indicated in the experimental section.
Comment 5: The discussion lacks depth, it needs to be improved. The measure of the temperature in the MW cavities is not problematic, but it is necessary to use the right devices and requires a study on how to position the probes. the authors should explain why an orange colour corresponds to a temperature above 700°C.
Reply 5: As replied to the first comment of Reviewer 1, the present work merely reports the outstanding nitrogen fixation data obtained using a simple reactor lay-out and does not intend to unravel the mechanistic details of the reaction. More fundamental work will be needed to clarify mechanisms (homogeneous, heterogeneous, mixed) taking place at highly challenging temperatures (this is not non-thermal plasma work taking place below 300 °C, we are here above 1700 °C, as indicated by the quartz wool melting). The picture and videos clearly show that the temperature is not homogeneous in the bed and hot-spot can occur at the interface between particles, where an optical pyrometer or optical fiber probe would not reach.
The 700 °C of the orange color is based on black body radiation color scheme as found, for instance, on Wikipedia. We have added the corresponding link https://en.wikipedia.org/wiki/Black-body_radiation
Comment 6. Lines 181-183. Th authors should explain why it is difficult to determine whether the reaction Is heterogeneous and/or homogeneous.
Reply 6: We have added the following paragraph to the Results and Discussion section :
“The density of hot spots and the temperature of those were not controlled and easily measurable. Therefore, it is difficult to relate catalyst weight, bed dead-volume and the apparent reaction rate. In addition, it is likely that bed packing also affected plasma generation, complicating the analysis of reaction rates and bed voids necessary to demonstrate the occurrence of gas-phase reactions. ”.
Comment 7: A number of interesting characterizations are listed on the spent catalyst; however the related comments are sketchy, e.g. lines 234-235 the authors stated: “The proportion of minor perovskite phases such as La2Ce2O6.72 and LaMnO3 somewhat evolved”. This needs to be discussed and clarified.
Reply 7: We have modified the discussion of the XRD analysis by rewriting the sentences as follows: “The proportion of minor perovskite phases LaMnO3 (PDF 01-076-9087) and La2Ce2O6.72 decreased, while that of LaMnO3.00 (PDF 00-035-1353) increased. This suggests a partial segregation of Ce and La.”.
Comment 8: The process described by the authors is on a laboratory scale, I ask if the authors hypothesized a scale up of the plant described in this article.
Reply 8: The simplicity of the reactor lay-out is promising, but it will be essential to investigate the energy balance of the process and durability of the catalyst before providing a sound answer to this question.
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsA careful evaluation of the authors' edits to the manuscript suggests that only some of the issues identified in the first round of revisions were addressed. The synthesi method was added to the supplementary material, the English was not improved, the discussion was not improved, and as for gaps to be filled, the authors inform us that this is the focus of this article: " ...... this work is rather a short communication aimed at reporting exciting new results". Short communications are manuscripts that report exceptional results, for which there is an urgent need for publication, however in my opinion the results reported in this manuscript are not sufficient to justify a short communication. Based on these considerations, I do not recommend publication of this manuscript.
Comments on the Quality of English LanguageEnglish should be improved
Author Response
Comment 1: A careful evaluation of the authors' edits to the manuscript suggests that only some of the issues identified in the first round of revisions were addressed. The synthesi method was added to the supplementary material, the English was not improved, the discussion was not improved, and as for gaps to be filled, the authors inform us that this is the focus of this article: " ...... this work is rather a short communication aimed at reporting exciting new results".
Reply 1: We have improved again the quality of the written English. The sentences are kept short and linear, so everybody should be able to understand the meaning of each of those.
Comment 2: Short communications are manuscripts that report exceptional results, for which there is an urgent need for publication, however in my opinion the results reported in this manuscript are not sufficient to justify a short communication. Based on these considerations, I do not recommend publication of this manuscript.
Reply 2: The present "short communication" comprises a main article of 10 pages and 7 pages of supplementary materials. This is already consequent. As far as the exceptional character of the data presented is concerned, this is somewhat arbitrary. We feel that there is no previous report in the literature that present such a simple reactor design to produce up to 2% of NOx from air in a flow system and an inexpensive commercial microwave oven. We would yet be happy to complete the reference section if important previous studies are missing.
