Low-Temperature NH3-SCR Performance and In Situ DRIFTS Study on Zeolite X-Supported Different Crystal Phases of MnO2 Catalysts
Round 1
Reviewer 1 Report
In this manuscript, the authors applied four MnO2 catalysts supported on X-type zeolite for the low-temperature NH3-SCR. The catalysts were characterized by SEM, NH3-TPD, H2-TPR. The reaction activity of four catalysts was studied, including the in-depth analysis via in-situ DRIFTS. Overall, the manuscript was well organized and analyzed. Thus, the reviewer suggests to accept it after a minor revision.
1. TEM should be measured, as the scale bar in SEM is too large and cannot reflect the aggregation of particles properly.
2. XRD of four catalysts should be provided.
3. Declination of band intensity in Line 147 and in line 210 should be explained properly.
4. The ER and LH mechanisms should be analyzed separately based on the results, and a reaction model can be provided for a better understanding.
5. Hydrothermal aging treatment and activity measurement should be carried out for catalysts, at least for the optimized one.
Author Response
Reviewer #1:
In this manuscript, the authors applied four MnO2 catalysts supported on X-type zeolite for the low-temperature NH3-SCR. The catalysts were characterized by SEM, NH3-TPD, H2-TPR. The reaction activity of four catalysts was studied, including the in-depth analysis via in-situ DRIFTS. Overall, the manuscript was well organized and analyzed. Thus, the reviewer suggests to accept it after a minor revision.
Comment 1: TEM should be measured, as the scale bar in SEM is too large and cannot reflect the aggregation of particles properly.
Response: Thanks for your good suggestion. From SEM images in Figure a1, b1, c1 and d1, it is clear to compare the aggregation of the four catalysts, also, combining with the EDS mapping, we could find that some Mn elements aggregated together in α-MnO2/X and σ-MnO2/X, whereas Mn element on β-MnO2/X and γ-MnO2/X distributed uniformly. Moreover, all of the catalysts possessed the regular octahedral zeolite X structures, and manganese dioxides species with different crystal types distributed on the support surface, which was in line with the results of our previous studies 1,2. Thereby, the microtopography of catalysts can be clearly observed via SEM measurements.
References:
- Chen, L.; Ren, S.; Jiang, Y.; Liu, L.; Wang, M.; Yang, J.; Chen, Z.; Liu, W.; Liu, Q., Effect of Mn and Ce oxides on low-temperature NH3-SCR performance over blast furnace slag-derived zeolite X supported catalysts. Fuel 2022, 320.
- Yang, J.; Ren, S.; Su, B.; zhou, Y.; Hu, G.; Jiang, L.; Cao, J.; Liu, W.; Yao, L.; Kong, M.; Yang, J.; Liu, Q., Insight into N2O formation over different crystal phases of MnO2 during low-temperature NH3–SCR of NO. Catalysis Letters 2021, 151, 2964–2971.
Comment 2: XRD of four catalysts should be provided.
Response: Thanks for your help suggestion. The XRD patterns of the four catalysts were shown in Figure 1d, and the analysis was added in red color in the revised manuscript.
Comment 3: Declination of band intensity in Line 147 and in line 210 should be explained properly.
Response: Thanks for your good suggestion. The description and analysis of bands intensity in Line 147 and in line 210 are modified in red color in the revised manuscript.
Comment 4: The ER and LH mechanisms should be analyzed separately based on the results, and a reaction model can be provided for a better understanding.
Response: Thanks for your kind suggestion. We add more analysis about the reaction mechanisms in Discussion section, and related reaction was proposed as well in red color in the revised manuscript.
Comment 5: Hydrothermal aging treatment and activity measurement should be carried out for catalysts, at least for the optimized one.
Response: Thanks for your comments. Although the hydrothermal aging treatment is important for zeolite-based catalysts, the aim of the research is to investigated the enhance of the zeolite X supported for different crystal phases of MnO2 basing on in situ DRIFTS study, thereby, the SCR performance and in situ DRIFTS spectra of the catalysts were focused on research.
Reviewer 2 Report
The authors present a comprehensive investigation of MnO2/X catalysts for NH3-SCR. The catalyst characterisation and the reaction testing (sections 2.1 and 2.2) are convincing. The infrared spectroscopy shown in section 2.3 is very hard work for the reader. This largely comes from insufficient information being provided to the reader as to what is being done.
In particular, the captions to Figures 5 and 6 do not provide enough detail to understand what is being shown. What are the plots shown in Figure 5a,b,c,d and 6a,b,c,d? They are not mentioned in the captions. From their appearance, they are clearly difference spectra, but what is the reference spectrum? ie what are the difference of? The red line in the figures is labelled as NH3 but there is no N-H stretch present. Why not?
The heat map plots shown also require more explanation. What is being shown? Are they difference plots or are they the actual spectra? In either case, what is used as a background? In Figure 5(a1) bands assigned to Lewis (L-NH3)and Bronsted (B-NH4+) sites are shown but these undergo shifts of several hundred wavenumbers at later times. This is not reasonable and needs explanation or justification.
At the end of section 3.3 they state "suggesting that the effective reaction pathway of SCR process happened" the authors should discuss how their data either supports or contradicts the existing knowledge of the mechanism of NH3-SCR.
The Conclusion also needs work. At present it reads like an Abstract. The authors should express an opinion as to which of their catalysts they consider to be the most promising for commercial use in NH3-SCR.
Overall, I believe that the work is certainly worthy of publication in Catalysts, however, major revision is required before that can happen.
Minor points
Line 41: "alpha" is missing from the first type of MnO2 listed.
Line 50: Should NH2 be NH3?
Lines 72-74 and Figure 1: The authors state that "some Mn elements aggregated together in α-MnO2/X and σ-MnO2/X, whereas Mn element on β-MnO2/X and γ-MnO2/X distributed uniformly, " I do not dispute the conclusions regarding the MnO2 distribution but this is very hard to see in Figure 1. Could the authors change the colour for the Mn distribution part of the figure to make the Mn more obvious?
Figure 2: In parts (a), (b), (c) the blue lines are labelled as delta-MnO2/X, surely these should be sigma-MnO2/X?
Figure 2: Part (c) has the y-axis as "N2O concentration (%)", however the scale goes to 300%! Presumably this should be ppm not %. Please correct.
Line 102: "concentration" not "centration"
Line 105: Surely this is section 2.2 not 2.3?
Lines 118-119: The authors state " the peaks of acid sites for γ-MnO2/X catalyst shifted to lower temperature, suggesting that γ-MnO2/X catalyst showed better acidity capacity than that of other catalysts," The acid capacity relates to the number of acid sites not their strength. Please clarify or justify.
Figure 3a: Why are there (apparently) two types of mediun acid sites as shown by the use two colours for these?
Figure 3b: What is being shown here? This should be explained in either (or both) the caption and the text. In the inset to b) labels for the colour coding and the axis labels are too small to read. Please correct.
Lines 133 and 257 "more reducible" not "stronger reduction capacity"
Figure 5 (c1), (c2), (c3) are all labelled as (c). Please correct.
The overall standard of English is acceptable, but would benefit from input by a native speaker.
Author Response
Reviewer #2:
Comment 1: The authors present a comprehensive investigation of MnO2/X catalysts for NH3-SCR. The catalyst characterization and the reaction testing (sections 2.1 and 2.2) are convincing. The infrared spectroscopy shown in section 2.3 is very hard work for the reader. This largely comes from insufficient information being provided to the reader as to what is being done.
Response: Thanks for your comments. In order to further investigate the reaction of α-MnO2/X, β-MnO2/X, γ-MnO2/X and σ-MnO2/X catalysts, the in situ DRIFTS studies were measured. Combined with the SCR activity of the catalysts, the reaction temperature was 200 oC, and the in situ DRIFTS spectra of only NH3 adsorption, reactions between NO+O2 and pre-adsorbed NH3 species, NO+O2 adsorption, reactions between NH3 and pre-adsorbed NO+O2 species and reactions between NH3+NO+O2 were tested to obtain the surface acidity, nitrate/nitrites species and related mechanisms over catalyst.
Comment 2: In particular, the captions to Figures 5 and 6 do not provide enough detail to understand what is being shown. What are the plots shown in Figure 5a, b, c, d and 6a, b, c, d? They are not mentioned in the captions. From their appearance, they are clearly difference spectra, but what is the reference spectrum? ie what are the difference of? The red line in the figures is labelled as NH3 but there is no N-H stretch present. Why not?
Response: Thanks for your kind suggestion. In Figure 5, the summary of in situ DRIFTS spectra of NH3 adsorption, NO+O2 reacted with pre-adsorbed NH3 species and NH3 +NO+O2 species at 200 oC over (a) α-MnO2/X, (b) β-MnO2/X, (c) γ-MnO2/X and (d) σ-MnO2/X catalysts, and in Figure 6, the summary of in situ DRIFTS spectra of NO+O2 adsorption, NH3 reacted with pre-adsorbed NO+O2 species and NH3 +NO+O2 species at 200 oC over (a) α-MnO2/X, (b) β-MnO2/X, (c) γ-MnO2/X and (d) σ-MnO2/X catalysts. We are sorry for the small mistake, and the caption was added in red color in the revised manuscript as well. Owing to the higher intensity of nitrate species bands, it could be hard to find the bands of adsorbed NH3 and N-H stretch, however, we could find the related bands at Figure 5a1, b1, c1, d1. The typical spectra of reactions between NO+O2 and pre-adsorbed NH3 species and reactions between NH3 and pre-adsorbed NO+O2 species on catalysts were chose in Figure 5a, b, c, d and 6a, b, c, d to better compare the changes of the surface species and conclude the reaction mechanism.
Comment 3: The heat map plots shown also require more explanation. What is being shown? Are they difference plots or are they the actual spectra? In either case, what is used as a background? In Figure 5(a1) bands assigned to Lewis (L-NH3) and Bronsted (B-NH4+) sites are shown but these undergo shifts of several hundred wavenumbers at later times. This is not reasonable and needs explanation or justification.
Response: Thanks for your good comment. In Figure 5 and Figure 6, the in situ DRIFTS spectra in every graph were actual spectra. In details, the different color showed the intensity of the spectra, and the background of blue color meant 0 value of the band intensity. It could be seen in Figure 5, the bands intensity of -NH2 species increased with NH3 introducing time, while the bands of Lewis (L-NH3) and Bronsted (B-NH4+) sites declined, suggested that adsorbed NH3 species reacted with adsorbed O or O2- species, and generated more -NH2 species.
Comment 4: At the end of section 3.3 they state "suggesting that the effective reaction pathway of SCR process happened" the authors should discuss how their data either supports or contradicts the existing knowledge of the mechanism of NH3-SCR.
Response: Thanks for your kind advise. The reaction mechanism was proposed in the Discussion section, and marked as red color in the revised manuscript.
Comment 5: The Conclusion also needs work. At present it reads like an Abstract. The authors should express an opinion as to which of their catalysts they consider to be the most promising for commercial use in NH3-SCR.
Response: Thanks for your kind suggestion. We added some mechanism discussion in the section 3 in red color in the revised manuscript. And the superior property of γ-MnO2/X catalyst was already summarized in section 3.
Comment 6: Overall, I believe that the work is certainly worthy of publication in Catalysts, however, major revision is required before that can happen.
Response: Thanks for your good comments. The related modification was added in red color in the revised manuscript.
Comment 7: Minor points
Comment 7.1: Line 41: "alpha" is missing from the first type of MnO2 listed.
Response: Thanks for your careful checking. We modify it in the revised manuscript.
Comment 7.2: Line 50: Should NH2 be NH3?
Response: Thanks for your careful checking. We modify it in the revised manuscript.
Comment 7.3: Lines 72-74 and Figure 1: The authors state that "some Mn elements aggregated together in α-MnO2/X and σ-MnO2/X, whereas Mn element on β-MnO2/X and γ-MnO2/X distributed uniformly, " I do not dispute the conclusions regarding the MnO2 distribution but this is very hard to see in Figure 1. Could the authors change the color for the Mn distribution part of the figure to make the Mn more obvious?
Response: Thanks for your kind suggestion. We change the contrast and brightness of the image, so that the distribution of Mn clearly.
Comment 7.4: Figure 2: In parts (a), (b), (c) the blue lines are labelled as delta-MnO2/X, surely these should be sigma-MnO2/X?
Response: Thanks for your comments. We check the experiment data result, and the color for all the catalysts is right.
Comment 7.5: Figure 2: Part (c) has the y-axis as "N2O concentration (%)", however the scale goes to 300%! Presumably this should be ppm not %. Please correct.
Response: Thanks for your careful checking. We modify it in the revised manuscript.
Comment 7.6: Line 102: "concentration" not "centration"
Response: Thanks for your careful checking. We modify it in the revised manuscript.
Comment 7.7: Line 105: Surely this is section 2.2 not 2.3?
Response: Thanks for your careful checking. We change the section number aright.
Comment 7.8: Lines 118-119: The authors state " the peaks of acid sites for γ-MnO2/X catalyst shifted to lower temperature, suggesting that γ-MnO2/X catalyst showed better acidity capacity than that of other catalysts," The acid capacity relates to the number of acid sites not their strength. Please clarify or justify.
Response: Thanks for your comments. Generally, the acid capacity depends on the amount of acid sites and the NH3 desorption amount. We are sorry for the unclear statement. In fact, this sentence aimed to summarized, we modify it in the revised manuscript.
Comment 7.9: Figure 3a: Why are there (apparently) two types of medium acid sites as shown by the use two colors for these?
Response: Thanks for your comments. There peaks belonged to medium acid sites could be divided into three fitting peaks, the different color could be separate clearly.
Comment 7.10: Figure 3b: What is being shown here? This should be explained in either (or both) the caption and the text. In the inset to b) labels for the color coding and the axis labels are too small to read. Please correct.
Response: Thanks for your kind suggestion. All the curves for NH3-TPD over catalysts were shown in the same baseline, which could be better to compare the peaks intensity and position. And the axis labels correct in the revised manuscript.
Comment 7.11: Lines 133 and 257 "more reducible" not "stronger reduction capacity"
Response: Thanks for your kind suggestion. We modify it in the revised manuscript.
Comment 7.12: Figure 5 (c1), (c2), (c3) are all labelled as (c). Please correct.
Response: Thanks for your careful checking. We modify it in the revised manuscript.
Comment 7.13: The overall standard of English is acceptable, but would benefit from input by a native speaker.
Response: Thanks for your comments. We modify it in the revised manuscript.
Round 2
Reviewer 2 Report
The authors have largely addressed the comments I raised. There are two minor items that require revision.
(i) In Figure 2 all of the blue lines are labelled as delta-MnO2/X, these should be sigma-MnO2/X
(ii) In response to my querying what was being shown in Figure 3b, the authors state: "All the curves for NH3-TPD over catalysts were shown in the same baseline, which could be better to compare the peaks intensity and position." This sentence should be included in the Figure caption.
When these minor points are addressed, I recommend publication in Catalysts.
Author Response
The authors have largely addressed the comments I raised. There are two minor items that require revision.
Response: Thanks for your comments.
(i) In Figure 2 all of the blue lines are labelled as delta-MnO2/X, these should be sigma-MnO2/X
Response: Thanks for your careful checking. We have modified it.
(ii) In response to my querying what was being shown in Figure 3b, the authors state: "All the curves for NH3-TPD over catalysts were shown in the same baseline, which could be better to compare the peaks intensity and position." This sentence should be included in the Figure caption.
Response: Thanks for your good suggestion. We have added the sentence under the Figure.
When these minor points are addressed, I recommend publication in Catalysts.
Response: Thanks for your comments.
