Reaction Kinetics and Mechanism of VOCs Combustion on Mn-Ce-SBA-15

Round 1

Reviewer 1 Report

The authors revised the original version of the manuscript and performed some changes according to the reviewing process. Therefore, I recommend this manuscript for publication in the present form.

Author Response

No further requests were specified.

Reviewer 2 Report

The article is focused on the study of three different catalysts, for abatement of VOCs in vehicles fueled with LPG. on that context, the emphasis is on the use of Mn and Mn-Ce catalysts as an cheaper alternative for established TWCs. The authors also claim that, according with literature,  Mn mixed with Ce catalysts present higher reducibility and better performance (activity, stability) than Mn monometallic ones. SBA-15 was selected as support due to its high surface specific area and pore structure. Furthermore, a kinetic study of the mechanism of the reaction is presented

According with this scope, I detect many controversial points in the work presented:

First, it is well known that LPG vehicles emits less VOCs than those running with gasoline or diesel, and, therefore, I don´t see the relation of using a formulation for VOcs abatement instead of a conventional TWC formulation, as a catalyst for vehicles should also meet the decrease of NOx and CO levels together with the VOCs oxidation. Could that be obtained with the proposed formulation?

Contrary to the literature results, the catalysts here studied present a lower reducibility when Ce is incorporated in the formulation. How can this interpreted and the conclusion compared with previous bibliographic studies?

The incorporation of Ce also results in a lower performance of the catalysts compared with the monometallic, in contradiction with the expectation of the authors expressed in the introduction. Could this be affected by the preparation recipe used? Would an alternative sequence of deposition of the components (Mn, Ce) result in a better activity?. Probably a comparison of a monometallic Ce –SBA-15 sample would be advisable to complete the study.( Apparently, that sample was studied by XPS, but no other results for this sample is presented in another part of the article).

Some aspects of the results presentation, analysis and discussion should be improved:

XRD: Although by TEM MnO2 phase is detected, in XRD Mn3O4 phase is cocluded by the presence of a broad and minor bump in the rane 20-40 degrees, arguing that the Mn3O4 main peaks lie in that zone. Nevertheless, CeO2 peaks are also present in that zone and, therefore, the presence of Mn3O4 can not be concluded from that bump in the XRD. Evenmore, opposite to exposed by authors “the lack of the XRD patterns for manganese oxides and CeO2 could NOT be taken as evidence for the formation of a solid solution between MnOx and CeO2”. Is much simple to deduce that that correspond to the presence of amorphous phases of Mn and Ce oxides, mixed or not.

In this context, XPS could be of great interest to help in the interpretation of the results, but the analysis should be improved and a more complete presentation is required. For instance, regarding the ftting analysis, is not clear what parameters (FWHM, position, relation of intensities between components) have been used for each of the proposed oxidation states (Mn4+, Mn3+). In addition, discussion in terms of formal oxidation states would not apply here. MnO2 and Mn3O4 components would contribute to two shapes, with different initial/final states satellites. In any case, the position of these satellites as well as the FMWH and their intensity ratio should be uniform when different fittings are done. It seems this has not been considered on the fittings here presented. Analysis of CeOx spectra is not consistent with the well discussed literature about this topic. Neither position, ratio of intensities or FMWH are maintained from one sample to another. Analysis of  surface % of Mn/Ce do not seem to support the enrichment of Ce in Mn/Ce samples, but on the contrary an enrichment of Mn vs CE. A different aspect is that in Mn/Ce samples, surface % are smaller than those corresponding to monometaliic Mn-SBA15 or Ce-SBA15 samples (probably due to a bigger particle size or a bigger amount of metals (Mn,Ce) inside the pores of SBA15. Finally, all the XPS corresponding to “after reaction” samples, correspond to samples exposed to atmosphere after been taken out from the reactor. Therefore, their MnO2/Mn3O4 or Ce3+/Ce4+ ratio can be seriously affected by this exposure to atmosphere.

TPR is presented in terms of qualitative analysis, but the technique is quantitative. Integration and calibration to obtain degree of reduction is required for a proper discussion.

EPR is presented, but no noticeable difference could be observed between samples or before/after reaction. What conclusions can be obtained from this study?

Finally, kinetic studies are presented for three samples (Although Ce_SBA15 was also studied in XPS), but only conclusions are extracted for propane oxidation case. Anyway, light-off curves analysis is probably not the best method for a detailed analysis of kinetic parameters, and a bigger number of points in th light off curves would be needed for the fitting of kinetic  parameters in the different samples.

Is not clearly argued from the results presented  that the reducibility of the samples or the Mn3+/Mn4+ surface ratio correlates with the catalytic activity presented. In addition, no stability results for the catalytic activity is presented.

Author Response

First, it is well known that LPG vehicles emits less VOCs than those running with gasoline or diesel, and, therefore, I don´t see the relation of using a formulation for VOCs abatement instead of a conventional TWC formulation, as a catalyst for vehicles should also meet the decrease of NOx and CO levels together with the VOCs oxidation. Could that be obtained with the proposed formulation?

Answer:

The reviewer’s comments are precise. Indeed, according to the literature the addition of Ce should improve the reducibility of the Mn-Ce catalyst. The results reported within the present study reveal some differences and could be explained by a difference in the point of view and the applied approach. We present the obtained data by the required objectiveness, taking into account the possible contradiction with the earlier reported data. The implemented into the practice TWCs are reliable solution to the problems related with the emissions from the LPG vehicles. The present study is aimed to contribute to the better understanding on the behavior metal oxide based catalysts (basic compositions of Mn-Ce supported on SBA-15) an alternative to the noble metal based systems. In general, the oxide based catalysts are more appropriate to operate at elevated temperatures (more typical for LPG and LNG engines). And here the reviewer’s comments are correct: at elevated temperatures the NOx are formed at higher concentrations. This is a subject of the continuation of the present study, together with the long – time thermal stability and the required resistance to SO₂ in presence of several ppm.

Contrary to the literature results, the catalysts here studied present a lower reducibility when Ce is incorporated in the formulation. How can this interpreted and the conclusion compared with previous bibliographic studies?

Answer:

According to literature data, the composition of Mn–Ce catalyst and the interaction between MnOx and CeO₂ strongly affected the catalyst activity [Sang Chai Kim (2002) J Hazard Mater 91:285; Papaefthiniou P, Ioannides T, Verykious XE (1997) Appl Catal B 13:17511; Rao T, Shen M, Jia L, Hao J, Wang J (2007) Catal Commun 8:1743]. The optimum composition was found to depend on the nature of the pollutant to be destroyed and the method of preparation [Chen H, Sayari A, Adnot A, Larachi F (2001) Appl Catal B 32:195, Azalim S, Franco M, Brahmi R, Giraudon J-M, Lamonier J-F (2011) J Hazard Mater 188:422]. Most probably, the current Mn–Ce catalyst has no optimum composition for methane, propane and butane oxidation, and for improved performance in this oxidation reaction, an optimization procedure should be applied with the current Mn–Ce compositions.

As was mentioned in the paper according the literature data the reduction of Mn-Ce catalysts depend of ratio Mn/Ce. When a Ce/Mn ratio greater than 1 the reduction maxima are shifted to lower reduction temperatures, and at a ratio less than 1 to higher reduction temperatures. In our cases, for the sample with Ce/Mn ratio less 1 (MnCe (1:0.5)), we observe overlap of the peaks and very slight shift to higher reduction temperatures, while for the MnCe(1:2) sample there is only one very wide peak with a maximum at 350 ⁰C.

The incorporation of Ce also results in a lower performance of the catalysts compared with the monometallic, in contradiction with the expectation of the authors expressed in the introduction. Could this be affected by the preparation recipe used? Would an alternative sequence of deposition of the components (Mn, Ce) result in a better activity? Probably a comparison of a monometallic Ce–SBA-15 sample would be advisable to complete the study. (Apparently, that sample was studied by XPS, but no other results for this sample are presented in another part of the article).

Some aspects of the results presentation, analysis and discussion should be improved:

Answer:

New experiments were carried out and the data about the temperature dependence of methane, propane and butane combustion are given in Figure 11.

Additionally, data from TPR studies are presented, because the addition of cerium affects the reducing properties of the samples. We do not consider it necessary to characterize this sample with the other methods, because it is with very low activity.

XRD: Although by TEM MnO₂ phase is detected, in XRD Mn₃O₄ phase is concluded by the presence of a broad and minor bump in the range 20-40 degrees, arguing that the Mn₃O₄ main peaks lie in that zone. Nevertheless, CeO₂ peaks are also present in that zone and, therefore, the presence of Mn₃O₄ can not be concluded from that bump in the XRD. Even more, opposite to exposed by authors “the lack of the XRD patterns for manganese oxides and CeO₂ could NOT be taken as evidence for the formation of a solid solution between MnOx and CeO₂”. Is much simple to deduce that that correspond to the presence of amorphous phases of Mn and Ce oxides, mixed or not.

The main XRD patterns for MnO₂ also appear in this interval. We added JCPD card for MnO₂ on the figures. We completely agree that the lack of the XRD patterns for manganese oxides and CeO₂ could be taken as evidence for the amorphous phases of Mn and Ce oxides. The chances are made in the text. The formation of a solid solution between MnOx and CeO₂ of where the replacement of Ce⁴⁺(0.97˚A) by Mn^x+(Mn⁴⁺= 0.53˚A; Mn³⁺= 0.65˚A; Mn²⁺= 0.83˚A) could not be excluded because of structure similarity of both CeO₂ and MnO_x [13].Additional explanations are added in the text.

In this context, XPS could be of great interest to help in the interpretation of the results, but the analysis should be improved and a more complete presentation is required. For instance, regarding the fitting analysis, is not clear what parameters (FWHM, position, relation of intensities between components) have been used for each of the proposed oxidation states (Mn⁴⁺, Mn³⁺). In addition, discussion in terms of formal oxidation states would not apply here. MnO₂ and Mn₃O₄ components would contribute to two shapes, with different initial/final states satellites. In any case, the position of these satellites as well as the FMWH and their intensity ratio should be uniform when different fittings are done. It seems this has not been considered on the fittings here presented. Analysis of CeOx spectra is not consistent with the well discussed literature about this topic. Neither position, ratios of intensities or FMWH are maintained from one sample to another. Analysis of surface % of Mn/Ce do not seem to support the enrichment of Ce in Mn/Ce samples, but on the contrary an enrichment of Mn vs. CE. A different aspect is that in Mn/Ce samples, surface % are smaller than those corresponding to monometallic Mn-SBA15 or Ce-SBA15 samples (probably due to a bigger particle size or a bigger amount of metals (Mn,Ce) inside the pores of SBA15. Finally, all the XPS corresponding to “after reaction” samples, correspond to samples exposed to atmosphere after been taken out from the reactor. Therefore, their MnO₂/Mn₃O₄ or Ce³⁺/Ce⁴⁺ ratio can be seriously affected by this exposure to atmosphere.

Answer:

Thank you for your considerable remarks. The curve fitting procedure is done by using a construction of peaks based on the standard spectra measured in the same apparatus. The standard spectra are recorded for bulk and pure materials. For example, in the paper attached (Lazarova et al. 2019, Applied Surface Science 496 (2019) 143571, fig. 7). We should not forget that there is a difference between bulk and supported materials, which could lead to a change in the binding energy, because of the surroundings and particle size. Moreover, the signal-to-noise ratio in our spectra is very bad, based on the surface concentration. This, of course, makes the curve fitting procedure more complicated and less accurate. In order to get better fits some parameters, like FWHM, position of the satellite (in our case Mn⁴⁺ possesses satellite), have been let a bit free, which results to observable change in the position. Nevertheless, a closer look at the line shape shows the existing of satellite structure, therefore, we have tried this curve fitting in order to have ratio between different oxidation states. Another parameter used in the curve fitting is related to Mn3s core level (V. R. Galakhov et al., Phys. Rev B 65 (2002) 113102), which is more sensible in obtaining oxidation state of manganese, but we used it in order to be sure that we have mixed oxides on the surface. The results, summarized in the table are promising considering that we have errors by concentration calculation as well as by curve fitting procedure.

Similar procedure have been used for Ce3d core level (V. Matolín at al., Surf. Interface Anal. 40 (3-4) (2008), 225 – 230). In this case, more informative is existence or lack of peak at about 916-918 binding energy. This peak exists only with Ce⁴⁺ oxidation state. Otherwise, the curve fitting is provided in order to obtain the ratio between two oxidation states 3+ and 4+ in cerium and, of course, it should be considered that the quality of Ce3d spectra is relatively bad. For Ce is always complicated to answer the question about changes in the oxidation state when exposing to atmosphere or even during the measurement by irradiation with X-rays. Therefore, we use a procedure during the measurement saving every scan separately in order to be able to follow changes. You are right that after catalysis, exposing in the air we could expect oxidation state changes, but with our setups, we cannot confirm that. According to our experience, we do not observe changes in the manganese oxidation states after catalysis and exposing samples to atmosphere. At least, we do not observe considerable changes in the spectra line shapes.

TPR is presented in terms of qualitative analysis, but the technique is quantitative. Integration and calibration to obtain degree of reduction is required for a proper discussion.

Answer:

The TPR data for mono component manganese sample and bi-component MnCe (1:0.5) are convoluted and new explanation are included in the text.

EPR is presented, but no noticeable difference could be observed between samples or before/after reaction. What conclusions can be obtained from this study?

Answer

There is no significant difference between EPR spectra of MnCe (1:2) samples before and after reaction which could be considered as further proof of the stability of the catalyst. In the case of catalysts with high stability, no changes should be observed due to the reaction conditions.

Finally, kinetic studies are presented for three samples (Although Ce_SBA15 was also studied in XPS), but only conclusions are extracted for propane oxidation case. Anyway, light-off curves analysis is probably not the best method for a detailed analysis of kinetic parameters, and a bigger number of points in the light off curves would be needed for the fitting of kinetic parameters in the different samples.

Answer:

In order to obtain data suitable for the kinetics calculations, the reaction temperature was kept almost constant (maximum deviation: +/–1 ^co). The isothermal operation of the reactor was approved by tests with reactions producing up to 45 ^co adiabatic rise – the gradient of 2 ^co was measured at conversion above 98 % by thermocouples before and inside the catalytic bed (the possible “channelling” effect was avoided by mobile installation of the thermo-couple in the catalytic bed). Therefore, for the present study conversions below 40 – 45 % were used. The reactor isothermal zone was measured to be 6.5 cm which can be considered a sufficient for the present catalyst volume. The pressure drop within the system was neglected (measured to be below 2 kappas). Preliminary measurements at GHSV above 40 000 h^-1 showed no significant differences in the values for the integrated reaction rate (first order kinetics). The radial concentration profile and the axial dispersion effect were neglected as the catalyst bed represents a chain of more than 10 ideal-mixing cells and therefore the geometrical characteristics and the flow conditions of the catalytic bed allow the consideration of the reactor to be close to the case of isothermal plug flow reactor. On this basis, the gaseous hourly space velocity (GHSV) was fixed at 150 000 h^–1. In order to minimize the internal diffusion effects, the experiments were performed at catalyst particle size of 0.45 ± 0.15 mm, thus the calculated effectiveness factor (assuming first order kinetics) showed values above 0.95 (conversions below 40 – 45 %), which limit was applied for calculation the kinetics parameters.

Is not clearly argued from the results presented  that the reducibility of the samples or the Mn³⁺/Mn⁴⁺ surface ratio correlates with the catalytic activity presented. In addition, no stability results for the catalytic activity is presented.

Answer:

As is described in the paper the kinetics calculations for propane combustion show that MVK - model is definitely more consistent with the experimental results than the LH - model. According to the Mars -van Krevelen mechanism the VOCs are oxidized by the solid, i.e. the oxygen species introduced in the organic molecule come from the lattice. Therefore, the catalytic behavior can be correlated with the lattice oxygen mobility, which is associated with the catalyst reducibility. Within the present study the most active catalyst, monometallic manganese, is reduced at the lowest temperature. As it can be seen, the addition of Ce leads to the decrease in catalytic activity when compared to the monometallic manganese catalyst, regardless of whether the reducibility of the samples improves or worsens. This effect could be attributed to the decrease of the manganese concentration on the surface (as can be seen from XPS data) on the one hand and on the other - to the fact that in bi-component catalysts the oxide particles are situated inside the channels and are less accessible to the reagents.

According to literature data the catalytic activity of Mn containing catalysts  in combustion reactions increased when the pair Mn⁴⁺–Mn³⁺ existed on the structure of the oxide [Figueroa SJA, Requejo FG, Lede EJ, Lamaita L, Peluso MA, Sambeth JE (2005) Catal Today 107–108:849].

The stability tests will be subject of future investigation, together with the resistance in presence SO₂ and high concentration of water vapor.

With kind regards,

The authors

Reviewer 3 Report

This paper reports reaction kinetics and mechanism of VOCs combustion on Mn-Ce - SBA-15. It seems as revised one from other manuscript, since there are many marks in the paper. I think it now has good quality and it is suitable for publication after some minor revision further.

1. authors should discuss more TPR results and provide more relevant papers.
2. table should put after its first citation

Author Response

Authors should discuss more TPR results and provide more relevant papers.

Answer:

The requested discussions and the corresponding citations were added to the manuscript.

Table should put after its first citation.

Answer:

The tables were put after the corresponding citation.

Round 2

Reviewer 2 Report

New modifications in the papaer improve the different sections, but still there is a lack of interpretation on correlation between different sections of the text.

TPR is now integrated, but not quantified (in terms of micromoles of hydrogen related with reducible species such as Mn or Ce). This is neccesary to understand how much of these species can be reduced and, therefore, may affect in terms of proposed hypothesis  by authors.

XPS fittings are not correctly done, once more.  Fitting parameters associated with XPS intrinsic parameters ( such as FWHM, separation or intensity ratio in a spin-orbit coupling) can not be override in terms of a better fitting quality. Moreover, the Ce3d is done with incorrect parameters in all the aspects mentioned above and is leading to a overstimation of the Ce4+ intensity in all the cases.

The Ce sample has been incorporated in TPR and catalitic activity, clarifying interpretation of results. THerefore, incorporation of this sample in some other sections of the text would also help in the interpretation. 

Author Response

Answers to Reviewer 2

1. TPR is now integrated, but not quantified (in terms of micromoles of hydrogen related with reducible species such as Mn or Ce). This is neccesary to understand how much of these species can be reduced and, therefore, may affect in terms of proposed hypothesis by authors.

Answer:

The total amount of hydrogen consumed is represented in Table 4. As it can be seen from the Table 4 the total amount of H₂ consumption decreases in the following order Mn-SBA-15> MnCe (1:0.5)> MnCe (1:2) indicating decreasing the reducible oxygen species amounts. Due to the fact that different oxide species can be reduced within the same reduction interval, the correct determination of the amount and the type of reduced species is practically impossible. Moreover, in the case of samples containing cerium, in addition to the manganese oxides, oxygen species from the surface of CeO₂ can be reduced within temperature ranges for manganese oxides reduction.

2. XPS fittings are not correctly done, once more.  Fitting parameters associated with XPS intrinsic parameters ( such as FWHM, separation or intensity ratio in a spin-orbit coupling) cannot be override in terms of a better fitting quality. Moreover, the Ce3d is done with incorrect parameters in all the aspects mentioned above and is leading to a overstimation of the Ce⁴⁺ intensity in all the cases

Answer:

The Binding energies and FWHM for new curve-fitting and a new Fig 7B is added. The tables are represented in the Supplementary Section. The curve fitting procedure is done by using a construction of peaks based on the standard spectra measured by the same apparatus. The standard spectra are recorded for bulk and pure materials.

3. The Ce sample has been incorporated in TPR and catalitic activity, clarifying interpretation of results. Therefore, incorporation of this sample in some other sections of the text would also help in the interpretation.

Answer

Due to the observed very low activity of the Ce-SBA-15 sample, it is not appropriate subject for some further detailed characterization. However, since the cerium contributes to the reducing properties of the bi-component samples, we considered as reasonable to include the data for this sample on the TPR figure.

Author Response File: Author Response.pdf

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

This work investigated Mn and Ce-based catalysts supported on SBA-15 aiming at possible application for abatement of volatile organic compounds (VOCs). Originality of the report as well as the discussion of the results is poor. Similar catalysts have been studied and they are widely reported, even among the authors of this work. Therefore, I do not recommend this work for publication in this Journal.

Nevertheless, some comments could be taken into account to improve this report:

- The introduction lacks solid arguments to show the novelty and strengths of the work. The bibliography must be updated with recent publications; many of the citations are not recent. There is a phrase related to the Co-Ce (cobalt) combination which does not seem referred to this work.

- The formation of a solid solution is assumed but there is not enough experimental evidence to support it. Only with the analysis of the X-ray diffraction results this cannot be confirmed.

- XPS discussion stated that enrichment in surface Ce amount is displayed in one catalyst and that in the other ones this behavior is not verified. Have the authors an explanation for this? How this fact is linked to the catalytic behavior? Some spectra did not show the corresponding fitting. For instance, the Mn:Ce (1:2) spectrum did not show Ce4+ signals? It is stated that only Ce3+ species are present.

This section must be rewritten and the results should be linked with the catalytic data.

- TPR results and their discussion are insufficient. The analysis of the reduction profiles is weak. Several literature reports could provide valuable information for the discussion of this point. A deconvolution of the profiles can be performed in order to better elucidate the present species.

- The results of the catalytic tests are only descriptive and they are not enough linked to the characterization data. What are the reasons of the cerium presence if the catalytic performances of the Ce-containing catalysts are worse?

- According to the authors, the reactions follow a MVK mechanism. This could be related with a change in the surface oxygen before and after reaction? XPS data could provide some information of this?

Reviewer 2 Report

This paper reports reaction kinetics and mechanism of VOCs combustion on Mn- 2 Ce - SBA-15. The following comments need to be considered to improve the quality of the manuscript.

1. Table 1 – Table consists of the * and ** symbols. What do these symbols indicate? Please provide the info at the bottom of the table
2. Figure 2 - the author should use the same range of y-axis for Figure (a) and (b) for better comparison.
3. Figure 3 – according to the caption, (a,b) is the TEM image of MnCe (1:0.5) before the reaction, while (c,d) is the TEM image after the reaction. Supposedly a,b is the same sample. Why are the images different? Why does the author need to provide two different images?
4. Figure 5 - It is unclear why the sample of MnCe(1:05)AR has Al contamination. The author should include XRD analysis without contamination.
5. Figure 7(b) – the author should provide the deconvoluted peaks of MnCe(1:2) for better analysis and discussion.
6. Figure 8(b) & Figure 10 – The author should provide the deconvoluted peaks for better analysis and discussion.
7. Table 6 - Table caption should be located on the top of the table
8. The correlation of properties and catalytic activities should be critically analyzed and discussed to improve the quality of the manuscript.
9. Majority of the references are outdated. The author should utilize recent publications to develop the research background, problem statement, research gap, and novelty.
10. Language deficiencies (poor language, grammar, word choice, tense, style of writing) and typing errors are observed in the manuscript. Please thoroughly revise the manuscript. In addition, the author should refrain from using the personal form (e.g we, they) in technical writing.