Optimizing Citrate Combustion Synthesis of A-Site-Deficient La,Mn-Based Perovskites: Application for Catalytic CH4 Combustion in Stoichiometric Conditions
Round 1
Reviewer 1 Report
LaMnO3-based perovskites are widely recognized as promising catalysts for a number of catalytic oxidation reactions. In this work, a series of site-deficient perovskites were prepared. The obtained catalysts were tested in CH4 oxidation to CO2 under stoichiometric O2. The manuscript certainly reported some useful information and may be considered for publication. However, many factors, such as cations reducibility, content of high valent cations, presence of mobile surface and lattice oxygen species, are related to the catalytic activity in oxidation reactions. Among them, oxygen vacancy may play important role. Unfortunately, this part of research is not showed in the current version of the manuscript. I suggest that relevant characterizations and discussion need to be added in the manuscript before the consideration of this manuscript to be accepted.
Author Response
Point 1: LaMnO3-based perovskites are widely recognized as promising catalysts for a number of catalytic oxidation reactions. In this work, a series of site-deficient perovskites were prepared. The obtained catalysts were tested in CH4 oxidation to CO2 under stoichiometric O2. The manuscript certainly reported some useful information and may be considered for publication. However, many factors, such as cations reducibility, content of high valent cations, presence of mobile surface and lattice oxygen species, are related to the catalytic activity in oxidation reactions. Among them, oxygen vacancy may play important role. Unfortunately, this part of research is not showed in the current version of the manuscript. I suggest that relevant characterizations and discussion need to be added in the manuscript before the consideration of this manuscript to be accepted.
Response 1: We thank the reviewer for this suggestion, which indeed was useful to further improve the quality of our work. As requested, we performed O2-TPD experiments to gain insight into the amount of oxygen desorbing from surface sites, associated to the presence of oxygen vacancies, as well as into the mobility of lattice oxygen. The results have been added and discussed in detail in section 2.1.4. Bulk reducibility and oxygen mobility (Figure 6 and Table 5), and further exploited for giving a better explanation of catalytic performances in section 2.2 Catalytic activity.
Reviewer 2 Report
This work claims A-site deficient La0.8MnO3 and La0.8Mn0.9BO3 (with B = Cu or Ni) perovskites as promising formulations for CH4 oxidation to CO2 under stoichiometric O2. Different parameters were varied during synthesis protocol (such as, citric acid/metal cations molar ratio (CA/M), either 1.1 or 1.5, and pH of the precursor solution, either acidic (given by HNO3 + citric acid) or neutral (after NH3 addition) in order to maximize catalytic activity. The subject matter itself is topical and relevant to an area of research which has recently gotten a great deal of attention. Although, many samples with different physicochemical properties and CH4 oxidation efficiencies were obtained, the reason of these differences it is not entirely clear. These issues could be addressed with some complementary experiments to achieve a more comprehensive study.
Specific Comments
1. The authors describe the preparation method as follows: “The selected amount of La2O3 (≥ 99.9% Sigma-Aldrich) and Mn(acetate)2·4H2O (≥ 99% Sigma-Aldrich) were dissolved in deionized water containing the stoichiometric amount of HNO3 (≥ 65% Sigma-Aldrich), as if the corresponding La(NO3)3 and Mn(NO3)2 salts were used as precursors.” Which are the real precursors of the perovskites nitrates or the formers?
2. Which is the pH range of acidic samples? Is it the same irrespectively the citric acid to nitrates molar ratio?
3. Why the authors selected different ignition temperatures for the synthesis under acidic and neutral pH?
4. Different calcination temperatures have been tested?
5. Significant differences have been observed on textural properties depending of the pH of the precursor, citric acid to nitrates molar ratio and Mn partial substitution by Cu or Ni. How the authors explain such differences? Complementary experiments, such as TG and/or thermo DRX during precursor decomposition should be performed to further understand more properly the obtained results since the authors claimed that these differences are derived from the combustion rate and heat generated upon burning step
6. The authors claimed that the CH4 combustion reaction is governed by Mars-Van Krevelen (MvK) mechanism, in which oxygen species on the perovskite surface are the primary active species for the reactant activation and complete oxidation to CO2 increase the number of oxygen vacancies, useful to improve reactant adsorption and to enhance oxygen mobility. Considering this aspect, why O2-TPD experiments have not been performed to understand more properly the differences observed in catalytic activity?
7. How the authors explain that the conversion curves of La0.8Mn0.9Cu0.1O3-H+-CA1.1 and La0.8MnO3-H+-CA1.1 intersection at around 575 °C in Figure 5a?
8. Which is the best formulation?
Minor editing of English language required.
Author Response
Point 1: The authors describe the preparation method as follows: “The selected amount of La2O3 (≥ 99.9% Sigma-Aldrich) and Mn(acetate)2·4H2O (≥ 99% Sigma-Aldrich) were dissolved in deionized water containing the stoichiometric amount of HNO3 (≥ 65% Sigma-Aldrich), as if the corresponding La(NO3)3 and Mn(NO3)2 salts were used as precursors.” Which are the real precursors of the perovskites nitrates or the formers?
Response 1: The real precursors are La2O3 and Mn(CH3COO)2, whereas nitrates are generated in solution by addition of HNO3. We made this choice because of the high hygroscopic nature of nitrate salts, which makes the precise amount of metal cations slightly less controllable if directly employing nitrates with respect to the used La2O3 and Mn(CH3COO)2. As a second reason, the precursors we selected are cheaper than the corresponding metal nitrates. For analogous reasons, a synthesis with NiO as precursor for Ni2+ was tried, but resulted in poorer perovskite phase purity, thus we directly employed Ni(NO3)2 in this case. We added these explanations in section 3.1. Synthesis protocol.
Point 2: Which is the pH range of acidic samples? Is it the same irrespectively the citric acid to nitrates molar ratio?
Response 2: The pH of acidic samples is surely < 1 as reported in Table 1, probably even negative, because of the presence of relatively concentrated HNO3 strong acid. The variation of citric acid amount is not expected to substantially change the pH, being well below its first pKa value (3.13). A sentence has been added at beginning of section 2.1.1. Structural and morphological features to explain this point.
Point 3: Why the authors selected different ignition temperatures for the synthesis under acidic and neutral pH?
Response 3: We selected different ignition temperatures for acidic and neutral pH synthesis mainly because of empirical observations in our laboratory: we noted that when employing acidic pH the gel decomposition seemed to start below 250 °C, whereas upon pH neutralization by NH3 the gel combustion usually started above that temperature. This point has been explained in section 2.1.2. Precursors decomposition, also in relation to the gel TGA profiles we recorded, as you kindly suggested us in Point 5.
Point 4: Different calcination temperatures have been tested?
Response 4: We did not test different calcination temperatures, in order not to add another variable which would have implied the preparation of several additional samples (with all the related characterizations and catalytic tests). The chosen temperature was a compromise between two necessities: the first was to approach the maximum temperature employed for the catalytic test; the second was to try to avoid excessive sintering induced by severe calcination temperatures. A sentence has been added at beginning of section 2.1.1. Structural and morphological features to explain this point.
Point 5: Significant differences have been observed on textural properties depending of the pH of the precursor, citric acid to nitrates molar ratio and Mn partial substitution by Cu or Ni. How the authors explain such differences? Complementary experiments, such as TG and/or thermo DRX during precursor decomposition should be performed to further understand more properly the obtained results since the authors claimed that these differences are derived from the combustion rate and heat generated upon burning step.
Response 5: We kindly thank the reviewer for this suggestion. As requested, we performed additional TGA analyses on some selected gel precursors, which have been added and discussed in section 2.1.2. Precursors decomposition (Figure 4) and correlated to the final perovskites textural properties whenever possible. Unfortunately, complementary DSC analyses was not possible with the instrumentation available in our department.
Point 6: The authors claimed that the CH4 combustion reaction is governed by Mars-Van Krevelen (MvK) mechanism, in which oxygen species on the perovskite surface are the primary active species for the reactant activation and complete oxidation to CO2 increase the number of oxygen vacancies, useful to improve reactant adsorption and to enhance oxygen mobility. Considering this aspect, why O2-TPD experiments have not been performed to understand more properly the differences observed in catalytic activity?
Response 6: We thank the reviewer for this suggestion as well, which indeed was useful to further improve the quality of our work. As requested, we performed O2-TPD experiments on five representative samples. The obtained results have been added and discussed in detail in section 2.1.4. Bulk reducibility and oxygen mobility (Figure 6 and Table 5), and further exploited for giving a better explanation of catalytic performances in section 2.2 Catalytic activity.
Point 7: How the authors explain that the conversion curves of La0.8Mn0.9Cu0.1O3-H+-CA1.1 and La0.8MnO3-H+-CA1.1 intersection at around 575 °C in Figure 5a?
Response 7: As we tried to explain in the manuscript (section 2.2 Catalytic activity), the intersection of CH4 conversion curves of both Cu- and Ni-doped samples with that of undoped La0.8MnO3-H+-CA1.1 (although at a slightly different temperature between the two dopants), is probably ascribed to the change of mechanism from suprafacial to intrafacial: the former is facilitated by the improved surface area and surface oxygen release achieved through doping, the latter is not improved by doping since lattice oxygen release is not enhanced. O2-TPD experiments proved this point at least on La0.8Mn0.9Ni0.1O3-H+-CA1.1, but we believe a similar explanation should be valid for La0.8Mn0.9Cu0.1O3-H+-CA1.1 as well, being the CH4 conversion curves very similar between the two different dopants.
Point 8: Which is the best formulation?
Response 8: The best formulation is La0.8Mn0.9Ni0.1O3-H+-CA1.1, as we highlighted with an additional sentence at the end of section 2.2. Catalytic activity.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
The subject of this review paper is of great interest in the area of perovskite catalysts for oxidation reactions. LaMnO3 perovskites were prepared by citrate combustion route and the effect of synthesis conditions and B-dopants (Cu, Ni) on the catalytic performance for methane combustion was evaluated. The authors should improve the discussion and comparison with other works in the literature.
I have some comments/suggestions for the authors:
1. Lines 83-88: “It was found that … without marked differences between Ni and Cu”. This part is more adequate for the Conclusions and not for the Introduction.
2. XRD results: Average crystallite sizes should be compared with other results of LaMnO3 perovskites synthesized under similar conditions in the literature.
3. The same must be done with specific surface areas.
4. “Depth composition” should be replaced by “bulk composition”.
5. Lines 362-364: “The selected amount of La2O3 (≥ 99.9% Sigma-Aldrich) and Mn(acetate)2·4H2O (≥ 99% Sigma-Aldrich) were dissolved in deionized water containing the stoichiometric amount of HNO3 (≥ 65% Sigma-Aldrich), as if the corresponding La(NO3)3 and Mn(NO3)2 salts were used as precursors.” Why did the authors not use La(NO3)3 and Mn(NO3)2 salts directly?
6. Catalytic tests were performed without any pre-treatment of the catalyst. Is it expected that a reduction treatment before the reaction modifies the catalytic activity for oxidation?
Minor editing of English language required.
Author Response
Point 1: Lines 83-88: “It was found that … without marked differences between Ni and Cu”. This part is more adequate for the Conclusions and not for the Introduction.
Response 1: We removed this part from the introduction.
Point 2: XRD results: Average crystallite sizes should be compared with other results of LaMnO3 perovskites synthesized under similar conditions in the literature.
Response 2: In section 2.1.1. Structural and morphological features, we provided comparison with some literature works on LaMnO3 perovskites prepared by a similar citrate combustion route, in which crystallite sizes were calculated by Williamson-Hall method. Please not that we chose not to cite works where the size was calculated by Scherrer equation, since the results might be quite different.
Point 3: The same must be done with specific surface areas.
Response 3: In section 2.1.1. Structural and morphological features, we also explicited the comparison of SSA values obtained by us to those of other literature works on LaMnO3-related perovskites, prepared by a similar citrate combustion method. A comparison with a work on Ni-doped LaMnO3, and another one on Cu-doped LaMnO3, was also included. Unfortunately, for the latter one the synthesis protocol was not the citrate combustion, but a similar increase in SSA was found both in our work and in the cited one.
Point 4: “Depth composition” should be replaced by “bulk composition”.
Response 4: In section 2.1.3 Bulk and surface composition, we replaced the term “depth” with “bulk” as suggested.
Point 5: Lines 362-364: “The selected amount of La2O3 (≥ 99.9% Sigma-Aldrich) and Mn(acetate)2·4H2O (≥ 99% Sigma-Aldrich) were dissolved in deionized water containing the stoichiometric amount of HNO3 (≥ 65% Sigma-Aldrich), as if the corresponding La(NO3)3 and Mn(NO3)2 salts were used as precursors.” Why did the authors not use La(NO3)3 and Mn(NO3)2 salts directly?
Response 5: We chose to generate nitrates in solution by HNO3 addition because of the high hygroscopic nature of these salts, which makes the precise amount of metal cations slightly less controllable if directly employing nitrates as precursors with respect to the used La2O3 and Mn(CH3COO)2. As a second reason, the precursors we selected are cheaper than the corresponding metal nitrates. For analogous reasons, a synthesis with NiO as precursor for Ni2+ was tried, but resulted in poorer perovskite phase purity, thus we directly employed Ni(NO3)2 in this case. We added these explanations in section 3.1. Synthesis protocol.
Point 6: Catalytic tests were performed without any pre-treatment of the catalyst. Is it expected that a reduction treatment before the reaction modifies the catalytic activity for oxidation?
Response 6: For sure some changes in the catalytic activity are expected if a reductive pre-treatment is performed, also depending on the specific conditions employed for the pre-treatment (temperature, etc.). This point has been mentioned with an additional sentence at beginning of section 2.2. Catalytic activity. In fact, several tests with pre-reduced catalysts have already been carried out in our laboratory but, unfortunately, we cannot disclose such data at this stage because we intend to publish the obtained results anytime soon.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have made substantial modification on this manuscript mainly based on reviewers' comments and suggestion. The revised version may be considered for the acceptance.
Reviewer 2 Report
Remove the tab symbol from inside Figure 4.
For comparison purposes, the y-axis on the right side must have the same scale in all figures that compose Figure 4
Some minor grammatical mistakes should be corrected before the publication of the Manuscript.
