Catalytic Combustion of Propane over Ce-Doped Lanthanum Borate Loaded with Various 3d Transition Metals

Round 1

Reviewer 1 Report

The paper “Catalytic Combustion of Propane over Ce doped Lanthanum borate loaded with various 3d transition metals” is interesting, on a high-interest topic. The research is well conducted and well discussed. Therefore I recommend that the paper be accepted after major revision. Some recommendations and comments are made below:

Section 1. Introduction:

- rows 45-47: “For example, Runduo Zhang et al. found that LaFe_0.8Cu_0.2O₃ showed nice activity toward acrylonitrile conversion (the temperature for 100% conversion of propane T100 = 523 K) …” – the paper discusses acrylonitrile conversion, not propane, so this needs to be corrected.

- rows 65-67: “(It always is involved to boost the ability of active molecular and enhance the stability of solid catalysts)” – the phrase is missing one/several words, please correct.

- rows 68-69: “TM modified Ce_0.05La_0.95BO₃ (TMs/Ce_0.05La_0.95BO₃) catalysts were synthesized using a sol-gel method.” There are several problems related with this phrase.

In the first place, the authors did not explain why they chose to modify the LaBO₃ solid by replacing only 5 % of the La cations. Did they have previous experience with other loadings? Was it a chance choice? Did they find information in the literature? Please elaborate on this.

Secondly, they do not offer information (until page 9, section 3. Materials and Methods, 3.1. Catalyst synthesis, 3.1.2. Preparation of Ce_0.05La_0.95BO₃ supported with 1 wt% metal oxide, row 274, and this is the only place in the manuscript) about the transition metal loading in the catalysts. In catalysis this is an important factor, and it should be mentioned. Also, why did they choose only 1 % loading of the transition metals? Please elaborate on this.

In the third place, the method they used, described in the section 3. Materials and Methods, 3.1. Catalyst synthesis, 3.1.1. Preparation of Ce_0.05La_0.95BO₃ support, rows 267-272, is not exactly a sol-gel method. The sol-gel method is different from the citrate precursor method, although they are commonly mixed up in most papers from the literature. The sol-gel method “is characterized by the formation of stable colloidal solutions (‘‘sol’’) in the first step, followed by anisotropic condensation of colloidal particles (micelles) producing polymeric chains with entrapped solution of condensation byproducts, resulting in the formation of a ‘‘liogel’’ or ‘‘hydrogel’’ or ‘‘monolith’’ when external solvent is not used. After washing out the byproducts, the solvent removal produces ‘‘xerogels’’ or ‘‘aerogels’’, depending on the drying mode, with distinct structures of the primary particles and their packing manner (texture)” [Landau M.V. (2008) Solid State Reactions. In: Ertl G., Knozinger H., Schuth F., Weitkamp J. (Eds.) Handbook of Heterogeneous Catalysis, 2nd ed, Wiley-VCH Verlag GmbH & Co., p. 119].

In the citrate method a stable amorphous or glassy precursor is prepared from the metallic nitrates (usually) and a hydroxyacid such as citric, malic, tartaric, lactic, or glycolic acid. The solution is concentrated in a Rotavapor until a “very stable syrup is obtained in this way which can be dehydrated under vacuum at about 80 °C to a rigid-foam (meringue) precursor, which is amorphous to X-rays, hygroscopic, and exhibits the color of the metallic ions present” [Delmon B. and Devillers M. (2008) Solid State Reactions. In: Ertl G., Knozinger H., Schuth F., Weitkamp J. (Eds.) Handbook of Heterogeneous Catalysis, 2nd ed, Wiley-VCH Verlag GmbH & Co., p. 308]. No gel (or “gel precursor” – row 271 – is obtained by this method.

In my opinion the method used by the authors is the citrate method.

 Section 2. Results and discussion

- rows 110-111: “The existence of the C 1s peak, 284.8 eV, could be attributed to the widespread presence of carbon in the environment.”The authors do not explain the source of the widespread carbon in the samples. We can assume that it comes from the citric acid (citrates) introduced during synthesis, but if the samples were calcined at 923 K it should have been eliminated. How do the authors explain the presence of carbon after calcination?

- rows 117-120: “The peaks belonging to boron species of Cu/ Ce_0.05La_0.95BO₃ shift to the lower BE (191.3 eV and 196.2 eV), in comparison to the other three materials. This suggests that the electronic structure of boron was greatly affected by Cu incorporation.” The differences between the BE are 0.1 eV (191.3 compared to 191.4, and 196.2 compared to 196.3, see rows 116-117). Therefore, it is difficult to understand how the electronic structure of B can be greatly affected by Cu incorporation. Also, the following explanation given in rows 120-126 does not involve the influence of Cu loading on boron solids, making the whole paragraph a little confusing. Please explain.

- rows 198-199: “nice interaction with metal species” – please change “nice” with another word.

- row 204: “cupreous species” – please change to cuprous.

- rows 220-222: “When 3d TMs were further introduced, the density of oxygen vacancy over these materials was thus improved. This led to the systematic variations over surface acidity …”. The authors did not measure the density of oxygen vacancies (e.g. by electrical conductivity experiments), therefore they can only suppose that it was improved, considering literature citations they provided. Also, they should explain how this increase of density in oxygen vacancies influences the surface acidity, because it is not a straightforward relationship.

- a comparison with literature results of propane combustion on other catalysts (activity, selectivity to CO₂) is necessary, in my opinion, to put the results of this research in perspective. The authors never mention if the combustion on the modified Ce_0.05La_0.95BO₃ catalysts leads to complete conversion to CO₂, or CO is also formed, and this is an important aspect of the combustion catalysis.

 - Figure 1 caption, row 231: the authors mention that BO₃ are figured as blue planar triangles, but there are no blue triangles in the figure.

- Figures 6 a), c) and d): the colors in the three plots are very similar for several samples, e.g. Mn/ and Cu/Ce_0.05La_0.95BO₃, or Fe/ and Co/ Ce_0.05La_0.95BO₃. Please modify the colors in order for the plot to be better understood.

 Section 3. Materials and Methods

- row 282: “Cu/Ce_0.05La_0.95BO₃, Cu/Ce_0.05La_0.95BO₃, …” – the same sample is mentioned twice.

- rows 298-301: “Titrate the solid acid acetonitrile solution with n-butylamine solution as the base titrant, and calculate the acid content of the catalyst according to the equivalence point determined by the first-order sudden change.” The phrase needs to be changed.

- rows 314-315: “The reaction gas, composed of C₃H₈ (0.1 vol%) and balance Ar, flows through the catalyst …” – the oxygen (or air that contains the oxygen) needed for the combustion, is missing from the mixture. Please correct.

 Section 4. Conclusions

- rows 332-333: “In the present, Ce_0.05La_0.95BO₃ was prepared by the sol-gel method …” – please correct the name of the method and fill in the missing word(s).

Author Response

Thanks for your valuable comments, which were helpful to improve the scientific rigor of our manuscript. Each comment was carefully addressed, please see below for details.

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

It is my pleasure to review your paper. From my point of view, this is a paper with a certain amount of work, which can clearly express the author's experimental content and purpose. But there are still some issues that I think the authors need to revise and note.

1. In the analysis of the catalyst acid sites, NH₃-TPD method be used to further elucidate the number and type of catalyst acid sites.

2. In Figure 6 (b), the Thermal stability should be tested for more samples. I personally think that the Thermal stability tests for Ce_0.05La_0.95BO₃, LaBO₃, and partially TM-loaded catalysts should be added to further discuss the impact of Cu on thermal stability.

3. In Figure 6 (c) and (d), The colors of the Ea lines of Mn and Cu, Fe and Co are so similar that I struggle to distinguish these lines. I suggest changing the colors to divide them.

4. The analysis of catalyst properties is not sufficient, more characterization methods (NH3-TPD, in-situ FTIR, etc) should be used to guide the conclusion.

5. Extensive editing of English language and style required. For some terms, such as: L-acid, B-acid, the expression is not uniform and accurate enough.

Author Response

Thanks for your positive comments for our work. We further improved this article based on your comments. Point-to-point responses are shown below and the corresponding changes in the manuscript are highlighted in blue for your convenience.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I agree with the publication of the manuscript, with minor corrections:

- row 80: "Bragg" instead of "bragg"

- row 119: "systematic" instead of "systemic"

- rows 122-123: "widespread presence of carbon in the environment" - if the explanation of the authors (in the cover letter) that even organic compounds from the people in the lab can be seen in the sample and "every sample has a layer of carbon on its surface, which leads to the presence of carbon after calcination" would be true, then every XPS spectrum would contain the carbon peak, and it simply is not so. I will leave it as it is.

- rows 351-357: "Titrate the solid suspended in acetonitrile with n-butylamine solution as the base titrant, and calculate the acid content of the catalyst according to the equivalence point determined by the first-order sudden change.  ...  Then use a newly prepared 0.1 mmol/L n-butylamine acetonitrile solution as a base titrant to titrate the suspension until a constant potential is reached." - the paragraph sounds like direction for students in the lab. It should be changed to describe the process, maybe something like "the solid suspended in acetonitrile was titrated with n-butylamine solution ..." and so on.

 

Author Response

Thanks for your comments. Each comment was carefully addressed, please see below for details.

• row 80: "Bragg" instead of "bragg"

Response: It has been corrected.

Added texts on page 2:

They all show diffraction peaks at 14.6, 22.8, 28.7, 29.5, 35.3, 43.2, 46.4, and 47.3°, which are characteristic Bragg diffractions for LaBO₃ material in monoclinic space group (PDF No. 73–1149) [43].

• row 119: "systematic" instead of "systemic"

Response: It has been corrected.

Added texts on page 3:

To understand the variations over the surface chemistry of these TM-loaded rare metal borates, Cu/Ce_0.05La_0.95BO₃, Ni/Ce_0.05La_0.95BO₃, Co/Ce_0.05La_0.95BO₃, and V/Ce_0.05La_0.95BO₃ were selected to perform systematic XPS measurements.

• rows 122-123: "widespread presence of carbon in the environment" - if the explanation of the authors (in the cover letter) that even organic compounds from the people in the lab can be seen in the sample and "every sample has a layer of carbon on its surface, which leads to the presence of carbon after calcination" would be true, then every XPS spectrum would contain the carbon peak, and it simply is not so. I will leave it as it is.

Response: Thanks for your comment. We corrected the response as following: XPS instrument is extremely surface sensitive as they detect the composition of the outmost atomic layers of a sample. Carbon compounds that are present in the air-for example, carbon dioxide, and even organic compounds from the people in the lab-are likely to adhere to the surface of most samples. In addition, our material was very thinly pasted on the carbon conductive tape for test preparing. As a result, the source of the widespread carbon can be detected for XPS characterization.

Added texts on page 12:

X-ray photoelectron spectroscopy (XPS) test was carried out with Physical Electronics Quantum 2000, equipped with monochromatic Al-Kα source (Kα = 1486.6 eV) at 300 W. The powder material was very thinly pasted on the carbon conductive tape for test preparing. Binding energy was corrected against the C 1s line at 284.8 eV.

• rows 351-357: "Titrate the solid suspended in acetonitrile with n-butylamine solution as the base titrant, and calculate the acid content of the catalyst according to the equivalence point determined by the first-order sudden change. ...  Then use a newly prepared 0.1 mmol/L n-butylamine acetonitrile solution as a base titrant to titrate the suspension until a constant potential is reached." - the paragraph sounds like direction for students in the lab. It should be changed to describe the process, maybe something like "the solid suspended in acetonitrile was titrated with n-butylamine solution ..." and so on.

Response: Thanks. This sentence has been corrected.

Added text on page 12：

N-butylamine potential titration method was adopted to measure the acidic prop-erties of samples on 904/906 Titrando (Metrohm, Switzerland). For instance, a mass of 50 mg solid was suspended in 50 mL of acetonitrile. Later this suspension was titrated with a solution of n-butylamine in acetonitrile (0.1 mmol/L) until a constant potential is reached.

Reviewer 2 Report

The manuscript has been carefully revised according to the comments of the reviewers, and some suggested experiments have been added. I believe that It is acceptable now.

Author Response

Response: Thank you very much!