Tungsten Oxide Modified V₂O₅-Sb₂O₃/TiO₂ Monolithic Catalyst: NH₃-SCR Activity and Sulfur Resistance

Round 1

Reviewer 1 Report

 

Modification of tungsten oxide on V2O5-Sb2O3/TiO2 monolithic catalyst: NH3-SCR activity and sulfur resistance

 

Liping Liu   et al

 

Submitted to « Processes »

 

General

The present proposal shows some interesting results but lacks from many technical and experimental details limiting the possibility to reproduce the results. The deepness of some results analysis is very limited, and some necessary results are lacking. The English used is sometimes strange and grammatical and writing problems exist, even in the title of the article. Problems are also seen in the data analysis as many comments are scarcely supported by literature references or specific experiments. The complexity of the catalysts is another point that deserves new attention: experiments with only TiO2, TiO2+W, TiO2+ Sb, TiO2+V, TiO2+V+Sb, TiO2+V+W, TiO2+Sb+W seem necessary before adding W to TiO2+V+Sb. As the authors have in the past published such studies, they must describe their previous findings and the limitations of these findings to justify the present proposal. Finally, limiting the situation to TiO2+Sb+V+W, variation in the W concentration would be welcome.

Some specific points among others will be detailed thereafter.

But, in the present situation, reviewer thinks that the proposal cannot be accepted without a complete reconsideration by the authors of their presentation and experimental approach of their research.

 

Detailed comments

P1         Line 2: the article title is confusing

Line 4: why using different character policies in the names of the different authors (Calibri and Times New Roman) and different sizes (10 and 10.5)

Line 20: …. sulfur tolerance ?       Although often used in the literature, the correct writing must be  “sulfate tolerance” or "sulphate tolerance"

Line 21:  Keywords         tugensten oxide           to be corrected

Line 34: …”posited”….find better expression

Lines 33-37: these sentences needs some references.

P2         Line 41: change “its” for “their”

              Lines 49-51: sentence needing new writing to increase understanding.

              Lines 54-56: “The introduction of WO3 can improve the NH3-SCR activity of catalyst through increasing the redox property and Brönsted acidity [25]” But Ref [25] is dealing with “Characterization of titanium sulfate supported on zirconia and activity for acid catalysis”. Then ??.

              Line 58: “They reported » … “They” refers to ref [25] or [26]?

              Lines 63- 64: “…the number of the Brönsted acid sites increase with the loading”     increases or increased

              Line 65: “low valance » »     correct « valence”

              Lines 69-71: “Kang et al. [29] reported V2O5-WO3/TiO2 catalysts physically mixed with Fe2O3  and the catalyst maintained better low-temperature activity than V2O5-WO3/TiO2 after a long-term reaction period in the presence of SO2.”  Sentence that needs better writing.

              Line 78: “promoter (Pb, B, Cu and P) contained catalysts”    “promoters (Pb, B, Cu and P) containing catalysts”  would be better

 

Experimental

 

P3         Line 90 +: “extrusion molding method”. This molding device or method needs a minimum description: what is the nature of material used? Metal, graphite? Are additives used to facilitate the extrusion process? Is there a selective retention of mineral species in the device? Origin and purity of the used chemical compounds? Size of the monolith macropores? Shape of the monolith (apparently cuboid). Direction of the porous structure? Physical-chemical properties of DP21 TiO2?

              Line 100-103: this sentence does not pertain to “catalyst preparation”

Line 104 +: in the catalytic tests, are complete monoliths used or only a fraction of them? What was the shape of the quartz reactor to deal with a cuboid monolith? What was the mass of the catalyst?

Line 115: “angle range of 10–90° and scanning rate of 6 °/min.” Two comments here

1/ In Figure S1, the angle scale is 20-80° and not 10-90°.

2/ the scanning rate is too high to observe potential lines due to V, Sb or W species. A scanning rate of 0.1 or 0.2°/min is necessary to be sure that the “supported oxides” do not present any crystallization.

Line 117: “adsorption and desorption isotherms »… If so, the authors also have access to meso-porosity of their materials, but nothing was said on such property. Anyway, the reader is not informed on the type of sample used for BET measurements: the whole monolith? A fraction of it (if so how was selected such fraction)?; grinded fraction of the monolith (if so grinding conditions)? The thermal and atmosphere pre-treatment before N2 adsorption were not informed.

 

P4         Line 119+: information on the type of sample used for UV-Vis characterization are also necessary as for BET data. The same is true for TPDC, H2-TPR, NH3 TPD and TG measurements.

              Lines 130-131: composition of O2/N2 mixture and information on the changes from O2/N2 to Ar/H2 (with or without volume purge).

 

Line 137+: “Afterwards, the sample was exposed to 1000 ppm NH3/N2 until saturation”. How was estimated “saturation”? What was the signal measured to estimate the desorption of NH3? FTIR? TCD? Other? How was calibrated the answer?

Line 140: what was the physical form of the catalyst used for SO2 oxidation?

 

Results

P5         Line 161 and Figure 1:

              In Figure 1a, were the catalysts changed for each new temperature or is it the same catalyst used, the results being obtained varying only the temperature? Are the authors giving initial conversion or equilibrium conversion in their Figure 1a? In Figure 1a and 1b, what is the reproducibility of an experiment? Is it possible to show an error bar?

In Fig 1a, at 220°C, the conversion seems close to 95% in the catalyst with W and close to 87% in absence of W. But in Figure 1b, all the values along the curve in function of time are below the values in Figure 1a. How is it possible?

              Lines 165-171 and Figure S1: as said before, the condition of Xray measurements do not allow possible detection of Sb, V or W species. But the authors wrote “diffraction features of crystalline sulfates were not observed on the sulfated catalyst”. Although this sentence can be accepted, a tiny detail was not considered by the authors. In Figure S1, TiO2 presents a doublet diffraction line close to 54-55°. In three cases, the line at 55° is more intense than the line close to 54°. But in the case of VSbWTi-S catalyst, the contrary is true. Do the authors can comment on this situation? Does this mean that SOx can be adsorbed on or reacts selectively with specific surface planes of TiO2?

              Line 173: what is the specific surface area of PURE TiO2? What is its porosity if any?

              Lines 175-176: The authors wrote …“may be mainly due to the reduced content of high-surface-area TiO2.”

1/ A TiO2 with some 30 m2/g cannot be considered as a “high surface area TiO2”.

2/ The decrease in TiO2 content is close to 2%, whereas the decrease in S BET is close to 10%. The explanation seems insufficient

.             Line 177: the authors try to explain the decrease in S BET after Sulfation by a process of pore blocking. But nothing is said along the draft on catalyst porosity. Or information on porosity is given or other hypotheses to explain the variations are necessary.

              Lines 181-182: these lines need a ref to support the absorbance edge at 410 nm for TiO2 anatase.

 

P6         Line 194: Figure 2: a clear difference appears in the catalysts containing S compared to the catalysts without S. The baseline in the first case is fully horizontal between some 600 and 800 nm, whereas in the latter cases this base line is slightly decreasing. Explanation??

              Line 199: “are ascribed to the water, NH3 and organic impurities adsorbed on the catalysts”. What is the origin of “organic impurities" adsorbed on the catalysts?

Lines 206-208: “The bi-modal feature of SO2 signal in the range of 330-460 °C is probably related to decomposition of ammonia hydrogen sulfates bonded to different metal sites”. This statement needs references on sulfates decomposition, an area where literature is rather rich. See for example Thermochimica Acta, 80 (1984) 23-33 and references given, and a work showing that TiO2 can be a very efficient species in SCR - Environ. Sci. Technol. 2019, 53, 11, 6462–6473

 

P7         Lines 209-210 and Figure 3: why pure S not followed in these experiments? In the case of catalyst containing W, the SO2 signal shows a high contribution at some 400°C, and a smaller one close to 600°C+. But when looking to the O signal, the situation is opposite, low contribution at around 400°C and huge contribution above 600°C: then O and SO2 do not follow the same desorption process. Can the authors comment on this situation?

              Line 213: no data are presented in Table 1

 

P8         Line 226: from the TG profiles, it seems that in the temperature range used for SCR, only a very limited amount of sulphated species are unstable.

              Line 227: as said before, in Table 1, no data are presented

 

P9         Line 245 and Figure 5. Why the TPR profiles are only shown in the 250-450°C temperature range. Is it observed the formation of WO3 bronzes at lower temperatures?

In Figure 5, the detected signal of “hydrogen consumption” is a TCD signal. But, with the quite complex catalysts presently used, TCD is not selective and other species than H2 can participate to TCD response and TPR profiles. How the authors considered this problem in their results?

              Line 257 and Figure 6: in the case of reaction or desorption under increasing temperature, it is expected that the higher the quantity in a specific element, the higher the temperature of the maximum reaction (desorption) rate. In the present case, the maximum rate of NH3 desorption is close to 400°C for VSbWTi-S, whereas it is close to 420 °C in the absence of W. Can the authors comment on this point, suggesting that if there is more acid sites when W is present, the strength of these acid sites is lower than in W absence.

 

P10       Line 277: “severer deposition »      more severe deposition?

              Line 280: in the conditions of low temperature SCR, i.e. below 300°C, the differences between both catalysts, with and without W are not significant for reviewer.

              Conclusions

Although in the abstract the authors allow the reader to expect important promotion of WO3 addition in their VSbTi monolith catalyst, the conclusions appear more negative than positive. Then, as suggested in its general comments, reviewer thinks that, in the absence of different W contents in the catalysts, definite conclusions cannot be drawn. Repeating, although some interesting results were obtained, the deepness of analysis, the design of research and the description of what was done do not allow the actual draft to be accepted as a publication in “Processes”.

Not all the existing problems were commented by reviewer in this written document.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The results obtained in the work are of great scientific and practical importance. After the introduction of minor refinements can be published in the journal.

 

1. In section 2.3 the formula (2) for calculation of SO₂ conversion is presented. It would be interesting to include in Figure 1 a graph of the change in SO₂ conversion as a function of temperature. If the authors report that catalyst stability depends on adsorption of sulfur-containing compounds on the catalyst surface, it would be informative to show the graph of dependence of SO₂ concentration in products on time (0-25 hours) of test duration.

 

2. Diagram 1.a shows stability of operation of catalysts for 25 hours. It is not clear whether this time is sufficient to estimate stability or not. Probably in the introduction the authors need to give the mode of operation of these catalysts in industry: interregeneration period of operation.

 

3. In section 3.1 it is recommended to present composition of products for two catalysts in optimum operating mode.

 

4. The text (line 206-207) shows that SO₂ release occurs in the region of                330-460 ⁰C as a result of decomposition of ammonium sulfate. The signal of SO₂ and NH₃ release occurs at different temperatures in Figure 3b. Ammonia is released at lower temperatures than sulfur oxide. Why do ammonium sulfate decompose release ammonia and sulfur oxide at different temperature ranges.

 

5. In section 3.2 it is desirable to present the scheme of transformations of SO₂, NH₃, H₂O, O₂ with the formation of ammonium sulfate and ammonium hydrosulfate. And their subsequent decomposition.

Author Response

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Reviewer 3 Report

Liu et.al., studied the effect of WO₃ addition to V₂O₅-Sb₂O₃/TiO₂ monolithic catalyst towards catalytic reduction of NO in presence of NH₃. Several characterization and reaction studies were undertaken to show that WO₃ addition improves the low temperature activity of V₂O₅-Sb₂O₃/TiO₂ catalyst but does not improve the SO₂ resistivity. This is an interesting and important study. I would accept the publication of the manuscript after the following concerns are addressed completely.

1.      I feel that the introduction section is not strong, and several in-situ/operando studies have been missing. For example: https://onlinelibrary.wiley.com/doi/abs/10.1002/ange.201904503, https://pubs.acs.org/doi/full/10.1021/acscatal.9b01514?casa_token=mCeDV2lgxs8AAAAA%3AiK00A2Zgr_Lvojkr8DKPoBsF-Qapn9Zmatzoe_wFC9tWL52PCf54af0JDi1qD-xsYqwu7X6R5-6veB4 etc. I recommend the authors should include all important studies in the literature.

2.      Table 1 does not have valid entry. Please check.

3.      The UV-vis analysis is not convincing. The effect of WO3 contribution towards the absorption band is completely ignored. Additionally, I suggest the authors to calculate the edge energy and make conclusion on vanadium local structure based on that.

4.      The discussion section is very poor. I strongly recommend adding a discussion section of couple of paragraphs connecting the structural and chemical probe findings with that of catalytic activity.

Author Response

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Reviewer 4 Report

Review of the Manuscript ID processes-1759170

 

1. Authors should correct English text and rewrite the Title, Abstracts, Introduction, Results and Dsicussion, Conclusions.

 

2.

3. Results and Discussion

3.2 Structural properties

“The BET surface areas of VSbTi, VSbWTi, VSbTi-S and VSbWTi-S are 37, 33, 30 and 173 27 m2/g, respectively.”

Please provide detailed BET and PSD results.

 

3.

Authors should correct according to journal requirements.

Ref. 7, 8, 18, 20, 26, 27 are missing authors, pages, volumes, etc.

Authors should add DOI of the references.

Comments for author File: Comments.docx

Author Response

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Round 2

Reviewer 1 Report

 

Processes

Tungsten oxide modified V2O5-Sb2O3/TiO2 monolithic catalyst: 2 NH3-SCR activity and sulfur resistance 3

Liping Liu 1,2, Xiaodong Wu 2,*, Yue Ma 2, Jinyi Wang 1, Rui Ran 2, Zhichun Si 3 and Duan Weng 2

 

Version 2

General comments

Reviewer appreciated the efforts made by the authors to discuss some of his considerations. The draft was then much better scientifically speaking, although no new experiment was performed to answer in a more precise way to some points (for example details on meso-porosity and reproducibility of experimental results). The English language was slightly better, but still numerous sentences need upgrading.

A new point needs clarification: now that we know the BET surface area of the starting TiO2 (this was not the case previously), i.e., 81 m2/g, a didactic comment must be done on the difference between this value and the values of the extrudate with addition of small amounts of V, Sb and V, Sb, W (37 and 33m2/g), this strong decrease in BET area being rather unexpected.

 

Some other points must be corrected or discussed.

1/ In the copy received by reviewer, the authors names are still appearing with different fonts

2/ In the whole text, the authors must write “sulfate” with the same spelling: use “sulfate” or use “sulphate” but only ONE writing

3/ in their answer regarding reactor (R18), the use of “stainless” is insufficient: please use “stainless steel”

4/ in their treatments preceding H2-TPR and NH3-TPD (R22), the authors used different O2/N2 mixtures (5 or 20% O2 content): why such a difference?

5/ after the answer relative to detection of organic compounds during gas evolution (Line 233), reviewer is still unable to understand: how such species can be released at a temperature well below the calcination temperature of the catalysts in their monolith form? Either the initial calcination conditions (550°C for 6 h) were insufficient to eliminate the additive fragments issued from the extrusion mixture containing stearic acid, polyethylene oxide and carboxymethyl cellulose (in unknown amount) together with inorganic anions and then the authors are working with unclean materials, or a contamination by the laboratory atmosphere is existing and then, the temperature of cleaning before experiments (200°C) is too low. When no pre-treatment is given to the samples before experiment, the “polluting” species can interfere with the results.

6/ in different section the words “ammonia sulfate” were used, but for reviewer “ammonium sulfate” would be better.

Author Response

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Reviewer 3 Report

I see that the authors have addressed all he concerns and I am satisfied with the revised version of the manuscript and recommend its publication.

Author Response

Thanks to the reviewer for the positive and encouraging comments.