NH₃-Selective Catalytic Reduction of NO_x to N₂ over Ceria Supported WO_x Based Catalysts: Influence of Tungsten Content

Round 1

Reviewer 1 Report

The authors have addressed the concernes of the Referee and have followed the suggestions proposed, including the performance of additional experiments.

In my opinion, the paper is now suitable for publication in Catalysts.

Author Response

The authors would like to thank reviewer 1 for the positive consideration of our manuscript

Reviewer 2 Report

The manuscript deals with the influence of Tungsten content on Ceria based catalysts for NH3-selective Catalytic Reduction of NOx to N2. The work is very interesting and well written. It needs some revisions before publication. Please address the following issues:

1) Abstract needs to be rewritten. It does not summarise in a proper way the  article.

2) The Introduction should focus also on the recent findings of using ammonia as a fuel for power generation. In this field the NOx reduction is a very important issue.  Please add some insights into recent works by reporting also relevant literature in this field. please see for example the following literature works:

• Franco, M. C., Rocha, R. C., Costa, M., & Yehia, M. (2021). Characteristics of NH3/H2/air flames in a combustor fired by a swirl and bluff-body stabilized burner. Proceedings of the Combustion Institute, 38(4), 5129-5138.
• Ariemma, G. B., P. Sabia, G. Sorrentino, P. Bozza, M. De Joannon, and R. Ragucci. "Influence of water addition on MILD ammonia combustion performances and emissions." Proceedings of the Combustion Institute 38, no. 4 (2021): 5147-5154

3) Results of figures 6 and 7 should be discussed better.

4) Conclusions need to summarise in a proper way the main findings of the work.

Author Response

The authors would like to thank reviewer for the useful comments. Issues raised from the reviewer have been addressed by the following reply and changes are accordingly made in the main text.
1) The abstract has been rewritten, where the findings of this work have been summarized.

2) We feel that the given references are irrelevant to this work. This work is directed to NO removal through a catalytic approach, and our results present the synthesis, characterization and catalytic activity of such catalyst. The suggested references deal with recent technology in ammonia combustion for power generation. Nevertheless, those references have been integrated.

3) Figure 6 has already been extensively discussed. On the other hand, a discussion about Figure 7 has been added.

4) The conclusion has been completed.

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 manuscript describes the preparation of W/CeO2 catalysts with different W loadings, their characterization and their catalytic behavior in NH3-SCR of NOx into N2. The subject is therefore, of interest to the readers of Catalysts and in the scope of the journal.  However, the paper needs significant improvement before being accepted for publication. Although there is a clear influence of the W content on the activity of the catalysts in the two different temperature ranges, no evidences are presented for the synergistic effect between W and CeO2. According to previous works, such as the 2019 paper from Gu et al., (ref. 27 of the manuscript), the formation of W-O-Ce as a results of the interaction between WOx and CeO2 will affect the reducibility and the acidity of WOx/CeO2 catalysts, due to a decrease of surface oxygen vacancies. In the manuscript, no reducibility or acidity data are presented, and according to the Raman results, the oxygen vacancies contents in the catalysts were similar close for all of them, even for those with high W amount, and “ in a fairly low value (Figure S3 and 104 S4, Table S1), indicating that the ceria preserved its surface morphology”. If the presence of W does not change the electronic properties of the CeO2 surface, the changes observed in the catalytic behavior may be due only to dispersion effects. Thus, additional characterization should be necessary to proof the existence of any type of synergy. Preparation of catalysts based on supports different from CeO2 but comparable W dispersions could also give information about this.

A second important issue that has not been approached in this work is the stability of the catalysts. Activity for several consecutive reaction-regeneration cycles or the study of hydrothermally treated catalysts should be included, as the different W species formed on the CeO2 surface for the different contents may have different thermal/hydrothermal stability.

Additional comments and suggestions are listed below:

1. Taking into account that HPW is only used as W precursor and that its structure is not present in the final catalysts, “ceria supported HPW based catalysts” may be misleading. I suggest changing this in the title and along the manuscript. Catalysts could be identified as WOx/CeO2.
2. Table 1: please specify that the wt% correspond to W, as has been done in other figures/tables along the manuscript.
3. Table 1 and figure 1: crystal sizes are estimated from the Scherrer equation. There are differences among the different catalysts but no clear trend. Moreover, the size obtained for the sample with 9 wt% W is smaller than the rest. Do the authors have an explanation for this exception or may it be due to experimental error regarding crystal size determination?
4. The authors relate the higher activity of catalyst with low W loadings to a synergistic effect between monomeric W species and CeO2. However, no synergistic effect has been demonstrated, only dispersion effect of HPW on textural properties of the CeO2 support, and polymerization degree of the W species by IR and Raman. How does this compare to similar dispersion on different supports, with different redox characteristics, less O vacancies? How would pure HPW behave? Additional characterization should be performed in order to study the effect of W and its loading on the redox and acidic properties of CeO2 and of the overall catalyst.
5. Catalyst thermal and hydrothermal stability is a very important issue in SCR processes and should be addresses, as catalysts with different W loadings and different W species are expected to present different aging behavior. Thus, consecutive reaction-regeneration cycles and/or hydrothermal treatments should be included.
6. When using HPW as W precursor, P is also added to the catalyst. What is the effect of this P? How is it interacting with the rest of the catalyst components?
7. Supporting Information: the figures of the pdf version present very low quality.

Author Response

Reviewer 1

This manuscript describes the preparation of W/CeO2 catalysts with different W loadings, their characterization and their catalytic behavior in NH3-SCR of NOx into N2. The subject is therefore, of interest to the readers of Catalysts and in the scope of the journal.  However, the paper needs significant improvement before being accepted for publication. Although there is a clear influence of the W content on the activity of the catalysts in the two different temperature ranges, no evidences are presented for the synergistic effect between W and CeO2. According to previous works, such as the 2019 paper from Gu et al., (ref. 27 of the manuscript), the formation of W-O-Ce as a results of the interaction between WOx and CeO2 will affect the reducibility and the acidity of WOx/CeO2 catalysts, due to a decrease of surface oxygen vacancies. In the manuscript, no reducibility or acidity data are presented, and according to the Raman results, the oxygen vacancies contents in the catalysts were similar close for all of them, even for those with high W amount, and “ in a fairly low value (Figure S3 and 104 S4, Table S1), indicating that the ceria preserved its surface morphology”. If the presence of W does not change the electronic properties of the CeO2 surface, the changes observed in the catalytic behavior may be due only to dispersion effects. Thus, additional characterization should be necessary to proof the existence of any type of synergy. Preparation of catalysts based on supports different from CeO2 but comparable W dispersions could also give information about this.

We do agree with this comment. Hence, additional experiment of HPW supported on silica has been included. It clearly showed that HPW/SiO2 exhibited a far lower catalytic performance than HPW/CeO2. Moreover, CeO2 (without tungsten) showed moderate activity under the same conditions.

The entire section 2.2 (catalytic activity) has been reformulated and improved.

A second important issue that has not been approached in this work is the stability of the catalysts. Activity for several consecutive reaction-regeneration cycles or the study of hydrothermally treated catalysts should be included, as the different W species formed on the CeO2 surface for the different contents may have different thermal/hydrothermal stability.

The reviewer raises an important question. We envisage to address this issue (hydrothermal stability and reaction regeneration cycles) on an upcoming manuscript, including a tentative determination of the active species and the deactivation mechanism.

Additional comments and suggestions are listed below:

1. Taking into account that HPW is only used as W precursor and that its structure is not present in the final catalysts, “ceria supported HPW based catalysts” may be misleading. I suggest changing this in the title and along the manuscript. Catalysts could be identified as WOx/CeO2.

We do agree with the reviewer and the title has been changed. At the beginning, it has been declared that all characterizations and catalytic investigations have been done with the calcined samples, which indeed are better described by WOx/CeO2. However, we prefer to keep HPW/CeO2 to nominate the catalysts in order to clarify the tungsten precursor for the audiences.

1. Table 1: please specify that the wt% correspond to W, as has been done in other figures/tables along the manuscript.

The caption in table 1 has been modified, as requested.

1. Table 1 and figure 1: crystal sizes are estimated from the Scherrer equation. There are differences among the different catalysts but no clear trend. Moreover, the size obtained for the sample with 9 wt% W is smaller than the rest. Do the authors have an explanation for this exception or may it be due to experimental error regarding crystal size determination?

It was indeed an error, the correct value should be 60 Å. Hence, the crystallite sizes of the samples are more or less similar.

1. The authors relate the higher activity of catalyst with low W loadings to a synergistic effect between monomeric W species and CeO2. However, no synergistic effect has been demonstrated, only dispersion effect of HPW on textural properties of the CeO2 support, and polymerization degree of the W species by IR and Raman. How does this compare to similar dispersion on different supports, with different redox characteristics, less O vacancies? How would pure HPW behave? Additional characterization should be performed in order to study the effect of W and its loading on the redox and acidic properties of CeO2 and of the overall catalyst.

Additional experiment of HPW supported on silica has been included. It clearly showed that HPW/SiO2 exhibited a far lower catalytic performance than HPW/CeO2. SiO2 can be regarded as an inert support, without redox characteristic and no O vacancy. It has been mentioned in the introduction that pure HPW has only low catalytic activity, due low surface area and thermal decomposition. As also mentioned above, we envisage to study the stability of the catalysts in an upcoming project, in which additional characterization will be employed.

1. Catalyst thermal and hydrothermal stability is a very important issue in SCR processes and should be addresses, as catalysts with different W loadings and different W species are expected to present different aging behavior. Thus, consecutive reaction-regeneration cycles and/or hydrothermal treatments should be included.

The reviewer raises an important question. We envisage to address this issue (hydrothermal stability and reaction regeneration cycles) on an upcoming manuscript, including a tentative determination of the active species and eventually the deactivation mechanism.

1. When using HPW as W precursor, P is also added to the catalyst. What is the effect of this P? How is it interacting with the rest of the catalyst components?
   The role of P has indeed been omitted due to the very low concentration of this component on the materials. Note that the P content is at least 1/12 of the tungsten content. It is reasonable to believe that eventually acidic properties originated from P can be ignored.
2. Supporting Information: the figures of the pdf version present very low quality.

The figures in supporting information has been improved.

Reviewer 2 Report

The manuscript is very interesting and it deals with NH3 SCR of NOx to N2 on Ceria Supported HPW Based Catalysts by analyzing the influence of Tungsten content.

The English level needs to be improved in the manuscript, some sentences are not clear and too tedious.

Introduction is somehow poor and weak, it needs a strong improvement before accepting for publication. Please see the following points:

1) The aim of the paper is not clear in the Introduction and it must be reported in a clearer manner

2) At the beginning of the introduction the authors analyzed the importance of NOx emissions when originated from combustion plants.  Despite that, several novel emission technologies depicted very low NOx emissions levels (such as MILD Combustion or low-NOx gas turbines).  Such technologies should be cited properly in the first part of the Introduction to frame the paper in the right context. Please cite the following articles:

- Funke, H. W., Beckmann, N., Abanteriba, S. (2019). "An overview on dry low NOx micromix combustor development for hydrogen-rich gas turbine applications". International Journal of Hydrogen Energy, 44(13), 6978-6990.

- Sorrentino, G., Sabia, P., Bozza, P., Ragucci, R., de Joannon, M. (2019). "Low-NOx conversion of pure ammonia in a cyclonic burner under locally diluted and preheated conditions". Applied Energy, 254, 113676.

- Okafor, E. C., Somarathne, K. K. A., Hayakawa, A., Kudo, T., Kurata, O., Iki, N., & Kobayashi, H. (2019). Towards the development of an efficient low-NOx ammonia combustor for a micro gas turbine. Proceedings of the Combustion Institute, 37(4), 4597-4606.

3) Please improve the discussion of results, especially of Figures 6 and 7.

4) Conclusions need to be enhanced, by reporting in a clearer way the main findings of the manuscript.

Author Response

Reviewer 2

The manuscript is very interesting and it deals with NH3 SCR of NOx to N2 on Ceria Supported HPW Based Catalysts by analyzing the influence of Tungsten content.

The English level needs to be improved in the manuscript, some sentences are not clear and too tedious.

The entire manuscript has been reviewed and improved.

Introduction is somehow poor and weak, it needs a strong improvement before accepting for publication. Please see the following points:

• The aim of the paper is not clear in the Introduction and it must be reported in a clearer manner

The aim of this work has been underlined by the following text:
The intention is to evaluate the nature of tungsten supported on ceria in the NH3-SCR efficiency. A series of different tungsten loading allows to determine the effect of the dispersion. Simultaneously, this study will also reveal the quantity of tungsten required to obtain the optimal catalyst.

2) At the beginning of the introduction the authors analyzed the importance of NOx emissions when originated from combustion plants.  Despite that, several novel emission technologies depicted very low NOx emissions levels (such as MILD Combustion or low-NOx gas turbines).  Such technologies should be cited properly in the first part of the Introduction to frame the paper in the right context. Please cite the following articles:

- Funke, H. W., Beckmann, N., Abanteriba, S. (2019). "An overview on dry low NOx micromix combustor development for hydrogen-rich gas turbine applications". International Journal of Hydrogen Energy, 44(13), 6978-6990.

- Sorrentino, G., Sabia, P., Bozza, P., Ragucci, R., de Joannon, M. (2019). "Low-NOx conversion of pure ammonia in a cyclonic burner under locally diluted and preheated conditions". Applied Energy, 254, 113676.

- Okafor, E. C., Somarathne, K. K. A., Hayakawa, A., Kudo, T., Kurata, O., Iki, N., & Kobayashi, H. (2019). Towards the development of an efficient low-NOx ammonia combustor for a micro gas turbine. Proceedings of the Combustion Institute, 37(4), 4597-4606.
The beginning of the introduction has been dramatically changed and improved, including the mentioned references.

3) Please improve the discussion of results, especially of Figures 6 and 7.
The entire section 2.2 (catalytic activity) has been reformulated and improved.

4) Conclusions need to be enhanced, by reporting in a clearer way the main findings of the manuscript.

The conclusion has been enhanced, the main findings have been further highlighted and a supplementary text has been added:
Current findings reveal that the tungsten dispersion will highly affect the NH3-SCR of NO, in which high dispersion is favorable. Importantly, low loading of tungsten is sufficient to obtain highly active catalyst.

Reviewer 3 Report

The authors report the NH3-SCR of NOx over CeO₂ supported HPW catalysts with varying W content. This concept is interesting but not new. Comparing the studies published in e.g. Ref. 11 and 15, the presented investigations do not have any novelty value, especially as important, mainly spectroscopic and/or NH3-TPD and TPR studies failed. Thus, most of the conclusions are speculative and not confirmed by respective experimental data.

Remarks

The authors claimed “12-tungstophosphoric acid H3PW12O40 (HPW) with the Keggin structure is a cheap and readily available tungsten source”; but is this really the case? For its preparation other W precursors are used which are also suitable to modify CeO2 (directly); especially as HPW is decomposed during calcination forming different WOx species. What is the nature and role of the also remaining P on the surface?

Figure 3 / Table 2: While the 2% and 4.5% HPW/CeO2 samples contain – as concluded from the W=O band position – monomeric [WO4] units, the band position around 950 cm-1 indicates the presence of polytungstate or polymeric species (see relevant literature, e. g. Mamede et al., J. Catal. 2004, 223, 1-12). For my opinion, there is no indication for monomeric species, rather for different polymeric species. Insofar, the deconvolution of the bands of the higher loaded samples (Table 2) makes no sense.

Page 5, lines 126-129: the authors claim “Hence, it is difficult to probe the tungsten forms on the surface based on UV-Vis.” That is right and can be omitted here, because no new aspects can be presented.

Page 5, lines 144-145: the authors claim “Once loaded with tungsten, the NH3 adsorption is increased, thus improving the catalytic performance of the materials. This conclusion is only based on results of Re. 11, own studies of NH3 adsorption are missing!

Figure 6: the applied GHSV is comparable low (Ref. 11 GHSV=120.000!). Did the authors test the catalysts also at higher GHSV?

Author Response

Reviewer 3

The authors report the NH3-SCR of NOx over CeO₂ supported HPW catalysts with varying W content. This concept is interesting but not new. Comparing the studies published in e.g. Ref. 11 and 15, the presented investigations do not have any novelty value, especially as important, mainly spectroscopic and/or NH3-TPD and TPR studies failed. Thus, most of the conclusions are speculative and not confirmed by respective experimental data.

Remarks

The authors claimed “12-tungstophosphoric acid H3PW12O40 (HPW) with the Keggin structure is a cheap and readily available tungsten source”; but is this really the case? For its preparation other W precursors are used which are also suitable to modify CeO2 (directly); especially as HPW is decomposed during calcination forming different WOx species. What is the nature and role of the also remaining P on the surface?
12-tungstophosphoric acid is a commercial available tungsten source and it is fairly cheap. This precursor is frequently used in many applications, particularly for the preparation of catalysts. One great advantage with this precursor is the solubility, and thus make it suitable for impregnation.

The nature of eventually remaining P has indeed been omitted due to the low concentration (1/12 with respect to W). You may imagine that the remaining P may create supplementary acidity, but this acidic contribution should be far lower than the contribution from tungsten. Ongoing experiments involving modification of this elements (for example with Si) are under investigations in order to determine the role of P.

Figure 3 / Table 2: While the 2% and 4.5% HPW/CeO2 samples contain – as concluded from the W=O band position – monomeric [WO4] units, the band position around 950 cm-1 indicates the presence of polytungstate or polymeric species (see relevant literature, e. g. Mamede et al., J. Catal. 2004, 223, 1-12). For my opinion, there is no indication for monomeric species, rather for different polymeric species. Insofar, the deconvolution of the bands of the higher loaded samples (Table 2) makes no sense.
The suggested reference is a quite old article in which the authors utilize Ce(III)tungstate (tetrahedral coordinated tungsten) as reference compound. This paper highlights that polymeric tungsten species are characterized by a peak around 950 cm-1. In our manuscript, the peak assignments are based on a very recent article published in 2020. These assignments are well-known, particularly in olefin metathesis studies (ref 35). Most importantly, there is no mismatch between our manuscript and the given reference (Mamede et al, J. Catal. 2004, 223, 1). In line 122-123, we clearly claim that: “the mono (915-930 cm-1) and poly-tungstate (945-960 cm-1)”
As this contribution appears as a broad peak, it has been submitted to deconvolution in order to estimate the monomeric and polymeric tungsten species.

Page 5, lines 126-129: the authors claim “Hence, it is difficult to probe the tungsten forms on the surface based on UV-Vis.” That is right and can be omitted here, because no new aspects can be presented.

We all agree that UV-Vis is not the most reliable technique to probe the tungsten forms. However, the tendency is coherent with results from Raman spectroscopy: Higher tungsten loading decreases the band gap. Hence, we believe that this work merits to be mentioned.

Page 5, lines 144-145: the authors claim “Once loaded with tungsten, the NH3 adsorption is increased, thus improving the catalytic performance of the materials. This conclusion is only based on results of Re. 11, own studies of NH3 adsorption are missing!
As the reviewer mentioned at the beginning, studies that have already been published do not have any novelty value and has initially been excluded. We do believe that no one will argue that adding the tungsten will enhance the acidity and thereby increase the NH3 adsorption. Nevertheless, this comment will be considered in our upcoming work involving detailed characterization of the most active catalyst, along with the stability and recycling tests.

Figure 6: the applied GHSV is comparable low (Ref. 11 GHSV=120.000!). Did the authors test the catalysts also at higher GHSV?

The reviewer raises an important remark that has unfortunately not been considered in the current manuscript. Working with higher GHSV allows better understanding of the catalyst and to identify the primary products. This comment will be integrated in our upcoming work.

Round 2

Reviewer 1 Report

Although the authors have addressed some of the suggestions of the reviewers, the actions taken are not sufficient, and the paper is not suitable for publication in Catalysts. The main concern, e.g., if the benefits of low W loaded Ceo2 are due to true synergistic effects or to higher W dispersion, has not been clarified, although different approaches were suggested in order to evidence the synergistic effect between W and CeO2:

1. Additional characterization was suggested to proof the existence of any type of synergy, such as for instance acidity or reducibility data that could show possible changes in the electronic properties of the CeO2 surface, which are not observed by Raman. No additional characterization has been presented
2. Preparation of catalysts based on supports different from CeO2 but comparable W dispersions could also give information about this. REgarding this suggestion, the authors present the results obtained with an additional catalyst, 2% HPW/SiO2, which performs better than the CeO2 support and worse than any of the HPW/CeO2 catalysts. However, I did not find any characterization of this new catalyst (BET surface area of the parent SiO2, of the final catalyst…) that would allow comparison of the W dispersion. This new experiment, in the way it is presented, does not provide any additional information.

Moreover, the hydrothermal stability has not been studied, and according to the authors’ response, “this issue (hydrothermal stability and reaction regeneration cycles) will be addressed on an upcoming manuscript, including a tentative determination of the active species and the deactivation mechanism.”

Comments of the first revision that have not been addressed:

1. The authors relate the higher activity of catalyst with low W loadings to a synergistic effect between monomeric W species and CeO2. However, no synergistic effect has been demonstrated, only dispersion effect of HPW on textural properties of the CeO2 support, and polymerization degree of the W species by IR and Raman. How does this compare to similar dispersion on different supports, with different redox characteristics, less O vacancies? How would pure HPW behave? Additional characterization should be performed in order to study the effect of W and its loading on the redox and acidic properties of CeO2 and of the overall catalyst.
2. Catalyst thermal and hydrothermal stability is a very important issue in SCR processes and should be addresses, as catalysts with different W loadings and different W species are expected to present different aging behavior. Thus, consecutive reaction-regeneration cycles and/or hydrothermal treatments should be included.

Reviewer 3 Report

The authors report the NH3-SCR of NOx over CeO₂ supported HPW catalysts with varying W content. The revised version is not significantly improved. The novelty is low because mainly spectroscopic and/or NH3-TPD and TPR studies failed and thus, most of the conclusions are speculative and not confirmed by respective experimental data. The hint to upcoming work is not constructive.

By inspecting some of the literature references it has been shown that they are often incomplete or not correct; all reference data must be checked and revised according the guidelines of “Catalysts”.

By the way (concerning Raman data), information from “quite old articles” are not necessarily worse!