High Performance Ru-CN_x/CeO₂ Catalyst for Catalytic Wet Oxidation of N-Methyldiethanolamine in Water

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript submitted by authors describes the synergetic effect between Ru and CN_x species for Ru-CNx/CeO₂ catalyst in the catalytic wet oxidation of N-methyldiethanolamine (MDEA) in wasterwater. The results are systematically described and clearly demonstrate the influence of CNx species on the creation of more oxygen vacancies and further activation of MDEA. Some points should be addressed in more details:

1. In the section of introduction, the authors did not give any introduction about the recent progresses in the catalysts for the removal of MDEA by CWO. Thus, it is very difficult to judge the superiority or inferiority of the catalyst and catalytic performance in this paper.
2. The real loadings of Ru and CN_x on the catalysts have a very large difference with the nominated loadings, please explain the reasons.
3. The authors concluded that the Ru-CNx/CeO₂ catalyst has excellent inorganic salt resistance. How about other reference samples, such as Ru/CeO₂ or CN_x/CeO₂?
4. For the stability test, the catalytic activity of Ru-CNx/CeO₂ is gradually decayed with the reuse times. One of the reasons is the leaching of the active Ru species. This is very fatal for the real application of the catalysts. It is necessary for authors to provide a solution to solve the problem now or in the future.
5. In this paper, it can be found that the metallic Ru can be the active sites for the removal of MDEA by CWO. However, why the catalyst was obtained by calcinations in a N₂ atmosphere rather than was treated by a reductive atmosphere to obtained more metallic Ru active sites?

Author Response

Dear reviewer,

We sincerely appreciate your efforts  in evaluating our manuscript entitled "High Performance of Ru-CNx/CeO₂ Catalyst for Catalytic Wet Oxidation of N-methyldiethanolamine in Water." The insightful suggestions provided have been instrumental in guiding substantive improvements to the manuscript. In accordance with the recommendations received, we have meticulously revised the content highlighted in the revised version. The following sections present a comprehensive response to each comment from the reviewers, accompanied by corresponding amendments implemented in the revised manuscript.

Best regards.

     Dr. Liuye Mo

Zhejiang Ocean University

 Zhoushan, China

 

Reviewer #1: 

Comment 1: In the section of introduction, the authors did not give any introduction about the recent progresses in the catalysts for the removal of MDEA by CWO. Thus, it is very difficult to judge the superiority or inferiority of the catalyst and catalytic performance in this paper.

Response: 

We agree with the reviewer’s comment. Normally, literature review should be performed extensively in the part of the introduction to emphasize the novelty of our work. Actually, we have reviewed the reported results in the introduction, although few research works have been done on the removal of MDEA by CWO.

Comment 2: The real loadings of Ru and CN_x on the catalysts have a very large difference with the nominated loadings, please explain the reasons.

Response: 

The reviewer is right that the nominated loadings of the supported Ru catalysts are quite difference with the real loadings. The preparation method in the manuscript named modified impregnation method is developed by our group, in which the low solubility Ce(OAc)₃.xH₂O was adopted instead of the thermal stable CeO₂. The Ce(OAc)₃.xH₂O precursor is a nonstoichiometric compound unlike the CeO₂. Furthermore, the promoter of CN_x derived from the melamine on the prepared catalysts is also nonstoichiometric compound different from the C₃N₄. Additionally, the pyrolysis of melamine in N2 atmosphere may produce low yield of CN_x. Therefore, it is not easy to accurately control the loading of Ru. These inherent characteristics of the precursor materials substantiate the observed discrepancies between nominal and actual Ru loading values in the catalytic system.

 

Comment 3 The authors concluded that the Ru-CN_x/CeO₂, catalyst has excellent inorganic salt resistance How about other reference samples, such as Ru/CeO₂, or CN_x/CeO₂?

 Response: 

We totally agree with the comment that inorganic salt resistance is very important property for a catalyst applied in wastewater treatment. Therefore, we performed the experiments using the best Ru-CN_x/CeO₂ catalyst. It is a pity that the reference samples have not been done to test the experiments as their activities are not satisfied in treating the MEDA-containing only water.

Comment 4 For the stability test, the catalytic activity of Ru-CN_x/CeO₂ is gradually decayed with the reuse times. One of the reasons is the leaching of the active Ru species. This is very fatal for the real application of the catalysts. It is necessary for authors to provide a solution to solve the problem now or in the future.

 Response: 

The reviewer raised a very good issue of the Ru leaching during the reaction. It is a big challenge to avoid the active metals leaching from a catalyst applying in a liquid phase reaction. In fact, we have achieved immature results that the strong interaction between Ru and Mn promotor on the Ru-MnO_x based catalysts can prevent the Ru leaching at certain extend.

Comment 5 In this paper, it can be found that the metallic Ru can be the active sites for the removal of MDEA by CWO. However, why the catalyst was obtained by calcinations in a N₂ atmosphere rather than was treated by a reductive atmosphere to obtained more metallic Ru active sites?

 

Response:

It is reasonable to use a reductive atmosphere to obtain more metallic Ru active sites based on the experiment results. In fact, we have used H₂ to treat the catalysts, but the activities are not good at all. The reason we think is that the CN_x species will be hydrogenated to depolymerize in presence of Ru which is very active in hydrogenation reaction.

 

Reviewer 2 Report

Comments and Suggestions for Authors

High Performance of Ru-CNx /CeO2 Catalyst for Catalytic Wet Oxidation of N-methyldiethanolamine in Wastewater

Review

This paper deals with the Ru-CNx /CeO2 catalyst for the wet oxidation of N-methyldiethanolamine in water. It provides satisfying characterization and the effect of some parameters on the N-methyldiethanolamine removal. However, it needs some thorough revision. The authors must clearly state the novelty and significance of their work. The paper lacks discussion and explanation of many phenomena it deals with; there are mostly the results, with little or no discussion. The present references are relevant, but it needs some more to back up some of the claims and explain some phenomena. Some figures need slight corrections. English, grammar, and typing need to be seriously revised before the acceptance for publication in Sustainability.

1. Abstract: It should be more informative. It contains mostly conclusions. The characterization techniques and catalytic methods should be mentioned. The most important findings should be quantified where possible (considering the word number limitation by a journal).
2. The word “Wastewater” in the title should be replaced by “water” since the authors work with model solutions, not with real wastewater.
3. Introduction, third paragraph: The authors mention active radicals, such as OH, which are very important in many types of catalytic processes, but they say almost nothing about them. These radicals should be properly introduced. They are well presented and explained in the: https://doi.org/10.1016/j.ces.2020.116209, https://doi.org/10.1016/j.dwt.2024.100384 and/or https://doi.org/10.1016/j.matchemphys.2015.03.030.
4. Introduction, the last paragraph: The authors should clearly state this study's novelty and significance. Why did you choose Ru-CNx /CeO2 type material? What were the expectations of it based on?
5. Section 2.3: The sentence “As the temperature was reached the reaction temperature…” is unclear and grammatically incorrect. Please edit it.
6. Section 2.3: The equation should be labeled by a number.
7. Section 3.1: The characteristic peaks positions for every crystalline phase (2 theta angles), as well as the lattice parameters, should be given.
8. Section 3.1: Basically, XRD detected only CeO2 and C3N4, and the absence of Ru or CNx peaks was attributed to the amorphous nature or high dispersion. This should be backed by a proper reference. Is it possible that Ru and the surface were covered by CeO2, so the XRD could not “see” their proper peaks?
9. Section 3.1: “the Ru-CNx/CeO2 catalyst exhibits the smallest CeO2 crystallite size, suggesting that species of CNx on the CeO2 supports may inhibits the growth of CeO2 crystallites”. This should be backed by a reference. Instead of “CeO2 crystallites”, it should be “CeO2 crystals”.
10. Section 3.2: “Figure.2a shows that the pure CeO2 catalyst mainly presents a nanosheet structure”. The average size should be given.
11. Section 3.2: the BET analysis needs discussion. Why does the CNx have such impact on the catalyst pore volume?
12. Section 3.3, 5^th row: references “33, 34” are written in different font. Please fix it.
13. Section 3.3: The average % of each element detected by EDS and the corresponding error should be given. The results obtained by the TEM and EDS should be compared to those obtained by XRD.
14. Figure 3: b: the font should be bigger or in other color to make it clearer for reading; c: The legend should be written in bigger font; d: the labels in the squares (with the elements) should be bigger, and the contrast between the letters and the color of the squares should be higher to make them properly visible.
15. Section 3.5, 3.6 and everywhere else it appears in the text: the square brackets must be separated from the word (“catalyst [39]” instead of “catalyst[39]”.
16.  What is the nature of the Ru-CNx/CeO2 catalyst, ie, what is the nature of the bonds in the catalyst (is it composite or what?). What conclusion can be drawn from all this characterization?
17. Section 3.7: It should be clearly noted in this place that Ru-10%CNx/CeO2 is Ru-CNx/CeO2 (the one that was subjected to detailed characterization).
18. Section 3.7: “It demonstrates that excessive CNx loading may cover the active sites of Ru leading to the decrease of the activity.” This should be backed by the experiment or proper literature.
19. Section 3.8: Why did the authors not apply a temperature over 200°C? The increase of the reaction rate is sharp up to 180°C, and it seemingly slows down after that. Why? This should be all discussed.
20. Chapter 3.9: Why did the authors not examine the effect of O2 pressure on the TOC removal? It should be done if possible.
21. Chapter 3.9, the end: “is >1.0Mpa[27].” The font should be uniform, with the word separated from the parentheses.
22. Section 3.10: Why do the authors examine the concentration of salt only up to 9%? TOC
23. Chapter 3.11: This chapter needs discussion. Why does the removal of COD increase with the increase of pH, and why does it suddenly drop from pH 10 to 12?
24. Figure 12, please fix the font.
25. Why did the authors measure both COD and TOC only in the temperature section and not for the rest of the parameters?
26. Section 3.13.1, 7^th row: “crystals” instead of “crystallites”. Fix in the conclusion as well.
27. Section 3.13.1: This part should be discussed in more detail, with a special focus on the bonds and interactions between Ru, CNx, and CeO2 and their mutual effects and possible synergy.
28. Section 3.13.2:”…of carboxylic acid or organic compounds.” The presence of a particular intermediate should be proven by some analytical technique. Otherwise, this part should be removed from the text.
29. The English and grammar need some serious corrections, especially in the “Experimental, results, and discussion part.

Author Response

Dear reviewer,

We sincerely appreciate your efforts  in evaluating our manuscript entitled "High Performance of Ru-CNx/CeO₂ Catalyst for Catalytic Wet Oxidation of N-methyldiethanolamine in Water." The insightful suggestions provided have been instrumental in guiding substantive improvements to the manuscript. In accordance with the recommendations received, we have meticulously revised the content highlighted in the revised version. The following sections present a comprehensive response to each comment from the reviewers, accompanied by corresponding amendments implemented in the revised manuscript.

Best regards.

     Dr. Liuye Mo

 

Reviewer #2:

Comment 1.

Abstract: It should be more informative. lt contains mostly conclusions. The characterization techniques and catalytic methods should be mentioned. The most important findings should be quantified where possible (considering the word number limitation by a journal).

 

Response: 

We appreciate the constructive comment. We have revised the abstract in the revised manuscript.

 

Comment 2.

The word "Wastewater" in the title should be replaced by“water" since the authors work with model solutions. not with real wastewater.

 

Response: 

We agree with this point. We have replaced the word of "Wastewater" by “water”.

 

Comment 3.

 Introduction, third paragraph: The authors mention active radicals, such as OH, which are very important in many types of catalytic processes, but they say almost nothing about them. These radicals should be properly introduced.

 

Response: 

We agree with this comment. We have revised this part accordingly in the revised manuscript.

 

Comment 4.

Introduction, the last paragraph: The authors should clearly state this study's novelty and significance. Why did you choose Ru-CN_x/CeO₂ type material? What were the expectations of it based on?

 

Response: 

   Thanks for the comment. We have strengthened this point based on the comment.

 Comment 5.

     Section 2.3: The sentence “As the temperature was reached the reaction temperature”is unclear and grammatically incorrect. Please edit it.

 

Response: 

Sorry for the mistake. We have removed this sentence.

 

Comment 6.

Section 2.3: The equation should be labeled by a number.

 

Response: 

Agree. We have revised it in the revised manuscript.

 

 Comment 7.

Section 3.1The characteristic peaks positions for every crystalline phase (2 theta angles), as well as the lattice parameters should be given.

 

Response: 

Agree. We have revised it.

 

Comment 8.

Section 3.1: Basically, XRD detected only CeO₂ and C₃N₄, and the absence of Ru or CN_x peaks was attributed to the amorohous nature or high dispersion. This should be backed by a proper reference. Is it possible that Ru and the surface were covered by CeO₂. so the XRD could not “see” their proper peaks?

 

Response: 

Question 1. We have cited references to back this point.

Question 2. The XRD technique is wildly used to characterize the phase composition on a solid material. Notably, XRD enables the detection of crystalline phases penetrating deeply into a material's interior [Epp, J. X-ray diffraction (XRD) techniques for materials characterization. In Materials characterization using nondestructive evaluation (NDE) methods, Elsevier: 2016; pp 81-124.], which is different from those surface analyzed instruments as XPS, SEM/TEM etc.

 

Comment 9.

Section 3.1:“the Ru-CN_x/CeO₂ catalyst exhibits the smallest CeO₂ crystallite size,suggesting that species of CN_x on the CeO₂ supports may inhibits the growth of CeO₂ crystalites". This should be backed by a reference. Instead of CeO₂ "crystallites", it should be“CeO₂ crystals”.

 

Response: 

We have revised in the revised version.

 

Comment 10.

Section 3.2: “Figure.2a shows that the pure CeO₂ catalyst mainly presents a nanosheet structure”. The average size should be given.

 

Response: 

Figure 2a shows that the pure CeO₂ catalyst mainly exhibits a nanosheet-like morphology. It should be noted that SEM characterizes the surface morphology but cannot be used to determine the actual particle size.

 

 

Comment 11.

Section 3.2: the BET analysis needs discussion. Why does the CN_x have such impact on the catalyst pore volume?

 

Response: 

We have revised according to the comment. The increase of pore size can be attributed to the pyrolysis effect of melamine evolving gases during thermal pretreatment to form larger pore structure [Rao, R.; Huang, Y.; Zhang, H.; Hu, C.; Dong, X.; Fang, W.; Zhou, Q.; Chen, Z.; Fang, S.; Jin, D. A simple melamine-assisted cellulose pyrolysis synthesis of magnetic and mesoporous N-doped carbon composites with excellent adsorption of Congo red. Sep Purif Technol 2024, 347, 127678.].

 

Comment 12.

Section 3.3, 5^th row: references “33, 34" are written in different font. Please fix it.

Response: 

Agree. We have revised it.

 

Comment 13.

Section 3.3: The average % of each element detected by EDS and the corresponding error should be given. The results obtained by the TEM and EDS should be compared to those obtained by XRD.

 

Response: 

 The average % of each element detected by EDS and the corresponding error are given. The Ru and CN_x species are not observed in the TEM images, possibly due to their high dispersion and low concentrations. And the particle size of CeO2 on the Ru-CNx/CeO2 determined by TEM is listed in the manuscript. These results ae consistent with the results of XRD.

 

 

Comment 14.

Figure 3: b: the font should be bigger or in other color to make it clearer for reading; c: The legend should be written in bigger font; d: the labels in the squares (with the elements) should be bigger, and the contrast between the letters and the color of the squares should be higher to make them properly visible.

Response: 

We have revised in the revised version.

 

Comment 15.

Section 3.5, 3.6 and everywhere else it appears in the text: the square brackets must be separated from the word (“catalyst [39]” instead of “catalyst[39]”.

 

Response: 

 We have revised it.

 

 

Comment 16.

What is the nature of the Ru-CN_x/CeO₂ catalyst, ie, what is the nature of the bonds in the catalyst (is it composite or what?). What conclusion can be drawn from all this characterization?

 

Response: 

It is complex issue that there are Ru-O-Ce species and interfaces among components in the catalyst. Regarding to the interface species between Ru species and CN_x are unknown. We believe there are interactions among the components but we can not “see” them. Therefore, it is difficult to definite the catalyst is a composite or not. The only conclusion we can drawn from the characterization is that the Ru-O-Ce species are formed in the interfaces.

 

Comment 17.

Section 3.7: lt should be clearly noted in this place that Ru-10%CN_x/CeO₂ is Ru-CN_x/CeO₂ (the one that was subjected to detailed characterization).

 

Response: 

We have revised it.

 

Comment 18.

Section 3.7. “lt demonstrates that excessive CN_x loading may cover the active sites of Ru leading to the decrease of the activity." This should be backed by the experiment or proper literature.

 

Response: 

We cited the reference (Yuan, C.; Zhang, Y.; Zong, Z.; Zhou, S.; Cui, H.; Tan, H. In situ growth of an ultrathin Cu/gC3N4 coating over SBA-15 for catalytic wet air oxidation of pollutants under extremely mild conditions. Green Chem 2024, 26, 6601-6615.) in the revised manuscript.

 

Comment 19.

Section 3.8: Why did the authors not apply a temperature over 200 ℃? The increase of the reaction rate is sharp up to 180 ℃, and it seemingly slows down after that. Why? This should be all discussed.

 

Response: 

Question .1. Only reactions at 200 °C were investigated in this study as it will take a long time to increase the temperature of 200 °C. Therefore, we prefer low reaction temperature rather than high temperature to evaluate the catalytic performance of the prepared catalysts.

Question .2. Furthermore, as the temperature increased from 180 to 200 °C, the removal of TOC and COD shown no significant change, which may be attributed to the formation of refractory intermediate products [68].

68. Delgado, J.J.; Chen, X.; Pérez-Omil, J.A.; Rodríguez-Izquierdo, J.M.; Cauqui, M.A. The effect of reaction conditions on the apparent deactivation of Ce–Zr mixed oxides for the catalytic wet oxidation of phenol. Catal Today2012, 180, 25-33.

 

Comment 20.

Chapter 3.9: Why did the authors not examine the effect of O₂ pressure on the TOC removal? It should be done if possible.

 

Response: 

The experimental findings demonstrated that TOC and COD exhibited analogous trends with a robust correlation coefficient. Consequently, a single parameter of COD was selected for catalytic activity measurements.

 

Comment 21.

 Chapter 3.9, the end: “is >1.0Mpa[27],." The font should be uniform, with the word separated from the parentheses.

 

Response: 

We have revised it.

 

Comment 22.

Section 3.10: Why do the authors examine the concentration of salt only up to 9%?

 

Response: 

The concentrations of salts in water is 9%, which is very high in wastewater [Zhang, W.; Sun, W.; Zhang, Y.; Yu, D.; Piao, W.; Wei, H.; Liu, X.; Sun, C. Effect of inorganic salt on the removal of typical pollutants in wastewater by RuO₂/TiO₂ via catalytic wet air oxidation. Chemosphere 2023, 312, 137194]. Therefore, a maximum salt concentration of 9% was used in this study.

 

Comment 23.

Chapter 3.11: This chapter needs discussion. Why does the removal of COD increase with the increase of pH, and why does it suddenly drop from pH 10 to 12?

 

Response: 

We have revised it.

Specifically, when pH is below 5, the overabundance of H⁺ ions may deplete OH^-, significantly suppressing the generation of ·OH and ultimately resulting in a noticeable reduction in the oxidation efficiency of MDEA. In contrast, at pH between 7 and 10, the increased OH^- may promote ·OH generation, which could enhance the degradation efficiency of MDEA [72]. However, the degradation efficiency markedly diminishes as the pH increases to 12 as depicted in Figure 12a, the phenomenon is likely attributable to the scavenging of free radicals under conditions of elevated alkalinity [73].

72. Ao, X.; Wang, W.; Sun, W.; Lu, Z.; Li, C. Degradation and transformation of norfloxacin in medium-pressure ultraviolet/peracetic acid process: An investigation of the role of pH. Water Res2021, 203, 117458.
73. Yadav, B.R.; Garg, A. Efficacy of fresh and used supported copper-based catalysts for ferulic acid degradation by wet air oxidation process. Ind Eng Chem Res2012, 51, 15778-15785.

 

Comment 24.

Figure 12, please fix the font.

 

Response: 

We have revised it.

 

Comment 25.

Why did the authors measure both COD and TOC only in the temperature section and not for the rest of the parameters?

 

Response: 

The reason is the same with the comment 20.

 

Comment 26.

Section 3.13.1, 7^th row: “crystals" instead of ”crystals" instead of "crystallites". Fix in the conclusion as well.

 

Response: 

 We have revised it.

Comment 27.

Section 3.13.1: This part should be discussed in more detail, with a special focus on the bonds and interactions between Ru, CN_x, and CeO₂ and their mutual effects and possible synergy.

 

Response: 

The issue is very important but difficult to solve because heterogeneous catalysis is so complex that we can not answer clear in the manuscript. The question we answer is as the question 16 you raised.

 

Comment 28. Section 3.13.2:"...of carboxylic acid or organic compounds." The presence of a particular intermediate should be proven by some analytical technique. Otherwise, this part should be removed from the text.

 

Response: 

Agree. We have removed them from the text.

 

Comment 29. The English and grammar need some serious corrections, especially in the “Experimental results, and discussion part.

 Response: 

Agree. We have revised it carefully.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have carefully revised the manuscript according to the reviewer's comments.  I recommend this revised paper to be published by Sustainability.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have improved the article's quality enough to be acceptable for publication in Sustainability.