Efficient Removal of Methylene Blue Using an Organic–Inorganic Hybrid Polyoxometalate as a Dual-Action Catalyst for Oxidation and Reduction
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript presented for review presents the study of properties of organic-inorganic hybrid polyoxometalate CoPMoV as a heterogeneous catalyst for potential dye wastewater treatment via oxidation and reduction. Important and Interesting multidisciplinary work that will be of interest to the Catalysts' readers. The authors conducted a wide-ranging study using a wide range of experimental analysis methods and carefully studied the mechanism of the process. I recommend the manuscript for publication after addressing a number of minor recommendations that may improve the work soundness:
1) It would be interesting to add in Introduction part figure with illustrative examples and some more references regarding on POMs application in materials, photochemistry and medicine felds.
2) It would be desirable to describe in more detail what determines the choice of this particular catalyst (CoPMoV). What is its possible advantage over others?
3) At the first mention of the CoPMoV structure in the text, it would be possible to insert for clarity and better presentation Figure S1 (a) from Supplementary.
4) Despite that fact that CoPMoV was prepared according to the literature procedure, it would be desirable to add in Experimental part its General preparation method.
5) Figures 6 and 10: the copies of mass spectra from Supplementary should be transferred to this figures.
6) The conclusion section should be rewritten in order to better indicating the novelty of the work.
Author Response
Reviewer 1
The manuscript presented for review presents the study of properties of organic-inorganic hybrid polyoxometalate CoPMoV as a heterogeneous catalyst for potential dye wastewater treatment via oxidation and reduction. Important and Interesting multidisciplinary work that will be of interest to the Catalysts' readers. The authors conducted a wide-ranging study using a wide range of experimental analysis methods and carefully studied the mechanism of the process. I recommend the manuscript for publication after addressing a number of minor recommendations that may improve the work soundness:
Comment 1): It would be interesting to add in Introduction part figure with illustrative examples and some more references regarding on POMs application in materials, photochemistry and medicine felds.
Response: Thanks for this useful suggestion. It is better to add Figure to the introduction of review article, so, we added several references regarding on POMs application in materials, photochemistry and medicine fields.
Comment 2): It would be desirable to describe in more detail what determines the choice of this particular catalyst (CoPMoV). What is its possible advantage over others?
Response: We have discussed in introduction. It’s advantage is the cooperative redox activity of cobalt and [PMoVI8VIV4VV2O42]5- anion.
Comment 3): At the first mention of the CoPMoV structure in the text, it would be possible to insert for clarity and better presentation Figure S1 (a) from Supplementary.
Response: We have moved Figure S1 (a) to the main text.
Comment 4): Despite that fact that CoPMoV was prepared according to the literature procedure, it would be desirable to add in Experimental part its General preparation method.
Response: We have added the preparation method.
Comment 5): Figures 6 and 10: the copies of mass spectra from Supplementary should be transferred to this figures.
Response: We have combined the corresponding Figures.
Comment 6): The conclusion section should be rewritten in order to better indicating the novelty of the work.
Response: Thanks for the suggestion. We have revised the conclusion.
Reviewer 2 Report
Comments and Suggestions for AuthorsThis paper deals with a conventional well studied system. The use of instrumental techniques especially XPS to the relevance of the study has not come out clearly. The difference between the two processes could be clearly stated.
Comments on the Quality of English LanguageThe system chosen namely MB degradation is well studied The positive side of this study is the steps taken to identify the intermediates and products of degradation. The use of hybrid POM is worth documenting.
The methodology followed is conventional. The use of XOS to the relevance of this study has not come out claarly.
Author Response
Reviewer 2
Comments and Suggestions for Authors
Comments: This paper deals with a conventional well studied system. The use of instrumental techniques especially XPS to the relevance of the study has not come out clearly. The difference between the two processes could be clearly stated.
The system chosen namely MB degradation is well studied The positive side of this study is the steps taken to identify the intermediates and products of degradation. The use of hybrid POM is worth documenting.
The methodology followed is conventional. The use of XOS to the relevance of this study has not come out claarly.
Response: I am not sure that I have understood the comments. We have revised the XPS related analysis.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe research explores the synthesis and dual catalytic activity of an organic-inorganic hybrid polyoxometalate (POM) for the remediation of methylene blue (MB) pollution. While the manuscript is well-prepared and relevant to the journal, a few modifications are needed before publication consideration.
- The authors propose a mechanism for MB degradation involving the generation of •OH and •O2H radicals through V4+/V5+ and Mo6+/Mo5+ redox cycles. While this is plausible, the evidence provided is not entirely convincing. The authors should provide more direct evidence for the involvement of these redox cycles, such as EPR spectroscopy or cyclic voltammetry. Additionally, the authors should discuss the possibility of other reactive species being involved in the degradation process, such as 1O2 or H2O2.
- Some of the figures in the manuscript could be improved in terms of clarity and presentation. For example, the font size in some figures is too small, especially Fig. 9, making it difficult to read the labels and data points. The authors should ensure that all figures are of high quality and easy to interpret.
- It is important to evaluate the performance of the catalyst in real wastewater samples, which may contain other contaminants and complex matrices that can affect the activity and stability of the catalyst. If the authors can present the results of such a measurement, the impact of the study could be greatly expanded.
Overall, this manuscript demonstrates a high standard of writing and presents noteworthy results. That said, attending to the identified issues is a prerequisite for the manuscript to be deemed suitable for publication.
Author Response
Reviewer 3
The research explores the synthesis and dual catalytic activity of an organic-inorganic hybrid polyoxometalate (POM) for the remediation of methylene blue (MB) pollution. While the manuscript is well-prepared and relevant to the journal, a few modifications are needed before publication consideration.
Comment 1- The authors propose a mechanism for MB degradation involving the generation of •OH and •O2H radicals through V4+/V5+ and Mo6+/Mo5+ redox cycles. While this is plausible, the evidence provided is not entirely convincing. The authors should provide more direct evidence for the involvement of these redox cycles, such as EPR spectroscopy or cyclic voltammetry. Additionally, the authors should discuss the possibility of other reactive species being involved in the degradation process, such as 1O2 or H2O2.
Response: The essence of MB degradation is Fenton reaction, and the function of CoPMoV is similar to that of Fe2+ ions. Co2+/Co3+, V4+/V5+ and Mo6+/Mo5+ redox cycles are similar to that of Fe2+/Fe3+. Of course, it will be more convincing if the redox cycles can be confirmed by EPR spectroscopy or cyclic voltammetry. Unfortunately, we can not reach such equipment. In addition, redox cycles are dynamic and it is difficult to accurately capture the changes in real time. To our knowledge, no such work has appeared in literature.
According to references, it is possible to generate 1O2 and H2O2.
Comment 2- Some of the figures in the manuscript could be improved in terms of clarity and presentation. For example, the font size in some figures is too small, especially Fig. 9, making it difficult to read the labels and data points. The authors should ensure that all figures are of high quality and easy to interpret.
Response:- We are sorry for the inconvenience. We have improve the quality of Figures.
Comment 3: It is important to evaluate the performance of the catalyst in real wastewater samples, which may contain other contaminants and complex matrices that can affect the activity and stability of the catalyst. If the authors can present the results of such a measurement, the impact of the study could be greatly expanded.
Response: This is a very meaningful suggestion. We will take the study when conditions permit.
Comment 4: Overall, this manuscript demonstrates a high standard of writing and presents noteworthy results. That said, attending to the identified issues is a prerequisite for the manuscript to be deemed suitable for publication.
Response: Thanks for your fair and objective evaluation.
Reviewer 4 Report
Comments and Suggestions for AuthorsThe presented manuscript is devoted to a study of a polyoxometalate-based catalyst (containing Co, Mo and V, and denoted as “CoPMoV”) for methylene blue removal via both Fenton-like reaction and catalytic reduction with using NaBH4 as reductive agent. CoPMoV was shown to demonstrate high stability and reusability both in Fenton-like reaction and catalytic reduction, with the latter being much active. The data obtained are interesting, actual and important. To characterize the studied material a set of physicochemical methods such as XRD, IR, SEM, EDS, XPS etc. has been used. The paper is written in quite clear way; however, it needs to pay careful attention the following comments and only then it can be published:
1) The authors should thoroughly reconsider their XPS data. First of all, raw (as-recorded) spectra must be added in Figures 4a-h. Secondly, an interpretation of each peak observed in Fig 4 should be also presented e.g. the authors say that “The two peaks at 398.2 eV and 399.4 eV were derived from N 1s” (page 4, line 143) however there is no information about nitrogen states corresponding to those peaks. The same situation is with C1s, P2p and O1s XPS spectra. Thirdly, it is well known that measurement error in XPS is about ±0.1 eV. Therefore, if the difference in binding energy less than 0.2 eV, such states could not be resolved. Thus, concerning O1s spectra, such decomposition is incorrect. It looks that there are not more than 3 oxygen states. Please, check O1s spectrum deconvolution.
2) Then, figure 4a. in survey XPS spectrum, the peak close to 190 eV should be also signed (as “P2s”); two peaks at ~ 400 eV and 415 eV ascribed as “N1s”, should be signed in survey XPS spectrum as “Mo3p+N1s”; the peak close to 521 eV ascribed as “V2p” should be written as “V2p+O1sAlKα3,4”. OX scales should be called as “Binding energy”.
3) Next, taking into account low content of nitrogen and the fact that N1s and Mo3p3/2 overlaps, the authors should provide the region N1s+Mo3p with their deconvolutions. The next question concerning the N1s spectrum is its deconvolution in two peaks. Taking into account the proposed structure (Figure S1a) it should be only one type of nitrogen atoms, since all of them are equivalent.
4) The similar situation with V2p spectra. Since V2p3/2 and O1sAlKα3,4 overlap, the authors are asked to show how O1sAlKα3,4 was taken into account during V2p spectra deconvolution. Besides, the V2p spectrum is deconvolved into two vanadium states (VIV and VV) with binding energies of V2p3/2 at 516.0 eV and 516.9 eV, respectively. However, in the previous authors’ paper (ref. 12 of the manuscript), where similar system has been studied (CuPMoV), only one vanadium state with binding energy of V2p3/2 at 516.7 eV was assigned to VIV. Why in the case of CoPMoV the V2p is deconvolved into two states, while for CuPMoV – into only one, and why the peak at 516.7 eV was assigned for VIV in case of CuPMoV, while the peak at 516.9 eV was already assigned to VV in case of CoPMoV. Could the author show where VIV and VV are in the proposed structure (Figure S1a). As I can see, all vanadium atoms look equivalent, and therefore V2p spectrum should be also deconvoluted into one state.
5) Concerning Co2p XPS spectrum, it is impossible now to judge about quality of its deconvolution, because raw Co2p XPS spectrum is absent. Nevertheless, the presented spectra could not be assigned to Co2+. Typical binding energy of Co2p3/2 corresponding to Co2+ is about 781.3 eV, and intensive shake-up satellite close to 785 eV should be present. The slight lower binding energy (like in Fig 4h) and the absence of shake-up satellite could inform us about Co3+. The authors referred to ref. 27, when Co2p spectra analyzing. I would recommend to find more reliable deconvolution of Co2p spectra as a reference. For example, ref. 16 in the manuscript is more suitable and reliable for Co2p spectra deconvolution and can be used by the authors here too.
6) Taking into account that there is a O1sAlKα3,4 one can conclude that non-monochromatized X-ray radiation has been used. This information should be added to the experimental part. Moreover, the authors are kindly asked to add information how XPS spectra were calibrated.
7) XPS is a powerful technique for surface composition determination. However, there is no information neither about atomic concentration (Co in at. %, V in at. % etc.) nor atomic ratios (Co/Mo, V/Co, C/O, N/Co etc.) typically used in XPS. Please, provide this information and compare it with EDS. The information how surface composition was calculated should be then also added to the Experimental part.
8) It is not clear from the manuscript about role of cobalt. This has been not discussed at all in the manuscript. In previous published authors’ paper, CuPMoV (ref. 12) has been studied. It was shown that Cu+ was participated in •OH and •OH2 radicals production. If cobalt does the same, it should be written in 3.2.2. part. Taking into account ref. 12, it can be concluded that CuPMoV is a more active system compared to CoPMoV one both in Fenton-like reaction and in catalytic reduction. This also should be discussed in the manuscript as well as such possible differences in their behavior. Moreover, in Introduction, it is not clear why exactly cobalt was chosen in the current manuscript.
9) It is not also clear about role of different oxygen, nitrogen and carbon species that were observed from XPS. This has been also not discussed at all in the manuscript.
10) Figure captions in Supporting information should be put directly under each figure like it the main part. It would make the Supporting information to be clearer and more reader-friendly.
11) An abbreviation of energy dispersive X-ray spectroscopy is given as EDX in Abstract and SI, while it is given as EDS in the main text of the manuscript. Please, use only one throughout the manuscript.
12) In Fig 1a and 1b, sizes of the measured objects are in (mm), while on the page 3, line 117 it says that “CoPMoV displayed a bulk morphology with a size range of 0.51 nm - 2.14 nm”. Please correct this and add scales in Fig 1a and Fig 1b.
13) Page 6, line 191. “Generally speaking, reactive species (ROS) represented…”. It looks that “oxygen” is missing between “reactive” and “species”.
14) Equation 2. There is a mistake in the right part. It should be “•OH + OH–”.
15) What confidence interval for K is? It should be added to the manuscript.
16) The authors say that “The possible interaction between Mo and V might contribute to the redox cycles and thus boost the degradation rate” (page 7, line 214). Taking into account the findings in 3.2.1 it can be concluded that •OH radicals are more active in comparison with •OH2. Eq. 6 informs us that Mo6+ interacts with V4+ resulting in Mo5+ and V5+. V5+ could produce •OH2 transforming into V4+ (Eq. 3). V4+ could produce •OH transforming into V5+ (Eq. 2). The same situation is with Mo5+ and Mo6+ (Eqs. 4-5). Therefore, the benefits of interaction between Mo6+ and V4+ are not obvious and the statement aforementioned above should be explained more clearly and in more detail.
17) There are some typos, e.g. page 8, line 220, “So, the the repeated…”; page 9, line 254 “LC-anlysis”; page 2, lines 91-92 “energy dispersive X-ray spectrometer (Oxford, X-Act) energy dispersive spectroscopy (EDS)”.
18) What accuracy of EDS data presented in Fig 1c is? Please, round percentage correctly.
19) Information in parts of “Supplementary Materials”, “Author Contributions”, “Funding”, “Data Availability Statement” and “Conflicts of Interest” are absent.
20) MB abbreviation given in the Title would be better to exclude providing full name as “methylene blue”.
Author Response
Reviewer 4
The presented manuscript is devoted to a study of a polyoxometalate-based catalyst (containing Co, Mo and V, and denoted as “CoPMoV”) for methylene blue removal via both Fenton-like reaction and catalytic reduction with using NaBH4 as reductive agent. CoPMoV was shown to demonstrate high stability and reusability both in Fenton-like reaction and catalytic reduction, with the latter being much active. The data obtained are interesting, actual and important. To characterize the studied material a set of physicochemical methods such as XRD, IR, SEM, EDS, XPS etc. has been used. The paper is written in quite clear way; however, it needs to pay careful attention the following comments and only then it can be published:
Comment 1)The authors should thoroughly reconsider their XPS data. First of all, raw (as-recorded) spectra must be added in Figures 4a-h. Secondly, an interpretation of each peak observed in Fig 4 should be also presented e.g. the authors say that “The two peaks at 398.2 eV and 399.4 eV were derived from N 1s” (page 4, line 143) however there is no information about nitrogen states corresponding to those peaks. The same situation is with C1s, P2p and O1s XPS spectra. Thirdly, it is well known that measurement error in XPS is about ±0.1 eV. Therefore, if the difference in binding energy less than 0.2 eV, such states could not be resolved. Thus, concerning O1s spectra, such decomposition is incorrect. It looks that there are not more than 3 oxygen states. Please, check O1s spectrum deconvolution.
Response: Thank you for your suggestion. We have revised the XPS section, reprocessed the XPS data for C, N, O, P, Mo, V, and Co elements, and reinterpreted the peak information.
Comment 2)Then, figure 4a. in survey XPS spectrum, the peak close to 190 eV should be also signed (as “P2s”); two peaks at ~ 400 eV and 415 eV ascribed as “N1s”, should be signed in survey XPS spectrum as “Mo3p+N1s”; the peak close to 521 eV ascribed as “V2p” should be written as “V2p+O1sAlKα3,4”. OX scales should be called as “Binding energy”.
Response: Thank you for your suggestion. We have added P 2s, Mo3p + N1s, and V2p + O1s to Figs. and made the corresponding modifications.
Comment 3) Next, taking into account low content of nitrogen and the fact that N1s and Mo3p3/2 overlaps, the authors should provide the region N1s+Mo3p with their deconvolutions. The next question concerning the N1s spectrum is its deconvolution in two peaks. Taking into account the proposed structure (Figure S1a) it should be only one type of nitrogen atoms, since all of them are equivalent.
Response: Thank you for your suggestion. We have made the necessary revisions to this section in our manuscript and reconsidered the N1s peak information.
Comment 4) The similar situation with V2p spectra. Since V2p3/2 and O1sAlKα3,4 overlap, the authors are asked to show how O1sAlKα3,4 was taken into account during V2p spectra deconvolution. Besides, the V2p spectrum is deconvolved into two vanadium states (VIV and VV) with binding energies of V2p3/2 at 516.0 eV and 516.9 eV, respectively. However, in the previous authors’ paper (ref. 12 of the manuscript), where similar system has been studied (CuPMoV), only one vanadium state with binding energy of V2p3/2 at 516.7 eV was assigned to VIV.Why in the case of CoPMoV the V2p is deconvolved into two states, while for CuPMoV – into only one, and why the peak at 516.7 eV was assigned for VIV in case of CuPMoV, while the peak at 516.9 eV was already assigned to VV in case of CoPMoV. Could the author show where VIV and VV are in the proposed structure (Figure S1a). As I can see, all vanadium atoms look equivalent, and therefore V2p spectrum should be also deconvoluted into one state.
Response: In the XPS spectra of the CoPMoV material, the binding energy range for the V 2p XPS spectrum was between 512 and 526 eV, while the effective range for the O 1s binding energy is between 527 eV and 536 eV. This difference effectively prevents overlap between the V 2p and O 1s XPS peaks, as illustrated in Fig.s 5(d) and 5(g).
The presence of VIV and VV was obtained by calculatation with bond valence sums (BVS) using parameters given by Brown. So, it is not reasonable to specify the valence states of vanadium.
- Brown, in Structure and Bonding in Crystals, ed. M. O’keeffe and A. Navrotsky, vol. 2, Academic Press, New York, 1981, p. 1.
The difference in the analysis of CoPMoV and CuPMoV was caused by the utilization of different references.
Comment 5) Concerning Co2p XPS spectrum, it is impossible now to judge about quality of its deconvolution, because raw Co2p XPS spectrum is absent. Nevertheless, the presented spectra could not be assigned to Co2+. Typical binding energy of Co2p3/2 corresponding to Co2+is about 781.3 eV, and intensive shake-up satellite close to 785 eV should be present. The slight lower binding energy (like in Fig 4h) and the absence of shake-up satellite could inform us about Co3+. The authors referred to ref. 27, when Co2p spectra analyzing. I would recommend to find more reliable deconvolution of Co2p spectra as a reference. For example, ref. 16 in the manuscript is more suitable and reliable for Co2p spectra deconvolution and can be used by the authors here too.
Response: Thank you for your suggestion. We have reconsidered the XPS spectra of Co 2p and reassigned the peaks accordingly. We have made the corresponding changes in the manuscript.
Comment 6) Taking into account that there is a O1sAlKα3,4 one can conclude that non-monochromatized X-ray radiation has been used. This information should be added to the experimental part. Moreover, the authors are kindly asked to add information how XPS spectra were calibrated.
Response: We have made the necessary additions to the manuscript. The XPS spectra were calibrated based on the C 1s peak at 284.8 eV, and this information has also been included in the revised manuscript.
Comment 7) XPS is a powerful technique for surface composition determination. However, there is no information neither about atomic concentration (Co in at. %, V in at. % etc.) nor atomic ratios (Co/Mo, V/Co, C/O, N/Co etc.) typically used in XPS. Please, provide this information and compare it with EDS. The information how surface composition was calculated should be then also added to the Experimental part.
Response: Thank you for your suggestion. By using Avantage software to analyze the fine spectra of each element, we obtained the atomic ratios of the elements in the material and have included this information in the supplementary materials. The elemental ratios obtained through XPS analysis differ from those obtained by EDS, which may be due to the fact that both methods only provide relative elemental content in the material.
Comment 8) It is not clear from the manuscript about role of cobalt. This has been not discussed at all in the manuscript. In previous published authors’ paper, CuPMoV (ref. 12) has been studied. It was shown that Cu+ was participated in •OH and •OH2 radicals production. If cobalt does the same, it should be written in 3.2.2. part. Taking into account ref. 12, it can be concluded that CuPMoV is a more active system compared to CoPMoV one both in Fenton-like reaction and in catalytic reduction. This also should be discussed in the manuscript as well as such possible differences in their behavior. Moreover, in Introduction, it is not clear why exactly cobalt was chosen in the current manuscript.
Response: Yes, this is a good comment. The corresponding Eqs. were added. The reson to choose cobalt was discussed in introduction.
The structure of POM anions in CoPMoV and CuPMoV are [PMoVI8VIV4VV2O42]5- and [PMoVI8VIV6O42]7-, respectively. The latter one was more reduced which affected the redox property of CuPMoV. We discussed it in conclusion.
Comment 9) It is not also clear about role of different oxygen, nitrogen and carbon species that were observed from XPS. This has been also not discussed at all in the manuscript.
Response: The oxygen, nitrogen and carbon species were belonged to the organic ligand which coordinated with the POM anion and Co ion. Please refer to Fig.1.
Comment 10) Figure captions in Supporting information should be put directly under each figure like it the main part. It would make the Supporting information to be clearer and more reader-friendly.
Response: OK, we have revised according to your suggestion.
Comment 11) An abbreviation of energy dispersive X-ray spectroscopy is given as EDX in Abstract and SI, while it is given as EDS in the main text of the manuscript. Please, use only one throughout the manuscript.
Response: We have unified the writing style.
Comment 12) In Fig 1a and 1b, sizes of the measured objects are in (mm), while on the page 3, line 117 it says that “CoPMoV displayed a bulk morphology with a size range of 0.51 nm - 2.14 nm”. Please correct this and add scales in Fig 1a and Fig 1b.
Response: We have corrected the scale in Fig 1a and added scale in Fig 1b.
Comment 13) Page 6, line 191.“Generally speaking, reactive species (ROS) represented…”. It looks that “oxygen” is missing between “reactive” and “species”.
Response: We are sorry for the omission. Thanks for your kind comment.
Comment 14) Equation 2. There is a mistake in the right part. It should be “•OH + OH–”.
Response: Yes, they are hydroxyl radical and hydroxide anion, respectively.
|
≡V4+ + H2O2 → ≡V5+ + •OH + OH- |
(2) |
Comment 15) What confidence interval for K is? It should be added to the manuscript.
Response: We have added.
Comment 16) The authors say that “The possible interaction between Mo and V might contribute to the redox cycles and thus boost the degradation rate” (page 7, line 214). Taking into account the findings in 3.2.1 it can be concluded that •OH radicals are more active in comparison with •OH2. Eq. 6 informs us that Mo6+ interacts with V4+ resulting in Mo5+ and V5+. V5+could produce •OH2transforming into V4+ (Eq. 3). V4+ could produce •OH transforming into V5+ (Eq. 2). The same situation is with Mo5+ and Mo6+ (Eqs. 4-5). Therefore, the benefits of interaction between Mo6+ and V4+ are not obvious and the statement aforementioned above should be explained more clearly and in more detail.
Response:Thanks for the useful suggestion. We have changed the explanation.
Comment 17) There are some typos, e.g. page 8, line 220, “So, the the repeated…”; page 9, line 254“LC-anlysis”; page 2, lines 91-92“energy dispersive X-ray spectrometer (Oxford, X-Act) energy dispersive spectroscopy (EDS)”.
Response: We are very sorry for the mistakes. They are correct now.
Comment 18) What accuracy of EDS data presented in Fig 1c is? Please, round percentage correctly.
Response: We have revised the writing.
Comment 19) Information in parts of “Supplementary Materials”, “Author Contributions”, “Funding”, “Data Availability Statement” and “Conflicts of Interest” are absent.
Response: Thanks! We have added the “Supplementary Materials”, “Author Contributions”, “Funding”, “Data Availability Statement” and “Conflicts of Interest”.
Comment 20) MB abbreviation given in the Title would be better to exclude providing full name as “methylene blue”.
Response: We have changed MB with methylene blue.
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsI have carefully reviewed the revised version of the manuscript. I focused specifically on the points I raised in my review.
The authors have successfully addressed my concerns. These revisions have enhanced the overall quality and clarity of the manuscript. I am satisfied with the author's response to my comments and recommend that the manuscript be accepted for publication in Catalysts.
Author Response
We would like thank you for your professional review work. Your time and efforts are greatly appreciated.
Reviewer 4 Report
Comments and Suggestions for AuthorsI thank the authors for their answers. The manuscript has been significantly improved. I have some more small comments and questions before it can be accepted for publication:
11) The authors claim that “In addition, the elemental ratios obtained through XPS analysis was different with the results of EDX (Table S1), which was caused by the fact that the two methods only provide relative elemental content in the material” (page 9). One of the true reasons for that is different dimension of presented values: XPS (Table S1) is presented in atomic %, whereas EDX is presented in weight %. If the authors want to compare them it should be recalculated and presented unified (everywhere in atomic % or everywhere in weight %).
22) The most crucial comment for now is concerning the new N1s XPS spectrum deconvolution. Looking at Survey XPS spectrum (Fig. 5a) it can be noticed that there is a pronounced doublet at ~ 395 eV and ~ 412 eV (∆E ~ 18 eV) corresponding to Mo3p3/2 and Mo3p1/2, respectively. The relative area Mo3p3/2 and Mo3p1/2 is 2:1 (it is also in good agreement with high intensity of Mo3d peak indicating high content of molybdenum). Therefore, a contribution of Mo3p3/2 should be higher than one presented in Fig. 5c. (N1s+Mo3p3/2). If the authors deconvolute N1s+Mo3p3/2+Mo3p1/2 correctly, they will find the data obtained by XPS and EDX are in even better agreement (since amount of nitrogen will be lower). Please, pay attention to N1s+Mo3p3/2+Mo3p1/2 spectra re-deconvolution (Fig. 5c).
33) The OX scale of Figs. 5a-5h should be called as “Binding energy”, not “Bind”, not “Bing”.
Otherwise, I have no more questions.
Author Response
Comments and Suggestions for Authors
I thank the authors for their answers. The manuscript has been significantly improved. I have some more small comments and questions before it can be accepted for publication:
Comment 11) The authors claim that “In addition, the elemental ratios obtained through XPS analysis was different with the results of EDX (Table S1), which was caused by the fact that the two methods only provide relative elemental content in the material” (page 9). One of the true reasons for that is different dimension of presented values: XPS (Table S1) is presented in atomic %, whereas EDX is presented in weight %. If the authors want to compare them it should be recalculated and presented unified (everywhere in atomic % or everywhere in weight %).
Response: Thanks for the suggestion. We have changed the description and made corresponding modifications to Table S1.
Comment 22) The most crucial comment for now is concerning the new N1s XPS spectrum deconvolution. Looking at Survey XPS spectrum (Fig. 5a) it can be noticed that there is a pronounced doublet at ~ 395 eV and ~ 412 eV (∆E ~ 18 eV) corresponding to Mo3p3/2 and Mo3p1/2, respectively. The relative area Mo3p3/2 and Mo3p1/2 is 2:1 (it is also in good agreement with high intensity of Mo3d peak indicating high content of molybdenum). Therefore, a contribution of Mo3p3/2 should be higher than one presented in Fig. 5c. (N1s+Mo3p3/2). If the authors deconvolute N1s+Mo3p3/2+Mo3p1/2 correctly, they will find the data obtained by XPS and EDX are in even better agreement (since amount of nitrogen will be lower). Please, pay attention to N1s+Mo3p3/2+Mo3p1/2 spectra re-deconvolution (Fig. 5c).
Response: Thank you for your suggestion. We have reconsidered the deconvolution of the N 1s+Mo 3p spectra, and based on this, we have also reevaluated the elemental content presented by XPS.
Comment 33) The OX scale of Figs. 5a-5h should be called as “Binding energy”, not “Bind”, not “Bing”.
Response: We are sorry for the mistake and has corrected it.
