A Systematical Comparison of Catalytic Behavior of NM/γ-Al₂O₃ (NM = Ru, Rh, Pt, Pd, Au, Ir) on 1,2-Dichloroethane Oxidation: Distributions of By-Products and Reaction Mechanism

Round 1

Reviewer 1 Report

Manuscript: A comparison study of 1,2-dichloroethane oxidation over γAl2O3 supported noble metals (Ru, Rh, Pt, Pd, Au, Ir) catalysts: Distributions of by-products and reaction mechanism. The work shows the mechanisms of destruction of 1,2-dichloroethane using different noble metals supported on alumina. General questions can be clarified. The paper needs minor revisions before publication, here are some comments:

 

1.       In the XRD, add the planes referring to the Al₂O₃ peaks.

2.       No characterization presents results for the raw alumina support, in this case it would show the differences when the support with noble metals is performed.

3.       As there are already published works using these metals supported on alumina, I suggest highlighting the innovation of this study in relation to what already exists in the literature.

4.       It would be interesting to show the stability of the catalysts, for example in successive cycles or a prolonged reaction cycle.

Comments for author File: Comments.docx

Author Response

A comparison study of 1,2-dichloroethane oxidation over γ-Al₂O₃ supported noble metals (Ru, Rh, Pt, Pd, Au, Ir) catalysts: Distributions of by-products and reaction mechanism. The work shows the mechanisms of destruction of 1,2-dichloroethane using different noble metals supported on alumina. General questions can be clarified. The paper needs minor revisions before publication, here are some comments:

Response: Thank you for your kind work and professional comments.

1. In the XRD, add the planes referring to the Al₂O₃ peaks.

Response: Thank you for the professional suggestion. The initial Al₂O₃ XRD pattern has been posted in Fig. S1 in Supporting Information document, and corresponding description was shown in “3.2. Crystalline and morphology” as “Figure S1 shows the XRD pattern of supporting material γ-Al₂O₃, in which the typical diffraction peaks of 2θ at 32.0°, 37.7°, 45.7°, and 66.6° are well agreed with PDF card # 97-003-9014.”, which were marked in yellow.

2. No characterization presents results for the raw alumina support, in this case it would show the differences when the support with noble metals is performed.

Response: We sincerely thank for your reminder. Some characterization for support material Al₂O₃, such as XRD, H₂-TPR and O₂-TPD are suppled and shown in Fig. S1, and Fig. 6a,b, respectively, along with the figures are their description, which were exhibited in “3.3. Catalyst reducibility, acid properties and oxygen species” as “At first, a H₂-TPR pattern of supporting material γ-Al₂O₃ is shown in Fig. 6a, where no obvious H₂ consumption appeared over γ-Al₂O₃ except a small peak centered at 735 ºC, suggesting a poor reduction property of supporting material γ-Al₂O₃.”, “It is shown that only a weak desorption peak at 503 ºC was emerged for γ-Al₂O₃, which is assigned to the surface lattice oxygen species.” and “Moreover, as compared with support γ-Al₂O₃, all noble metal catalysts showed an obvious increasing desorption amount during the temperature region of 300–700 ºC, as well as a significant low-temperature shift of oxygen desorption, suggesting an promotion of active oxygen and mobility.”, which were highlighted in yellow & blue. Additionally, the activity of 1,2-DCE on γ- Al₂O₃ was also illustrated in Fig. S2, as well as the description was replenished in “3.1 Catalyst activity performance” as “By the way, the catalytic ability of γ-Al₂O₃ was tested and illustrated in Fig. S2, where a 90% of 1,2-DCE conversion occurred at 419 ºC, the efficiency is far lower than that of noble metal supporting catalysts, indicating the enhancement of oxidation ability is mainly ascribed to the participation of noble metals.” and marked in yellow.

3. As there are already published works using these metals supported on alumina, I suggest highlighting the innovation of this study in relation to what already exists in the literature.

Response: Thank you. Noble metal materials, as the most proposed catalysts for industrial application for years, are still the superior candidate in chlorine-containing VOCs catalytic oxidation, such as 1,2-dichloroethane. Due to high cost on noble metals, the strategy of supporting on a low-cost material is the common way to reduce the cost and make it more efficiency of active phase by dispersing well. Interestingly, as a widely-used support material, γ-Al₂O₃ is low-cost and environmentally friendly, which is in accordance with universal application and commercial operation. As a result, noble metal supported on γ-Al₂O₃ is more practical than other types of catalysts. The main purpose of this article is to discuss the different catalytic behavior of typical noble metal-based catalysts systematically with a comparison investigation on byproducts formation, which was not studied as an overall six kinds of noble metals in the same reaction conditions. Hopefully, a positive and practical view on better selection of operable candidate of noble metal-based catalysts should be concluded by a systematical study. To better illustrate this point more clearly, a supplement explanation was posed in the introduction section as “As a widely-used support material, γ-Al₂O₃ is low-cost and environmentally friendly, which is in accordance with universal application and commercial operation. As a result, noble metal supported on γ-Al₂O₃ is more practical than other types of catalysts and should be systematically discussed on various noble metal-based catalysts.” and highlighted in yellow.  

4. It would be interesting to show the stability of the catalysts, for example in successive cycles or a prolonged reaction cycle.

Response: Thank you for the suggestion. The stability tests of NM/Al₂O₃ were operated and illustrated in Fig. 2, with supplementary description in the end of section “3.1. Catalyst activity performance” and marked in yellow.

Reviewer 2 Report

see comments

Comments for author File: Comments.docx

Author Response

This manuscript presents the oxidation of DCM on noble metals supported on Al₂O₃ and several characterizations to explain the results and propose a reaction mechanism. As seen the activity was determined for T₅₀ and T₉₀ and the Ru/ Al₂O₃ was the most active sample. Besides, the selectivity was also determined. The characterizations showed the influence of acidity, oxygen in the lattice that are the most important results to explain the proposed mechanism. The article is well presented; however, there are some questions in following comments:

Response: Thank you for your positive evaluation and professional comments.

1. The activity of the different catalysts for T₅₀ indicates that they depend on the dispersion of the metal and not on the support Al₂O₃. Authors didn’t measured the dispersion, needing reduction and chemisorption results for determining the TOF value as function of the metallic sites. The conversion alone and the rate determined are not sufficient to explain the activity. Moreover, the selectivity in figures 1c and 1d, indicate total oxidation by forming CO₂, which apparently is high, but why CO for Pd and Ir present maximum at about 300 ºC? and how authors explain the formation of CO. On the other hand, the selectivity or distribution of different compounds, which are the most important results for this system showed the highest yield for C₂HCl₃ for Ir and Au and Ru the maximum CO₂. Thus from this results it seems that the Au and Ir are the most important products and Ru only for total oxidation? Later authors showed through TPD of NH₃ that the acidities are medium and strong and indicate B and L sites, depending on the temperature range. The TPD of NH₃ determines the total acidity, but specifically for the Al₂O₃ support, but not due to the presence of metal. Therefore the acidity may influence significantly the distribution of products, as commented before.

Response: Thank you very much for the professional suggestion.

Q1: The dispersion of noble metal of prepared catalysts has been measured by using a pulsed CO chemisorption technique, the experiment process is added and detailed in Supporting Information and marked in blue. As a result, each dispersion (%) of NM on NM-based catalyst was calculated and listed in Table 2, which is also highlighted in blue.

Q2: The turnover frequency (TOF) was also supplemented in the article, where the equation was added in “2.2. Reactivity evaluation”, as well as the final results were calculated and tabulated in Table 1 (including conversion efficiency and E_a values), finally, the TOF description was replenished in “3.1. Catalyst activity performance” as “At the same time, TOF were also estimated at low conversion efficiency range (took 200 °C as representation) and listed in Table 1. Obviously, the TOF value of NM/Al₂O₃ catalysts for 1,2-DCE oxidation at 200 °C is ranked as: Ir/Al₂O₃ > Pd/Al₂O₃ > Ru/Al₂O₃ > Rh/Al₂O₃ > Au/Al₂O₃ > Pt/Al₂O₃, indicating the superior low-temperature activity of Ir and Pd-based catalyst and poorest performance of Pt-based sample. However, a slightly different trend is found with that of E_a, where Ru and Rh-based catalyst exhibited higher TOF value compared to that of Au, which is opposite to E_a tendency, suggesting the intrinsic potential activity of Ru/Al₂O₃ and Rh/Al₂O₃ on 1,2-DCE oxidation reaction [20].”, which is highlighted in blue.

Q3: Actually, the CO₂ yield for all NM/Al₂O₃ catalysts was evaluated under 400 ºC, where Ru, Rh, Pt and Pd based catalysts can achieve over 90% of CO₂ formation at 400 ºC that has been mentioned in the article. Commonly, the CO yield for various catalysts exhibits increasing with reaction temperature and then decreasing at higher temperature range, in this work, the highest CO yield for Ir/Al₂O₃ is 34.22% at 370 ºC, as well as CO₂ yield is just 37.21% at the same reaction temperature, meaning that the total CO_x yield for Ir/Al₂O₃ is just 71.43% at 370 ºC. Another catalyst Pd/ Al₂O₃, the maximum CO yield occurs at 345 ºC with 23.34%, along with the CO₂ yield of 39.23% at the same temperature, the total CO_x yield is 62.57% at the maximum CO formation condition of 345 ºC. In summary, the highest CO yield condition does not mean a superior mineralization efficiency. A higher CO formation is still mainly ascribed to a poor oxidation capacity, especially deep oxidation capacity. The similar CO yield trend can be observed by other studies, for instance, Di et al. [1] found that the maximum CO yield was observed at 250-300 ºC by 1,2-DCE oxidation with different loading of VO_x/CeO₂ catalysts, which was decreased gradually with the increasing reaction temperature. Zhang et al. [2] prepared a SO₄^2-/Fe₂O₃ catalyst for dichloromethane oxidation, and a higher CO selectivity was found in the range of 200-350 ºC, still it was sharply decreased with continuously elevated the reaction temperature.

Q4: About the organic byproduct distribution, thank you for reminder. This section should be re-concluded as “Along with the analysis of CO_x yield tendency, it is observed that although Au and Ir-based catalyst obtained a higher 1,2-DEC conversion efficiency at lower temperature, while the conversion just stops at “degradation” process over higher temperature, with higher organic compounds generation and lower mineralization efficiency. In contrast, a superior “destruction” ability was found for Ru/γ-Al₂O₃ that maximum emergency of organic byproducts at 300 ºC and then decreased significantly with increasing temperature, which shows the lowest organic generation over its T₉₀ reaction condition compared with that of other catalysts.” to reveal the advantage of Ru-based catalyst and point out a conversion ability does not mean destruction capacity, which is already highlighted in blue in corresponding area of “3.1. Catalyst activity performance”.

Q5: The NH₃-TPD measurement of supporting material Al₂O₃ has been replenished and illustrated in Fig. 7c. The corresponding description was also added in section “3.3. Catalyst reducibility, acid properties and oxygen species” as “For supporting material γ-Al₂O₃, only a significant peak at 506 ºC was observed, indicating most acidity of γ-Al₂O₃ belongs to strong acidity.”, which is highlighted in blue. At the same time, the acid properties description are modified and revised, as “After supported NM, the desorption over strong acidity range of Pt/γ-Al₂O₃ was strengthened significantly, Au, Ir and Pd-based material were also increased extensively. Interestingly, the desorption peaks over moderate and weak acid range were emerged in all NM/γ-Al₂O₃ catalysts, especially the occurrence of moderate acidity, suggesting a possible existing of Brønsted acid sites, where Ru/γ-Al₂O₃ was observed more moderate acid emergency than that of other samples. Strong acid sites generally refer to Lewis acid sites [41], 1,2-DCE usually is easily trapped on the Lewis acid sites, causing the dehydrochlorination effect and inducing the formation of the main by-product (vinyl chloride). On the other hand, the Brønsted acid sites can efficiently promote the chlorine species desorption from the catalysts surface, further relieving the chlorination effect during 1,2-DCE degradation as well as hindering the formation of chlorine byproducts [26]. As a result, the intensive of Brønsted acid sites and suitable amount of Lewis acid sites may all contribute to the excellent performance of Ru/γ-Al₂O₃ on 1,2-DCE destruction.” and also marked in blue.

References:

1      Dai Q.; Bai S.; Li H.; Liu W.; Wang X.; Lu G. Catalytic total oxidation of 1,2-dichloroethane over highly dispersed vanadia supported on CeO₂ nanobelts. App. Catal. B: Environ., 2015, 168-169, 141-155.

2      Zhang Z.; Huang J.; Xia H.; Dai Q.; Gu Y.; Lao Y.; Wang X. Chlorinated volatile organic compound oxidation over SO₄^2-/Fe₂O₃ catalysts. J. Catal. 2018, 360, 277-289.

 

2. The TPO and XPS results presented the oxygen in the lattice O_α (529 ev) and adsorption O_β (531eV), that authors attribute to the oxidation of DCM. However, the Al₂O₃ support does not present vacancies. Where are located the lattice oxygen? Is it because of the metals that provoke defects at the surface? Authors didn’t discuss or give any reference to explain?

Response: Thank you for the professional comment. The O₂-TPD of Al₂O₃ was supplemented in Fig. 7b, and the corresponding description has been added in “3.3. Catalyst reducibility, acid properties and oxygen species” as “It is shown that only a weak desorption peak at 503 ºC was emerged for γ-Al₂O₃, which is assigned to the surface lattice oxygen species.” and “Moreover, as compared with support γ-Al₂O₃, all noble metal catalysts showed an obvious increasing desorption amount during the temperature region of 300–700 ºC, as well as a significant low-temperature shift of oxygen desorption, suggesting an promotion of active oxygen and mobility.”, which were highlighted in yellow & blue. The supplementary content is trying to explain the oxygen species of original supporting material γ-Al₂O₃, and the result shows the main oxygen species of γ-Al₂O₃ is surface lattice oxygen, no descriptions of “the Al₂O₃ support does not present vacancies” appears in the article. In summary, the lattice oxygen is still an important species for Al₂O₃, which gets stronger and more active after noble metal participation.

3. DRIFTS results showed interesting results and one is the formation of acetate. It was not observed in the proposed mechanism. The acetate is an intermediate compound that authors can explore to explain the reaction mechanism.

Response: Thank you. Actually, the intermediate acetate has been mentioned in section “3.5. Catalyst intermediate species and oxidation mechanisms”, the second sentence at the second paragraph, as “Path 1: 1,2-DCE occurs dehydrochlorination reaction to produce vinyl chloride, vinyl chloride reacts with H· to generate carbon dioxide positive ion, carbon dioxide positive ion reacts with O^2- to create acetate”, where the intermediate formation of acetate has been referred.

Reviewer 3 Report

Dear Editor

Thank you for your e-mail and for choosing me as a reviewer for your valuable journal.

I do not agree with the authors about the suitability of the work for publication in your journal. My opinion is based on the lack of novelty.

There are many reports in different sources that have done similar exercises and it is not clear how exactly the present plan differs from the twenty articles published in this field.

Examples: 

Journal of Catalysis Volume 291, July 2012, Pages 104-109

Journal of Environmental Sciences Volume 29, 1 March 2015, Pages 199-209

ACS Omega 2018, 3, 8, 8460–8470

The titles used are not suitable for catalysts. Because the prepared catalysts are not supported metals on Alumina and in my opinion transition metals are doped in the alumina structure.

The title of the work is wrong and it is not clear which is the main part and which is the second part.

Therefore, the work is returned to the authors.

Author Response

1. There are many reports in different sources that have done similar exercises and it is not clear how exactly the present plan differs from the twenty articles published in this field. Examples:

Journal of Catalysis Volume 291, July 2012, Pages 104-109

Journal of Environmental Sciences Volume 29, 1 March 2015, Pages 199-209

ACS Omega 2018, 3, 8, 8460–8470

Response: Thank you for the comment. Noble metal materials, as the most proposed catalysts for industrial application for years, are still the superior candidate in chlorine-containing VOCs catalytic oxidation, such as 1,2-dichloroethane. Due to high cost on noble metals, the strategy of supporting on a low-cost material is the common way to reduce the cost and make it more efficiency of active phase by dispersing well. Interestingly, as a widely-used support material, γ-Al₂O₃ is low-cost and environmentally friendly, which is in accordance with universal application and commercial operation. As a result, noble metal supported on γ-Al₂O₃ is more practical than other types of catalysts. The main purpose of this article is to discuss the different catalytic behavior of typical noble metal-based catalysts systematically with a comparison investigation on byproducts formation, which was not studied as an overall six kinds of noble metals in the same reaction conditions. Hopefully, a positive and practical view on better selection of operable candidate of noble metal-based catalysts should be concluded by a systematical study. To better illustrate this point more clearly, a supplement explanation was posed in the introduction section as “As a widely-used support material, γ-Al₂O₃ is low-cost and environmentally friendly, which is in accordance with universal application and commercial operation. As a result, noble metal supported on γ-Al₂O₃ is more practical than other types of catalysts and should be systematically discussed on various noble metal-based catalysts.” and highlighted in yellow.

On the other hand, a universal application of catalyst should be developed for various organic gas stream purification, a simple, environmentally friendly synthesis method is necessary for practical usage. Some of the preparation method mentioned above experienced complex preparation process, or required participation of additional metal cation, some of the doping cations are toxic or expensive, such as Cr, V, et al. By the way, the status of noble metals in our study are focused on zero-valence metal particles, the catalyst Ru/Ce_xAl_y designed by Gu et al. is paid attention on its oxide status, which is RuO₂ phase.

In conclusion, the study of this manuscript is paid more attention on the universal application comparison, which needs to lower the cost, simplifies the synthesis operation, more environmental and economic process, et al. To more clearly representing our point, the description of “For universal application, the synthesis routine should be simple, economic, and environmentally friendly. On the other hand, at present, only the reaction mechanism of specific pollutants on specific catalysts has been explored, but a universal reaction mechanism on various noble metal catalysts should be investigated to find out whether there is a common route for CVOCs oxidation on noble metal materials.” was added in the “1. Introduction” section and marked in green color.

2. The titles used are not suitable for catalysts. Because the prepared catalysts are not supported metals on Alumina and in my opinion transition metals are doped in the alumina structure.

Response: Thank you for the suggestion. Due to the γ-Al₂O₃ applied in the synthesis process was a commercial material that was purchased from Sigma-Aldrich (USA), which was already a metal oxide status from the beginning of catalytic synthesis. So, the noble metal precursors were deposited on the surface of γ-Al₂O₃ by reducing agents such as NaBH₄ and hydrazine hydrate, which is also a kind of supporting process due to the structure of was hardly changed during reducing and calcination operations.

3. The title of the work is wrong and it is not clear which is the main part and which is the second part.

Response: Thank you for the suggestion. The title of this manuscription has been revised into “A systematical comparison of catalytic behavior of NM/γ-Al₂O₃ (NM = Ru, Rh, Pt, Pd, Au, Ir) on 1,2-dichloroethane oxidation: Distributions of by-products and reaction mechanism” and highlighted in green, hope it could more suitable to generalize the purpose and viewpoint of the study.

Round 2

Reviewer 2 Report

Comments for author File: Comments.docx

Author Response

Reviewer #2

This work presents the effect of V and Sr supported on NbAl mixed oxides on the ODH reaction augmenting the propene selectivity, where V-based catalysts favored the H–C bonds of adsorbed propane which is activated by the V–O bonds to produce propyl radicals and OH groups (1). The addition of V and Sr were obtained by coimpregnation and successive impregnation. Authors state that the V and Sr affect the surface acid-base properties and conclude that the 10Sr-4V-10NbAl - SI catalyst presented the highest selectivity. 

My comments:

1. The catalysts were prepared with 5 and 10% of Nb and Sr by Coimpregnation (COI)and successive impregnation (SI). The XRD results presented the respective oxides even for low contents, which is not expected to detect low contents. Can authors determine quantitatively by FRX the real contents?
2. The surface areas of the base samples yNbAl are very high around 275 m²/g (Table 1) while the promoted with Sr and Nb content for both methods decayed more than a half, which indicated blocking effect or large Sr and Nb crystallites, not observed even through the SEM images. Please explain?
3. Figure 2 shows the TPR profiles of the of the COI and SI catalysts, which are very similar. However, authors state that “The addition of alkaline earth metals shifted these peaks to higher temperatures, which indicated a decrease in reducibility”  and interaction “acidic V sites and basic alkaline-earth metals can hinder V reduction”, which is not proven. Authors should determine the degree of reduction of V and Sr oxides and compare with the theoretical value.
4. The Raman spectra showed the presence of V=O bond in isolated monovanadate/polyvanadate surface species and Nb18V4O55, which proves the active sites of V and the other species are inactive. In fact, they can affect the surface acid-basic sites. Please discuss.
5. Table 2 presents the decomposition of isopropanol at 300 ⁰C for identifying the acid sites propene and basic sites acetone. In fact, they have high acidity and low basicity, however, the acidity falls significantly with the Sr and Nb content of the SI sample compared to the COI. Why? In fact, comparing with the SA it seems that the Sr and Nb particles are great and blocking the acid sites. Is it possible to correlate later with the ODH rates? Please discuss and compare with the literature data.
6. Figures 8-10 display the conversion of propane and selectivity of propene. In fact, the selectivity of propene in Fig.10 is relatively higher for the SI samples, decreasing with higher temperature, which indicates deactivation. To prove its deactivation authors can determine the acidity using the spent catalyst, or TGA if carbon was formed.

Response: Thank you for the comment. However, it looks like some mistake on the reviewer suggestions due to the theme is not related to our manuscript. The main theme and content of our study is about noble metal catalysts and their performance on 1,2-dichloroethane oxidation, not related to V, Sr supported on NbAl material and ODH reaction.

Reviewer 3 Report

Dear Editor

Thank you for your e-mails and for choosing me as a reviewer.

As the authors commented on my previous comments, the manuscript could be having a chance to be published.

"There are many reports in different sources that have done similar exercises and it is not clear how exactly the present plan differs from the articles published in this field. " Authors should perform a comparison table between their obtained results and the reported literature results.

 Some  factors should be compared:

1) Yield/Time of the selected reactions

2) Reaction conditions

3) Catalysts

4) Methods for the preparation of the catalysts

5)Methods for performing the reactions.

Please note that similar reports that use noble metals supported or combined with Alumina should be noticed in the comparison tables.

Updated references are mandatory according to the reported literature in comparison tables.

I hope your life is beautiful and your hearts beat warmly.

I hope you always feel the merciful God by your side and take comfort in his memory.

Indeed, it is with the remembrance of God that hearts find peace.

With Kind Regards

 

 

Author Response

As the authors commented on my previous comments, the manuscript could be having a chance to be published.

"There are many reports in different sources that have done similar exercises and it is not clear how exactly the present plan differs from the articles published in this field. " Authors should perform a comparison table between their obtained results and the reported literature results.

 Some  factors should be compared:

1) Yield/Time of the selected reactions

2) Reaction conditions

3) Catalysts

4) Methods for the preparation of the catalysts

5)Methods for performing the reactions.

Please note that similar reports that use noble metals supported or combined with Alumina should be noticed in the comparison tables.

Updated references are mandatory according to the reported literature in comparison tables.

I hope your life is beautiful and your hearts beat warmly.

I hope you always feel the merciful God by your side and take comfort in his memory.

Indeed, it is with the remembrance of God that hearts find peace.

Response: Thank you for the professional suggestion. A comparison summary of previous works about various noble metal-based catalysts for CVOC oxidation were collected and listed in Table S1 of Supplementary Information, which was highlighted in yellow color. Additionally, a description of “In fact, many reports have recently pointed out the catalytic performance of noble metal-based catalysts in combustion of CVOCs can be improved by introducing different supports, as summarized in Table S1 of Supplementary Information.” was still added in section “1. Introduction”, as well as “On the other hand, a simple synthesis method of NaBH₄ and hydrazine hydrate reduction could create superior NM supported catalysts compared with most of other kind of samples, as shown in Table S1.” in section “3.1. Catalyst activity performance”.

The supporting information has been attached to this message.

 

Author Response File: Author Response.docx