Synthesis of Palladium and Copper Nanoparticles Supported on TiO₂ for Oxidation Solvent-Free Aerobic Oxidation of Benzyl Alcohol

Round 1

Reviewer 1 Report

I do not recommend the publication of this manuscript as many characterization and studies are lacking, some of them are summarized below:

- Strong oxidizing agents such as potassium permanganate and dichromate are used in industries to produce benzaldehyde due to the feasibility of controlling the reaction conditions with high yield: In which industries? Ref. should be included

- Titanol (TiOH) group is present on the surface of TiO2 which is amphoteric in nature and helps in acid-base equilibrium: How does this property affect your catalytic activity and/or property? What are the advantages? Please elaborate it in the text

- Introduction part is not well-organized. Should be re-written. The reader is left with no conclusion why Pd and Cu are used as the combination. What are the previous studies on this combination, and which supports have been used, What other bimetallic systems have been used for benzyl alcohol oxidation?

- Why TiO2 has been applied? This is mostly the support for photocatalytic reactions. The author should provide more references on the use of TiO2 as support for thermal oxidation reactions.

- Absorption properties of the support mainly depend on the pH value. What author means by absorption? Absorption or adsorption? Is this not related to photoreactions? How does this influence your study?

- In comparison to monometallic catalysts, bimetallic NPs have greater activity and are thus preferentially used in academic research and industries: please provide some examples of this in aerobic alcohol oxidation and explain them in detail.

- Keywords need modification: words nanocatalysis, metal oxide should be removed.

- Based on these previous studies, we have selected bimetallic catalyst Pd-Cu and use support TiO2 benzyl alcohol oxidation in an aqueous solution in the current study: This is not a complete definition of your work: please write a paragraph and explain your work, novelty and achievements.

- The English of the whole manuscript needs correction. There are many grammar and typo errors.

-Many formulas should be written as a subscript.

- 0.1 M, NaBH4/Au, mol/mol = 5: This is not clear, please explain it well in the experimental part

- In the experimental part of the catalysis please mention reaction temp. and time.

- The Pd-Cu/TiO2 catalyst has not been well characterized. FT-IR to show the interaction between particles and support is essential.

- EDX mapping should be provided.

- ICP-OES analysis must be performed to show the exact amount of metals.

- XRD analysis must be presented. The crystalline phase of TiO2 should be mentioned based on XRD as well as the metallic Pd and Cu.

- Based on TEM the author applied materials with different ratios of metals loading. This has not been emphasized so far and even not in the experimental part. Please write this clearly and explain the reason and outcome of using different ratios.

- The peaks in Fig. 2 should be labeled.

- XPS data for the TiO2 should be presented and should be compared with the catalyst and explain any difference due to the interaction with metals.

- References should be inserted in the XPS explanation for all peaks to confirm the correlation with reports.

- A table showing the elemental composition for all peaks and elements from the XPS analyses should be included.

- Author should explain how Pd0 can be obtained and why it has a high amount as high as Pd(II).

- Author should explain how CuO Can be obtained.

- Please remake Fig.3d and use the same fonts as the previous ones.

- The correlation between ICP-OES and XPS on the number of metals should be observed in good accordance with the experimental ratio.

- Caption of Fig. 3 should be modified. For each part, an individual caption is required, (…) XPS Cu2p…, etc.

- Conversion of the blank reaction, as well as the reaction using only titania, were both minima at 120 °C, and hence this temperature was selected as the reaction temperature: This is not a good reason. The reaction would be minimum in other Temps as well ranging from RT to higher Temps. The author should perform this reaction at RT, 50, 80, and 120 and check the activity of the bimetallic system. To my knowledge, Pd catalysts are very active even at RT.

- Based on the text the caption of Fig.4 is not correct and should be monometallic Cu…

- Please present a nice table showing all catalytic activities (Conv. Sel.) of all the conditions and results on blank, TiO2, Pd/TiO2, Cu/TiO2, Pd-Cu/TiO2 with all ratios.

-Increasing Pd content in the monometallic Pd/TiO2 (1wt%) showed less catalytic activity: Looks wrong

- There are many errors in the catalytic parts. The author needs to modify and rewrite it all again and present the data nicely and be understandable.

- an increase in Pd content led to a slight decrease in aldehyde selectivity due to disproportionation reaction, resulting in the formation of toluene, reaching a maximum of approximately 2% with the monometallic 1wt% Pd/TiO2: Please provide GC and GC-MS data for this statement in the SI.

- An interesting observation is that catalysts exhibit a high activity towards Cu-rich compounds till the mol ratio of copper-palladium is 0.25:0.75 Cu:Pd. On the other hand, a slight decrease in activity was observed in Pd-rich compositions, followed by a gradual increase: It is not clear what the author means. Please explain clearly and present the data in a Table.

- The part on the effect of support should be completely removed. The author has not explained how these catalysts are prepared. There is no characterization data on those catalysts and no comparison can be made in this way. If the author would like to investigate this, all characterization data for all catalysts (FT-IR, TEM, XRD, ICP-OES) should be performed. Moreover, the activity should be compared in terms of TON and TOF and not the Conv. and Sel.

- Title of Table 2 is wrong.

- Effect of temperature: There is no point to study these high Temp and no conclusion can be made as to the sel. Towards toluene is low in all cases. This part should be removed and instead a better optimization is needed where lower amounts of catalysts and lower reaction Temp are investigated.

- The catalyst shows unsatisfactory reusability and the reason that the author presented does not look to be probable in this system and there is no benzoic acid detected in this system and the amount of toluene was very low. Possible leaching tests and hot filtration should be performed. The reused catalyst should be characterized by means of ICP-OES, XRD, HRTEM and XPS.

- TON and TOF data should be presented for all catalytic tests.

- No comparison is made with previous bimetallic systems for the same reaction.

- The scope of catalyst activity is very narrow and limited only to benzyl alcohol. Linear and non-aromatic substrates should be investigated.

Author Response

please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

The authors report the preparation of palladium and copper nanoparticles supported on TiO2, and their use as heterogeneous catalysts in the oxidation of alcohols. The topic is of interest of the catalytic community, and the work is well described, and the discussion is supported by their data. Thus, I recommend it for publication after few clarifications: 

1) How does the author's results compare to the homogeneous catalysis used for this type of reaction? 

2) Were the experiments performed at least in triplicate? Can authors add the error bars in the graphs?

3) In section 3.2, could authors calculate and determine the “catalyst activity” of their systems? 

4) In the experimental part, authors only mention "The catalyst was separated by simple filtration". Could authors give more details about the activation of the catalyst for the reusability tests? 

5) In lines 237-238, it's mentioned that "Suitable heat treatments may reverse phase alteration and the adsorption of organic materials", however it's not shown any data or detailed info about the heat treatment to reactivate the catalysts. Can authors clarify it and add data about this statement?

6) Please, check typos such as 'benzyladhyde' in the tables,  '2ed' in Fig 6, etc.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 3 Report

This is a well-written paper which I am happy to recommend for publication in Processes.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The manuscript lacks many characterization data, suitable English and not clear science in some parts. The catalysts are compared only based on Conv. And Selc. and not the TON and TOF which is scientifically not correct. It is highly recommended that the authors request an extension for the time of a revision to fully address the comments. Many comments are left with no answer and modification. Below are the points:

1- Regarding the comment below, the author has not included the explanation in the manuscript. A reference should be included as well. The English need modification

Titanol (TiOH) group is present on the surface of TiO2 which is amphoteric in nature and helps in acid-base equilibrium: How does this property affect your catalytic activity and/or property? What are the advantages? Please elaborate it in the text

2 - Regarding the comment below and the authors' reply, the referee does not see this study in the previous work of the authors. Please bring a short explanation in the manuscript that the reactions are done at lower Temp. (50-100) and cite that work. Otherwise, this experiment must be performed.

“Conversion of the blank reaction, as well as the reaction using only titania, were both minima at 120 °C, and hence this temperature was selected as the reaction temperature: This is not a good reason. The reaction would be minimum in other Temps as well ranging from RT to higher Temps. The author should perform this reaction at RT, 50, 80, and 120 and check the activity of the bimetallic system. To my knowledge, Pd catalysts are very active even at RT.

Response: The choice of reaction temperature based on our previous experience for this type of reaction.“

3- Regarding the comment below, Fig. 4 does not show the selectivity.

“Please present a nice table showing all catalytic activities (Conv. Sel.) of all the conditions and results on blank, TiO2, Pd/TiO2, Cu/TiO2, Pd-Cu/TiO2 with all ratios.”

4- Increasing Pd content in the monometallic Pd/TiO2 (1wt%) showed smaller catalytic activity than the catalyst containing 0.75% Pd: As shown in Fig.4 the monometallic Pd catalyst has even higher catalytic conversion.

5- Regarding the comment below, Table 4 does not present previous bimetallic systems.

“No comparison is made with previous bimetallic systems for the same reaction. Response: Thank you very much for pointing out. A comparative assessment with other works was established in Table 4, in the revised manuscript.”

6- Without ICP measurements, and without calculating TON and TOF, the comparison of different catalysts using various supports is scientifically wrong (Table 3). Moreover, no characterization is performed on those catalysts. This way of comparison is not suitable and must be modified (based on ICP and other characterizations) or removed.

Following comments have not been addressed and responded to in the cover letter:

7- an increase in Pd content led to a slight decrease in aldehyde selectivity due to disproportionation reaction, resulting in the formation of toluene, reaching a maximum of approximately 2% with the monometallic 1wt% Pd/TiO2: Please provide GC and GC-MS data for this statement in the SI.

8-  An interesting observation is that catalysts exhibit a high activity towards Cu-rich compounds till mol ration of copper-palladium is 0.25:0.75 Cu:Pd. On the other hand, a slight decrease in activity was observed in Pd-rich compositions, followed by a gradual increase: It is not clear what the author means. Please explain clearly and present the data in a Table.

9 - The part on the effect of support should be completely removed. The author has not explained how these catalysts are prepared. There is no characterization data on those catalysts and no comparison can be made in this way. If the author would like to investigate this, all characterization data for all catalysts (FT-IR, TEM, XRD, ICP-OES) should be performed. Moreover, the activity should be compared in terms of TON and TOF and not the Conv. and Sel.

10- The part on the effect of support should be completely removed. The author has not explained how these catalysts are prepared. There is no characterization data on those catalysts and no comparison can be made in this way. If the author would like to investigate this, all characterization data for all catalysts (FT-IR, TEM, XRD, ICP-OES) should be performed. Moreover, the activity should be compared in terms of TON and TOF and not the Conv. and Sel.

11- Effect of temperature: There is no point to study these high Temp and no conclusion can be made as to the selectivity. Towards toluene is low in all cases. This part should be removed and instead a better optimization is needed where lower amounts of catalysts and lower reaction Temp are investigated.

12- The catalyst shows unsatisfactory reusability and the reason that the author presented does not look to be probable in this system and there is no benzoic acid detected in this system and the amount of toluene was very low. Possible leaching tests and hot filtration should be performed. The reused catalyst must be characterized by means of ICP-OES, XRD, HRTEM and XPS.

13- TON and TOF data should be presented for all catalytic tests. A catalytic study without TON and TOF has no meaning and does not reflect the activity of a catalyst.

Comments for author File: Comments.pdf

Author Response

please see the attachment

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Can be accepted in the present form