Solvent-Free Oxidation of Benzyl Alcohol Derivatives by In Situ Generated Redox Couple Pd(0)/PdO_x Supported on Ceria Nanorods

Round 1

Reviewer 1 Report

This manuscript concerns the conversion of several benzylic alcohol derivatives over catalysts made from PdO/CeOx. The manuscript is well written and the study is adequately comprehensive. The literature is well cited. The title and other sentences should be made more accurate, I think.

I think materials and methods should be moved higher in the article. I don't know if Catalysts has a guideline that is different, but I woudl expect materials and methods to be the second section (near top of article).


Though the catalyst particles were initially PdO, the active sites are likely Pd(0), as was shown for Ref 23. Accordingly, the title should probably say "Pd catalysts derived from PdOx", or similar wording.

For Figure 5 interpretation, I think the authors should be a bit more careful. Table 3 includes the authors interpretations. It would be better if Table 3 only listed the temperature ranges, and the authors could include their interpretation in the text.

For Table 3, the Ce(IV) to Ce(III) column should make clear the meaning and/or just be in the text. Whether it is the %Ce(IV) compared to the stoichiometric CeO2 or %Ce(IV) relative to Ce(IV) at start of temperature ramp should be made more clear.

For Table 4, the authors wrote "the hindering effect of the group", I think the authors mean "steric hindering effect". If this is what the authors mean, I suggest to add the word hindering.

The authors choice to make further characterization of the products from 4-hydroxybenzyl alcohol are impressive!

I do not know if this statement is correct: "This is observable during the reaction where the catalyst color changes from light yellow to dark grey."  I think the yellow color is from teh cerium oxide. I think probably the sample looks yellow because of cerium oxide (yellow) and white PdO. Then, perhaps as the authors say, Pd becomes reduced then the Pd metal might make the full sample look gray. If the authors see that the CeO2 without Pd is already yellow, the authors should make the description about the color change and interpretation more clear.

The conclusions section is too short. It should be longer and more clear what the conclusions are -- with more details, so the reader can understand more clearly what is usefula bout the study.

I am confused that the conclusions mention the ionic mechanism, but do not mention Scheme 1 mechanism.  The main feedback I have on this work is that the conclusions section should be worked on more, so that it can better emphasize what was learnt about the work. Even if the conclusions section is technically correct, it will be better if the emphasis of the conclusions section matches what the reader should learn from the paper.

I recommend an additional short round of review.

Author Response

Solvent-free oxidation of benzyl alcohol derivatives by in situ generated redox couple Pd(0)/PdO_x supported on Ceria nanorods

Answers to Reviewer 1

Reviewer 1:

This manuscript concerns the conversion of several benzylic alcohol derivatives over catalysts made from PdO/CeOx. The manuscript is well written and the study is adequately comprehensive. The literature is well cited. The title and other sentences should be made more accurate, I think.

I think materials and methods should be moved higher in the article. I don't know if Catalysts has a guideline that is different, but I would expect materials and methods to be the second section (near top of article).

Reply:

We thank the reviewer for this comment. The order of the sections in our manuscript has been set according to the instructions of the MDPI’s Catalysts journal, which rules the “Materials and Methods” section being before the “Conclusions.”

Reviewer 1:

Though the catalyst particles were initially PdO, the active sites are likely Pd(0), as was shown for Ref 23. Accordingly, the title should probably say "Pd catalysts derived from PdOx", or similar wording.

Reply:

We appreciate this suggestion by the reviewer. XPS proved that the palladium was in oxide state before it’s used in the reactions. The XPS also showed that the oxidation state decreased after the catalyst was used. We added the recyclability test of our catalyst and observed that the result of the second cycle was even better than the first one. Therefore, appreciating the interesting comment of yours, we changed the title of our manuscript to “Solvent-free oxidation of benzyl alcohol derivatives by in situ generated redox couple Pd(0)/PdO_x supported on Ceria nanorods.”

Reviewer 1:

For Figure 5 interpretation, I think the authors should be a bit more careful. Table 3 includes the authors interpretations. It would be better if Table 3 only listed the temperature ranges, and the authors could include their interpretation in the text.

For Table 3, the Ce(IV) to Ce(III) column should make clear the meaning and/or just be in the text. Whether it is the %Ce(IV) compared to the stoichiometric CeO2 or %Ce(IV) relative to Ce(IV) at start of temperature ramp should be made more clear.

Reply:

Thank you for the very useful comments. The Ce(III) percentage is relative to the total stoichiometric CeO₂; we have explained it better in the revised text. We have changed Table 3 as follows and we have completely revised the TPR section, improving the interpretation of the profiles.

Reviewer 1:

For Table 4, the authors wrote "the hindering effect of the group", I think the authors mean "steric hindering effect". If this is what the authors mean, I suggest to add the word hindering.

Reply:

Thank you for this suggestion. We have changed the phrase as “the steric hinderance of the group seemed more influential than the electron donating effect;”

Reviewer 1:

The authors choice to make further characterization of the products from 4-hydroxybenzyl alcohol are impressive!

Reply:

We greatly appreciate your heart-warming compliment.

Reviewer 1:

I do not know if this statement is correct: "This is observable during the reaction where the catalyst color changes from light yellow to dark grey."  I think the yellow color is from teh cerium oxide. I think probably the sample looks yellow because of cerium oxide (yellow) and white PdO. Then, perhaps as the authors say, Pd becomes reduced then the Pd metal might make the full sample look gray. If the authors see that the CeO2 without Pd is already yellow, the authors should make the description about the color change and interpretation more clear.

Reply:

Thank you for this comment. To clarify the issue of the color, we added the images of the ceria, PdO_x/CeO₂-NR, and the reduced PdO_x/CeO₂-NR to the Supplementary Materials Figure S2. The color of the ceria was light-yellow, which changed to dark yellow (or light brown) for the PdO_x/CeO₂-NR.

We change the phrase as ”the catalyst color changes from light-brown of the PdO_x/CeO₂ to a dark grey due to the presence of Pd(0) (Figure S3).”

Reviewer 1:

The conclusions section is too short. It should be longer and more clear what the conclusions are -- with more details, so the reader can understand more clearly what is useful about the study.

Reply:

We appreciate your advice and we have extensively modified the “Conclusions” section.

Reviewer 1:

I am confused that the conclusions mention the ionic mechanism, but do not mention Scheme 1 mechanism.  The main feedback I have on this work is that the conclusions section should be worked on more, so that it can better emphasize what was learnt about the work. Even if the conclusions section is technically correct, it will be better if the emphasis of the conclusions section matches what the reader should learn from the paper.

Reply:

Thank you for this comment. We tried to put the main results of the work in a clearer manner in the new “Conclusions” section.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments to the Author

This manuscript reported the utilization of palladium oxide (PdO_x) supported on ceria nanorods (CeO₂-NR) for aerobic solvent-free oxidation of BnOH derivatives to corresponding aldehydes. The PdO_x/CeO₂NR was characterized by N₂ adsorption-desorption analysis, temperature-programmed reduction with hydrogen (H₂-TPR), and X-ray Photoelectron Spectroscopy (XPS), proving that PdO can affect the electronic state of the support have a strong interaction with the support. The PdO_x/CeO₂-NR catalyst showed the aerobic oxidation of various benzyl alcohol derivatives with good conversion, and high selectivity towards the corresponding aldehydes.

From my point of view, major revisions are requested for the current manuscript, and particularly, the following points have to be addressed.

1.     In this manuscript, the author draw the conclusion of high dispersion of palladium oxide species based on the absence of the PdO_x reflections in XRD pattern. However, this is indirect evidence, and direct evidence, e.g., TEM images, should be provided to prove the well-dispersed PdO_x.

2.     The authors demonstrated the palladium in the PdO_x/CeO₂-NR was 2.01 wt% by energy-dispersive spectroscopy, in good agreement with the nominal 2 wt% palladium in the catalyst synthesis procedure. However, EDS results of SEM are too local to explain the overall information of the sample. Please provide the macroscopic quantitative results of the sample, such as ICP.

3.     In the manuscript on page 12, line 394-395, the author claimed that “The Pd-loading process does not affect the crystallite structure and size of CeO₂-NR support excluding any solution of Pd into CeO₂ lattice”. But explanation of CeO₂ lattice is not mentioned in the manuscript, please explain further.

4.     The proposed mechanism for the PdO_x/CeO₂-NR catalyst was not supported by any experimental evidence except for the change of solution color during the experiment. No experiment data or theoretical calculation can be found to determine the reaction route. The reaction mechanism in Scheme 1 was not convincing.

Author Response

Solvent-free oxidation of benzyl alcohol derivatives by in situ generated redox couple Pd(0)/PdO_x supported on Ceria nanorods

Answers to Reviewer 2

Reviewer 2:

This manuscript reported the utilization of palladium oxide (PdO_x) supported on ceria nanorods (CeO₂-NR) for aerobic solvent-free oxidation of BnOH derivatives to corresponding aldehydes. The PdO_x/CeO₂NR was characterized by N₂ adsorption-desorption analysis, temperature-programmed reduction with hydrogen (H₂-TPR), and X-ray Photoelectron Spectroscopy (XPS), proving that PdO can affect the electronic state of the support have a strong interaction with the support. The PdO_x/CeO₂-NR catalyst showed the aerobic oxidation of various benzyl alcohol derivatives with good conversion, and high selectivity towards the corresponding aldehydes.

From my point of view, major revisions are requested for the current manuscript, and particularly, the following points have to be addressed.

1. In this manuscript, the author draw the conclusion of high dispersion of palladium oxide species based on the absence of the PdO_x reflections in XRD pattern. However, this is indirect evidence, and direct evidence, e.g., TEM images, should be provided to prove the well-dispersed PdO_x.

Reply:

Thank you for this comment. It is true that information from the XRD analysis or TPR analysis are indirect evaluations of metal dispersion, and that TEM is one of the effective techniques for the direct analysis of supported metal dispersion. For this reason, we revised the discussion about dispersion evaluation from XRD. In fact, we also performed some TEM analysis of pure support and PdOx/CeO_2-NR sample and we did not detect any clear evidence of palladium particles. We are aware that estimating the palladium dispersion by TEM requires a deeper analysis of the images, that must be performed also by a local EDS micro-analyzer. This study is of interest for the characterization of the material, but currently is beyond the scope of this paper, that focused on the effect of the oxidation state of the active metal on the catalytic activity.  We consider your suggestion, and we are working on doing it in the continuation of the study of PdOx/ceria materials. Here we provide a couple of the TEM images.  

Reviewer 2:

2. The authors demonstrated the palladium in the PdO_x/CeO₂-NR was 2.01 wt% by energy-dispersive spectroscopy, in good agreement with the nominal 2 wt% palladium in the catalyst synthesis procedure. However, EDS results of SEM are too local to explain the overall information of the sample. Please provide the macroscopic quantitative results of the sample, such as ICP.

Reply:

We thank the reviewer for pointing out this important issue. We admit that the EDS results cannot be attributed to the whole sample. However, the value that we noted (2.01 wt.%) was acquired as the mean value resulted from analysis of the sample in six areas. We added the below sentence to the Materials and Methods section to clarify this issue:

The EDS analysis was performed in six different areas, and the Pd wt. % was obtained as the mean value.

Furthermore, considering the wet impregnation process used, we expect that all amount of the initial palladium remains in the solid because, at the end of the process, we only evaporated the solvent leaving all the supposedly non-deposited palladium together with the rest of the solid.

Unfortunately, we couldn’t manage to have our sample analyzed by ICP but, to provide more information about the chemical composition of the sample, in the XPS paragraphs, we add the data about the atomic percentage of palladium with respect to cerium on the surface of PdOx/CeO₂NR:

“In addition, the palladium atomic ratio i.e., Pd / (Pd + Ce) on the surface of PdOx/CeO2-NR was 10.3 %.”

Reviewer 2:

3. In the manuscript on page 12, line 394-395, the author claimed that “The Pd-loading process does not affect the crystallite structure and size of CeO₂-NR support excluding any solution of Pd into CeO₂ lattice”. But explanation of CeO₂ lattice is not mentioned in the manuscript, please explain further.

Reply:

Thank you for the comment: we used the term “lattice” in the mere meaning of “reticular crystalline structure”, as generally used in the crystallographic theory and in the scientific community reporting XRD data, so we think it is unnecessary to add an extended explanation of the term “lattice”.  Anyway, to improve the clarity we specified “crystal lattice” in the text.

Reviewer 2:

4. The proposed mechanism for the PdO_x/CeO₂-NR catalyst was not supported by any experimental evidence except for the change of solution color during the experiment. No experiment data or theoretical calculation can be found to determine the reaction route. The reaction mechanism in Scheme 1 was not convincing.

Reply:

We appreciate your comment. It is true that theoretical calculations were not performed, and the mechanism presented is only hypothesized, but we have presented more information other than catalyst color change, that can support our thesis. The mechanism proposed considers both our experimental and literature data.  The reduction of PdO_x to Pd(0) was confirmed by XPS of “used” catalyst and previously demonstrated in literature even if on TiO₂ (ref 23). Recent literature has evidenced the tendence of the PdOx/Pd(0) to participate synergistically to the catalysis (ref. 39) at the temperature in which the reactions are performed. The recyclability test (new data) evidenced that the obtained PdOx/Pd(0)/CeO₂-NR catalyst isuven more reactive than the previous one. Furthermore, the strong substituent effect observed in EWG or ERG is more typical of ionic rather than radical reactions. However, in line with your advice, we preferred to delete Scheme 1 explaining in the chapter only the steps confirmed by literature data. For this reason, the “Mechanistic aspects” chapter has been thoroughly revised.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, PdO_x supported on ceria nanorods (PdO_x/CeO₂-NR) is presented as a catalyst for solvent-free oxidation of benzyl alcohol derivatives. The PdO_x/CeO₂-NR were investigated using XRD, SEM, EDS, N₂ adsorption-desorption analysis, H₂-TPR, XPS, and catalytic measurements. The topic is within the journal scope and the manuscript is worth publishing. Before the further consideration of the manuscript for the possible publication in Catalysts, the authors must address the following concerns:

1. What is the exact composition of PdO_x? How much palladium is in the catalyst?

2. What is the actual size of PdO_x nanoparticles? The absence of PdO_x maxima in XRD pattern can be explained by small PdO_x content, not only by a small particle size. SEM at low accelerating voltages can help to see more details in images.

3. The H₂-TPR profile for obtained PdO_x/CeO₂-NR is strange. Does the presence of PdOx (even the small amount) fully suppresses the reduction of ceria? By the way, in the authors’ previous report (DOI: 10.3390/catal9100847), the H₂-TPR profile for the same (?) catalyst significantly differs from the one in the current manuscript. Please, discuss.

4. Is the catalytic reaction proceeds only on PdO_x nanoparticles? The ceria or ceria/PdO_x interface is very likely to take part in the process. Please refer to the book “Catalysis by Ceria and Related Materials”.

5. Did the authors try to fit the Ce XPS data by Ce(IV) peaks only? Perhaps, the fit based on the presence of Ce(III) is odd. Please discuss whether XPS was conducted in vacuum, under low pressures Ce(IV)-Ce(III) transition is likely, there is a rich literature on the topic.

6. XPS background is quite noisy. Please, provide the Abbe criterion for XPS data. In the case of Ce XPS, the positions of binding energy in Figure 4(b) do not correspond to Table 2: the first peak in Figure is at ~887 eV, while in the Table at 882.5 eV. Thus, the equipment calibration must be done. By the way, the Binding energy axis should be inverted.

7. The catalyst recycling problem is of particular interest. Please comment on the possible catalyst inactivation upon the reaction? Can the catalyst be recovered and how?

8. Please comment on the colour of precipitate (purple CeO2). CeO₂ is surely of a yellow colour.

9. Please comment on the mechanism of nanorod formation? Does the shape of nanoparticles play any role in a catalytic process? In view of the chosen synthetic route, a thorough discussion of this point is needed.

10. Raman spectroscopy is recommended for a deeper insight in CeO2 role in the catalytic process.

11. How was the lattice parameter calculated? Please, provide the corresponding errors for unit cell parameter and crystallite size. The full refinement of XRD data is necessary.

12. Why do authors consider the method of impregnation as “improved”? There is no evidence of improvement taking in mind the previous reports.

Taking into account the above-mentioned concerns and questions, I suppose the manuscript needs the major revision.

Author Response

Solvent-free oxidation of benzyl alcohol derivatives by in situ generated redox couple Pd(0)/PdO_x supported on Ceria nanorods

Answers to Reviewer 3

Reviewer 3:

In this work, PdOx supported on ceria nanorods (PdOx/CeO2-NR) is presented as a catalyst for solvent-free oxidation of benzyl alcohol derivatives. The PdOx/CeO2-NR were investigated using XRD, SEM, EDS, N2 adsorption-desorption analysis, H2-TPR, XPS, and catalytic measurements. The topic is within the journal scope and the manuscript is worth publishing. Before the further consideration of the manuscript for the possible publication in Catalysts, the authors must address the following concerns:

1. What is the exact composition of PdOx? How much palladium is in the catalyst?

Reply:

Thank you for the question. As we have already explained in the Materials and Methods section, the initial amount of palladium precursor used for wet impregnation corresponded to 2.00 wt.% Pd in the final sample. We expect that all amount of the initial palladium remains in the solid because, at the end of the process, we only evaporated the solvent, leaving all the supposedly non-deposited palladium together with the rest of the solid. This amount was confirmed by EDS analysis (2.01 wt %, see SEM paragraph) in six different areas of the sample.

We added this detail in the Materials and Methods as below:

“The EDS analysis was performed in six different areas, and the Pd wt. % was obtained as the mean value.”

However, we used the XPS spectra to calculate the Pd% / (Pd+Ce) on the surface of the sample (where the catalytic reaction occurs) and added the below sentence to the XPS paragraph:

“In addition, the palladium atomic ratio i.e., Pd/(Pd + Ce) on the surface of PdOx/CeO2-NR was 10.3 %.”

Reviewer 3:

2. What is the actual size of PdOx nanoparticles? The absence of PdOx maxima in XRD pattern can be explained by small PdOx content, not only by a small particle size. SEM at low accelerating voltages can help to see more details in images.

Reply:

Thank you. We agree with your valid comment, and we have corrected the XRD paragraph as follows:

 “The absence of the PdOx diffractions is due to the low content.”

In addition, we performed TEM analysis on the PdOx/CeO2-NR sample in order to measure the size of the PdOx nanoparticles, but the particles couldn’t be detected in the TEM images. This could be due to high dispersion or low concentration. As the analysis did not add new information, we do not add this data to the revised version of the manuscript.

Reviewer 3:

3. The H2-TPR profile for obtained PdOx/CeO2-NR is strange. Does the presence of PdOx (even the small amount) fully suppresses the reduction of ceria? By the way, in the authors’ previous report (DOI: 10.3390/catal9100847), the H2-TPR profile for the same (?) catalyst significantly differs from the one in the current manuscript. Please, discuss.

Reply:

Thank you very much for your valid comment that helped us to improve the interpretation of the profile. The TPR profile is clearly different from that reported in the previous paper [1]. Both XPS and TPR characterization techniques showed different chemical-physical characteristics and the catalytic tests showed different catalytic activity for the catalysts studied in the previous in the current paper, although they were prepared with only a slightly different procedure (impregnation with home-made or commercial Pd-nitrate solutions); i.e.  XPS show a higher oxidation state of Pd in the current sample. In fact, we attentively re-considered the TPR discussion concluding that: 1) the ceria reduction cannot be limited to that observed in the range of 300-500 °C, 2) the high vacuum XPS analysis may overestimate Ce(III) [2]. The TPR profile presents only a very weak H2 consumption in the range of the pure ceria reduction (300-500°C), so the reduction should be promoted by Pd at a lower temperature (lower than in the previously published paper) and occurred together with Pd reduction. The TPR results have been deeply revised in the revised manuscript.

Reviewer 3:

4. Is the catalytic reaction proceeds only on PdOx nanoparticles? The ceria or ceria/PdOx interface is very likely to take part in the process. Please refer to the book “Catalysis by Ceria and Related Materials”.

Reply:

We thank the reviewer for raising this issue. In our previous work [1], we showed that the pure CeO₂-NR alone was inactive in BnOH oxidation in ethanol or toluene solvents. However, since in the present manuscript, the solvent-free conditions are administered, we performed a blank experiment with CeO₂-NR as catalyst, which led to no BnOH conversion. Accordingly, we added the below sentence to the second paragraph of Section 2.2 Catalytic Tests:

“In the blank experiment in which CeO₂-NR was used as catalyst, no BnOH conversion occurred after 18 h.”

In addition, in the 4^th paragraph of the Introduction of our previous work [1], we explained the advantages of ceria as support for heterogeneous catalysts:

“CeO₂ is a reductive oxide, efficient as support for metals such as Pd, Pt and Au to catalyze reactions like amination of alcohols, and oxidation of CO and alcohols. The reason is the enhanced surface area and high amount of surface active sites (oxygen vacancies) on nano ceria comparing with other common supports.”

Therefore, we didn’t repeat the same information here in the present manuscript to avoid redundancy.

Reviewer 3:

5. Did the authors try to fit the Ce XPS data by Ce(IV) peaks only? Perhaps, the fit based on the presence of Ce(III) is odd. Please discuss whether XPS was conducted in vacuum, under low pressures Ce(IV)-Ce(III) transition is likely, there is a rich literature on the topic.

Reply:

The Ce3d XPS spectrum requires five spin-orbit pairs for the fit, coherently with the literature indicating two Ce(III) and three Ce(IV)-related contributions. XPS was carried out in UHV (page 50, lines 4-6: “Measurements were carried out in UHV, typical vacuum pressure in the analysis chamber during measurements was in the 10^-9 – 10^-10 Torr range.”). All samples showed a reproducible percentage of Ce(III) and Ce(IV) (about 50%), that is not related to the sample pretreatment.

Reviewer 3:

6. XPS background is quite noisy. Please, provide the Abbe criterion for XPS data. In the case of Ce XPS, the positions of binding energy in Figure 4(b) do not correspond to Table 2: the first peak in Figure is at ~887 eV, while in the Table at 882.5 eV. Thus, the equipment calibration must be done. By the way, the Binding energy axis should be inverted.

Reply:

All XPS spectra plotted with peak fitting residuals have been reported in the supporting information as Figure S3 a), b) and c).

We thank the reviewer for pointing out the calibration error in Ce3d spectrum. The correctly calibrated spectrum is now reported in Figure 4b.

Binding Energy axis can be reported both ways (increasing values from left to right or other direction), for coherence with previous works, [1], we prefer to let it as it is.

Reviewer 3:

7. The catalyst recycling problem is of particular interest. Please comment on the possible catalyst inactivation upon the reaction? Can the catalyst be recovered and how?

Reply:

Thank you for the answer that prompted us to perform recycling tests and the XPS analysis after 3 runs. The data obtained have been added to the article and the results added considerable value to the work. As a matter of fact, the redox couple formed during the first catalytic process proved to be stable and more performant than the starting catalyst as the conversion of BnOH even increased in the second cycle compared to the first one (see Table 5).  These new findings were added to the mechanistic and conclusion paragraphs.

Reviewer 3:

8. Please comment on the colour of precipitate (purple CeO2). CeO2 is surely of a yellow colour.

Reply:

Thank you for noticing this point. We observed that after 24 h in the oven, the color is initially light purple, but changes immediately to light yellow by decrease of the pH when being washed by distilled water.

To avoid any ambiguity, we re-wrote the respective sentences as below, and added the image of the solid samples in the Supplementary Materials Figure S2.

“Then the obtained precipitate was vacuum filtered, washing with distilled water and ethanol. To fully evaporate the remaining solvent, the solid was left in oven over the night at 120 °C. Finally, the prepared solid was uniformly milled and then calcined at 400 °C for 5 hours, resulting in the light yellow ceria powder (Figure S2a).”

Reviewer 3:

9. Please comment on the mechanism of nanorod formation? Does the shape of nanoparticles play any role in a catalytic process? In view of the chosen synthetic route, a thorough discussion of this point is needed.

Reply:

We appreciate the concerns of the reviewer. In our previous work [1], in the 4^th paragraph of the Introduction section we referred to the research works in which, the nanorod morphology of ceria provided better results compared to other ceria morphologies for reactions such as CO oxidation, dry reforming of methane and in the synthesis of dimethyl carbonate from methane and methanol.

We also highlighted the better outcome of BnOH oxidation with noble metals supported on ceria nano rods in comparison with other ceria morphologies.

We thought that repeating these issues in our present manuscript would be redundant, specifically since the morphology of the support has not been the focus of our work, if not out of its scope.

Reviewer 3:

10. Raman spectroscopy is recommended for a deeper insight in CeO2 role in the catalytic process.

Reply

Thank you for your advice but the main scope of this article is to provide a new catalyst for benzyl alcohol oxidation, easy to prepare by wet impregnation process and to be used directly without reduction. Many other articles have already studied the role of ceria in the catalytic process, so we didn’t want to further investigate on this field. May be this could be material for further research.

Reviewer 3:

11. How was the lattice parameter calculated? Please, provide the corresponding errors for unit cell parameter and crystallite size. The full refinement of XRD data is necessary.

Reply:

Thank you for this useful comment. In the revised version we have refined the XRD data.

We are sorry for the omission of the formula by which lattice parameter was calculated, and we have added it to the text.

The previously reported cell parameter was calculated from the most intense XRD line of ceria. Now we have reported in the table the “a” average value calculated from the eight XRD peaks with the corresponding error (5.408 ± 0.001 and 5.405 ± 0.003). The cell parameter of pure CeO₂ and of Pd/CeO₂ sample were used for a comparison, to check an eventual variation, not as absolute values; for this reason, we think that compare values obtained with the same instrument and calculation procedure is sufficient to obtain this information.

Paper about catalysis generally report particle dimension of materials without the error because Scherrer formula is a rough estimation, and the real error is hard to estimate. To improve the reliability of the particle dimension values (12.8 and 12.1 nm) obtained by the Scherrer equation from the most intense XRD line, we also calculated the particle dimension from all the eight XRD reflections of CeO2 by the straight line of the Size-Strain plot, obtaining 13.2 and 12.2 nm. We substituted the previous values with these new values in the table and added the plots in the Supplementary Materials Figure S1 of the revised manuscript. 

Reviewer 3:

12. Why do authors consider the method of impregnation as “improved”? There is no evidence of improvement taking in mind the previous reports.

Reply:

We thank the reviewer for this comment. We considered the method of preparation of the PdOx catalyst improved because of the using the commercial Sigma Aldrich palladium(II) nitrate solution (10 wt.% in 10 wt. % nitric acid; 99.999 % trace metal) solution whereas in the previous method, Alfa Aesar’s solid Palladium(II) nitrate dehydrate (40 % Pd) was used as palladium precursor which had to be dissolved by us. The difficulty of solubilizing the Palladium(II) nitrate dehydrate in water and the necessity of addition of unknown amount of nitric acid to obtain a transparent solution made the synthesis of the catalyst less reproducible. But with the Sigma Aldrich commercial solution, that problem was resolved. Furthermore, with this new Pd precursor, the catalyst obtained resulted different from that previously produced as can be seen by the XPS, TPR experiments, etc. However, to avoid misunderstanding we decided to cancel the adjective in the manuscript and wrote “PdO_x/CeO₂-NR was prepared through a slightly modified wet Ce(NO₃)₃·6H₂O impregnation process using commercial Pd(NO₃)₂, solution, and subsequent calcination.”

1. Moeini, S.S.; Battocchio, C.; Casciardi, S.; Luisetto, I.; Lupattelli, P.; Tofani, D.; Tuti, S. Oxidized Palladium Supported on Ceria Nanorods for Catalytic Aerobic Oxidation of Benzyl Alcohol to Benzaldehyde in Protic Solvents. Catalysts 2019, 9, 847, doi:10.3390/catal9100847.
2. Zhang, F.; Wang, P.; Koberstein, J.; Khalid, S.; Chan, S.-W. Cerium Oxidation State in Ceria Nanoparticles Studied with X-Ray Photoelectron Spectroscopy and Absorption near Edge Spectroscopy. Surf. Sci. 2004, 563, 74–82, doi:10.1016/j.susc.2004.05.138.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I am happy to see the authors addressed the issues I have raised in the first round of revision. The current form can be publised. 

One suggestion, the TEM images can be incorporated in the manuscript. 

Reviewer 3 Report

During the revision, the manuscript was enhanced considerably. Authors have addressed my main concerns satisfactorily enough. The other are either debatable or do not touch the main findings.

Now, the manuscript can be published in Catalysts.