An Investigation into the Bulk and Surface Phase Transformations of Bimetallic Pd-In/Al₂O₃ Catalyst during Reductive and Oxidative Treatments In Situ

Round 1

Reviewer 1 Report

This manuscript attempts to prove that Pd-In bimetallic catalyst nanoparticles form core shell structures utilizing EXAFS/XANES and FTIR.  Their experimental evidences seem to support their claim of the core shell structure formation as a result of surface segregation in CO environment. However, they should present microscopic data on their Pd-In nanoparticles supported by Al2O3. They do not present any data on how large/small their nanoparticles are. Are the PdIn particles spherical, faceted, or of any other forms? Do the nanoparticles change the shapes during the heating? Such information is critical since the surface crystal orientations may play an important role in catalysis.

Author Response

We have added TEM micrographs with description on pages 2, 3 and 9 of manuscript. On page 2 added: TEM micrographs of bimetallic PdIn/Al2O3 catalyst are presented in Figure 1. The freshly reduced catalyst (Figure 1a) contains nearly spherical nanoparticles with size distribution in the range of 2-6.5 nm. The morphology of the supported particles after oxidative treatment (Figure 1, c and d) does not change significantly. The mean particle diameter slightly decreased from 5.1 to 4.2 nm, which may be related with the loss of indium in metal particles.

Reviewer 2 Report

The manuscript aims at studying the structural stability of an IMC bimetallic PdIn (a promising catalysts for different reactions). In details, the authors investigated the oxidation and reduction of PdIn catalysis supported over Al₂O₃ through in situ analysis. The results suggest the formation of a “core-shell” structure after the reduction and oxidation steps.

The manuscript is interesting and deserve to be published in Catalysts after addressing some issues:

• I suggest enriching the introduction with a few more examples of IMCs (and their potential application in industrial productions).
• Please check some minor typing errors (e.g., “Figure” sometimes is abbreviated in “Fig.” sometimes is not). In the conclusion section check “…66 at.% out of the..” it seems that there is a double space between “of” and “the”, etc.
• Figure 1: according to x-scale (ranging from 0 to 6) it’s quite difficult to observe the shifting of Pd peak (ranging around 2.70 angstrom). Maybe a supporting figure zoomed on the zone of the analyzed peak would be useful (or reporting additional data – i.e., values - on the figure)
• One may hypothesize that 250^oC has been selected as maximum oxidation T according to the average T (and maxima temperatures) of the industrial processes where the catalysts can be used. However, a justification must be added (e.g., by adding the operative T of the processes in the intro section). Otherwise, higher T should be considered.
• Please justify the use of a PdIn bimetallic catalysts prepared using a 50:50 equimolar solution of Pd (were other ratios also considered? Maybe in previous studies?).
• The conclusions should be enriched with some more relevant data.
• I recommend adding some comments related to future investigations of the results in the conclusions section.

Author Response

• I suggest enriching the introduction with a few more examples of IMCs (and their potential application in industrial productions).

We have extended the Introduction section and added more examples of IMCs and their specific catalytic applications with appropriate referencing.

• Please check some minor typing errors (e.g., “Figure” sometimes is abbreviated in “Fig.” sometimes is not). In the conclusion section check “…66 at.% out of the..” it seems that there is a double space between “of” and “the”, etc.

The requested corrections have been made in the text.

• Figure 1: according to x-scale (ranging from 0 to 6) it’s quite difficult to observe the shifting of Pd peak (ranging around 2.70 angstrom). Maybe a supporting figure zoomed on the zone of the analyzed peak would be useful (or reporting additional data – i.e., values - on the figure)

The corresponding corrections have been made in Figure 2a.

• One may hypothesize that 250oC has been selected as maximum oxidation T according to the average T (and maxima temperatures) of the industrial processes where the catalysts can be used. However, a justification must be added (e.g., by adding the operative T of the processes in the intro section). Otherwise, higher T should be considered.

As the last sentence of Introduction, we have added the following phrase: The oxidative treatments are presently limited to a maximum temperature of 250°C in order to avoid progressive sintering and thus clearly differentiate between surface segregation and particle growth effects.

• Please justify the use of a PdIn bimetallic catalysts prepared using a 50:50 equimolar solution of Pd (were other ratios also considered? Maybe in previous studies?).

A concise discussion on the Pd-In phase diagram and occurrence of different IMCs has been added in Introduction. Our plans to extend the study to IMCs of stoichiometry other than 1:1 are now mentioned at the end of Conclusions.

• The conclusions should be enriched with some more relevant data.
• I recommend adding some comments related to future investigations of the results in the conclusions section.

We have supplemented Conclusions with an additional paragraph emphasizing the potential impact of the results reported and giving an outlook for expanding our work in the future.

Round 2

Reviewer 1 Report

I am happy with the addition of TEM images before and after annealing. In the future, they can do STEM studies to verify their claim of surface segregation.

Reviewer 2 Report

The authors did a good job and addressed all the comments. The paper now deserves to be published in Catalysts.