Graphene Oxide-Supported Metal Catalysts for Selective Hydrogenation of Cinnamaldehyde: Impact of Metal Choice and Support Structure

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

his study compared most of the well-known active metals in the hydrogenation of cinnamaldehyde, focusing on the impact of metal choice and support structure. Plenty of data on the catalyst characterizations and catalytic performance are presented. However, the novelty of this study is not clearly discussed. Please discuss the necessity of doing so. Also, the new findings on this topic could be addressed. Other suggestions to the authors are as follows:

1. In ESI TEM images, “(a), (b), (c)” should be added to the pictures.
2. In the XRD part, lines 181-182, the authors claimed that “Diffraction patterns typical for Rh, Ru as well as Co are not visible in the XRD patterns of the prepared catalysts suggesting fine dispersion of small metal´s nanoparticles”. According to the TEM results, this statement is not correct, please revise it.
3. The scale bar in TEM images (Figure 6) could be redrawn.
4. rGO prepared by different methods show distinct catalytic performance. The reason behind is hardly mentioned. As shown Figure 2, different organic groups are present. Besides, different acids are used in GO preparation. How do the composition and chemical nature of rGO/AC influence the electronic properties of Pd nanoparticles?
5. What is the major factor controlling ether production vs. hydrogenation products?

Author Response

Reviewer 1 Overall Comment:
This study compared most of the well-known active metals in the hydrogenation of cinnamaldehyde, focusing on the impact of metal choice and support structure. Plenty of data on the catalyst characterizations and catalytic performance are presented. However, the novelty of this study is not clearly discussed. Please discuss the necessity of doing so. Also, the new findings on this topic could be addressed.

Response: We thank the reviewer for these valuable and constructive comments. Below we provide a detailed, point-by-point response, with revisions incorporated in the revision manuscript and Supplementary file, where all changes made during the revision process are marked yellow.
First, we have expanded the discussion of novelty and motivation in the Introduction section. The key novelties are now stated as (i) a direct and systematic comparison of Pt, Pd, Rh, Ru, and Co catalysts on two types of 2D rGO supports (HUGO and TOGO) and on conventional 3D activated carbon under identical reaction and preparation conditions; (ii) discovery and analysis of a rarely reported side product, 3-isopropoxy-propan-1-yl benzene, arising from reductive etherification of hydrocinnamaldehyde (HCAL); and (iii) correlation of Pd nanoparticle size and dispersion with catalytic performance and selectivity via newly added regeneration study.

 

Reviewer 1 Comment 1:
In ESI TEM images, “(a), (b), (c)” should be added to the pictures.

Response:
The labels "(a), (b), and (c)" have been added to all TEM images and corresponding figure captions for clarity.

Reviewer 1 Comment 2:
In the XRD part, lines 181–182, the authors claimed that “Diffraction patterns typical for Rh, Ru as well as Co are not visible in the XRD patterns of the prepared catalysts suggesting fine dispersion of small metal nanoparticles.” According to the TEM results, this statement is not correct; please revise it.

Response:
We thank the reviewer for pointing out this inconsistency. The section has been revised to reflect the actual TEM findings and provide a more accurate explanation:

„Diffraction patterns typical for Rh, Ru as well as Co are not visible in the XRD patterns of the prepared catalysts. For the Ru and Rh catalysts, the absence of characteristic metal diffraction peaks in the XRD patterns can be attributed to the fine dispersion of small metal nanoparticles, as confirmed by SEM and TEM analyses (See below section 2.1.4. and 2.1.5.). Although the Co particles were relatively large (~20 nm, determined below by TEM, section 2.1.5.), no Co-related diffraction peaks were observed in the XRD patterns. This can be explained by a combination of factors: (i) the low metal loading (1 wt.%), (ii) strong background scattering from the carbon supports, and (iii) the poor crystallinity or partial amorphous character of the cobalt nanoparticles, which suppresses sharp XRD reflections.“

 

Reviewer 1 Comment 3:
The scale bar in TEM images (Figure 6) could be redrawn.

Response:
All TEM images have been updated with redrawn, clearly visible scale bars to ensure accurate interpretation.

 

Reviewer 1 Comment 4:
rGO prepared by different methods show distinct catalytic performance. The reason behind is hardly mentioned. As shown Figure 2, different organic groups are present. Besides, different acids are used in GO preparation. How do the composition and chemical nature of rGO/AC influence the electronic properties of Pd nanoparticles?

Response:
We thank the reviewer for highlighting this point. We have addressed this question in Section 2.2.1 by the following discussion:

„The differences between Pd/rHUGO and Pd/rTOGO stem not only from particle size and dispersion but also from differences in the surface functional groups of the rGO supports.OGO, rich in carboxyl groups, can induce stronger metal–support interactions and potentially influence the electronic density of the supported Pd nanoparticles. Such electronic modulation could affect the adsorption energies of intermediates and thus alter activity and selectivity.“

 

Reviewer 1 Comment 5:
What is the major factor controlling ether production vs. hydrogenation products?

Response:
Based on the time-resolved selectivity data and the characterization of regenerated catalysts, we conclude that Pd nanoparticle size plays a key role in controlling ether formation. This conclusion is supported by Figures 10 and 12, and further reinforced by the newly added Section 2.3 (Catalyst Regeneration and Reuse, 4 pages), which provides detailed analysis of catalyst morphology and performance after reuse. In particular, we observed that smaller Pd particles formed after the first regeneration cycle were associated with a marked increase in ether selectivity and a concurrent decrease in HCAL selectivity. These findings suggest that Pd particle size not only affects overall catalytic activity but also shifts the product distribution, likely by promoting secondary transformation of HCAL to ether.

Reviewer 2 Report

Comments and Suggestions for Authors

1.    Please check the manuscript for typos (see page 4, line 139 “carbo-based”, etc)
2.    Are the surface area values in Table 1 obtained as an average of several measurements or just a single sample determination. It would be nice to have the standard deviation of these measurements. 
3.    From my point of view the content of Table 2 does not reflect its title, as the values are not CAL conversions. Please be more specific. 
4.    No information regarding the method implemented in Shimadzu GC to analyze the composition of the reaction mixture is provided. No information about the GC calibration for the main and by-products is given.
5.    Are the continuous lines in Figure 7 representing some modeling/fitting results. Otherwise, dots should be enough to provide information about the experimental results. 
6.    Given that the time range used to determine the initial reaction rates is relatively large can you comment about the accuracy of the numerical values? Moreover, we are discussing catalytic reactions, and the reaction rate should be expressed in respect to the catalyst weight. Please justify your choice. 
7.    Figure 7 caption mentions that the reactions were carried out at 80 °C, whereas in Table 3 for Rh lines, the temperature is 110°C. Please justify these results. 
8.    Are the reactions taking place in CAL hydrogenation reversible? The authors mention (lines 460-461) that “COL formation reaches equilibrium with its further hydrogenation to HCOL”.
9.    The Conclusion section is relatively large. Please make it more concise. 
10.    No information about catalyst reuse in several batches is provided in the manuscript, in order to bring information about catalytic activity.  

 

Author Response

We thank the reviewer for these valuable and constructive comments. Below we provide a detailed, point-by-point response, with revisions incorporated in the revision manuscript and Supplementary file, where all changes made during the revision process are marked yellow.

 

Reviewer 2 Comment 1:
Please check the manuscript for typos (see page 4, line 139 “carbo-based”, etc.)

Response:
Thank you. We have carefully gone through the manuscript to remove all remaining typos.

 

Reviewer 2 Comment 2:
Are the surface area values in Table 1 obtained as an average of several measurements or just a single sample determination? It would be nice to have the standard deviation of these measurements.

Response:
Nitrogen physisorption measurements were performed as single determinations per sample, and thus no standard deviation values are available. This limitation is now clarified in the manuscript text near Table 1.

 

Reviewer 2 Comment 3:
From my point of view, the content of Table 2 does not reflect its title, as the values are not CAL conversions. Please be more specific.

Response:
Thank you for your idea. Eventually, we changed the title of Table 2 to:
“Table 2. Metal particle size distribution on different supports determined by TEM (reduced catalysts).”

 

Reviewer 2 Comment 4:
No information regarding the method implemented in Shimadzu GC to analyze the composition of the reaction mixture is provided. No information about the GC calibration for the main and by-products is given.

Response:
We have added a detailed description of the GC method and calibration in the revised manuscript:

“The GC oven temperature was initially set at 70 °C and held for 5 minutes. Subsequently, the temperature was increased at a rate of 20 °C/min to 250 °C, where it was held for an additional 1 minute. Hydrogen was used as the carrier gas at a constant flow rate of 3.19 mL/min. Calibration for cinnamaldehyde (CAL), cinnamyl alcohol (COL), and hydrocinnamaldehyde (HCAL) was performed using mesitylene as an internal standard.”

 

Reviewer 2 Comment 5:
Are the continuous lines in Figure 7 representing some modeling/fitting results? Otherwise, dots should be enough to provide information about the experimental results.

Response:
We agree to remove the continuous lines in Figure 7 (and in Figures 9, 10, and 12). The plots now show only experimental data points.

 

Reviewer 2 Comment 6a:
Given that the time range used to determine the initial reaction rates is relatively large, can you comment about the accuracy of the numerical values?

Response:
We acknowledge this point. The initial reaction rates were calculated based on the concentration difference of CAL between 0 and 30 minutes. While no intermediate sampling occurred in this interval, CAL conversion remained below 20% in most cases at 30 minutes, suggesting the reaction was in the kinetic regime where rates can be considered quasi-constant. Therefore, we consider these values suitable for comparative purposes, while recognizing the limitations of this approach. A corresponding clarification has been added in the manuscript.

 

Reviewer 2 Comment 6b:
The reaction rate should be expressed with respect to catalyst weight. Please justify your choice.

Response:
We chose not to normalize reaction rates to catalyst mass because all experiments were conducted under identical conditions, with the same catalyst mass (10 mg) and metal loading (1 wt.%). Therefore, the reported rates still enable meaningful comparison across catalysts. This justification has been added below the table reporting the reaction rate values.

 

Reviewer 2 Comment 7:
Figure 7 caption mentions that the reactions were carried out at 80 °C, whereas in Table 3 for Rh lines, the temperature is 110 °C. Please justify these results.

Response:
Thank you for noting this. The temperature increase from 80 °C to 110 °C for Rh catalysts after 120 minutes has now been clearly indicated in the caption of Figure 7 and reflected in the graph itself (using empty dots for the higher temperature segment).

 

Reviewer 2 Comment 8:
Are the reactions taking place in CAL hydrogenation reversible? The authors mention (lines 460–461) that “COL formation reaches equilibrium with its further hydrogenation to HCOL”.

Response:
We thank the reviewer for this note and we apologize for a misleading statement. The formation of COL and its subsequent hydrogenation to HCOL in our system should not be described as an equilibrium process. These reactions proceed consecutively and irreversibly under the applied reaction conditions. Once COL is formed, it is continuously hydrogenated to HCOL without significant contribution of reverse reaction. Therefore, the sentence in the manuscript has been revised to clarify that COL is gradually converted to HCOL over time.

 

Reviewer 2 Comment 9:
The Conclusion section is relatively large. Please make it more concise.

Response:
We thank the reviewer for this valuable suggestion. In response, the Conclusion section has been thoroughly revised to improve clarity and focus while retaining all key findings.

 

Reviewer 2 Comment 10:
No information about catalyst reuse in several batches is provided in the manuscript, in order to bring information about catalytic activity.

Response:
We fully agree. A new section (2.3 Catalyst Regeneration and Reuse, 4 pages) has been added to the Results and Discussion. In this section, we report two reuse cycles for the best-performing catalyst (Pd/rTOGO), along with characterization (XRD, SEM, EDS, and TEM) and performance evaluation. The study confirms retained catalytic activity and selectivity, and correlates changes in Pd particle size with performance differences between fresh and reused catalysts.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed my concerns well, and I recommend acceptance.

Reviewer 2 Report

Comments and Suggestions for Authors

Good luck.