Investigating the Catalytic Deactivation of a Pd Catalyst during the Continuous Hydrogenation of CO2 into Formate Using a Trickle-Bed Reactor
, Sungho Yoon 3 and Kwang-Deog Jung 1,2,*Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this manuscript, the authors reported the commercial supported Pd catalyst-catalyzed hydrogenation of CO2 to formate in a trickle-bed reactor. A good productivity was obtained although the stability of the catalyst was not well. Based on this issue, the authors carried out a serial of characterizations, and a comprehensive analysis between the fresh and spent catalysts revealed that the diminished catalytic activity is attributed to the partial sintering and leaching of Pd nanoparticles during the reaction process. The results are well described and deepen discussed. In my opinion, it is suitable for publication in Catalysts.
In addition, there remain be minor questions needing to be considered.
1) In this study, the experimental part is limited (only the parameters e.g., temperature and time are referred). The current optimal condition may be gained on the basis of the previous research. The center in the manuscript is the characterization study on the stability on the catalytic activity. In my opinion, the title “Continuous hydrogenation of CO2 to formate on Pd catalyst using a Trickle-bed reactor” is too wide and inaccurate. The similar statement problem is also existing in the Abstract and Conclusions.
2) The condition of the spent catalyst obtained should be clearly illustrated.
Author Response
Ms. Ref. No.: catalysts-2894698
Title: Continuous hydrogenation of CO2 to formate on Pd catalyst using a Trickle-bed reactor
We would like to express our gratitude for the constructive feedback provided by the reviewers of our manuscript. Their insights have allowed us to improve the quality and precision of our research. We have highlighted the revisions made to the manuscript in yellow background for your ease of identification.
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Reviewer #1
In this manuscript, the authors reported the commercial supported Pd catalyst-catalyzed hydrogenation of CO2 to formate in a trickle-bed reactor. A good productivity was obtained although the stability of the catalyst was not well. Based on this issue, the authors carried out a serial of characterizations, and a comprehensive analysis between the fresh and spent catalysts revealed that the diminished catalytic activity is attributed to the partial sintering and leaching of Pd nanoparticles during the reaction process. The results are well described and deepen discussed. In my opinion, it is suitable for publication in Catalysts.
In addition, there remain be minor questions needing to be considered.
(Reviewer #1’s Comment 1) In this study, the experimental part is limited (only the parameters e.g., temperature and time are referred). The current optimal condition may be gained on the basis of the previous research. The center in the manuscript is the characterization study on the stability on the catalytic activity. In my opinion, the title “Continuous hydrogenation of CO2 to formate on Pd catalyst using a Trickle-bed reactor” is too wide and inaccurate. The similar statement problem is also existing in the Abstract and Conclusions.
(Author’s Response) We deeply appreciate the insightful comment from the reviewer and agree with that the title of this manuscript is too much wide. In direct response to this comment, we have modified the title on the basis of the characterization study on the stability on the catalytic activity.
In the revised manuscript , The title is modified as follows,
Investigating the catalytic deactivation of Pd catalyst during continuous hydrogenation of CO2 to formate using a Trickle-bed reactor
(Reviewer #1’s Comment 2) The condition of the spent catalyst obtained should be clearly illustrated.
(Author’s Response) In accordance with your valuable suggestions, we have added the conditions for the catalyst recover in the manuscript as follows,
In the revised manuscript,
After the reaction, the spent catalyst was retrieved from the reactor and washed with water (100 mL x 3) through filtration. Then the recovered catalyst was dried under vacuum at 80 °C before further characterizations.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript describes hydrogenation of CO2 to formate over Pd/active carbon catalyst. The article is devoted to current topics and looks quite interesting, however, upon careful study, a number of minor comments arise.
1. Why was palladium chosen as the deposited metal, and not, for example, platinum or ruthenium? Why was only one mass fraction of Pd, 4.5%, chosen and studied?
2. XRD patterns of the catalysts before and after reaction should be provided.
3. Table 1. Too many significant digits in SBET and Vpore values.
4. Fig. 2. The error of measurements should be shown.
5. Fig. 3. The measurements shown are routine and should be removed from the article.
6. Fig. 4 e,f. The dispersion of the obtained particle size values shall be shown.
Comments on the Quality of English Language
Minor editing of English language required
Author Response
Ms. Ref. No.: catalysts-2894698
Title: Continuous hydrogenation of CO2 to formate on Pd catalyst using a Trickle-bed reactor
We would like to express our gratitude for the constructive feedback provided by the reviewers of our manuscript. Their insights have allowed us to improve the quality and precision of our research. We have highlighted the revisions made to the manuscript in yellow background for your ease of identification.
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Reviewer #2
The manuscript describes hydrogenation of CO2 to formate over Pd/active carbon catalyst. The article is devoted to current topics and looks quite interesting, however, upon careful study, a number of minor comments arise.
(Reviewer #2’s Comment 1) Why was palladium chosen as the deposited metal, and not, for example, platinum or ruthenium? Why was only one mass fraction of Pd, 4.5%, chosen and studied?
(Author’s Response) We acknowledge the reviewer's point regarding metal types. It is important to note that we have investigated other types of metal catalysts such as ruthenium and iridium, which are known for the high catalytic ability for CO2 hydrogenation. However, at the initial tests under batch reaction, Ru and Ir based catalyst showed a negligible activity for the low temperature hydrogenation, and especially, they are not working under condition with ammonium bicarbonate as a reactant. Besides, this study is the first attempt for the continuous hydrogenation to formate using a Pd-based catalysts, so that we have targeted to give an insight for the feasibility for the Pd based catalysts for the trickle bed reactor systems. Thus, we exploited commercially available Pd/AC catalyst which have mass fraction of Pd, 4.5 %.
(Reviewer #2’s Comment 2) XRD patterns of the catalysts before and after reaction should be provided.
(Author’s Response) XRD analysis was performed for the fresh and spent catalysts. The result showed that the diffraction pattern for the metallic Pd was clearly discernable for both catalysts, but the peak intensity was increased, which indicates the sintering of the Pd particles during the hydrogenation. Based on this observation, we supplemented the explanation on XRD analysis as follows,
In the revised manuscript,
Figure 5. XRD analysis for the fresh and spent Pd/AC catalysts
The microstructural characteristics of the fresh and spent Pd/AC catalysts were meticulously analyzed through the wide-angle X-ray diffraction (XRD) to gain insights into the crystallinity of support materials and phase change of the supported Pd species (Figure 5). XRD analysis results, as depicted in Figure 5, showcases distinct patterns that are crucial for understanding the structural properties of the catalysts. The patterns feature broad and low-intensity peaks at 2θ values of approximately 23° and 42°. These specific angles correspond to the (002) and (100) planes of carbon, respectively. The broadness and low intensity of these peaks suggest the amorphous nature of the carbon material, highlighting a lack of long-range crystalline order typically associated with graphitic structures. Furthermore, the analysis revealed distinct and strong peaks at 2θ values of approximately 40° and 47°, marked with diamonds in the figure. These peaks are attributed to the (111) and (200) planes of metallic Pd species. The presence of these peaks is in line with the TEM analysis results. Further, the diffraction peaks at 2θ values of 34° and 43° were observed, marked with circle, indicating the presence of PdO species for the fresh catalyst. In particular, the XRD analysis further reveals notable changes in the Pd/AC catalyst before and after use. Specifically, an increase in the intensity of peaks at 2θ values of 40° and 47° and disappearance of the peaks at 34° and 43° was observed for the spent Pd/AC catalyst in comparison to the fresh catalyst. This increase in peak intensity is a clear indication of the sintering of the supported Palladium nanoparticles, which suggests that these nanoparticles coalesce into larger particle sizes, and simultaneously reduction of the PdO species during the continuous hydrogenation process.
(Reviewer #2’s Comment 3) Table 1. Too many significant digits in SBET and Vpore values. (Author’s Response) The author appreciate for the meticulous comment of the reviewer. Significant digit for SBET and Vpore values were modified following the reviewer’s comment.
(Reviewer #2’s Comment 4) Fig. 2. The error of measurements should be shown.
(Author’s Response) Please understand that it is challenging to display error bars for these experiments since the results presented are not the average of multiple independent experiments repeated several times. However, in the case of Figure 2a, it represents the results of formic acid productivity measured at 1-hour intervals during continuous reaction operation, with each condition being measured three times and all three measurements showing similar productivity. This demonstrates the reproducibility of catalyst productivity under those conditions. Similarly, for Figure 2b, analyses of each sample were conducted at 1-hour intervals over a 20-hour period, during which a gradual decrease in activity was observed. Since these results also do not stem from independent experimental repetitions, it was deemed inappropriate to include error bars.
(Reviewer #2’s Comment 5) Fig. 3. The measurements shown are routine and should be removed from the article.
(Author’s Response) As a direct response to the reviewer’s comment, the measurements are removed from the Fig. 3 for the clearness.
(Reviewer #2’s Comment 6) Fig. 4 e,f. The dispersion of the obtained particle size values shall be shown.
(Author’s Response) Dispersion values were supplemented to Figure 4e and f.
Author Response File: Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsIn this study, Park et al. present their findings on the continuous hydrogenation of CO2 to formate using a Pd catalyst in a Trickle-bed reactor. They achieved maximum formate productivity at 150 ℃ and 8 MPa, with a H2/CO2 ratio of 1:1. The authors also identified that catalyst deactivation at high temperatures was attributed to the partial sintering and leaching of Pd nanoparticles. The manuscript is well-prepared and suitable for publication in Catalysts after minor revisions.
1. In Figure 1, there is a significant negative peak observed at 500-650 °C. Could you elaborate on the process to which this peak can be attributed?
2. It would be beneficial to include a comparison of the catalytic performance of the Pd/AC catalysts in this study with relevant published studies.
3. Previous research has demonstrated that Pd-based catalysts exhibit high selectivity in the electrochemical reduction of CO2 to formate (References: 10.1021/acscatal.2c03842; 10.1016/j.apcatb.2022.121659; 10.1016/j.xcrp.2022.100949;). It is recommended that the authors discuss the similarities and differences between thermal and electrochemical reductions to broaden the appeal to a wider audience.
4. Typically, catalysts undergo an activation process lasting from a few to tens of hours. Could you provide insights into the performance of the Pd/AC catalyst after a 20-hour test?
5. It would be valuable if the authors could offer a discussion on potential strategies to enhance the stability of the catalyst.
Author Response
Ms. Ref. No.: catalysts-2894698
Title: Continuous hydrogenation of CO2 to formate on Pd catalyst using a Trickle-bed reactor
We would like to express our gratitude for the constructive feedback provided by the reviewers of our manuscript. Their insights have allowed us to improve the quality and precision of our research. We have highlighted the revisions made to the manuscript in yellow background for your ease of identification.
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Reviewer #3
In this study, Park et al. present their findings on the continuous hydrogenation of CO2 to formate using a Pd catalyst in a Trickle-bed reactor. They achieved maximum formate productivity at 150 ℃ and 8 MPa, with a H2/CO2 ratio of 1:1. The authors also identified that catalyst deactivation at high temperatures was attributed to the partial sintering and leaching of Pd nanoparticles. The manuscript is well-prepared and suitable for publication in Catalysts after minor revisions.
(Reviewer #3’s Comment 1) In Figure 1, there is a significant negative peak observed at 500-650 °C. Could you elaborate on the process to which this peak can be attributed?
(Author’s Response) Our research team is also making efforts to analyze the peaks in question. We conducted mass-assisted analysis to elucidate the cause of H2 generation in the temperature range of 500-650 °C, as shown in the figure below (Figure R1). Regrettably, while it was observed that H2 consumption occurs above 500 °C, along with the generation of CH4 and CO due to the decomposition of the carbon support, accurately identifying the cause of the negative peak proved challenging. However, to minimize confusion for the readers, we have decided to modify the temperature range of the TPR data in Figure 1 to include only temperatures up to 500 °C, as meaningful results from our study can be analyzed below this threshold.
Figure R1. Mass spectroscopy analysis on outlet gas from TPR analysis for fresh Pd/AC catalyst
(Reviewer #3’s Comment 2) It would be beneficial to include a comparison of the catalytic performance of the Pd/AC catalysts in this study with relevant published studies.
(Author’s Response) Most studies on formic acid production using Pd-based catalysts have been reported through batch reactor systems. To our knowledge, this paper represents the first attempt in the field of carbon dioxide hydrogenation for formic acid production using the Pd based catalysts. Therefore, we request understanding for the difficulty in comparing with other reported catalyst systems.
(Reviewer #3’s Comment 3) Previous research has demonstrated that Pd-based catalysts exhibit high selectivity in the electrochemical reduction of CO2 to formate (References: 10.1021/acscatal.2c03842; 10.1016/j.apcatb.2022.121659; 10.1016/j.xcrp.2022.100949;). It is recommended that the authors discuss the similarities and differences between thermal and electrochemical reductions to broaden the appeal to a wider audience.
(Author’s Response) Electrochemical systems are also highly innovative and hold promising potential for the future. An explanation related to this has been added to the introduction to aid readers' understanding, and we agree that it contributes significantly to the comprehension of the topic. We appreciate such insightful comments, and the relevant papers have been cited along with the explanation as follows,
The recent LCA-TEA analysis reports highlight the positive impact on the CO2-hyrogenated FA production in thermo- or electrochemical catalytic systems, especially with the rapidly emerging technology for the integration of renewable energy [9-14]. Electrochemical systems for producing formate are gaining prominence due to their efficient integration with renewable energy and potential to significantly reduce global warming impact, showcasing promising electrolyzer performance [15-17]. However, the scalability of these systems to produce concentrated formate in a continuous manner and the cost implications of the separation process remain areas for further development. In thermo-catalytic systems also face remaining challenges in the economical aspects, such as high reactant costs, significant energy demands for purification, and lower catalytic efficiencies, hindering its competitiveness against traditional fossil fuel-based processes.
Ref. 15 Guo, S.; Liu, Y.; Murphy, E.; Ly, A.; Xu, M.; Matanovic, I.; Pan, X.; Atanassov, P. Robust palladium hydride catalyst for electrocatalytic formate formation with high CO tolerance. Appl. Catal. B-Environ. 2022, 316, 121659.
Ref. 16 Koolen, C.D.; Luo, W.; Züttel, A. From single crystal to single atom catalysts: Structural factors influencing the performance of metal catalysts for CO2 electroreduction. ACS Catal. 2022, 13, 948-973.
Ref. 17 Zhang, J.; Pham, T.H.M.; Ko, Y.; Li, M.; Yang, S.; Koolen, C.D.; Zhong, L.; Luo, W.; Züttel, A. Tandem effect of Ag@ C@ Cu catalysts enhances ethanol selectivity for electrochemical CO2 reduction in flow reactors. Cell Rep. Phys. Sci. 2022, 3, 100949.
(Reviewer #3’s Comment 4) Typically, catalysts undergo an activation process lasting from a few to tens of hours. Could you provide insights into the performance of the Pd/AC catalyst after a 20-hour test?
(Author’s Response) Upon investigating the causes of catalyst deactivation up to 20 hours, it was found that 1) leaching of Pd and 2) gradual sintering of Pd nanoparticles are responsible. However, these effects are irreversible. Therefore, continuous deactivation of the catalyst is expected as the reaction progresses beyond 20 hours.
(Reviewer #3’s Comment 5) It would be valuable if the authors could offer a discussion on potential strategies to enhance the stability of the catalyst.
(Author’s Response) As explained in Comment 4 by Reviewer #3, the deactivation of the Pd catalyst is due to the leaching and sintering of Pd nanoparticles. This is attributed to excessive reduction of Pd nanoparticles under reductive reaction conditions. To suppress this, it is anticipated that enhancing the interaction strength between the catalyst support and Pd nanoparticles, as well as precisely controlling the support's three-dimensional structure to confine Pd nanoparticles, could mitigate the causes of deactivation. Research related to this is currently being actively pursued by our team and is expected to be reported shortly. We are grateful for the very insightful comment related to this study and have added a discussion on the future development directions of Pd catalysts in the manuscript, in response to contents of Reviewer's comments 4 and 5.
Based on the results of the continuous carbon dioxide hydrogenation reaction using a trickle bed reactor and the in-depth analysis of the catalyst recovered after the reaction, this study confirmed that the activity of the Pd/AC catalyst gradually decreases over long reaction durations. This decline in activity was attributed to the leaching of Pd nanoparticles due to the continuous feed flow and their sintering caused by exposure to a reducing environment. The sintering of nanoparticles, due to leaching and reduction, results in irreversible deactivation that cannot be reversed by catalyst regeneration. Therefore, from a practical perspective, this catalyst is not suitable for long-term reaction conditions. The fundamental reason for these outcomes is believed to be the limitations of the activated carbon support. Recent trends in research on formic acid production through carbon dioxide hydrogenation have shown that in catalyst systems based on Pd and various single atom catalysts, strong interactions between the support surface and the supported metals significantly enhance the stability of the supported metal catalysts under reducing conditions. Furthermore, catalyst supports based on metal oxides and organic materials have also been reported to improve the stability of the catalyst during hydrogenation reactions by confining Pd nanoparticles within their unique pore structures. Continuous hydrogenation reactions have the advantage of being simple and operable under scalable conditions within a thermodynamic conversion system. However, the process still faces challenges, such as low reaction efficiency due to continuous reaction feed flow and short residence time. As discussed earlier, the development of innovative synthetic methods that can precisely control the chemical state of the support surface is crucial. Simultaneously, developing supports that can effectively confine Pd nanoparticles within a co-structure while enhancing the mass transfer efficiency of the reactants is of great importance.
Author Response File: Author Response.docx
