Pd-Decorated 2D MXene (2D Ti₃C₂Ti_x) as a High-Performance Electrocatalyst for Reduction of Carbon Dioxide into Fuels toward Climate Change Mitigation

Round 1

Reviewer 1 Report

I am confused about several results of this work:

1.The XRD pattern of Pd-MXene 127
catalysts shown in Fig.1c is much more clear than the MXene pattern in Fig.1b?

2. High resolution de-convoluted spectrums are inaccurate, please double check.

3.Fig.5a and Fig.5b need error bars.

 

4. Too too much keywords.

 

5. NO-Electrocatalysts studies.

 

6. Where is the part 3??

Author Response

Reviewer-1 response sheet

Reviewer-1: I am confused about several results of this work:

Response: We thank the reviewer for appreciating the content of the manuscript and providing us with valuable comments about the article. All changes/updates made in response to the review comments are highlighted in “Red” color in the revised manuscript.

1.The XRD pattern of Pd-MXene catalysts shown in Fig.1c is much more clear than the MXene pattern in Fig.1b?

Response: This is an important comment from the reviewer, which we appreciate. In the SAED pattern, we show the individual SAED patterns for smaller Pd NPs and MXene sites. An analysis of SAED in Figure 1j confirms that the smaller Pd NPs with high crystallinity can be formed from few of the larger Pd NPs which is confirmed by the XRD pattern. SAED patterns obtained are consistent with articles recently published on Pd and MXene. Example; Nanoscale Research Letters 4(2):181-186; ACS nano, 15(9), pp.14071-14079; Sensors and Actuators B: Chemical. 2021 Jan 1;326:128969.

2. High resolution de-convoluted spectrums are inaccurate, please double check.

Response: Thank you for this important suggestion and comment, reviewer-1. The deconvoluted XPS spectrums are double-checked and presented based on the literature.

3.Fig.5a and Fig.5b need error bars.

Response: I appreciate your comment and suggestion. In Fig.5, the error bar is shown as instructed by the reviewer.  

4. Too too much keywords.

Response: Thank you. As instructed by the reviewer, keywords can be revised.

5. NO-Electrocatalysts studies.

Response: Thank you. The manuscript presents electrocatalytic studies. You can find the lines 221-245 on page no.6

6. Where is the part 3??

Response: The typos have been corrected. Page 8-9 contains part.3.

Reviewer 2 Report

MANUSCRIPT – CATALYSTS - 1908968

In this manuscript, Pd-decorated 2D MXene (2D Ti3C2Tix) as a high-performance electrocatalyst for reducing carbon dioxide into fuels toward mitigation of climate change, the authors have studied the application of the designed electrocatalyst by combining MXene and palladium nanoparticles in the CO2 reduction into methanol, thereby giving a novel way to tackle the negative impact of fuel affecting climate change. In the last five years, the exploitation of MXene has been widely investigated in various areas because of its inherent properties. Thereby the authors suggest overcoming the drawbacks of palladium nanoparticles and also increasing efficiency as a catalyst-  

General comments

The language must be checked with the native English speaker.

Word spelling must be checked.

Check the subscript for the chemical formula

CO2RR refers to a CO2 reduction reaction, not an activity; please change

The title can be modified as a Pd-decorated 2D MXene (Ti3C2Tix) as a high-performance electrocatalyst for CO₂ reduction in fuels toward climate change mitigation.

Keywords: can be limited to 5

Most of the acronym is missing the abbreviations; it is better at least to mention them once

Major comments that should be addressed

Introduction

Line 34 – 42 must be removed as it is the content of the manuscript template.

Line 48 – authors refer to studies; some more references can be added instead of 1

Line 69 – format?

Line 66 – 76. The authors highlight only the studies of 2D graphene, which could compare with other 2D materials too.

Materials Methods

Synthesis of Pd-MXene nanocomposites

Line 335 - Synthesis method reference is missing

Line 342 – Centrifugation parameters are missing like rpm & time

Line 348 – Better to explain on what basis the microwave parameters were chosen or to cite the work

Results

Physicochemical properties of the electrocatalysts

Line 127 – Ti3c2 (110) peak is missing

Line 132 – MXene 2θ = 18.2° is for the (006) hkl plane, which is not seen in the composite (c) and 2θ = 27.7° is for the (008) plane. Apart from Pd NPs and MXene peaks, there are other peaks (103, 104 & 107), too; please explain.

Line 134 – Usually Scherrer formula is employed for spherical or quasi-spherical particles. This is a 2D composite, so the calculation is not recommendable and unacceptable.

Usually, after the formation of the composite, the peaks will be shifted, but here the 2θ is precisely similar to that of individual components; please explain why.

Line 141 -172 – Better to explain SEM and TEM analysis in a separate paragraph instead of explaining both to avoid confusion

Line 160 – The author mentioned the uniform decoration of Pd NPs. It is better to show TEM instead (for example, more than 100 nm scale) of HR-TEM. Also, image f is a mere cluster of NPs, not a uniform decoration. So need a better explanation.

Figure 2f, g & j – is not mentioned in the explanation paragraph

Figure 2j – interpretation is missing

Line 173 – 178 – The explanation needs to be checked as the image alphabets are mentioned wrongly

Line 185 - Pd3d5/2 & Pd3d3/2 peaks locations are mentioned wrong, please check it

Figure 3a – the x-axis is binding energy, not 2θ

Line 216 – Please mention the recently reported SSA values

CO2RR activity

Line 234 - MXene current density (j) is expressed in mA/cm-2 instead of mV/cm-2? I believe it is -11 from the graph, not 16.5. Check it

Line 237 - Pd-MXene electrode current density (j) is -10.5 not -17.5 mA cm-2 from the graph. Or something wrong with the figure?

Check the interpretation CO2RR activity paragraph because the explanation does not match the figure.

Figure 5d – The mechanism is explained well

Conclusion

The author concludes that the Pd NPs size of 6 nm significantly impacts CO2 reduction.

Why has the study not compared different sizes of NPs to conclude in a more prospective way? Or why is it limited to 6 nm NPs?

Author Response

Reviewer-2 response sheet

In this manuscript, Pd-decorated 2D MXene (2D Ti₃C₂Ti_x) as a high-performance electrocatalyst for reducing carbon dioxide into fuels toward mitigation of climate change, the authors have studied the application of the designed electrocatalyst by combining MXene and palladium nanoparticles in the CO₂ reduction into methanol, thereby giving a novel way to tackle the negative impact of fuel affecting climate change. In the last five years, the exploitation of MXene has been widely investigated in various areas because of its inherent properties. Thereby the authors suggest overcoming the drawbacks of palladium nanoparticles and also increasing efficiency as a catalyst-  

Response: We thank Reviewer-2 for providing us with valuable comments on our article. All changes/updates made in response to the review comments are highlighted in red in the revised manuscript.

General comments

The language must be checked with the native English speaker.

Response: We thank the reviewer for this valuable suggestion. This manuscript has been reviewed by a native English speaker.

Word spelling must be checked.

Response: Spelling has been checked throughout the manuscript.

Check the subscript for the chemical formula

Response: All subscripts have been checked and corrected.

CO₂RR refers to a CO₂ reduction reaction, not an activity; please change

Response: Thank you for this important suggestion, reviewer. The revised manuscript includes all necessary suggestions.

The title can be modified as a Pd-decorated 2D MXene (Ti3C2Tix) as a high-performance electrocatalyst for CO₂ reduction in fuels toward climate change mitigation.

Response: We thank the reviewer suggestion. According to the reviewer's instructions, the title has been revised. 

New title: Pd-decorated 2D MXene (2D Ti₃C₂Ti_x) as a high-performance electrocatalyst for reduction of carbon dioxide into fuels toward climate change mitigation

Keywords: can be limited to 5

Response: A maximum of 5 keywords has been revised.

Keywords: Pd Nanoparticles; Electrocatalysts; 2D MXene Nanosheets; CO₂ Reduction; Carbon Neutrality.  

Most of the acronym is missing the abbreviations; it is better at least to mention them once

Response: We thank the reviewer for this important suggestion. A revised version of the manuscript includes all abbreviations.

Major comments that should be addressed

Introduction

Line 34 – 42 must be removed as it is the content of the manuscript template.

Response: Thanks to the reviewer for this observation. We have removed the inappropriate lines mentioned above.

Line 48 – authors refer to studies; some more references can be added instead of 1

Response: Line 41-43 has been updated with more references.

 

Line 69 – format?

Response: Changed format to formate intermediates. Please see line 68.

Line 66 – 76. The authors highlight only the studies of 2D graphene, which could compare with other 2D materials too.

Response: 2D carbides-based CO₂ reduction studies have been included in the revised text.

Materials Methods

Synthesis of Pd-MXene nanocomposites

Line 335 - Synthesis method reference is missing

Response: We thank the reviewer for this important suggestion. We have included the reference in the synthesis method section.

Line 342 – Centrifugation parameters are missing like rpm & time

Response: In the experimental section, centrifugation parameters are included. Please see lines 332-334.  

Line 348 – Better to explain on what basis the microwave parameters were chosen or to cite the work

Response: For microwave parameters, a reference has been cited. Please see Line 341.

Results

Physicochemical properties of the electrocatalysts

Line 127 – Ti3c2 (110) peak is missing

Response: We appreciate the reviewer's important observation. The revised text includes the (110) plan. Please see page no 125.

Line 132 – MXene 2θ = 18.2° is for the (006) hkl plane, which is not seen in the composite (c) and 2θ = 27.7° is for the (008) plane. Apart from Pd NPs and MXene peaks, there are other peaks (103, 104 & 107), too; please explain.

Response: Thank you, reviewer, for pointing out this important point. The XRD pattern has been revised as follows: 

In the diffraction pattern, Pd exhibits the broadened and high intense diffraction peaks at 40.2°, 46.5°, and 68.1°, are associated with the (111), (200), (220) crystal planes, respectively which corresponds to fcc Pd (JCPDS no. 05-0681), while the well-defined peaks of (002), (008), (103), (104), (107), and (110) at 2θ = 8.9°, 26.8°, 34.1°, 36.2°, 45.2° and 61.5° respectively can be assigned to the diffraction of the functionalized hexagonal 2D MXene nanosheets.[21] A non-appearance of the (006) plane and the appearance of (103), (104), and (107) indicate that MXene nanosheets were functionalized and exfoliated with DMSO. The DMSO forms hydroxyl and oxygen functional groups on the surfaces of the MXene (Ti3C2Tix) nanosheets.

Line 134 – Usually Scherrer formula is employed for spherical or quasi-spherical particles. This is a 2D composite, so the calculation is not recommendable and unacceptable.

Response: Thank you for pointing out this important suggestion. We completely agree with you. To determine the crystallite size of the Pd NPs, we used the major plan of (111).

A plane of (111) of Pd in the nanocomposites was used to estimate the crystallite size of the Pd NPs, which was estimated to be 5.8 nm by the Scherrer formula, which indi-cates that MXene nanosheets reduce the size of the Pd NPs. Particularly, MXene nanosheets prevent aggregation of Pd metallic nuclei, resulting in smaller Pd particles and enhancing the uniform distribution of Pd NPs on the surfaces of MXene nanosheets.

Usually, after the formation of the composite, the peaks will be shifted, but here the 2θ is precisely similar to that of individual components; please explain why.

Response: Thank you for pointing out this important suggestion. The XRD section is revised as follows,

 In the diffraction pattern, Pd exhibits the broadened and high intense diffraction peaks at 40.2°, 46.5°, and 68.1°, are associated with the (111), (200), (220) crystal planes, respectively which corresponds to fcc Pd (JCPDS no. 05-0681), while the well-defined peaks of (002), (008), (103), (104), (107), and (110) at 2θ = 7.5°, 27.1°, 34.1°, 36.2°, 45.2° and 61.4° respectively can be assigned to the diffraction of the functionalized hexagonal 2D MXene nanosheets.[21] A non-appearance of the (006) plane and the appearance of (103), (104), and (107) indicate that MXene nanosheets were functionalized and exfoliated with DMSO. MXene nanosheets exhibited a shift in peak positions due to functionalization and composite effects. The DMSO forms hydroxyl and oxygen functional groups on the surfaces of the MXene (Ti₃C₂Ti_x) nanosheets. A plane of (111) of Pd in the nanocomposites was used to estimate the crystallite size of the Pd NPs, which was estimated to be 5.8 nm by the Scherrer formula, which indicates that MXene nanosheets reduce the size of the Pd NPs.

Line 141 -172 – Better to explain SEM and TEM analysis in a separate paragraph instead of explaining both to avoid confusion

Response: The reviewer made an important suggestion, which we appreciate. To avoid any confusion, we have revised this section. 

Line 160 – The author mentioned the uniform decoration of Pd NPs. It is better to show TEM instead (for example, more than 100 nm scale) of HR-TEM. Also, image f is a mere cluster of NPs, not a uniform decoration. So need a better explanation.

Response: We agree with the reviewer comments. The SEM image (100 nm scale) in Fig.2e shows that Pd NPs were formed on the surfaces of MXene nanocomposites. The uniform decoration has been removed and the text has been revised accordingly.

Figure 2f, g & j – is not mentioned in the explanation paragraph

Response: These are typos. The section has been revised and the appropriate Figure caption has been added to the manuscript. Please see lines; 155- 177

The revised text as follows,

SEM micrographs of Fig.2e show that the Pd NPs were successfully incorporated into the exfoliated MXene nanosheets. Furthermore, TEM micrographs with different magnification revealed that spherical-like Pd NPs with a diameter of 5 to 6.5 nm were decorated on the surfaces of MXene nanosheets, as shown in Fig.2f&g. Furthermore, Fig.2h shows the magnified HR-TEM image of the Pd NP and its lattice interplanar distance is estimated to be 0.22 nm in the crystal plane (111) of the fcc Pd NPs.[22] As shown in Fig.2i, HR-TEM images of hexagonal MXene nanosheets shows their lattice fringes with d-spacing of 0.27 nm, which is in good agreement with previous reports.[21] Moreover, in the SAED pattern shown in Fig.2j, the Pd catalytic sites exhibit the lattice planes of (111), (200), and (220), which is more consistent with the XRD results. Furthermore, the MXene nanosheets in hybrid nanocomposites have a hexagonal structure which is confirmed by the SAED pattern in Fig.2k.[21]

Figure 2j – interpretation is missing

Response: This is a typo error and all typographical errors have been corrected.  

Line 173 – 178 – The explanation needs to be checked as the image alphabets are mentioned wrongly

Response: Thank you very much for observing these typo errors. All typographical errors have been corrected.    

Line 185 - Pd3d_5/2 & Pd3d_3/2 peaks locations are mentioned wrong, please check it

Response: We appreciate the reviewer's important observations. Additionally, peak binding energies for Pd3d_5/2 & Pd3d_3/2 were corrected.

The revised text as follows,

In Fig.3b, the deconvoluted Pd 3d XPS spectrum of the Pd-MXene nanocatalysts denotes that Pd3d_5/2 and Pd3d_3/2 double peaks located at 336.69 and 342.13 eV correspond to metallic Pd^o, whereas a small peak at 338.5 eV corresponds to Pd-O, indicating NaBH₄ is capable of completely reducing Pd²⁺ ions into metallic Pd^o under microwave irradiation

 

Figure 3a – the x-axis is binding energy, not 2θ

Response: We really thank the reviewer for this observation. The caption for the x-axis has been corrected. Figure 3a in the revised manuscript has been updated.

Line 216 – Please mention the recently reported SSA values

Response: Thank you so much for this important comment. Relevant literature is cited for the SSA values.

CO₂RR activity

Line 234 - MXene current density (j) is expressed in mA/cm-2 instead of mV/cm-2? I believe it is -11 from the graph, not 16.5. Check it

Response: We appreciate your important observation. Using a 3-electrode channel, we repeated LSV studies to determine the exact electrocatalytic behavior toward CO2RR. All graphs are corrected for IR and plotted against RHE. Revisions are made based on the onset potentials of electrodes and revisions of the text as follows.

The electrocatalytic properties of the Pd NPs, MXene, and Pd-MXene were carried out in a gas-tight 3-electrode cell at room temperature. Fig. 4(a) shows the LSV curves of the Pd NPs, MXene, and Pd-MXene electrodes at 10 mV s^-1 as the scan rate in 1.0 M KHCO₃ as the electrolyte solution. A Pd NP exhibits an overpotential of -229 mV, while MXene exhibits an overpotential of -502 mV, indicating that Pd is a more active metallic electrocatalyst than MXene. However, a hybrid Pd-MXene electrode exhibited similar electrocatalytic behavior to Pd NPs at an overpotential of -381 mV, suggesting that metallic Pd and MXene nanosheets are combined to perform superior electrocatalytic activity than MXene nanosheets. Electrocatalytic performance has been enhanced dramatically due to the large surface area of 2D MXene nanocomposites and the high conductivity of the metallic Pd phase. A CO₂-saturated KHCO₃ electrolyte solution was used as the electrolyte solution for LSV experiments over Pd nanoparticles, MXene, and Pd-MXene nanocomposites at room temperature. As shown in Fig.4b, the Pd-MXene electrode exhibited a lower onset potential (E_o) of -154 mV than the Pd (E_o=-278 mV) and MXene (E_o=-365 mV) electrodes. A lower E_o is caused by the CO₂ reduction process occurring on the electrode surfaces.   Specifically, the Pd-MXene exhibits synergistic electrochemical effects, as CO₂ is largely accommodated on the surface of the MXene nanosheets, while the metallic sites of Pd are primarily involved in the reduction of CO₂, which enhances the CO₂RR process as compared to pure Pd and MXene based electrodes. Moreover, Pd-MXene also displayed a higher difference in current density (j=-4.1 mV cm^-2) than previously reported Cu/Ti₃C₂T_x (-0.12 mA cm²), Ti₃C₂T_x/g-C₃N₄ (-2.9 mA cm²), and Ti₃C₂T_x/Bi₂WO₆ nanosheets (-3.3 mA cm²), and was also better than most MXenes reported in the literature.[27-29]          

Line 237 - Pd-MXene electrode current density (j) is -10.5 not -17.5 mA cm-2 from the graph. Or something wrong with the figure?

Response: We appreciate your important observation. The text has been revised based on the Figure 4.

Check the interpretation CO2RR activity paragraph because the explanation does not match the figure.

Response: We appreciate your important observation. The text has been revised based on the Figure 4.

Figure 5d – The mechanism is explained well

Response: Thank you. Furthermore, the mechanism has been revised in more detail.   

Conclusion

The author concludes that the Pd NPs size of 6 nm significantly impacts CO₂ reduction. Why has the study not compared different sizes of NPs to conclude in a more prospective way? Or why is it limited to 6 nm NPs?

Response: This is an important suggestion from the reviewer, which we thank him for. A further study will be conducted to determine the performance of CO₂RR versus different sizes of Pd NPs. Taking this perspective into account will be the basis for our future research.

Round 2

Reviewer 1 Report

 

1.The figures in full article need to be reformatted, due to the text in some figures was stretched out of shape. 2.Fig.5b need error bars. 3..Several related review and paper are suggested to cite in this work in the background,such as DOI: 10.1039/d0cs01599g, https://doi.org/10.1016/j.envres.2021.112651. doi: 10.3390/membranes12040368

 

 

     

 

 

Author Response

Reviewer-1

Response: We thank reviewer-1 for providing us with valuable comments on our article. All changes/updates made in response to the review comments are highlighted in red in the revised manuscript.

1.The figures in full article need to be reformatted, due to the text in some figures was stretched out of shape.

Response: All the figures qualities are updated

2.Fig.5b need error bars. 

Response: Figure 5b is updated with error bars

3..Several related review and paper are suggested to cite in this work in the background, such as DOI: 10.1039/d0cs01599g, https://doi.org/10.1016/j.envres.2021.112651. doi: 10.3390/membranes12040368

Response: Thank you. All the suggested articles are cited. Please see References 7-9

 

Reviewer 2 Report

The authors have considered all the suggestions and made the necessary changes/modifications. So my suggestion would be to accept in the present form.

Author Response

 I am grateful for reviewer-2's recommendation to publish this revised manuscript