Methanol Oxidation on Graphenic-Supported Platinum Catalysts
, Elena Pastor * and Gonzalo García *Round 1
Reviewer 1 Report
Authors tested the methanol oxidation reaction (MOR) with Pt/GO catalysts before and after thermal treatment or nitrogen doping. They characterized there with the physicochemical techniques required and measured their catalytic performance to check any influence on the surface electronic states. In general, however, the performance of Pt/rGO-TT isn't so much impressive for reader's attention. Questions and comments are the following.
1. Introduction
Further intensive literature survey is recommended for the reason why they selected rGO-TT and N-rGO as support. In addition, they need to compare their catalytic performance to other Pt/rGO catalysts with and without doping from literatures.
2. EDS analysis in Table 2.
XRD patterns of N-rGO has a peak at 10.7 degree in Figure 1. Authors interpreted a partial reduction of GO when caffeine was employed as a reduction agent for N-rGO. If so, whey the oxygen content of N-rGO is lower than rGO-TT in Table 2? It also has to be related to the XPS results, but not clear in Figures 4 and 5.
3. TEM
When they measured the nanoparticle sizes, did they also count them at the aggregated area in the TEM image of Pt/N-rGO (Figure 3)? There are a lot of particles, but it seems to be challenging to measure the size distribution precisely. If they didn't, it might increase the error bar. In addition, they didn't mention the actual loading amount of Pt, which is important for activity, but only theoretical value 20%. What if the catalysts have different amount of Pt loading, which caused the main difference in MOR? ICP-MS is strongly recommended. Otherwise, they may estimate it from XPS or EDS.
4. XPS
C 1s and N 1s spectra for Pt/N-rGO are missing in Figures 4 and 5. Legends for C 1s are imprecise in Figure 4. In addition to the deconvolution results, relative atomic concentration obtained by applying atomic sensitivity factors to the peak area should be reported together. The deconvoluted components need to be matched to the corresponding elements or species. For example, if Pt 2+ is assigned to PtO in Figure 4, its peak area has to be matched to the area of the corresponding component in O 1s spectra in Figure 5. This is the fundamental in deconvolution process. All other elements or species should be matched for more reliable deconvolution results in Table 3.
5. The current transients were measured between 0.05 and 1.0 V with a rate of 0.02 V/s in Figure 8. What do the symbols at 0, 100, 200 and 300 s indicate for?
6. On page 11, any reference for the mechanisms for MOR and CO oxidation is required.
7. Pt/rGO-TT shows slight enhancement of CO tolerance and better performance than Pt/C, but not that much. Authors need to explain why Pt-rGO-TT is still important.
Author Response
Ms. Ref. No.: surfaces-401326
Title: Methanol oxidation on graphenic-supported Pt catalysts
December 7th, 2018
Dear N. Alonso-Vante and G. Granozzi,
First of all, we would like to thank you and the reviewers for the valuable comments and detailed revision, which have certainly been really helpful to improve our work. Please find attached the revised version of the manuscript entitled “Methanol oxidation on graphenic-supported Pt catalysts” (Ref. No.: surfaces-401326). We have considered all the comments made by the referees and introduced the necessary amendments into the revised version of the paper. All changes have been highlighted in yellow in the revised manuscript. The answers to the referees’ comments and the description of the changes made accordingly are listed below.
Please, accept my best personal regards
Gonzalo García
Reviewers' comments:
Reviewer 1:
Authors tested the methanol oxidation reaction (MOR) with Pt/GO catalysts before and after thermal treatment or nitrogen doping. They characterized there with the physicochemical techniques required and measured their catalytic performance to check any influence on the surface electronic states. In general, however, the performance of Pt/rGO-TT isn't so much impressive for reader's attention. Questions and comments are the following:
1. Introduction: Further intensive literature survey is recommended for the reason why they selected rGO-TT and N-rGO as support. In addition, they need to compare their catalytic performance to other Pt/rGO catalysts with and without doping from literatures.
Response: we really appreciate the reviewer’s suggestion. The following paragraphs and references have been added to “introduction” and “results and discussion” sections:
“Many works have described chemical exfoliation methods of graphite to obtain the so-called graphene oxide (GO) as catalysts support. However, the conductivity of such materials is lower than the highly desirable two-dimensional graphene because of the high amount of oxygen groups on the surface. To fulfill conductivity problems, a great diversity of physical and chemical methods have been described in the literature for GO reduction to obtain reduced GO (rGO) with higher conductivity than GO itself 15–18. On the other hand, it has been demonstrated that the surface chemistry of the catalytic support plays an essential role in the activity of the catalyst toward MOR 19-23.”
“Figure 7 shows the lowest performance for Pt/N-rGO toward the MOR, which is in disagreement with previously reported by other authors (Liu, D., Li, L., & You, T. (2017). Superior catalytic performances of platinum nanoparticles loaded nitrogen-doped graphene toward methanol oxidation and hydrogen evolution reaction 33,34. This different catalytic performance may be ascribed to the elevated amount of Pt surface oxide species (PtO and PtO2 observed by XPS analysis) by using the caffeine route for the synthesis of the N-rGO that inhibits the methanol adsorption step (see below).”
2. EDS analysis in Table 2: XRD patterns of N-rGO has a peak at 10.7 degree in Figure 1. Authors interpreted a partial reduction of GO when caffeine was employed as a reduction agent for N-rGO. If so, why the oxygen content of N-rGO is lower than rGO-TT in Table 2? It also has to be related to the XPS results, but not clear in Figures 4 and 5.
Response: The peak at 10.7 degree was already reported in reference 24 in the original version of the manuscript. Nevertheless, the text was modified in the new version of the manuscript: Page 8, first paragraph: “On the other hand, it is also important to note that N-rGO reveals a small diffraction peak at 10.7° associated to GO, which suggests a partial reduction of GO when caffeine is employed as a reducing agent 24.”
The presence of this diffraction peak may arise from nitrogen oxide species but we don’t have evidence to state the presence of this species. Thus, we believe that the best option is just to suggest that it is related to oxide species.
On the other hand, XPS is a surface sensitive technique, while XRD provides bulk analysis. Therefore, it is not so simple the analysis and correlation between both techniques.
3. TEM: When they measured the nanoparticle sizes, did they also count them at the aggregated area in the TEM image of Pt/N-rGO (Figure 3)? There are a lot of particles, but it seems to be challenging to measure the size distribution precisely. If they didn't, it might increase the error bar. In addition, they didn't mention the actual loading amount of Pt, which is important for activity, but only theoretical value 20%. What if the catalysts have different amount of Pt loading, which caused the main difference in MOR? ICP-MS is strongly recommended. Otherwise, they may estimate it from XPS or EDS.
Response: The actual loading of Pt is mentioned in the original version of the manuscript. Page 9, first paragraph: “Scanning electron spectroscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) was employed to study the morphology, distribution, surface topography and the semi-quantitative bulk composition of the catalysts. The amount of Pt incorporated into each sample is close to the nominal value (19.0 ± 1.5 wt. %).”
On the other hand, particle sizes were not obtained from aggregate area. Nevertheless, the particle sizes are in agreement with crystallite sizes achieved by XRD. Indeed, TEM images were selected to show the effect of the catalyst support not only on the particle size but also on the agglomeration degree and homogeneity of Pt particles.
4. XPS: C 1s and N 1s spectra for Pt/N-rGO are missing in Figures 4 and 5. Legends for C 1s are imprecise in Figure 4. In addition to the deconvolution results, relative atomic concentration obtained by applying atomic sensitivity factors to the peak area should be reported together. The deconvoluted components need to be matched to the corresponding elements or species. For example, if Pt 2+ is assigned to PtO in Figure 4, its peak area has to be matched to the area of the corresponding component in O 1s spectra in Figure 5. This is the fundamental in deconvolution process. All other elements or species should be matched for more reliable deconvolution results in Table 3.
Response: we really appreciate the reviewer’s suggestion. Actually, it seems that an error happened during the manuscript uploading in the journal platform. In the new version of the manuscript Figures 4 and 5 were amended. Also, the deconvolution process was redone according to the reviewer’s suggestion and the results included in the table 3. Additionally, the text was modified in the new version of the manuscript. Pages 10 and 11: text highlighted in yellow.
5. The current transients were measured between 0.05 and 1.0 V with a rate of 0.02 V/s in Figure 8. What do the symbols at 0, 100, 200 and 300 s indicate for?
Response: Figure 8 shows a chronoamperometry study, in which the current along the time is recorded at different applied potentials (0.50 V ≤ Ef ≤ 0.90 V). The symbols are to make reading easier for readers. In this sense, the currents are recorded every 0.1 second.
6. On page 11, any reference for the mechanisms for MOR and CO oxidation is required.
Response: Following referee recommendation the following references were introduced in the new version of the manuscript (page 11):
“Guillén-Villafuerte, O., García, G., Guil-López, R., Nieto, E., Rodríguez, J. L., Fierro, J. L. G., Pastor, E. Carbon monoxide and methanol oxidations on Pt/X@ MoO3/C (X= Mo2C, MoO2, Mo0) electrodes at different temperatures. J. Power Sources 2013, 231, 163-172.”
“Gisbert, R., García, G., Koper, M. T. Oxidation of carbon monoxide on poly-oriented and single-crystalline platinum electrodes over a wide range of pH. Electrochim. Acta 2011, 56(5), 2443-2449.”
“Gilman, S. The mechanism of electrochemical oxidation of carbon monoxide and methanol on platinum. II. The “Reactant-Pair” mechanism for electrochemical oxidation of carbon monoxide and Methanol1. J. Phys. Chem. 1964, 68(1), 70-80”
7. Pt/rGO-TT shows slight enhancement of CO tolerance and better performance than Pt/C, but not that much. Authors need to explain why Pt-rGO-TT is still important.
Response: This work is devoted to study the effect of the catalyst-support in the whole catalytic activity of the catalysts. In this regard, it was observed that Pt/C and Pt/rGO-TT revealed similar catalytic performance toward the MOR, although a slight enhancement of the CO tolerance at Pt/rGO-TT was perceived. The last seems to be related to electronic effects (charge transfer from rGO-TT to Pt) and to higher amount of C-OH species onto the graphenic-based catalyst (bifunctional effect). Therefore, we believe that a better understanding of the factors governing the catalytic performance will help to researchers to develop catalysts with enhanced activity towards the MOR.
Reviewer 2 Report
The paper systematically investigated the catalytic properties of Pt nanoparticles coated GO, rGO, N-rGO, and carbon Vulcan composite catalysts in the methanol oxidation reaction. Extensive characterizations were employed to identify their differences in elemental composition, electronic structure, element valence state, and catalytic performance, etc. Results demonstrated that Pt/rGO-TT show the outstanding catalytic propensity in methanol oxidation. The result is convincing, clear, and concise, and it presented insight into the significant roles of the graphitic based supports in affecting the catalytic activity of MOR. The paper is well written and organized. I’m glad to accept a minor revised paper which has considered the following questions:
1. The author should further highlight the significance of MOR in terms of their widely used catalysts. Some important papers are recommended: Nanoscale 7 (4), 1250-1269; Chemical reviews 112 (11), 6027-6053; Chemical Society Reviews 39 (8), 3157-3180
2. From the XRD results, it is advisable to calculate the size of Pt nanoparticles by Scherrer Equation. The results are highly recommended to compare with their measurements presented in Figure 3.
3. Even in single atomic noble metal catalysts, metal-O bonds are unavoidable. Deconvolution peaks of Pt(II) and Pt(IIII) can be observed from Figure 4. Taken together, the author should clarify the presence of Pt-O interaction in their serial catalysts.
4. The results indicated Pt/N-rGO exhibited the lowest activity in MOR, which is opposite to the consensus that nitrogen-doped graphene-based catalysts provided more active sites for electron transfer. Can the author explain this phenomenon?
5. I’d like to let the author add one more paragraph to discuss how the supports result in distinct catalytic activities.
Author Response
Ms. Ref. No.: surfaces-401326
Title: Methanol oxidation on graphenic-supported Pt catalysts
December 7th, 2018
Dear N. Alonso-Vante and G. Granozzi,
First of all, we would like to thank you and the reviewers for the valuable comments and detailed revision, which have certainly been really helpful to improve our work. Please find attached the revised version of the manuscript entitled “Methanol oxidation on graphenic-supported Pt catalysts” (Ref. No.: surfaces-401326). We have considered all the comments made by the referees and introduced the necessary amendments into the revised version of the paper. All changes have been highlighted in yellow in the revised manuscript. The answers to the referees’ comments and the description of the changes made accordingly are listed below.
Please, accept my best personal regards
Gonzalo García
Reviewer 2:
The paper systematically investigated the catalytic properties of Pt nanoparticles coated GO, rGO, N-rGO, and carbon Vulcan composite catalysts in the methanol oxidation reaction. Extensive characterizations were employed to identify their differences in elemental composition, electronic structure, element valence state, and catalytic performance, etc. Results demonstrated that Pt/rGO-TT show the outstanding catalytic propensity in methanol oxidation. The result is convincing, clear, and concise, and it presented insight into the significant roles of the graphitic based supports in affecting the catalytic activity of MOR. The paper is well written and organized. I’m glad to accept a minor revised paper which has considered the following questions:
1. The author should further highlight the significance of MOR in terms of their widely used catalysts. Some important papers are recommended: Nanoscale 7 (4), 1250-1269; Chemical reviews 112 (11), 6027-6053; Chemical Society Reviews 39 (8), 3157-3180.
Response: we appreciate the positive feedback from the reviewer. All changes have been highlighted in yellow in the revised manuscript. According to the reviewer’s suggestion the following works were introduced in the new version of the manuscript:
i) “Nanoscale 7 (4), 1250-1269”, ii) “Chemical reviews 112 (11), 6027-6053” and iii) “Chemical Society Reviews 39 (8), 3157-3180”.
2. From the XRD results, it is advisable to calculate the size of Pt nanoparticles by Scherrer Equation. The results are highly recommended to compare with their measurements presented in Figure 3.
Response: We agree with the reviewer that crystallite size is an important factor. However, the Pt crystallite sizes were already reported and compared with particle sizes obtained by TEM in the original version of the manuscript: i) Table 1; ii) Page 8, last paragraph: “The most important crystallographic parameters of Pt-based catalysts are depicted in Table 1, in which similar interplanar spacing for all catalysts, smaller crystallite size for Pt/N-rGO than Pt/C and Pt/rGO-TT and a small but visible lattice contraction in graphenic-based catalysts are discerned.”; iii) Page 10, first paragraph: “Morphology, particle size and agglomeration degree were studied by transmission electron microscopy (TEM). Figure 3 shows TEM images of Pt/C, Pt/rGO-TT and Pt/N-rGO catalysts. Spherical Pt nanoparticles are homogeneously dispersed on carbon sheets, however, the agglomeration degree increases with the amount of oxygen into the catalyst in the subsequent way: Pt/C < Pt/rGO-TT < Pt/N-rGO. Therefore, Pt nanoparticles seem to nucleate and growth preferentially at surface oxygenated sites. Regarding the particle sizes, Figure 3 includes histograms for each catalyst and it is found an increment in the following way: Pt/ N-rGO (3.14 ± 0.50 nm) < Pt/C (3.28 ± 0.25 nm) < Pt/rGO-TT (3.32 ± 0.43 nm). The last is in agreement with the trend observed for the crystallite size values calculated by XRD, although the values are slightly lower than those achieved by TEM, which is the expected, i.e. crystallite size ≤ grain size ≤ particle size.”
3. Even in single atomic noble metal catalysts, metal-O bonds are unavoidable. Deconvolution peaks of Pt(II) and Pt(IIII) can be observed from Figure 4. Taken together, the author should clarify the presence of Pt-O interaction in their serial catalysts.
Response: According to the reviewer’s suggestion all species (not only Pt-O interaction) are reported in the text, table 3, figure 4 and figure 5 in the new version of the manuscript.
4. The results indicated Pt/N-rGO exhibited the lowest activity in MOR, which is opposite to the consensus that nitrogen-doped graphene-based catalysts provided more active sites for electron transfer. Can the author explain this phenomenon?
Response: we really appreciate the reviewer’s suggestion. The following paragraph and references have been added to the “results and discussion” section:
“Figure 7 shows the lowest performance for Pt/N-rGO toward the MOR, which is in disagreement with previously reported by other authors (Liu, D., Li, L., & You, T. (2017). Superior catalytic performances of platinum nanoparticles loaded nitrogen-doped graphene toward methanol oxidation and hydrogen evolution reaction 33,34. This different catalytic performance may be ascribed to the elevated amount of Pt surface oxide species (PtO observed by XPS analysis) by using the caffeine route for the synthesis of the N-rGO that inhibits the methanol adsorption step (see below).”
5. I’d like to let the author add one more paragraph to discuss how the supports result in distinct catalytic activities.
Response: Several paragraphs, including those discussing about the different catalytic activity, were added and highlighted in yellow in the new version of the manuscript.
Round 2
Reviewer 1 Report
Authors answered the reviewer's questions and revised the manuscript according to them with additional references. Comments and questions are the following.
1. Table 2
Atom. % is recommended to be included also for EDX analysis, even though Pt loading is considered in weight %. The relative atomic concentrations could be calculated form their XPS peak area and atomic sensitivity factors. They already have data's o it would be helpful if included.
2. Figure 3
Their high resolution images are recommended to be included also. Lower resolution images are good enough for dispersion, not average nanoparticle sizes.
3. Figure 4
A legend for C 1s is required on the right column as Pt 4f on the left. In addition, the x format is in reverse order, so C 1s spectra may be better at the left side to follow the x scale.
4. Figures 4 and 5
Only Figures 4 and 5 use different sample names from others. They have to be matched for consistency.
5. Table 3
The components in Table 3 don't match to them in Figures 4 and 5. They need to be the same names and numbers. They already assigned them well, but not used in Table 3.
Author Response
Dear N. Alonso-Vante and G. Granozzi,
First of all, we would like to thank you and the reviewer for the valuable comments and detailed revision, which have certainly been really helpful to improve our work. Please find attached the revised version of the manuscript entitled “Methanol oxidation on graphenic-supported Pt catalysts” (Ref. No.: surfaces-401326). We have considered all the comments made by the referees and introduced the necessary amendments into the revised version of the paper. All changes have been highlighted in yellow as well as with "Track Changes" function in Microsoft Word in the revised manuscript. Additionally, high-resolution TEM images for all catalysts are included as Supporting Information in the revised version of the manuscript. The answers to the referees’ comments and the description of the changes made accordingly are listed below.
Please, accept my best personal regards
Gonzalo García
Reviewers' comments:
Reviewer 1:
1. Table 2: Atom. % is recommended to be included also for EDX analysis, even though Pt loading is considered in weight %. The relative atomic concentrations could be calculated form their XPS peak area and atomic sensitivity factors. They already have data's o it would be helpful if included.
Response: we really appreciate the reviewer’s suggestion. We agree that this information may be helpful, although we don’t believe that it is crucial for the understanding of the main idea behind this work. On the other hand, we are in holiday-time and our working place is closed, and at the same time the Editorial Office gave us only 5 days to upload the new version of the manuscript.
Thus, no action will be taken on this question.
2. Figure 3: Their high resolution images are recommended to be included also. Lower resolution images are good enough for dispersion, not average nanoparticle sizes.
Response: We already answer this question to the First Reviewer Report: “Particle sizes were not obtained from aggregate area. Nevertheless, the particle sizes are in agreement with crystallite sizes achieved by XRD. Indeed, TEM images were selected to show the effect of the catalyst support not only on the particle size but also on the agglomeration degree and homogeneity of Pt particles.”
Nevertheless, high-resolution TEM images for all catalysts are included as Supporting Information in the revised version of the manuscript.
3. Figure 4: A legend for C 1s is required on the right column as Pt 4f on the left. In addition, the x format is in reverse order, so C 1s spectra may be better at the left side to follow the x scale.
Response: The figure was amended and replaced in the revised version of the manuscript.
4. Figures 4 and 5: Only Figures 4 and 5 use different sample names from others. They have to be matched for consistency.
Response: Figures were amended and replaced in the revised version of the manuscript.
5. Table 3: The components in Table 3 don't match to them in Figures 4 and 5. They need to be the same names and numbers. They already assigned them well, but not used in Table 3.
Response: Table 3 was amended in the revised version of the manuscript.
