CO Oxidation Reaction by Platinum Clusters on the Surface of Multiwalled Carbon Nanotubes: Experimental and Theoretical Study of Kinetics in a Wide Range of O2/CO Ratios
, Lidiya Kibis 1, Andrey Stadnichenko 1, Olga Stonkus 1, Daniil Zhuravlev 1, Andrey Zadesenets 2, Sergey Korenev 2, Olga Podyacheva 1 and Andrei Boronin 1,*
Round 1
Reviewer 1 Report
The paper by Lashina et al. addresses CO oxidation on Platinum. Despite the fact that this system has been studied frequently, it is still of interest to the community, especially under the aspect of cluster size dependent reaction mechanisms.
I cannot say much about the experimental methods. They sound reasonable to me. I also agree with the conclusions drawn here.
My expertise is modeling. Here, I think the paper would need some improvement.
- -The model description could be more precise. On page 8, line 257, “iterative algorithms” were mentioned. This is quite generic. Can you describe in more detail the solution of the differential equation system?
- -The model parameter \gamma was not given anywhere. I understand that this is the adsorption capacity, here measured in units of pressure. Did you model it as a temperature dependent parameter? Usually, I would expect that the amount of catalyst correlates to a number of moles. When you convert that into a pressure according to ideal gas law, you get a linear dependence on temperature.
- -The units of k_2 (lines 293 and 374) should be 1/(s Torr)
- -By the way, why do you use the old-fashioned unit Torr? I would prefer SI units or something compatible like bar.
- -The weakest point for me is the missing of explanation how the mechanism parameters were obtained. Were they fitted manually to the experimental data? Did you use some parameter estimation tool?
Interesting for me was that the authors are able to describe the aging of the nanoparticle clusters despite the simplicity of the model. I know that this would go beyond the scope of the paper, but can you think of some dynamic switching between mechanisms according to aging conditions?
It would also be good to have this paper proofread by a native English speaker. There were some language errors, especially in the use of articles the/a, which made the paper a bit difficult to read fluently. Some minor spelling mistakes (e.g. line 33: oxygen-rich, or the word dependencies) should be corrected. From the title, it is clear what the abbreviation MWCNT means, but nevertheless it should be defined on first appearance.
In summary, I think these issues can be fixed in a minor revision.
Author Response
R1.1. The model description could be more precise. On page 8, line 257, “iterative algorithms” were mentioned. This is quite generic. Can you describe in more detail the solution of the differential equation system?
Authors reply:
Thank you very much for your question. To describe the kinetic data, we develop both the reaction mechanisms and the mathematical models. The parameter values are chosen to describe the data of the kinetic experiment. In the present paper we specified not only the rate constants but also the reaction mechanism. The reaction of CO oxidation over platinum surface has been extensively studied. In particular, we studied the reaction in the following works: Lashina et al doi: 10.1016/j.ces.2022.118328; Lashina et al. doi: 10.1016/j.cej.2009.02.017. The classical mechanism of CO oxidation over metallic platinum (Langmuir-Hinshelwood mechanism) consists of three stages. At elevated temperatures, the oxidation of the Pt surface occurs, and the reaction proceeds according to the Mars-van Krevelen mechanism. In this work we refined the values of the corresponding kinetic parameters to describe the kinetic data.
Thus, we consider a comprehensive approach, which includes the selection of the minimal set of the stages of the reaction mechanism based on the literature data and the results of the experimental studies. Then values of the kinetic parameters were refined using the directional descent method.
In accordance with your comment we clarified the phrase about “iterative algorithms” in the revised manuscript on p. 9:
“The limitation of the reaction by heat and mass transfer processes was not taken into account. To describe the kinetic experimental data and estimate the rate constants of the individual stages the qualitative methods of dynamical systems theory, the methods of solving the initial problems for stiff differential equations implemented in the MATLAB package, and iterative algorithms the directional descent approach were used.
R1.2. The model parameter \gamma was not given anywhere. I understand that this is the adsorption capacity, here measured in units of pressure. Did you model it as a temperature dependent parameter? Usually, I would expect that the amount of catalyst correlates to a number of moles. When you convert that into a pressure according to ideal gas law, you get a linear dependence on temperature.
Authors reply:
Thank you for the question. We added the description of the parameter into the revised version of the manuscript on p. 10 as follows:
“where is the residence time, and the coefficient depends on the adsorption capacity of the catalyst in the unit of the reactor volume. The coefficient was calculated as , where is the amount of Pt on the surface of the catalyst, mol; R is the universal gas constant; T is the temperature and is the volume of the gas in the reactor.”
R1.3. The units of k_2 (lines 293 and 374) should be 1/(s Torr). By the way, why do you use the old-fashioned unit Torr? I would prefer SI units or something compatible like bar.
Authors reply:
We agree with the Reviewer. To unify with the units accepted in the literature we recalculated the rate constants in bar. See corrections on p.10 and p.13:
“The rate constants were estimated as: k1 = 6.75´104 s-1mbar-1, s-1, k2 = 750 s-1 mbar-1, s-1, s-1, s-1bar-1, kcal/(mol´K), and T, K – temperature.”
“To describe the data of the kinetic experiments, the values of the rate constants of the stages were estimated as: s-1 bar-1, s-1, s-1bar-1, s-1, s-1, s-1, s-1 bar-1, kcal/(mol´K) and T, K – temperature.”
R1.4. The weakest point for me is the missing of explanation how the mechanism parameters were obtained. Were they fitted manually to the experimental data? Did you use some parameter estimation tool?
Authors reply:
Since the aim of the present paper was to develop both the reaction mechanism and the corresponding simple kinetic model, which are able to describe the TPR-CO+O2 kinetic data and the dynamics of CO conversion under isothermal conditions we did not consider the optimization problem. Thus, we did not use the estimation tools to determine the rate constants. To describe the experimental data, we stated the reaction mechanism and fined the estimations of the rate constants. Please, see our reply on the question R1.1.
R1.5. Interesting for me was that the authors are able to describe the aging of the nanoparticle clusters despite the simplicity of the model. I know that this would go beyond the scope of the paper, but can you think of some dynamic switching between mechanisms according to aging conditions?
Authors reply:
Thank you very much for your question. The switching between mechanisms is presented in Figure 13. The high CO conversion during the initial interval of CO+O2 exposure is related to the reaction over the oxidized Pt centers (MvK mechanism). When the critical concentration of the reduced centers is achieved the fast switching to the CO assisted O2 dissociation mechanism takes place. The following decrease of the CO conversion is related to the slow accumulation of the adsorbed CO species.
In accordance with your comment the following sentence was inserted on p. 16:
“The sharp decrease in the CO conversion is due to a sharp fast decrease in the rate of CO consumption as a result of the concerted mechanism (stage 1) (see Figure 13). So, this decrease of CO conversion is related to the switching between the MvK mechanism and the CO assisted O2 dissociation mechanism.”
R1.6. It would also be good to have this paper proofread by a native English speaker. There were some language errors, especially in the use of articles the/a, which made the paper a bit difficult to read fluently. Some minor spelling mistakes (e.g. line 33: oxygen-rich, or the word dependencies) should be corrected. From the title, it is clear what the abbreviation MWCNT means, but nevertheless it should be defined on first appearance.
Authors reply:
Thank you for pointing out the errors in the text. We carefully checked the manuscript and corrected the errors. We have also defined the abbreviation MWCNT in the text (page 2).
In summary, I think these issues can be fixed in a minor revision.
Thank you very much for your useful comments.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript titled CO “Oxidation Reaction by Platinum Clusters on the Surface of Multi-Walled Carbon Nanotubes. Experimental and Theoretical Study of Kinetics in a Wide Range of O2/CO Ratios"
In this manuscript, Authors reported that the Catalytic Activity for CO Oxidation can be enhanced by Pd nanoparticles supported on multi-walled carbon nanotube. However, the authors failed to explain and draw out the novelty of the work, this aspect needs to be improved. This work is worthwhile to be publish in this journal after major revision. The following issues should be addressed:
1. Introduction is well-organized and well-written, but the importance and novelty of the research should be highlighted and more clearly stated. The authors give some examples of works in the bibliography, but which is the advantage of their work in comparison with those works.
2. Maybe the author should compare their results clearly with other reported works, highlighting the advantage and disadvantages of their novel composite.
3. The authors are responsible for the English, which should be polished throughout the manuscript to clear some minor typo/grammar errors.
4. Introduction part, if possible, some important and relative reports references could helped: https://doi.org/10.1007/s10562-022-04026-y, DOI: 10.1039/d0ra09970h, https://doi.org/10.1016/j.jtice.2021.08.034.
5. Abstract not targeted; the authors should rephrase it.
6. There is a lack in characterization techniques that should be added like XRD, BET, .. etc.
Author Response
R2.1. Introduction is well-organized and well-written, but the importance and novelty of the research should be highlighted and more clearly stated. The authors give some examples of works in the bibliography, but which is the advantage of their work in comparison with those works. Maybe the author should compare their results clearly with other reported works, highlighting the advantage and disadvantages of their novel composite.
Authors reply:
Following the Reviewer’s comment, we modified the Introduction part and added the necessary information on importance and novelty of the presented study. Also, we extended the reference list with short description of the literature results for comparison with the experiments presented in our study.
The additions are presented below as follows:
On p.3:
“The similar approach for the stabilization of the dispersed noble metal species using the supports based on the metal-organic frameworks (MOF-74) or the reduced graphene oxides (rGO@Cu-BTC, rGO@Mg-BTC) was described in the works [19-21]. The catalysts comprising Au0 or Pd0 nanoparticles as active components demonstrated the high activity in CO oxidation reaction at temperatures below 100oC [19-21]. The authors pointed an important role of the dispersion of the active component and the influence of the surface oxygen species on the activity of the systems in CO oxidation reaction [10.1007/s10562-022-04026-y; 10.1016/j.jtice.2021.08.034]. However, the authors did not perform the modeling of the kinetic data to describe the mechanism of the reaction.”
On p.3:
“In this work we performed a detailed investigation of the kinetic features in the CO oxidation reaction over the Pt clusters supported on the MWCNTs. We considered the CO oxidation reaction proceeded in the temperature-programmed reaction mode and in isothermal regime with variation of the O2/CO ratio in a wide range. Such detailed study would provide the deep insight in understanding the reaction mechanism on the Pt clusters. The investigation of the reaction in the temperature-programmed reaction mode demonstrated for the first time the high activity of the Pt/MWCNTs catalysts at low temperatures even below 0oC. The comparison with the similar experiments obtained on the bulk Pt foil showed the significantly different behavior of CO conversion depending on temperature. The main difference was connected with inability of Pt foil to catalyze CO oxidation at temperatures below 100oC that was confirmed by simulation of CO conversion curves on the base of both Langmuir-Hinshelwood and Mars-van Krevelen mechanisms. The comparison of the experimental and simulated data provided insights into the understanding of the reaction mechanisms on the dispersed platinum species showing that the description of the kinetic data is possible only within the framework of a combination of several mechanisms including CO assisted O2 dissociation mechanism which is responsible for CO2 formation at low-temperature. The developed mathematical model allowed us to simulate the kinetic curves obtained in the isothermal experiments in a wide range of O2/CO ratio that has a significant practical value for air purification in the enclosed spaces.”
R2.2. The authors are responsible for the English, which should be polished throughout the manuscript to clear some minor typo/grammar errors.
Authors reply:
We have corrected typo/grammar errors.
R2.3. Introduction part, if possible, some important and relative reports references could helped: https://doi.org/10.1007/s10562-022-04026-y, DOI: 10.1039/d0ra09970h, https://doi.org/10.1016/j.jtice.2021.08.034.
Authors reply:
Please see the reply on the comment R2.1.
R2.4. Abstract not targeted; the authors should rephrase it.
Authors reply:
We have rephrased Abstract as follows:
This work presents a systematic study of the kinetic aspects of the CO oxidation reaction catalyzed by platinum nanoparticles (NPs) supported on the surface of multi-walled carbon nanotubes (MWCNTs). The presented investigation is closely related to the actual practical task of air purification in enclosed spaces.. Therefore, the catalytic reaction was carried out in the presence of an excess of oxygen (5 vol.%) and over a wide range of CO concentrations from 50 ppm to 1600 ppm. For the catalyst characterization, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were applied. Kinetic modelling based on the Langmuir-Hinshelwood and Mars-van Krevelen mechanisms was taken as a basis, using the results obtained on Pt foil. Simulation of the CO oxidation reaction on platinum NPs at temperatures above 90 °C was carried out using a kinetic model describing the reaction mechanism on bulk platinum. The description of the kinetics of the CO oxidation reaction on Pt NPs over the entire temperature range, including the low temperatures down to -40 °C, required the introduction of the steps characterizing an additional concerted mechanism related to CO assisted O2 dissociation. Using the presented model, some predictions of the kinetic behaviour of the system were made.
R2.5. There is a lack in characterization techniques that should be added like XRD, BET, .. etc.
Authors reply:
Our paper is focused on the kinetic modeling of the CO oxidation reaction on the Pt-based systems. To compare theoretical results with the experimental data we studied the catalytic activity of two different systems: massive Pt foil and Pt nanoparticles. In characterization of the samples by physicochemical methods we were focused on the parameters most important for the comparison with the theoretical data: the oxidation state of platinum and the size of the Pt nanoparticles. The oxidation state of platinum was studied by XPS method (see Figure 2 of the manuscript). The size of the Pt nanoparticles was analyzed by TEM method. The TEM results confirmed the formation of the dispersed Pt nanoparticles. In our earlier paper [doi:10.1134/S0012501622700038] we have studied the Pt nanoparticles deposited on the MWCNTs by XRD method and also showed the formation of the dispersed Pt nanoparticles.
We added the reference to our earlier paper on p.4:
“According to TEM data, nanoparticles of the platinum with an average size of 2.5 nm are formed on the surface of the MWCNTs (Figure 1a). The interplanar distances measured from high-resolution images correspond to metallic platinum (Figures 1b,c). The particles are located near tube defects such as breaks or bends of the graphene layers. These data are in good agreement with our previous results [24]. We have shown earlier by X-ray diffraction method that deposition of the platinum species on the MWCNT surface from the (Me4N)2[Pt2(µ-OH)2(NO3)8] complexes results in the formation of the metallic Pt crystallites of about 3 nm in size.”
Information about SBET of the supported Pt nanoparticles (the Pt-C sample) is given in the experimental part. The surface area of the Pt foil is rather hard to measure, but it can be estimated as 0.003 m2/g. We added this information in the experimental part on p. 18:
“The Pt foil (20×15mm, 99.99% purity, and weight of 0.2 g, and the estimated SBET~0.003m2/g) was rolled around the thermocouple pocket loaded into the reactor so that the space between the lap of the foil, the thermocouple pocket, and the reactor walls was filled up with the a-Al2O3 particles of 0.25−0.5 mm in size.”
We believe that the physicochemical characterization provided in our paper should be sufficient because any additional experiments might shift the focus of the readers from the results of the kinetic modeling.
Author Response File: Author Response.pdf
Reviewer 3 Report
1. The figure title should contain more information about the conditions.
2. What is the rate of stage 1 and stage 6 the same at the initial in Fig 13?
3. It would be good to make comparsion for the calculated results if there were experimental results.
4. For Fig 9, how is the inlet CO concentration at 0.2% with O2/CO=5000?
5. For Fig 12, given the CO conversion is 1.0, does it mean the overall conversion of CO2 should be 1.0 and the area when CO2 production <1.0 and the area when CO2 production >1.0 should have a sum at 0?
Author Response
R3.1. The figure title should contain more information about the conditions.
Authors reply:
In the revised manuscript we provided the additional information in the figures’ captions.
R3.2. What is the rate of stage 1 and stage 6 the same at the initial in Fig 13?
Authors reply:
Thank you for your question. The rate of stage 1 is higher than the rate of stage 6 during the whole exposure range. In the revised version of the manuscript we modified Figure 13. Indeed, at the exposition value about 250 ppm´min these rates are very similar, but they are not the same. The rate of stage 1 is 3.00´10-6 s-1 and the rate of stage 6 is 2.97´10-6 s-1.
R3.3. It would be good to make comparsion for the calculated results if there were experimental results.
Authors reply:
We compared the results of the modeling with the experimental data in Figures 8 and 10. Also, the dynamics of CO conversion under isothermal conditions in Figure 11 can qualitatively describe the experimental results in Figure 5. Moreover, Figure 12 reflects the dependence of CO conversion on the temperature presented in Figure 7.
R3.4. For Fig 9, how is the inlet CO concentration at 0.2% with O2/CO=5000?
Authors reply:
The results of the modeling presented in Figure 9 are predictive. They were used to demonstrate that the CO conversion cannot be observed at low temperature even if we supply the significant increase of the O2 concentration in the gas phase. We did not experimentally study the reaction under these conditions. For clarity we changed the part of the text on p.12 as follows:
“Thus, the modeling data presented in Figure 9b show predict that even with a significant increase of the number of the active centers and the O2/CO ratio, the low-temperature activity in the CO oxidation reaction on massive platinum cannot be achieved.”:
R3.5. For Fig 12, given the CO conversion is 1.0, does it mean the overall conversion of CO2 should be 1.0 and the area when CO2 production <1.0 and the area when CO2 production >1.0 should have a sum at 0?
Authors reply:
The following sentence was inserted on p.15:
The mean value of CO2 production during the whole temperature interval equals 1.
The results presented in Figure 12 agree with the equations for , and , which are:
So we consider the following sum of these three equations:
,
which means that the sum of CO and CO2 concentrations in the reactor depends on the flow of the reagents, but not on the chemical reactions.
Thus, in the case when there is no CO in the gas phase and the CO2 production depends on CO surface coverage, i.e.
.
We can consider two time intervals: and with and . Moreover suppose that |CO2 production takes the following values: for and for , and
Then the following equality takes place:
.
As a result, the mean value of CO2 production equals to 1.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Accepted in the present form
