Methane Combustion over the Porous Oxides and Supported Noble Metal Catalysts
Round 1
Reviewer 1 Report
This review paper describes the combustion of methane over porous oxide-supported noble metal catalysts. A major revision is required for publication.
1. The title of the paper needs to be clarified. What does the author mean by “porous oxides and supported noble metal catalysts?
2. Why the authors selected noble metal-based catalysts to write the review? Whereas there is an increasing concern about replacing noble metal-based catalysts with less expensive non-noble metal-based catalysts. The authors should emphasize the importance of noble metal catalysts. The authors should also make a comparison table between the efficiency difference of noble metal catalysts and non-noble metal catalysts by giving examples of already published data in the literature.
3. The authors should describe the importance and need for this review on methane decomposition in the introduction section.
4. The authors should cite important references such as https://doi.org/10.1016/j.rser.2022.112774
5. The authors should describe the mechanism for methane combustion on various metal catalysts. What different metals or metal oxides contribute to the combustion process? What are the side products and intermediates?
6. The authors should add future perspectives and the possibility of the applicability of noble metal catalysts at an industrial scale.
7. The authors should describe the limitations of noble metal-based catalysts and the possibilities to overcome these shortcomings. How the cost of noble metal-based catalysts can be decreased.
Comments for author File: 
Comments.pdf
Author Response
Reviewer #1: This review paper describes the combustion of methane over porous oxide-supported noble metal catalysts. A major revision is required for publication.
Question 1: The title of the paper needs to be clarified. What does the author mean by “porous oxides and supported noble metal catalysts?
Response: Thank the Reviewer for the good suggestion. The key issue of methane combustion is to develop highly efficient catalysts. The high activity of a catalyst is related to a number of factors, such as redox property, oxygen vacancy concentration, high thermal stability, active component content and its dispersion state, crystal phase structure, and specific surface area. Thus, it is necessary to prepare catalysts with large specific surface areas, suitable supports, ultrahigh component dispersion, and well-ordered pore structures. The pore structure of a catalyst with a large surface area can accelerate the adsorption and activation of CH4 and O2 molecules, thus facilitating mass transfer of CH4 in the reaction system to reach a better contact with the surface active centers. This review article summarizes the recent advances in the preparation of ordered porous oxides which include transition-metal oxides, hexaaluminate-type oxides, and perovskite-type oxides as well as their supported (single or alloyed) noble metal nanocatalysts and their applications for catalytic methane combustion.
Question 2: Why the authors selected noble metal-based catalysts to write the review? Whereas there is an increasing concern about replacing noble metal-based catalysts with less expensive non-noble metal-based catalysts. The authors should emphasize the importance of noble metal catalysts. The authors should also make a comparison table between the efficiency difference of noble metal catalysts and non-noble metal catalysts by giving examples of already published data in the literature.
Response: Thank the Reviewer for the good suggestion. Most of the works reported in the literature are based on noble metals catalysts for methane combustion. Actually, we have compared catalytic activities of the noble metal catalysts and non-noble metal catalysts, as shown in several tables of the manuscript.
Modification: Top/Page 4: Methane is the most stable hydrocarbon and the breakdown of C-H bond is considered to be the rate-determining step for methane combustion. Noble metals possess special electron states of d-band, which is beneficial for the activation of the C−H bond to decrease the activation energy of methane dissociation for low-temperature oxidation. Therefore, noble metals catalysts are most widely investigated in catalytic methane combustion due to its lower light-off temperature and high resistance to carbon deposition and sulfur dioxide.
Question 3: The authors should describe the importance and need for this review on methane decomposition in the introduction section.
Response: Thank the Reviewer for the good suggestion.
Modification: Middle/Page 5: Generally speaking, CH4 can be adsorbed and activated on the transition-metal oxides and supported noble metals catalysts. In a view of relevant studies, significant efforts have been emphasized to synthesize numerous novel catalysts with excellent performance and protected active sites. Generating a porous structure is one of the most available strategies to develop the high-surface-area materials, so that the number of active sites can be increased and the catalytic activity is thus improved significantly. Up to now, however, a systematic and comprehensive review on catalytic materials with well-ordered meso- or macroporous structure and large-surface-area supports to facilitate uniform and ultrahigh noble metal dispersion and low loadings has not been seen in the literature. Hence, such a review that summarizes the recent advances in ordered meso- and macroporous oxides and supported noble metals catalysts for methane complete oxidation is highly needed.
Question 4: The authors should cite important references such as https: //doi.org/10.1016/j.rser.2022.112774.
Response: Thank the Reviewer for the good suggestion. According to the Reviewer’s suggestion, we have added the mentioned reference to the revised manuscript.
Modification: Middle/Page 2: Thermo-catalytic methane decomposition reactor system is thoroughly discussed to achieve optimized heat management and better methane conversion. Raza et al. [5] proposed a methane decomposition reactor system configuration for industrial and commercial application, and gave the involved working principle, technical advantages, and limitation. A suitable catalyst used in a reactor system can decrease the activation energy and accelerate the reaction process. However, catalyst deactivation and huge energy consumption are still big challenges to be overcome.
[5] Raza, J.; Khoja, A.H.; Anwar, M.; Saleem, F.; Naqvi, S.R.; Liaquat, R.; Hassan, M.; Javaid, J.; Qazi, U.Y.; Lumbers, B. Methane decomposition for hydrogen production: A comprehensive review on catalyst selection and reactor systems. Renew. Sustain. Energ. Rev. 2022, 168, 112774.
Question 5: The authors should describe the mechanism for methane combustion on various metal catalysts. What different metals or metal oxides contribute to the combustion process? What are the side products and intermediates?
Response: Thank the Reviewer for the good suggestion. The related modifications have been made in the revised manuscript.
Modification: Middle/Page 29-Top/Page 30: Supported noble metals catalysts are generally divided into two forms: metallic state and oxidized state. The oxygen on the surface of metal phase is usually adsorbed oxygen which exhibits a low adsorption energy. Methane dissociation is generally considered to be a rate-determining step, reaction kinetics has been discussed according to the Eley-Rideal (E-R) [101] and Langmuir-Hinshelwood (L-H) mechanisms [102]. Metal oxides always coordinate with metal atoms and lattice oxygen, where the adsorption energy of lattice oxygen is higher than that of the adsorbed oxygen. The consumed lattice oxygen during the catalytic methane oxidation process is then made up by gaseous oxygen. In this case, the Mars-van Krevelen (MvK) mechanism is proposed to discuss the redox process [103]. Methane and oxygen are adsorbed on the surface of a catalyst according to the L-H mechanism. Different intermediates are formed by adsorption and activation of methane during the dehydrogenation process. The dehydrogenation process from CH4 to the C* species on the Pd surface was revealed by the first-principles simulation [104]. The reaction process of the adsorbed methane and oxygen species can be described using the following steps [105]：
CH4 + * → CH4*
O2 + 2* → 2O*
CH4* + O* → CH3* + OH*
CH3* + O* → HCHO* + H*
HCHO* + O* → CHO* + H*
CH* + * → C* + OH*
C* + O* → CO* + *
CO* + O* → CO2* + *
CO2* → CO2 + *
OH* + OH* → O* + H2O*
H2O* → H2O + *
* represents the active adsorption sites, X* represents the adsorbed X species at the active sites.
The reaction process of the gaseous methane and adsorbed oxygen species in the E-R mechanism can be described using the following steps:
O2 + 2* → 2O*
CH4 + 2O* → HCHO* + OH*
HCHO* + O* → CHO* + OH*
CHO* + O* → CO* + OH*
CO* + O* → CO2* + *
CO2* → CO2 + *
OH* + OH* → * + H2O*
H2O* → H2O + *
The mechanism of methane oxidation with lattice oxygen vacancy formation via the MvK redox steps over PdO (101) has been investigated. Rate-controlling analysis reveals that the dissociative CH4 adsorption via hydrogen abstraction over the Pdcus–Ocus site-pairs was the rate-determining step during the light-off process. Methane is first adsorbed on the surface of a catalyst and then reacts with lattice oxygen to form CO2 and H2O in the MvK redox mechanism. Detailed methane oxidation reaction path analysis (RPA) has been discussed for the dry and the wet reaction mixture with 12 vol% water in the feedstock at 1 bar to elucidate the preferential pathway. Methane is adsorbed and reacted with lattice oxygen to generate the side products in sequence under the dry condition and temperatures of 200-400 oC: CH4 → CH3 → CH2OH → CH2O → CHO → CO → CO2. Molecular H2O adsorption or desorption contributes to vacancy formation through O2 adsorption from gas-phase and hydroxide formation or decomposition. The preferential path under the dry condition and temperatures above 400 oC is similar to that under the wet condition: CH4 → CH3 → CH2 → CH2O → CHO → CO → CO2. The major difference is reaction sequence of the hydroxyl-methyl intermediates which is due to water competitive adsorption [106].
[101] Yao, Y., Giapis, K.P. Direct hydrogenation of dinitrogen and dioxygen via Eley–Rideal reactions. Angew. Chem. Int. Ed. 2016, 128, 11767–11771.
[102] Kumar, K.V., Porkodi, K., Rocha, F. Langmuir–Hinshelwood kinetics – A theoretical study. Catal. Commun. 2008, 9, 82–84.
[103] Widmann, D., Behm, R.J. Dynamic surface composition in a Mars-van Krevelen type reaction: CO oxidation on Au/TiO2. J. Catal. 2018, 357, 263–273.
[105] Tang, Z., Zhang, T., Luo, D., Wang, Y.J., Hu, Z., Yang, R.T. Catalytic combustion of methane: From mechanism and materials properties to catalytic performance. ACS Catal. 2022, 12, 13457–13474.
[104] Jørgensen, M., Grönbeck, H. First-principles microkinetic modeling of methane oxidation over Pd(100) and Pd(111). ACS Catal. 2016, 6, 6730–6738.
[106] Stotz, H., Maier, L., Boubnov, A., Gremminger, A.T., Grunwaldt, J.-D., Deutschmann, O. Surface reaction kinetics of methane oxidation over PdO. J. Catal. 2019, 370, 152–175.
Question 6: The authors should add future perspectives and the possibility of the applicability of noble metal catalysts at an industrial scale.
Response: Thank the Reviewer for the good suggestion. The brief future perspectives and possibility of the applicability of noble metals catalysts at an industrial scale have been included in the revised manuscript.
Modification: Middle/Page 40: The biggest challenge for commercial application of noble metals catalysts is deactivation caused by agglomeration and sintering at high temperatures. Supports with high surface areas, well-ordered pore structures, and narrow pore-size distributions provide a large number of catalytic active sites and favor to stabilize noble metal NPs. Noble metal NPs are anchored on the porous skeleton with regular arrangements to improve the dispersion of the active centers, and reduce the exposure extent to get high resistance to carbon deposition and sulfur dioxide tolerance. The pore structure can give rise to a large active surface area and better structure stability of active components to increase possibility of the interaction between reactants and active sites. This review provides a theoretical support for the design and synthesis of high-thermal-stable catalysts. Noble metals supported on ordered porous oxides would be the suitable catalysts for the commercial and industrial applications of methane combustion.
Question 7: The authors should describe the limitations of noble metal-based catalysts and the possibilities to overcome these shortcomings. How the cost of noble metal-based catalysts can be decreased.
Response: Thank the Reviewer for the good suggestion. We have added the corresponding statements in the revised text.
Modification: Bottom/Page 24−Top/Page 25: Noble metals catalysts have been studied for low light-off temperatures and good C−H activation ability. However，uniformly dispersed noble metals NPs are easily sintered and aggregated to form nanoclusters at high temperatures with increased sizes, leading to poor thermal stability and irreversible catalyst deactivation. The design and preparation of the active components with specific structures can greatly improve thermal stability of the catalysts, the addition of a base metal to noble metal(s) presents an optimized noble metals utilization efficiency with a less cost. The noble metal reduction has become one of the most promising prospects for commercial applications by enhancing the efficiency of noble metal utilization, such as monoatomic, noble metal-transition metal alloy, core-shell structure design and suitable support selection. Recently, Dai’s group successfully embedded partially Pt NPs in the skeleton of 3DOM Mn2O3 support, which exhibited an excellent thermal stability and catalytic performance compared with the catalyst derived from the PVA-protecting reduction approach [77]. Another innovative strategy is developed by coating noble metals with metal oxides to construct the shell (metal oxide)-core (noble metal) structure to protect the active component. Xie et al. [33] designed a core-shell-structured Pd@Co (core: Pd; shell: Co) NPs to successfully stabilize Pd NPs and avoid sintering of the noble metal. Multiple core-shell confinement was used to fabricate the confined ultrathin Pd-CeOx nanowire (2.4 nm in diameter) catalysts with superior hydrothermal stability below 800 oC [85]. The multiple core-shell confinement effectively enhanced the interaction between metal and oxide with a ultrahigh component dispersion. The confinement and shielding effect significantly restricted the active center in the support to avoid sintering and agglomeration. The use of rare earths or transition-metal oxides as catalyst modifiers can improve the dispersion and thermal stability of noble metals and reduce their use quantities, while the use of the more thermally stable and catalytically active oxides (e.g., perovskites and hexaaluminates) as supports can achieve the same target.
Author Response File: 
Author Response.docx
Reviewer 2 Report
H. Lin and col. presented a review of the methane combustion over the porous oxides and supported noble metal catalysts. Although there are many reviews on this subject, I believe that the author's approach is very interesting because hits review article summarizes the recent advances in the preparation of ordered meso -and macroporous oxides and supported noble metal catalysts for complete methane oxidation. They present a review of the discussing the properties of various types of catalysts in detail. This review has a clear presentation and detailed content, and it is very complete. However, there are still some details that need to be dealt with.
1- In my opinion, reference to previous reviews (before 2000) on this subject is lacking. Of course, it is impossible to cite all the papers on this subject, but you could consider quoting any earlier reviews. I propose this so that young readers can see that it is a topic of high interest in catalysis and that it has been studied for decades.
2- Page 1:” …incomplete CH4 combustion of methane.. “.
Please, review this sentence.
3- Page 2: “CH4 is weakly adsorbed on the catalyst surface, while H2O and SO2 display strong adsorption, which induces poisoning and deactivation of the catalyst”...
NOx can also be strongly adsorbed on the catalytic surface. Could the authors add a short discussion about it?
4- “Transition-metal oxides display poor thermal stability, whose phase transitions induced by treating with the fluctuating reaction conditions significantly affect their catalytic activities::”
Not all transition metal oxides display poor thermal stability. Could the authors expand on this point? It is recommended to add new references.
5- Page 3: “… Structural defects and lattice oxygen mobility that is in relation with the Mars–van Krevelen mechanism are also critical factors governing the catalytic activity. “
It is suggested to briefly introduce or explain the Mars-van Krevelen mechanism.
6- Please order the citations in the text. For example, on Page 4 the citations do not follow a numerical order.
7- It is suggested to indicate that some of the referenced catalytic systems (citations 24-34) have also been used for the combustion of other VOCS. And also, since the review is oriented to the combustion of methane, it is suggested to indicate in the following paragraph the specific citations for the materials used with methane: “The same group has demonstrated that large surface area and well-developed pore architecture were the key factors for the high performance of the catalysts in methane combustion.”
8- Page 7:” Non-silicon mesoporous materials include mesoporous metal oxides/composites, mesoporous carbon, mesoporous metals, and mesoporous aluminum phosphate. The synthesis of non-silicon mesoporous materials (especially mesoporous metal oxides) is more difficult than the silicon-based materials due to the diverse components, uncontrolled hydrolysis rates, and variable chemical valence states.”
Please add bibliographical references. In addition, it is suggested to broaden the discussion on this point. There are bibliographic reports that show the preparation of very good catalysts for the combustion of methane based on mesopore oxides, for example, zirconia or silica- zirconia.
9- Please improve the quality of figure 11.
10-Could the authors indicate the effect of support acidity on catalytic performance?
11-Could the authors add a short discussion related to the effect of the presence of NOX on catalytic performance?
12-Could the authors add a short discussion about the effect of the presence of alk ali metals on catalytic performance? This point may be of interest in those cases where it is necessary to eliminate the template with bases.
Author Response
Reviewer #2: H. Lin and col. presented a review of the methane combustion over the porous oxides and supported noble metal catalysts. Although there are many reviews on this subject, I believe that the author's approach is very interesting because hits review article summarizes the recent advances in the preparation of ordered meso- and macroporous oxides and supported noble metal catalysts for complete methane oxidation. They present a review of the discussing the properties of various types of catalysts in detail. This review has a clear presentation and detailed content, and it is very complete. However, there are still some details that need to be dealt with.
Question 1: In my opinion, reference to previous reviews (before 2000) on this subject is lacking. Of course, it is impossible to cite all the papers on this subject, but you could consider quoting any earlier reviews. I propose this so that young readers can see that it is a topic of high interest in catalysis and that it has been studied for decades.
Response: Thank the Reviewer for the good suggestion. The related information has been included in the revised text.
Modification: Middle/Page 5: In the past years, several reviews on catalytic methane combustion have been reported in the literature [30–34]. For example, Ciuparu et al. [30] reviewed the Pd-based catalysts for methane conversion and discussed the catalytic performance, redox mechanism, and CH4 activation at the PdO site. The kinetics of methane combustion and the sulfur dioxide-poisoning behaviors as well as the effect of metal NPs on catalytic performance of the noble metal catalysts were reported [31]. Yang et al. summarized the outstanding methane oxidation activities of nanostructured perovskite oxides and proposed the novel catalysts design strategy via lattice oxygen activation, lattice oxygen mobility, and materials morphology engineering [32]. Bashan et al. reported the perovskite preparation methods and the substitution effects of the doped perovskites as well as the sulfur dioxide-poisoning behaviors of the perovskite catalysts [33]. Nkinahamira et al. summarized the noble metals- and transition-metal-based catalysts for methane activation and discussed physicochemical properties of the promoters, such as reduction/oxidation potential, acidity/basicity, reducibility, and oxygen storage capacity [34].
[30] Ciuparu, D.; Lyubovsky, M.R.; Altman, E.; Pfefferle, L.D.; Datye, A. Catalytic combustion of methane over palladium-based catalysts. Catal. Rev. – Sci. Eng. 2002, 44, 593–649.
[31] Gelin, P.; Primet, M. Complete oxidation of methane at low temperature over noble metal based catalysts: A review. Appl. Catal. B 2002, 39, 1–37.
[32] Yang, J.; Guo, Y. Nanostructured perovskite oxides as promising substitutes of noble metals catalysts for catalytic combustion of methane. Chinese Chem. Lett. 2017, 29, 48–56.
[33] Bashan, V.; Ust, Y. Perovskite catalysts for methane combustion: applications, design, effects for reactivity and partial oxidation. Int. J. Energy Res. 2019, 43, 7755–7789.
[34] Nkinahamira, F.; Yang, R.; Zhu, R.; Zhang, J.; Ren, Z.; Sun, S.; Xiong, H.; Zeng, Z.; Current progress on methods and technologies for catalytic methane activation at low temperatures. Adv. Sci. 2022, 2204566.
Question 2: Page 1: "… incomplete CH4 combustion of methane.. ". Please review this sentence.
Response: The mentioned sentence has been corrected in the revised text.
Modification: Middle/Page 1: ... As the main component of natural gas, incomplete CH4 combustion causes resource waste and aggravates air pollution ...
Question 3: Page 2: “CH4 is weakly adsorbed on the catalyst surface, while H2O and SO2 display strong adsorption, which induces poisoning and deactivation of the catalyst”...
NOx can also be strongly adsorbed on the catalytic surface. Could the authors add a short discussion about it?
Response: Thank the Reviewer for the good suggestion. We have added short discussion on the related statements in the revised manuscript.
Modification: Bottom/Page 1−Top/Page 2: The adsorption of NO on the active component (MnO2) and support (TiO2) was studied by the density functional theory calculations [2]. NO was adsorbed at the top sites of Mn Lewis site in the way of the O-down orientation. Although there was an electron transfer between NO and Mn, NO adsorption belonged to a weak physisorption. In the case of NO adsorption, the hybridization between Ti and NO could be found, corresponding to a strong chemisorption. Striking is the positive influence of the catalyst regeneration by introducing NOx for methane conversion in the transient re-activation experiment [3]. The oxidation of NO and reduction of NO2 on the catalyst surface during the methane conversion process could be able to re-activate the catalyst.
[2] Wei, L.; Wang, Z.W.; Liu, Y.X.; Guo, G.S.; Dai, H.X.; Cui, S.P.; Deng, J.G. Support promotion effect on the SO2 and K+ co-poisoning resistance of MnO2/TiO2 for NH3-SCR of NO. J. Hazard. Mater. 2021, 416, 126117.
[3] Gremminger, A.T.; Carvalho, H.W.P.; Popescu, R.; Grunwaldt, J.D.; Deutschmann, O. Influence of gas composition on activity and durability of bimetallic Pd–Pt/Al2O3 catalysts for total oxidation of methane. Catal. Today 2015, 258, 470–480.
Question 4: “Transition-metal oxides display poor thermal stability, whose phase transitions induced by treating with the fluctuating reaction conditions significantly affect their catalytic activities:”
Not all transition metal oxides display poor thermal stability. Could the authors expand on this point? It is recommended to add new references.
Response: Thank the Reviewer for the good comment. Some explanations and cited references have been included in the revised manuscript.
Modification: Middle/Page 19: The transition metal (e.g., Cu, Fe, Co, Ni, Mn, and Ce) oxides with typical multiple valence states help to form redox cycles of the catalytic process between the high and low oxidation states, thereby restoring and releasing the lattice oxygen species. Phase transition of the active components may affect thermal stability and catalytic performance of the catalyst [68]. Co3O4 has the weakest metal–oxygen bond, and the ease reduction of Co3+ in Co3O4 to Co2+ could accelerate formation of the oxygen vacancies at low temperatures [69]. CeO2 is one of the most effective metal oxides due to its excellent redox property and oxygen storage ability [70]. However, the high-temperature redox cycle presents a remarkable challenge to structure and reactivity of the catalyst and limits the oxygen-carrying capacity of pure CeO2. MnOx has flexible valence states (i.e., Mn2+, Mn3+, and Mn4+), among which the nanocubic MnO2 sample exhibited the best low-temperature reducibility [71]. ZrO2 as an excellent stabilizer can effectively overcome the sintering challenge caused by a sharp decrease of surface area in the high-temperature redox cycle [72]. However, thermal stability of the unmodified active NiO catalysts was poor, which could be combined with other metals for the utilization of catalytic lean methane combustion [73]. For methane combustion in the presence of sulfur dioxide, metal oxide catalysts can form sulfates at a high temperature (e.g., 550 oC), which gives rise to an irreversible deactivation of the catalysts.
[68] Paredes, J.R.; Díaz, E.; Díez, F.V.; Ordóñez, S. Combustion of methane in lean mixtures over bulk transition-metal oxides: Evaluation of the activity and self-deactivation. Energy Fuels 2009, 23, 86–93.
[69] Li, H.F.; Lu, G.Z.; Qiao, D.S.; Wang, Y.Q.; Guo, Y.; Guo, Y.L. Catalytic methane combustion over Co3O4/CeO2 composite oxides prepared by modified citrate sol-gel method. Catal. Lett. 2011, 141, 452–458.
[70] Li, Y.; Li, K.Z.; Xu, R.D.; Zhu, X.; Wei, Y.G.; Tian, D.; Cheng, X.M.; Wang, H. Enhanced CH4 and CO oxidation over Ce1–xFexO2−δ hybrid catalysts by tuning the lattice distortion and the state of surface iron species. ACS Appl. Mater. Interfaces 2019, 11, 19227–19241.
[71] Zhang, K.; Peng, X.B.; Cao, Y.N.; Yang, H.G.; Wang, X.Y.; Zhang, Y.Y.; Zheng, Y.; Xiao, Y.H.; Jiang, L.L. Effect of MnO2 morphology on its catalytic performance in lean methane combustion. Mater. Res. Bull. 2018, 111, 338–341.
[72] Liotta, L.F.; Carlo, G.D.; Pantaleo, G.; Deganello, G. Catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 composite oxides for methane combustion: influence of catalyst pretreatment temperature and oxygen concentration in the reaction mixture. Appl. Catal. B 2007, 70, 314–322.
[73] Tao, F.F.; Shan, J.-J.; Nguyen, L.; Wang, Z.; Zhang, S.; Zhang, L.; Wu, Z.; Huang, W.; Zeng, S.; Hu, P. Understanding complete oxidation of methane on spinel oxides at a molecular level. Nat. Commun. 2015, 6, 7798–7807.
Question 5: Page 3: "… Structural defects and lattice oxygen mobility that is in relation with the Mars–van Krevelen mechanism are also critical factors governing the catalytic activity"
It is suggested to briefly introduce or explain the Mars-van Krevelen mechanism.
Response: Thank the reviewer for the good suggestion. The MvK mechanism has been introduced in the revised text.
Modification: Middle/Page 30: The mechanism of methane oxidation with lattice oxygen vacancy formation via the MvK redox steps over PdO (101) has been investigated. Rate-controlling analysis reveals that the dissociative CH4 adsorption via hydrogen abstraction over the Pdcus-Ocus site-pairs was the rate-determining step during the light-off process. Methane is first adsorbed on the surface of a catalyst and then reacts with lattice oxygen to form CO2 and H2O in the MvK redox mechanism. Detailed methane oxidation reaction path analysis (RPA) has been discussed for the dry and the wet reaction mixture with 12 vol% water in the feedstock at 1 bar to elucidate the preferential pathway. Methane is adsorbed and reacted with lattice oxygen to generate the side products in sequence under the dry condition and temperatures of 200-400 oC: CH4 → CH3 → CH2OH → CH2O → CHO → CO → CO2. Molecular H2O adsorption or desorption contributes to vacancy formation through O2 adsorption from gas-phase and hydroxide formation or decomposition. The preferential path under the dry condition and temperatures above 400 oC is similar to that under the wet condition: CH4 → CH3 → CH2 → CH2O → CHO → CO → CO2. The major difference is reaction sequence of the hydroxyl-methyl intermediates which is due to water competitive adsorption [106].
[106] Stotz, H.; Maier, L.; Boubnov, A.; Gremminger, A.T.; Grunwaldt, J.-D.; Deutschmann, O. Surface reaction kinetics of methane oxidation over PdO. J. Catal. 2019, 370, 152–175.
Question 6: Please order the citations in the text. For example, on Page 4 the citations do not follow a numerical order.
Response: Thank the Reviewer for the good suggestion. We are sorry to make such errors, and have corrected the citations with a numerical order in the revised manuscript.
Modification: Please see the citations in a numerical order.
Question 7: It is suggested to indicate that some of the referenced catalytic systems (citations 24-34) have also been used for the combustion of other VOCS. And also, since the review is oriented to the combustion of methane, it is suggested to indicate in the following paragraph the specific citations for the materials used with methane: “The same group has demonstrated that large surface area and well-developed pore architecture were the key factors for the high performance of the catalysts in methane combustion.”
Response: Thank the Reviewer for the good suggestion. We have added a summary to the revised text.
Modification: Middle/Page 5: The same group has demonstrated that large surface area and well-developed pore architecture were the key factors for the high performance of the catalysts in methane combustion.
Question 8: Page 7: “Non-silicon mesoporous materials include mesoporous metal oxides/composites, mesoporous carbon, mesoporous metals, and mesoporous aluminum phosphate. The synthesis of non-silicon mesoporous materials (especially mesoporous metal oxides) is more difficult than the silicon-based materials due to the diverse components, uncontrolled hydrolysis rates, and variable chemical valence states.”
Please add bibliographical references. In addition, it is suggested to broaden the discussion on this point. There are bibliographic reports that show the preparation of very good catalysts for the combustion of methane based on mesopore oxides, for example, zirconia or silica- zirconia.
Response: Thank the Reviewer for the good suggestion.
Modification: Top/Page 8: The Pd/Al2O3 catalyst with adjustable and uniform pore sizes was prepared via the sol-gel route by adjusting the template (deoxycholic acid and polyvinyl pyrrolidone acid) amounts. The as-obtained Pd/Al2O3 catalyst with relatively large pore sizes (ca. 12 nm) exhibited an efficient and sustained catalytic methane combustion performance under a variety of operating conditions compared with the catalyst with small pore sizes (5−7 nm) [41]. Cobalt-doped ordered mesoporous alumina with intrinsic activity for methane combustion were synthesized using the sol-gel method with a P123 template. The as-obtained Pd−Co bimetallic catalyst exhibited a lower activation energy and a higher activity, which was attributed to the abundant active oxygen species for stabilizing the active PdO phase [42].
[41] Lin, J.; Zhao, L.; Zheng, Y.; Xiao, Y.H.; Yu, G.T.; Zheng, Y.; Chen, W.; Jiang, L.L. Facile strategy to extend stability of simple component-alumina-supported palladium catalysts for efficient methane combustion. ACS Appl. Mater. Interfaces 2020, 12, 56095–56107.
[42] Lin, J.; Chen, Y.L.; Liu, X.J.; Chen, X.H.; Zheng, Y.; Huang, F.; Xiao, Y.H.; Zheng, Y.; Jiang, L.L. Microstructural property regulation and performance in methane combustion reaction of ordered mesoporous alumina supported palladium-cobalt bimetallic catalysts. Appl. Catal. B 2020, 263, 118269.
Question 9: Please improve the quality of figure 11.
Response: Thank the Reviewer for the good suggestion. We have improved the quality of Figure 11 in the revised manuscript.
Modification: Please see the revised Fig. 11 in the revised manuscript.
Figure 11. (a–c) HADDF–STEM images and (d) particle size distribution of Au–Pd–3.61CoO/3DOM Co3O4, (e) hydrogen consumption and Co2+/Co3+ molar ratio, (f) Pd2+/Pd and Auδ+/Au molar ratios of (a) Au–Pd/3DOM Co3O4, (b) Au–Pd–0.19CoO/3DOM Co3O4, (c) Au–Pd–0.40CoO/3DOM Co3O4, (d) Au–Pd–0.90CoO/3DOM Co3O4, and (e) Au–Pd–3.61CoO/3DOM Co3O4, (g) methane conversion, and (h) methane conversion as a function of reaction time in the presence of 5.0 vol% water vapor in the feedstock over the samples at 340 °C. Reprinted with permission from Ref. [32]. Copyright 2017, copyright American Chemical Society.
Question 10: 10-Could the authors indicate the effect of support acidity on catalytic performance?
Response: Thank the Reviewer for the good suggestion. The support acidity effect on catalytic activity of the catalysts has been discussed in detail in the revised text.
Modification: Middle/Page 14: Acidity and basicity are mainly discussed in zeolite supports, and the positive effect of acidic sites is to anchor the active PdO component. Migration of PdO NPs on external surface of the zeolitic support and formation of larger PdO NPs during the hydrothermal treatment process may also contribute to the catalyst deactivation. The presence of the Brønsted acid sites in zeolite may facilitate the ion-exchange of Pd and stabilize the dispersed PdO NPs, however, the presence of the Brønsted acid sites in the catalyst would eventually result in poor stability of the catalyst under the reaction conditions due to conversion of the PdO NPs into the Pd cations through “protonolysis”at the acid sites [55].
[55] Lou, Y.; Ma, J.; Hu, W.D.; Dai, Q.G.; Wang, L.; Zhan, W.C.; Guo, Y.L.; Cao, X.M.; Guo, Y.; Hu, P.; Lu, G.Z. Low-temperature methane combustion over Pd/H-ZSM-5: Active Pd sites with specific electronic properties modulated by acidic sites of H-ZSM-5. ACS Catal. 2016, 6, 8127–8139.
Question 11: Could the authors add a short discussion related to the effect of the presence of NOX on catalytic performance?
Response: Thank the Reviewer for the good suggestion. A short discussion related to the effect of the presence of NOx on catalytic performance has been included in the revised text.
Modification: Top/Page 2: Re-activation of a catalyst deactivated under the lean-burn conditions could be achieved by adding NO and/or NO2. Öcal et al. [4] investigated the influence of high NO amount (2.3 vol%) on catalytic performance of the Pd/hexaaluminate catalyst. Increased activity was found in the presence of NO above 400 oC. The authors attributed such a behavior to the increased re-oxidation of Pd during the CH4 oxidation process or to the formation of the highly reactive O* species on the catalyst surface from the dissociation of NO2.
[4] Öcal, M.; Oukaci, R.; Marcelin, G.; Jang, B.W.L.; Spivey, J.J. Steady-state isotopic transient kinetic analysis on Pd-supported hexaaluminates used for methane combustion in the presence and absence of NO, Catal. Today 2000, 59, 205–217.
Question 12: Could the authors add a short discussion about the effect of the presence of alkali metals on catalytic performance? This point may be of interest in those cases where it is necessary to eliminate the template with bases.
Response: Thank the Reviewer for the good suggestion. We have added a short discussion on the effect of alkali metals presence on catalytic performance in the revised manuscript.
Modification: Bottom/Page 14: The addition of alkali metal ions could significantly improve the activity and on-stream stability of aluminosilicates or siliceous zeolitic catalysts with a relatively low Si/Al ratio (< 100) [56]. The promotion effect of Na+ was confirmed to significantly enhance stability of the catalyst in the wet feedstock, especially in the step after Pd was already deposited on the zeolitic support [57]. The incorporation of Co2+ ions into Pd/BEA (Si/Al = 13) promoted the formation of PdO NPs, thus significantly improving methane combustion performance [58].
[56] Petrov, A.W.; Ferri, D.; Kröcher, O.; van Bokhoven, J.A. Design of stable palladium-based zeolite catalysts for complete methane oxidation by postsynthesis zeolite modification. ACS Catal. 2019, 9, 2303–2312.
[57] Luo, L.; Wang, S.; Fan, C.; Yang, L.; Wu, Z.W.; Qin, Z.F.; Zhu, H.Q.; Fan, W.B.; Wang, J.G. Promoting effect of alkali metal cations on the catalytic performance of Pd/H-ZSM-5 in the combustion of lean methane. Appl. Catal. A 2020, 602, 117678.
[58] Chen, J.J.; Giewont, K.; Walker, E.A.; Lee, J.K.; Niu, Y.B.; Kyriakidou, E.A. Cobalt-induced PdO formation in low-loading Pd/BEA catalysts for CH4 oxidation. ACS Catal. 2021, 11, 13066–13076.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors revised the manuscript following the reviewer's comments.
Reviewer 2 Report
The review has been substantially improved and can be accepted for publication.
