In situ generation of active ⋅OH on Co-SrTiO3 tandem catalyst for conversion of methane to methanol

Recently in Chem Catalysis, He and co-workers proposed the efficient conversion of methane to methanol under moderate conditions via the active O species in situ generated from photo-splitting of H2O over Co-SrTiO3 tandem catalysts. Using characterization techniques, the authors determined the intermediates and mechanism of the reaction. Recently in Chem Catalysis, He and co-workers proposed the efficient conversion of methane to methanol under moderate conditions via the active O species in situ generated from photo-splitting of H2O over Co-SrTiO3 tandem catalysts. Using characterization techniques, the authors determined the intermediates and mechanism of the reaction. Methane is a clean energy carrier and the main component of natural gas, shale gas, and coalbed gas.1Li Q. Ouyang Y. Li H. Wang L. Zeng J. Photocatalytic conversion of methane: Recent advancements and prospects.Angew. Chem. Int. Ed. 2022; 61e202108069Google Scholar Because methane is an Earth-abundant carbon resource, its conversion to liquid fuels has attracted great attention in catalysis because the transportation of liquid fuels has fewer investment requirements. Methane can be used to produce syngas (CO/H2) and then converted to value-added liquid fuels, such as hydrocarbons and alcohols, through syngas chemistry.2Zhai P. Li Y. Wang M. Liu J. Cao Z. Zhang J. Xu Y. Liu X. Li Y.-W. Zhu Q. et al.Development of direct conversion of syngas to unsaturated hydrocarbons based on Fischer-Tropsch route.Chem. 2021; 7: 3027-3051Abstract Full Text Full Text PDF Scopus (33) Google Scholar However, the above process is energy intensive and prone to be over-oxidized, producing by-products such as CO2.3Qi G. Davies T.E. Nasrallah A. Sainna M.A. Howe A.G. Lewis R.J. Quesne M. Catlow C.R.A. Willock D.J. He Q. et al.Au-ZSM-5 catalyses the selective oxidation of CH4 to CH3OH and CH3COOH using O2.Nat. Catal. 2022; 5: 45-54Crossref Scopus (41) Google Scholar The direct conversion of methane to value-added chemicals under mild conditions is therefore promising. However, the C–H bond energy of methane is high (439 kJ mol−1), so breaking it requires high temperature and high pressure for CH4 activation.4Schwach P. Pan X. Bao X. Direct conversion of methane to value-added chemicals over heterogeneous catalysts: Challenges and prospects.Chem. Rev. 2017; 117: 8497-8520Crossref PubMed Scopus (740) Google Scholar Therefore, the direct conversion of methane to value-added chemicals under mild conditions remains a formidable challenge. Solar energy can generate photons under mild conditions to drive catalytic reactions. Via the photocatalysis process, high-energy charge carriers can pre-activate methane and greatly reduce the activation energy.5Li X. Wang C. Tang J. Methane transformation by photocatalysis.Nat. Rev. Mater. 2022; https://doi.org/10.1038/s41578-022-00422-3Crossref PubMed Scopus (43) Google Scholar Typically, the photocatalytic conversion of methane involves three steps: the separation of charge carriers (electrons or holes), the activation of the C–H bond of methane, and the participation of methyl radicals (⋅CH3 or CH3−). Currently, the widely used catalysts include doped semiconductors and hybrid semiconductors (hybrid with semiconductors or metal). For example, using the binary Au/Cu hybrid semiconductor strategy (utilizing the electron-capture and hole-filling effect of binary Au/Cu), Luo et al. reported a method for the efficient and selective photocatalytic conversion of methane to methanol and C1 oxygenates at ambient temperature.6Luo L. Gong Z. Xu Y. Ma J. Liu H. Xing J. Tang J. Binary Au–Cu reaction sites decorated ZnO for selective methane oxidation to C1 oxygenates with nearly 100% selectivity at room temperature.J. Am. Chem. Soc. 2021; 144: 740-750Crossref PubMed Scopus (43) Google Scholar However, depending on the catalysts and reaction conditions (aerobic or anaerobic), the products in the activation of methane via photocatalysis can be CH3CHO, CH3COOH, CH3OH, CO, and CO2. Among these products, methanol is a versatile platform molecule that can be converted to other important chemicals via methanol-to-olefin (MTO) or carbonylation processes.7Snyder B.E. Bols M.L. Rhoda H.M. Plessers D. Schoonheydt R.A. Sels B.F. Solomon E.I. Cage effects control the mechanism of methane hydroxylation in zeolites.Science. 2021; 373: 327-331Crossref PubMed Scopus (35) Google Scholar Therefore, a lot of work has been carried out in the development of new photocatalysts for the direct conversion of methane to methanol with high selectivity. In this case, by comparing the photocatalytic efficiency of Bi2WO6, BiVO4, and the coupled Bi2WO6/TiO2-P25 in the direct conversion of methane to CH3OH, Murcia-Lopez et al. demonstrated the BiVO4 material to be a promising photocatalyst in that it displayed the highest CH3OH selectivity of ∼48% and had excellent stability.8Murcia-Lopez S. Villa K. Andreu T. Morante J.R. Partial oxidation of methane to methanol using bismuth-based photocatalysts.ACS Catal. 2014; 4: 3013-3019Crossref Scopus (97) Google Scholar As recently published in Chem Catalysis, He and co-workers improved the CH3OH selectivity to >98% via the indirect formation of an ⋅OH radical generated from the decomposition of H2O2, which was derived from photo-oxidization of H2O, on a self-generated Co-SrTiO3 tandem catalyst in the direct photocatalysis of methane to methanol under mild conditions (Figure 1).9Zhang Z. Zhang J. Zhu Y. An Z. Shu X. Song H. Wang W. Chai Z. Shang C. Jiang S. et al.Photo-splitting of water toward hydrogen production and active oxygen species for methane activation to methanol on Co-SrTiO3.Chem Catal. 2022; 2https://doi.org/10.1016/j.checat.2022.04.008Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar In this work, Co-SrTiO3 tandem catalysts with Co sizes from 3.6 to 10.7 nm were prepared via in situ reduction of SrTiCoO3 in H2 at 800°C with a ramping rate of 20°C/min, 10°C/min, and 2°C/min, corresponding to catalysts 3.6-Co-SrTiO3, 8.3-Co-SrTiO3, and 10.7-Co-SrTiO3, respectively. In comparison, a SrTiO3-r catalyst was prepared via the same procedure but without the addition of a Co precursor. The Co-SrTiO3 materials presented the dominant {110} facets as exposed and thus had excellent migration of photoexcited electrons and activity for the photo-splitting of water. In this photocatalysis (Figure 1A), H2O was first photo-reduced to H2 on the photocathode and photo-oxidized to H2O2 on the photo-anode. Then, H2O2 decomposed to an active ⋅OH radical on Co particles. The in-situ-generated intermediate ⋅OH radical attacked the C–H bond of CH4, which was adsorbed on Co particles, so CH3OH was obtained with a high selectivity of 98.7%. According to UV-visible measurement, the presence of the Co particle attains significantly higher adsorption intensity (400–600 nm) than the reference SrTiO3-r, indicating a higher light-source utilization. With the presence of the Co particle, the band gap of the Co-SrTiO3 catalyst obviously narrows from 3.15 to 2.86 eV (Figure 1B). The valence band for SrTiO3-r locates lower than the potential for H2O to H2O2 but higher than the oxidation level for H2O to the ⋅OH radical, indicating that the photo-generated holes could oxidize H2O into H2O2 through a two-electron pathway instead of producing ⋅OH directly. The presence of the Co particle further lowers the position of the valence band from 1.80 to 1.88–2.04 eV (Figure 1B), indicating that the Co particle could promote the photo-oxidation of H2O to H2O2 but not to the ⋅OH radical. Compared with SrTiO3-r, Co-SrTiO3 produced a significantly increased formation rate of the ⋅OH radical (up to 60.6 μmol g−1 h−1), whereas the rate of H2O2 decreased from 26.8 to 5.4 μmol g−1 h−1 (Figure 1C). In addition, the formation rate of H2O2 increased, whereas that of ⋅OH decreased as Co particle size increased. This is explained by the fact that the in-situ-generated H2O2 is decomposed to the ⋅OH radical on the Co particle, resulting in the lower formation rate of H2O2 but higher formation rate of the ⋅OH radical. Moreover, the DMPO-OH signal detected on 3.6-Co-SrTiO3 was higher than that on SrTiO3-r, implying that the Co particle catalyzes the H2O2 decomposition to the ⋅OH radical. After the introduction of CH4, the in situ electron paramagnetic resonance (EPR) experiments indicated that the ⋅OH and ⋅CH3 radicals simultaneously emerged and the intensity of the ⋅CH3 signal obviously became dominant during irradiation (Figure 1D), indicating that ⋅CH3 is produced through the attack of the C–H bond of CH4 by the ⋅OH radical. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) demonstrated that two weak bands attributed to chemisorbed CH4 were observed at 2,820–2,830 cm−1, and the intensity of the chemisorbed CH4 increased as Co particle size decreased, which was not observed on SrTiO3-r. This indicates that the Co particles also provide active sites for the adsorption of CH4. The adsorbed CH4 on the Co particle provides more chances to be activated by the in-situ-generated ⋅OH radical. The Co particle size is crucial, and the CH3OH selectivity increases as Co particle size decreases (Figure 1E), indicating that the CH4 adsorption also suppresses the deep oxidization of CH3OH. In summary, this work introduces a strategy for generating the Co-SrTiO3 tandem catalyst for highly selective methanol production from methane under mild conditions. The authors also extend this strategy to other metals (iron, nickel, and copper) supported on SrTiO3, showing excellent reactivity in the reaction. The work provides insight into the design of a tandem catalyst for the efficient activation of the C–H bond of CH4 with high methanol selectivity under mild conditions. We expect that the methanol yield can be improved further through optimization of the catalyst structure in the future. Furthermore, photocatalytic activation of methane for the production of other value-added chemicals might also benefit from this work. This work was supported by the National Natural Science Foundation of China (22121001 and 22072118). The authors declare no competing interests. Photo-splitting of water toward hydrogen production and active oxygen species for methane activation to methanol on Co-SrTiO3Zhang et al.Chem CatalysisMay 4, 2022In BriefA strategy for activation of the C-H bond in CH4 via the active O species generated from photo-splitting of H2O is proposed, simultaneously generating value-added CH3OH and H2. Full-Text PDF