Hydroxyl Radical Formation on Metal-Loaded Ga2O3 Photocatalysts for Dehydrogenative Coupling of Methane to Ethane with Water
F. Amano, M. Ishimaru
Energy Fuels 2022, 36, 10, 5393-5402, https://doi.org/10.1021/acs.energyfuels.2c00401
Photocatalytic non-oxidative coupling of methane (photo-NOCM; 2CH4 → C2H6 + H2) can directly convert methane into ethane and hydrogen at room temperature. However, the apparent quantum efficiency (AQE) of photo-NOCM is very low in the absence of oxidants. We observed that photocatalytic dehydrogenative coupling of methane (photo-DHCM) proceeds in the presence of water vapor using Ga2O3-based photocatalysts under ultraviolet (UV) light irradiation. Photo-DHCM is efficiently induced over Pt/Ga2O3 and Pd/Ga2O3 photocatalysts, accompanied by steam reforming of methane (photo-SRM). For C2H6 production, the AQE of photo-DHCM with water vapor was more than 2 orders of magnitude higher than that of conventional photo-NOCM. However, the role of the metal co-catalyst supported on Ga2O3 for the production of C2H6 and H2 in photo-DHCM is unclear. The reaction mechanism accelerated by water vapor was assumed in which CH4 was activated by a hydroxyl radical (•OH) and homocoupling occurred by the two methyl radicals. Therefore, •OH formation affects the productivity and selectivity of the photocatalytic CH4 conversion. In this study, we investigated the effect of metal co-catalysts (Au, Ag, Ru, Rh, Pd,and Pt) supported on a Ga2O3 photocatalyst on •OH formation by an electron spin resonance (ESR) method using a spin-trapping agent. ESR measurements upon UV irradiation revealed that the •OH concentration was high in Au/Ga2O3, which exhibited a high C2H6 production rate (1140 μmol gcat-1 h-1; AQE = 4.3% at 254 nm) and high C2H6 selectivity (92.2% on a carbon basis). In contrast, Rh/Ga2O3, which had the lowest •OH concentration, exhibited high selectivity for photo-SRM and water-splitting reactions compared to photo-DHCM.