Hydroxyl Radical Formation on Metal-Loaded Ga2O3 Photocatalysts for Dehydrogenative Coupling of Methane to Ethane with Water

Photocatalytic non-oxidative coupling of methane(photo-NOCM; 2CH4 -> C2H6+H2) can directly convert methaneinto ethane and hydrogen at room temperature. However, the apparentquantum efficiency (AQE) of photo-NOCM is very low in the absenceof oxidants. We observed that photocatalytic dehydrogenative couplingof methane (photo-DHCM) proceeds in the presence of water vaporusing Ga2O3-based photocatalysts under ultraviolet (UV) lightirradiation. Photo-DHCM is efficiently induced over Pt/Ga2O3andPd/Ga2O3photocatalysts, accompanied by steam reforming of methane(photo-SRM). For C2H6production, the AQE of photo-DHCM withwater vapor was more than 2 orders of magnitude higher than that ofconventional photo-NOCM. However, the role of the metal co-catalystsupported on Ga2O3for the production of C2H6and H2in photo-DHCM is unclear. The reaction mechanism accelerated by watervapor was assumed in which CH4was activated by a hydroxyl radical (center dot OH) and homocoupling occurred by the two methyl radicals.Therefore,center dot OH formation affects the productivity and selectivity of the photocatalytic CH4conversion. In this study, we investigatedthe effect of metal co-catalysts (Au, Ag, Ru, Rh, Pd, and Pt) supported on a Ga2O3photocatalyst on center dot OH formation by an electronspin resonance (ESR) method using a spin-trapping agent. ESR measurements upon UV irradiation revealed that the center dot OHconcentration was high in Au/Ga2O3, which exhibited a high C2H6production rate (1140 mu mol gcat-1h-1; AQE = 4.3% at 254 nm)and high C2H6selectivity (92.2% on a carbon basis). In contrast, Rh/Ga2O3, which had the lowest center dot OH concentration, exhibitedhigh selectivity for photo-SRM and water-splitting reactions compared to photo-DHCM