Methane coupling and hydrogen evolution induced by palladium-loaded gallium oxide photocatalysts in the presence of water vapor
M. Ishimaru, F. Amano, C. Akamoto, S. Yamazoe, J. Catal., accepted.
Non-oxidative coupling of methane (NOCM, 2CH4 → C2H6 + H2) is a reaction that can directly produce ethane and hydrogen at the same time, and gallium oxide (Ga2O3) powder has been reported as an effective photocatalyst for NOCM at room temperature. In this study, we investigated the reaction conditions for Pd-loaded Ga2O3 photocatalysts to improve the production rate of C2H6 and H2. We found that the 0.1 wt% Pd/Ga2O3 exhibited high selectivity of C2H6 (75.8%, carbon-based) under the conditions of steam reforming of methane. Photocatalytic NOCM seems to proceed in the presence of water vapor. An increase in water vapor pressure (PH2O) was essential for the steady production of C2H6 and H2. The C2H6 production rate was 0.79 μmol min−1 for 50 mg of Pd/Ga2O3 powder at PH2O = 3.6 kPa. The apparent quantum efficiency (AQE) for H2 production was 11.6%, which is much higher than that of conventional NOCM in the absence of water vapor. The importance of water adsorbates on the photocatalyst surface was suggested by water vapor adsorption isotherm and Fourier transform infrared (FT-IR) spectroscopy. It is revealed that multilayered water molecules adsorbed on the photocatalyst surface play a role as a reaction field that promotes the dehydrogenative coupling of CH4.