Methane Activation through Single-Electron Transfer from Water Molecules to the Surface States of Semiconductor Photocatalysts

Photocatalytic transformation of methane to ethane and hydrogen (2CH4 → C2H6 + H2) is enhanced over metallic cocatalyst-loaded Ga2O3 particles under gas-flow conditions with water vapor. We examined the photocatalytic activity of oxide materials (Ga2O3, NaTaO3:La, SrTiO3:Al, and AgTaO3) loaded with Pd or Rh–Cr oxides, which are hydrogen-evolving cocatalysts. Electron spin resonance analysis revealed that the hydroxyl radical (•OH) generated through the single-electron oxidation of water is the active species for methane activation to produce C2H6 and carbon dioxide over Pd/Ga2O3 and Pd/NaTaO3:La photocatalysts. In contrast, the photocatalysts active for water vapor splitting rather than methane transformation generate a surface peroxyl radical (superoxo) moiety as an intermediate of oxygen evolution. For methane activation, the ionization potentials (valence band maximum and midgap surface states) of the photocatalysts should be sufficiently high to promote single-electron transfer from the water to generate •OH since methane exhibits high oxidation potential. This study unravels the crucial role of surface energy levels of photocatalysts for the activation of methane with water vapor.