Trapping of Intermediates of a Photocatalytic Oxygen Evolution Reaction in Overall Water Splitting

Intermediates in the overall water splitting on particulate photocatalysts were investigated by electron paramagnetic resonance (EPR) measurements using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap. In situ EPR spectra under ultraviolet light showed the exclusive formation of a hydroxyl radical adduct (•DMPO-OH) over Ga2O3 and NaTaO3:La without a Rh–Cr oxide cocatalyst. In contrast, the Rh–Cr oxide cocatalyst-loaded photocatalysts active for water splitting formed a 5,5-dimethyl-2-oxo-pyrroline-1-oxyl radical (•DMPOX). •DMPOX is suggested as an indicator of the surface peroxyl radical intermediate (M-OO•) in the oxygen evolution reaction pathway.