Oxygen-mediated water splitting on metal-free heterogeneous photocatalyst under visible light

We report a visible-light-driven, photocatalytic partial water splitting process that proceeds via an oxygen-mediated, sequential single-electron transfer mechanism over heterogeneous photocatalysts. In the absence of any cocatalyst, hydrogen evolution efficiency of 78.9 and 643.0 μmol h⁻¹ g_catalyst⁻¹ were achieved over a simple carbon nitride catalyst from methanol and formaldehyde solutions, respectively, in 1.0 bar O₂ atmosphere at room temperature. On the carbon nitride catalyst, molecular O₂ is photocatalytically transformed into reactive oxygen species, which acted both as an oxidant and as a homogeneous catalyst for the partial photocatalytic water splitting reaction, resulting in the two-step oxidation of methanol to formaldehyde and subsequently formic acid. DFT calculations reveal that the new oxygen mediated photocatalytic hydrogen evolution pathway proceeds with virtually zero activation energy. Further improvement in hydrogen evolution efficiency was attained by doping the carbon nitride catalyst with phosphorous and sulfur, which promoted the transport of photogenerated charge carriers.