A Protocol for Unveiling the Nature of Photocatalytic Hydrogen Evolution Reactions: True Water Splitting or Sacrificial Reagent Acceptorless Dehydrogenation?

Photocatalytic water splitting for hydrogen evolution is a highly topical subject in academic research and a promising approach for sustainable fuel production from solar energy. Due to the mismatched energy diagram of the photosensitizer (especially semiconductor-based materials where band-edge engineering is not trivial) and the redox potential of the half-reactions of water splitting, photocatalytic H₂ generation from water splitting is usually accelerated by the addition of hole scavengers, i.e. sacrificial reagents such as alcohols, amines, and thiols. However, the source of the protons of the evolved H₂ is often neglected, and it is questionable whether such systems are really water splitting. Here, we discuss recent reports on sacrificial reagent-assisted photocatalytic water splitting and present our recent findings, which showcase that the sacrificial reagent in the investigated photocatalytic water splitting systems inherently undergoes acceptorless dehydrogenation, with H₂O serving as the proton shuttle, the amount of which doesn't change during the course of the reaction.