Beyond Hydrogen Evolution: Solar-Driven, Water-Donating Transfer Hydrogenation over Platinum/Carbon Nitride

Hydrogen-rich organic molecules such as alcohols are widely used as hydrogen donors in transfer hydrogenation. Nevertheless, water as a more abundant and ecofriendly hydrogen source has hardly been used due to the high difficulty in splitting water molecules. Herein, we designed a photocatalytic water-donating transfer hydrogenation (PWDTH) technique, in which hydrogen was extracted from water under light illumination and then in situ added to different unsaturated bonds (C-C, C-O, N-O) for chemical synthesis. Platinum-loaded carbon nitride (Pt/CN) was used as the model catalyst for this cascade reaction, which is beyond its normal applications for water splitting. This approach was highly accessible to efficiency optimization, either by modifying CN for extended light absorption and enhanced charge transfer or by alloying Pt with another metal for better catalytic activities. Remarkably, a quantum efficiency of up to 21.8% was achieved for nitrobenzene hydrogenation under 380 nm irradiation, which is 3 times higher than that obtained in the single water-splitting reaction, indicating that the PWDTH can be more rewarding than hydrogen evolution for solar energy harvesting. Deep insights into the underlying mechanism were provided by detailed measurements and interpretations of femtosecond transient absorption spectra, action spectra (quantum efficiency as a function of excitation wavelength), and reaction kinetic profiles under varied conditions including the variation of light intensities, temperatures, and water isotopes. The mild reaction conditions, simple processing, and broad substituent group tolerance endow this approach with a high potential toward a general solar-to-chemical conversion technique.