Atomic Scale Engineering of Multivalence-State Palladium Photocatalyst for Transfer Hydrogenation with Water as a Proton Source.

Hydrogenation reactions are fundamental in the fine chemical, pharmaceutical, and petrochemical industries, however heavily relying on H gas at high temperatures and pressures, incurring large energy and carbon costs. Photocatalytic transfer hydrogenation, using water as a proton source, offers a greener alternative, but existing photocatalysts often suffer from modest yields, limited selectivity, and narrow substrate scope. Additionally, they often require co-activation, such as Mg-activated water or non-sustainable hydrogen feeds. Here, a photocatalyst is introduced that offers high yields and selectivities across a broad spectrum of organic compounds. The developed photocatalyst is a multivalence palladium superstructure with ultrasmall Pd nanoparticles enveloped by isolated Pd/Pd atoms within a carbon-nitride matrix. Mechanistic studies reveal that the redox-flexible Pd single atoms, with triethylamine as an electronic modulator, attract photogenerated holes for water oxidation, while Pd nanoparticles facilitate hydrogen transfer to the unsaturated bonds of the organic molecules. The cooperative and dynamic behavior of Pd centers during catalysis, involving transitions among Pd, Pd, and Pd states, is validated using operando electron paramagnetic resonance spectroscopy. This multivalent palladium catalyst represents a conceptual advance in photocatalytic transfer hydrogenation with water as a hydrogen source, holding promise for sustainable hydrogenation processes in the chemical industry.