Solar-Driven Reduction of Aqueous Protons Coupled to Selective Alcohol Oxidation with a Carbon Nitride-Molecular Ni Catalyst System

Solar water-splitting represents an important strategy toward production of the storable and renewable fuel hydrogen. The water oxidation half-reaction typically proceeds with poor efficiency and produces the unprofitable and often damaging product, O-2. Herein, we demonstrate an alternative approach and couple solar H-2 generation with value-added organic substrate oxidation. Solar irradiation of a cyanamide surface-functionalized melon-type carbon nitride ((CNx)-C-NCN) and a molecular nickel(II) bis(diphosphine) H-2-evolution catalyst (NiP) enabled the production of H-2 with concomitant selective oxidation of benzylic alcohols to aldehydes in high yield under purely aqueous conditions, at room temperature and ambient pressure. This one-pot system maintained its activity over 24 h, generating products in 1:1 stoichiometry, separated in the gas and solution phases. The (CNx)-C-NCN-NiP system showed an activity of 763 mu mol (g CNx)(-1) h(-1) toward H-2 and aldehyde production, a Ni-based turnover frequency of 76 h(-1), and an external quantum efficiency of 15% (lambda = 360 +/- 10 nm). This precious metal-free and nontoxic photocatalytic system displays better performance than an analogous system containing platinum instead of NiP. Transient absorption spectroscopy revealed that the photoactivity of (CNx)-C-NCN is due to efficient substrate oxidation of the material, which outweighs possible charge recombination compared to the nonfunctionalized melon type carbon nitride. Photoexcited (CNx)-C-NCN in the presence of an organic substrate can accumulate ultralong-lived trapped electrons, which allow for fuel generation in the dark. The artificial photosynthetic system thereby catalyzes a closed redox cycle showing 100% atom economy and generates two value-added products, a solar chemical, and solar fuel.