Photoredox Cascade Catalyst for Efficient Hydrogen Production with Biomass Photoreforming

Biomass photoreforming is a promising method to provide both a clean energy resource in the form of hydrogen (H2) and valuable chemicals as the results of water reduction and biomass oxidation. To overcome the poor contact between heterogeneous photocatalysts and biomass substrates, we fabricated a new photoredox cascade catalyst by combining a homogeneous catalyst, 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), and a heterogeneous dual-dye sensitized photocatalyst (DDSP) composed of two Ru(II)-polypyridine photosensitizers (RuP6 and RuCP6) and Pt-loaded TiO2 nanoparticles. During blue-light irradiation (lambda=460 +/- 15 nm; 80 mW), the DDSP photocatalytically reduced aqueous protons to form H2 and simultaneously oxidized TEMPO center dot radicals to generate catalytically active TEMPO+. It oxidized biomass substrates (water-soluble glycerol and insoluble cellulose) to regenerate TEMPO center dot. In the presence of N-methyl imidazole as a proton transfer mediator, the photocatalytic H2 production activities for glycerol and cellulose reforming reached 2670 and 1590 mu mol H2 (gTiO2)-1 h-1, respectively, which were comparable to those of state-of-the-art heterogeneous photocatalysts. A new photoredox cascade photocatalyst consisting of dual-dye sensitized Pt-TiO2 nanoparticle photocatalyst and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) molecular catalyst was constructed. The system produced H2 efficiently under blue-light to simultaneously generate one-electron oxidized TEMPO+ that oxidized various biomass substrates ranging from water-soluble glycerol to insoluble cellulose.image