Catalytic routes and oxidation mechanisms in photoreforming of polyols

•Photoreforming and oxidation mechanisms of polyols on Rh/TiO2 for H2-generation.•Anodic reaction network for glycerol photoreforming was quantitatively elucidated.•Oxidative C-C cleavage is attributed to direct transfer of photogenerated holes.•Formation of carbonyl moieties and dehydration result from indirect hole transfer.•C-C bond rupture is favored with increasing polyol carbon number.

Photocatalytic reforming of biomass-derived oxygenates leads to H2 generation and evolution of CO2via parallel formation of organic intermediates through anodic oxidations on a Rh/TiO2 photocatalyst. The reaction pathways and kinetics in the photoreforming of C3-C6 polyols were explored. Polyols are converted via direct and indirect hole transfer pathways resulting in (i) oxidative rupture of C-C bonds, (ii) oxidation to α-oxygen functionalized aldoses and ketoses (carbonyl group formation) and (iii) light-driven dehydration. Direct hole transfer to chemisorbed oxygenates on terminal Ti(IV)-OH groups, generating alkoxy-radicals that undergo ß-C-C-cleavage, is proposed for the oxidative C-C rupture. Carbonyl group formation and dehydration are attributed to indirect hole transfer at surface lattice oxygen sites [Ti⋯O⋯Ti] followed by the generation of carbon-centered radicals. Polyol chain length impacts the contribution of the oxidation mechanisms favoring the C-C bond cleavage (internal preferred over terminal) as the dominant pathway with higher polyol carbon number.

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