Overcoming the Rate-Limiting Reaction during Photoreforming of Sugar Aldoses for H2-Generation

Photoreforming of sugars on metalloaded semiconductors is an attractive process for H-2-generation. However, the reaction proceeds typically with rapidly decreasing rates. We identified that this decrease is due to kinetic constraints rather than to catalyst deactivation. Thus, the nature of the rate-limiting reaction was elucidated by investigation of the reaction pathways and oxidation mechanisms during photoreforming of sugar aldoses on TiO2 decorated with Rh, Pd, or Pt. Using selective isotope labeling it is shown that ring opening of aldoses via direct hole transfer to the chemisorbed oxygenates yields primary formate esters. Under pH-neutral and acidic conditions, formates convert to the consecutive aldose intermediate through light-driven, redox-neutral hydrolysis. The slow kinetics of this step, which requires interaction with negative and positive photogenerated charges, leads to blocking of active sites at the photoanode and enhanced electron hole recombination. Stable H-2-evolution and sugar conversion over time is achieved through alkalinization of the aqueous-phase due to fast OH--induced hydrolytic cleavage of formate intermediates.