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

Photoreforming of sugars on metalloaded semiconductors is an attractive process for H₂-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 H₂-evolution and sugar conversion over time is achieved through alkalinization of the aqueous-phase due to fast OH^–-induced hydrolytic cleavage of formate intermediates

Kinetic Coupling of Water Splitting and Photoreforming on SrTiO₃‑Based Photocatalysts

Coupling the proton reduction of overall water splitting with oxidation of oxygenated hydrocarbons (photoreforming) on Al-doped SrTiO₃ decorated with cocatalysts enables efficient photocatalytic H₂ generation along with oxygenate conversion, while decreasing the accumulation of harmful byproducts such as formaldehyde. Net H₂ evolution rates result from the contributions of the individual rates of water oxidation, oxygenate oxidation, and the back-reaction of H₂ and O₂ to water. The latter reaction is suppressed by a RhCrO_<i>x</i> cocatalyst or by high concentrations of oxygenates in the case of Rh cocatalyst, whereas the rates of organic oxidation depend on their molecular structure. In the absence of the back-reaction to water, the H₂ evolution rates are independent of the oxygenate type and concentration because the rates of water splitting compensate the variations in the rates of oxygenate conversion. Under such conditions of suppressed back-reaction, the selectivities to water and oxygenate oxidation, both occurring with the same quantum efficiencies, depend on the oxygenate type and concentration. The dominant pathways for organic transformations are ascribed to the action of intermediates generated at the semiconductor during water oxidation and O₂ evolution. On a semiconductor without cocatalyst, the O₂ produced during overall water splitting is reductively activated to participate in oxidation of organics without consuming evolved H₂

Enabling Overall Water Splitting on Photocatalysts by CO-Covered Noble Metal Co-catalysts

Photocatalytic overall water splitting requires co-catalysts that efficiently promote the generation of H₂ but do not catalyze its reverse oxidation. We demonstrate that CO chemisorbed on metal co-catalysts (Rh, Pt, Pd) suppresses the back reaction while maintaining the rate of H₂ evolution. On Rh/GaN:ZnO, the highest H₂ production rates were obtained with 4–40 mbar of CO, the back reaction remaining suppressed below 7 mbar of O₂. The O₂ and H₂ evolution rates compete with CO oxidation and the back reaction. The rates of all reactions increased with increasing photon absorption. However, due to different dependencies on the rate of charge carrier generation, the selectivities for O₂ and H₂ formation increased in comparison to CO oxidation and the back reaction with increasing photon flux and/or quantum efficiency. Under optimum conditions, the impact of CO to prevent the back reaction is identical to that of a Cr₂O₃ layer covering the active metal particle

Cluster Size Effects in the Photocatalytic Hydrogen Evolution Reaction

The photocatalytic water reduction reaction on CdS nanorods was studied as function of Pt cluster size. Maximum H₂ production is found for Pt₄₆. This effect is attributed to the size dependent electronic properties (e.g., LUMO) of the clusters with respect to the band edges of the semiconductor. This observation may be applicable for the study and interpretation of other systems and reactions, e.g. H₂O oxidation or CO₂ reduction