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_x 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.