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

Coupling the proton reduction of overall water splitting with oxidation of oxygenated hydrocarbons (photo reforming) on Al-doped SrTiO3 decorated with cocatalysts enables efficient photocatalytic H-2 generation along with oxygenate conversion, while decreasing the accumulation of harmful byproducts such as formaldehyde. Net H-2 evolution rates result from the contributions of the individual rates of water oxidation, oxygenate oxidation, and the back-reaction of H-2 and O-2 to water. The latter reaction is suppressed by a RhCrOx 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 H-2 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 O-2 evolution. On a semiconductor without cocatalyst, the O-2 produced during overall water splitting is reductively activated to participate in oxidation of organics without consuming evolved H-2.