Photoreforming of ethylene glycol over Rh/TiO2 and Rh/GaN:ZnO

Article abstract of DOI:10.1016/j.jcat.2016.02.021

Photoreforming of diols, such as ethylene glycol, proceeds through a sequence of anodic oxidations, which enable the parallel formation of H₂ by reduction of H⁺ at the cathode. The anodic oxidation of ethylene glycol on Rh/TiO₂ leads to glycolaldehyde, formaldehyde and acetaldehyde as primary products. Glycolaldehyde is further converted via oxidative C-C-cleavage to formaldehyde and formic acid. Formaldehyde is oxidized to formic acid forming CO₂ and H₂. Acetaldehyde is oxidized to acetic acid, which decarboxylates to CO₂ and CH₄. Two catalytically active sites are proposed. On terminal Ti^IV-OH groups, oxygenates are oxidized via direct hole transfer to alkoxy-radicals prior to β-C-C-bond cleavage. Bridged [Ti··O··Ti]⁺ sites, in contrast, cleave a C-H bond, forming carbon centered radicals, which are further oxidized by transferring an electron to the conduction band of the semiconductor. On Rh/GaN:ZnO, glycolaldehyde is the main product, forming higher oxidized C2-oxygenates in turn by reaction with free oxygen radicals formed as product of OH^- photocatalytic oxidation. The overall rates of photoreforming and, hence, H₂ evolution, depend mainly on the surface concentration of the compounds which are oxidized, while the nature of the oxygenate is of less importance.