SELECTIVE PHOTOREDUCTION OF CO2/HCO3- TO FORMATE BY AQUEOUS SUSPENSIONS AND COLLOIDS OF PD-TIO2

Immobilization of Pd onto TiO2 powders and colloids results in active photocatalysts for the selective reduction of CO2/HCO3- to formate. The active photocatalysts are prepared by adsorption of either aqueous Pd-β-cyclodextrin colloids (Pd(β-CD)), synthesized at 60°C, or aqueous Pd(citrate) colloids onto TiO2 powders or colloids. Illuminated Pd-free TiO2 suspensions, in the presence of CO2/HCO3- and the electron donor oxalate, generate H2 and a small amount of formate; formate is formed at a rate of 3.7 × 10-3 μmol min-1. With Pd(β-CD,60°C)-TiO2 and Pd(citrate)-TiO2 suspensions, the photoconversion of CO2/HCO3- to formate occurs at rates of 2.3 × 10-2 and 3.7 × 10-2 μmol min-1, respectively. The photocatalytic reduction of CO2/HCO3- to HCO2- is inhibited by mercaptoethanol. Concomitant with the decline in formate production, the rate of H2 production increases. The inhibition of CO2/HCO3- reduction is attributed to the binding of mercaptoethanol to the HCO3--activation sites of Pd. The Pd(β-Cd,60°C)-TiO2 particles also catalyze the dark hydrogenation of CO2/HCO3- by H2 and the oxidation of formate by methyl viologen. A mechanism is invoked in which the photogenerated conduction-band electrons of TiO2 either directly reduce protons to H2 on the semiconductor surface or else reduce protons on the Pd surface to produce palladium hydride. The latter species reduces the Pd-activated HCO3- to formate. © 1990 American Chemical Society.