Yield of Carboxyl Anion Radicals in the Photocatalytic Degradation of Formate over TiO2 Particles

Carboxyl anion radicals formed in the photocatalytic decomposition of formate over TiO2 particles were trapped using 5,5-dimethyl-1-pyrroline N-oxide (DMPO). Quantitative kinetic determinations of the -O2C−DMPO• radical formation by electron paramagnetic resonance spectroscopy indicate that 30% of formate oxidation occurs through CO2•-. Absolute quantum yields of HCO2- disappearance obtained under steady-state monochromatic irradiation of TiO2 sols at pH = 5.2 and [HCO2-] ≥ 3 mM approach a limiting value Φ0-FA = 7.5%. In these conditions, the absolute value quantum yield for the carboxyl anion radical production is Φ0CO2•− = 2.4%. The implications of these results are analyzed in relation to the potential use of the photogenerated CO2•- radicals (E0(CO2/•CO2-)= −1.8 V) to mediate reductions that cannot be achieved directly by conduction band electrons. Particularly, we showed that nitrate to nitrite conversion becomes feasible by the addition of formate to irradiated TiO2 suspensions. We probed that nitrite production determined in air-equilibrated TiO2 suspensions using HCO2- as a sacrificial species is compatible with the quantitative scavenging of CO2•- anion radicals produced during the photocatalytic oxidation of formate.