Yield of carboxyl anion radicals in the photocatalytic degradation of formate over TiO2 particles

Carboxyl anion radicals formed in the photocatalytic decomposition of formats over TiO(2) particles were trapped using 5,5-dimethyl-1-pyrrolineN-oxide (DMPO). Quantitative kinetic determinations of the -O(2)C-DMPO(.) radical formation by electron paramagnetic resonance spectroscopy indicate that 30% of formate oxidation occurs through CO(2)(.-). Absolute quantum yields of HCO(2)(-) disappearance obtained under steady-state monochromatic irradiation of TiO(2) sols at pH = 5.2 and [HCO(2)(-)] greater than or equal to 3 mM approach a limiting value Phi (-FA)= 7.5%. In these conditions, the absolute value quantum yield for the carboxyl anion radical production is Phi (0)(CO2 .-) = 2.4%. The implications of these results are analyzed in relation to the potential use of the photogenerated CO(2)(.-) radicals (E((CO2/. CO2-))(0) = -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 TiO(2) suspensions using HCO(2)(-) as a sacrificial species is compatible with the quantitative scavenging of CO(2)(.-) anion radicals produced during the photocatalytic oxidation of formats.