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

Abstract

Carboxyl anion radicals formed in the photocatalytic decomposition of formate over TiO 2 particles were trapped using 5,5-dimethyl-1-pyrroline N -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 - ] ≥ 3 mM approach a limiting value Φ 0 - FA = 7.5%. In these conditions, the absolute value quantum yield for the carboxyl anion radical production is Φ 0 CO 2 •− = 2.4%. The implications of these results are analyzed in relation to the potential use of the photogenerated CO 2 • - radicals ( E 0 ( CO 2 / • CO 2 - ) = −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 TiO 2 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 formate.