Heterogeneous Photocatalytic Reduction of Cr(VI) in UV-Irradiated Titania Suspensions: Effect of Protons, Ammonium Ions, and Other Interfacial Aspects

Heterogeneous photocatalytic reactions in UV-irradiated TiO₂ suspensions are comprised of two conjugate reaction pathways involving the photogenerated electrons and holes, respectively. The role of the hole reaction pathway in the dynamics of the photocatalytic reduction of Cr(VI) is a major focus of this study. It is shown that proton supply plays a crucial role in this reduction reaction. Thus, the Cr(VI) photoreduction kinetics switch from first order to zero order as the proton concentration is systematically increased in the aqueous suspensions. Ammonium ions are also shown to exert a dramatic accelerating influence on Cr(VI) reduction in media of initial pH 6. This new observation is rationalized by considering that these species act as hole scavengers. The consequent improvement in quantum yield combines with the facile proton generation upon NH₄⁺ photooxidation, to result in the observed rate enhancement. Other interfacial aspects (i.e., adsorption) are also discussed.