Surface-catalyzed chromium(VI) reduction: The TiO2-Cr-VI-mandelic acid system

As part of a study exploring the pathways and rates of Cr-VI reduction under environmentally relevant conditions, this paper examines the kinetics and mechanism of Cr-VI reduction by mandelic acid in the presence of 1.0 g/L TiO2. The redox reaction is dramatically catalyzed by the oxide surface, and benzoylformic acid and benzaldehyde are the major products of mandelic acid oxidation. Comparisons of adsorption and reaction among mandelic acid, methyl mandelate, and atrolactic acid suggest that Cr-VI adsorption is required for the reaction to take place, while the adsorption of organic reductant may be unnecessary. The reaction, to our surprise, is zero order with respect to mandelic acid, but fractional order with respect to the ester methyl mandelate. This change in reaction order indicates that Cr-VI reduction by mandelic acid and methyl mandelate occurs via different rate-limiting steps.