Revisiting the mechanism of hexavalent chromium ion reduction: The parallel photodecomposition and photocatalytic reduction of chromate(VI) ester

The formation and homogeneous photodecomposition of chromate(VI) ester are often overlooked during the studies of photocatalytic reduction of hexavalent chromium, Cr(VI), in the presence of sacrificial electron donors such as methanol. When photoexcited (λ ≤ 525 nm), the chromate(VI) ester, formed by the spontaneous reaction of aqueous Cr(VI) with methanol, undergoes homogeneous stepwise decomposition to trivalent chromium, Cr(III). The addition of semiconductor photocatalysts, i.e., TiO₂ and WO₃, at relatively low concentrations (−1) was detrimental to the overall rate of Cr(VI) reduction. This was traced to the occurrence of interfacial electron injection from chromate ester to the photocatalyst conduction band, that in turn interrupted inner sphere electron transfer, i.e., photodecomposition. Since it is an interfacial process, the extent of suppression of Cr(VI) reduction is more significant for highly adsorbing TiO₂ than on weakly adsorbing WO₃. At the same time, even though chromate ester adsorbs strongly on Al₂O₃, the interfacial electron injection to the insulator particle was unfavorable, and hence no suppression in Cr(VI) reduction was observed. When the TiO₂ photocatalyst concentration was increased to 0.5 g L⁻¹ and beyond, the photocatalytic reduction path became dominant. Further increase in the photocatalyst concentration saw the linear dependency in the apparent rate of Cr(VI) reduction, as expected for reaction in the non-mass diffusion and non-light scattering limited region. As followed by the electron paramagnetic spectroscopy, both the photodecomposition and photocatalytic reactions follow the stepwise reduction from Cr(VI) to Cr(V) and eventually to Cr(III). For non-photoexcited TiO₂ (λ ≥ 420 nm), suppression in the rate of Cr(VI) reduction was observed at all concentrations tested. Importantly, the work showcases the parallel but non-independent effects of homogeneous photodecomposition of chromate(VI) ester and its photocatalytic reduction over semiconductor photocatalysts.