DFT study on TiO2 facet-dependent As(III) oxidation process: Importance of As(IV) species

The identification of active arsenic (As) intermediates is of importance in heterogenous As(III) oxidation to As(V) at the solid-water interface. However, the speciation and redox transformation of intermediate As(IV) is not fully understood. Herein, the molecular level As(III) oxidation on anatase {100}, {101}, and {001} facets were studied using density functional theory (DFT) calculations. Our DFT results show that As(IV) formation step is the thermodynamic obstacle in the hole-induced oxidation process as evidenced by its high free energy change in the range 1.21 -1.83 eV. The theoretical overpotential on facets follows {100} (1.21 -1.58 eV) -{101} (1.24 -1.78 eV) < {001} (1.61 -1.83 eV), suggesting the facet-dependent reactivity. Theoretical overpotential linearly correlated to the adsorption energy of As(IV) species, highlighting the impact of adsorption affinity to oxidation reactivity and implying a quantitative structure-activity relationship (QSAR). This facet-dependent oxidation is attributed to the electronic structure of the frontier orbitals of the adsorbed As(III/IV) on each facet. The gained knowledge will be instructive to design effective catalysts for the oxidation of arsenic and similar oxyanions.