Arsenic(III) oxidation over TiO2-supported nanocatalysts: Size effects and electronic regulation, and associated mechanisms

Abstract

Supported nanoclusters exert intriguing catalysis for many environmental and engineered processes such as As(III) oxidation to As(V) that not only reduces toxicity but also enhances adsorption. Disparate (MnO₂)_xH_y nanoclusters (x = 5, 2, 1; y = 0, 1, 2) are loaded over TiO₂ substrate, and their interactions and catalytic As(III) oxidation are addressed by the dispersion-corrected density functional theory (DFT+D3) approach. Interactions of (MnO₂)_xH_y with substrate are corroborated by multiple Ti-O_Mn and Mn-O_Ti bonds, and electron accumulation onto O atoms promotes electrostatic interactions and enhances (MnO₂)_xH_y stability. Although all supported (MnO₂)_xH_y nanoclusters can induce catalytic As(III) oxidation, mechanisms and catalytic activities rely strongly on size effects and electronic regulation. With reduction of (MnO₂)_x sizes, the tri-coordinated O_Mn atom, dangling O_Mn atom (x = 5), and then preferably (x = 2) and solely (x = 1) O_Mn atom at interface with rutile participate in As-O_Mn bond formation, respectively. Catalytic activities ascend rapidly and then mildly, and size reduction causes As(III) oxidation to occur favorably. Albeit with similar structural changes along reaction coordinates, the lower Mn oxidation states cause marked reduction of catalytic activities, and oxidizing agents vary: Mn for MnO₂, Mn and Ti for MnO(OH), and two Ti sites for Mn(OH)₂, suggesting the critical role of Ti(IV)/Ti(III)^• redox coupling. Catalytic activities are combined results of multiple factors, and mainly As(OH)₃ adsorption energies for size effects while numbers of Ti(III)^• radicals for electronic regulation. Emergence of dangling O_Mn atom is essential for catalytic As(III) oxidation by large (MnO₂)_xH_y nanoclusters. A systematic understanding is thus provided for interactions of (MnO₂)_xH_y nanoclusters with substrates and catalytic As(III) oxidation, which feeds back As(III) pollution control and architecture of supported nanocatalysts for environmental remediation.