Unravelling the facets-dependent behavior among H2O2, O3 and oxygen vacancies on CeOx and the promotion of peroxone reaction at under acidic conditions

Abstract

In this study, CeO_x with different exposed facets was prepared to investigate their potential effect on activating the peroxone reaction (H₂O₂/O₃) and breaking through the restriction of solution pH on peroxone. By controlling the concentration of hydroxide, cube CeO_x with a (100) facet, rod CeO_x with a (110) facet and octahedron CeO_x with a (111) facet were prepared by the hydrothermal precipitation method, and they all promoted heterogeneous peroxone for oxalic acid (OA) under acidic conditions. The catalytic activity of CeO_x was highly related to their surface oxygen vacancies (OVs) and the interfacial electron transfer process. Both experiments and theoretical calculations showed that CeO_x with different exposed facets activated peroxone in different ways and (111) CeO_x possessed better activity than (100) or (110) CeO_x. The low efficiencies of (100) and (110) CeO_x were due to their inefficient use of H₂O₂ to reduce surface Ce⁴⁺ and their large energy gap to generate surface peroxide (OVs-OOH). (111) CeO_x was stable with surface-adsorbed H₂O₂, and the rich electrons at OVs could be donated to H₂O₂ and generate OVs-OOH, which then triggered O₃ decomposition into hydroxyl radical (˙OH) and superoxide radical (˙O₂⁻). These electron migration processes depended on the electron-donating ability of the OVs rather than solution pH, therefore, still greater amounts of reactive species were generated under acidic conditions. The flexibility of the (111) CeO_x/H₂O₂/O₃ process was further investigated for removing 20 kinds of pharmaceuticals. The synergy effect of (111) CeO_x/O₃ and H₂O₂ varied with the structure of the pharmaceuticals.