Mesoporous Ce-Ti-Zr ternary oxide millispheres for efficient catalytic ozonation in bubble column

Abstract

Most heterogeneous catalysts available for catalytic ozonation are present as powders and not ideal for column operation in advanced water treatment. In this study, a mesoporous Ce-Ti-Zr ternary oxide (CTZO) millisphere of large size (0.8–1.0 mm in diameter) and high surface area (180 m2 g−1) was synthesized as a highly stable catalyst to meet the hydrodynamic demand in column ozonation. The fraction of Ce(III) is much higher on the surface of CTZO (49.2%) than CeO2 (27.5%), resulting in its high catalytic activity in ozone decomposition and mineralization of oxalic acid (OA), an ozone-refractory probe compound. The most favorable catalytic ozonation of OA was observed at pH 3.0 and the adsorption of OA by CTZO via surface complexation is a requisite step for its ozonation. The EPR analysis and radical scavenging experiment confirmed that OH was the dominating reactive species in catalytic ozonation of OA by CTZO. XPS revealed that the oxidation of OA was mediated by the Ce(III)/Ce(IV) redox cycle, which continously accepts the electron supply from OA ligand and meanwhile donates electron to activate O3 into OH. The catalytic ozonation of OA by CTZO was enhanced in the presence of sulfate due to the generation of sulfate radical. Cyclic runs demonstrated that the CTZO millispheres exhibited high stability for sustainable catalytic ozonation of OA without noticeable release of Ce or change of Ce valence state. This study will provide new insight for the development of efficient ozonation catalysts towards practical application in advanced water treatment.