CeO2/H2O2 system catalytic oxidation mechanism study via a kinetics investigation to the degradation of acid orange 7

Abstract

Nano ceria with cubic lattice was prepared and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Degradation kinetics of acid orange 7 (AO7) was investigated to understand the catalytic oxidation mechanism of the CeO2 with H2O2. The degradation of AO7 relies significantly on its adsorption on the surface of CeO2 with an apparent order of 2.40. Increasing the concentration of H2O2 increases the degradation reaction rate constant of AO7 in a positive linear relationship at the initial stage, but later hinders further degradation of AO7 due to over-complexation of Ce3+ with H2O2. The catalytic kinetics of CeO2 in pre-adsorbed mode (AO7 pre-adsorbed on CeO2 before the addition of H2O2) and pre-mixed mode (CeO2 pre-mixed with H2O2 before the addition of AO7) is quite different. The pre-mixed mode is unfavorable for AO7 degradation, as almost all of surface Ce3+ are pre-oxidized into surface peroxide species with H2O2 and hence reduces adsorption of AO7 as well as the activation of AO7 degradation. Reversely, the reactivity of CeO2 can slowly be recovered by adsorption of AO7, which competitively adsorbs on the CeO2 and gradually initiates the release of the surface Ce3+ by consuming the surface peroxide species. The AO7 degradation kinetics investigation in this work verifies that the degradation of organics in CeO2/H2O2 system is adsorption-triggered and the Ce3+ in reduced state is essential for activating the catalytic oxidation activity of surface peroxide species. EPR studies show that the surface peroxide species oxidizes the organics via hydroxyl adduct route.