Mixed-valent manganese oxide for catalytic oxidation of Orange II by activation of persulfate: heterojunction dependence and mechanism

The coexistence of aliovalent cations may facilitate the formation of electronic and ionic defects, altering the catalytic properties of manganese oxide. In this paper, a series of mixed-valent manganese oxides (MnOx) were prepared by a hydrothermal route and tested for heterogeneous activation of peroxydisulfate for Orange II degradation in aqueous solution. The morphology, crystal structure and surface chemistry of the MnOx were adjusted by the addition of ethanol in the hydrothermal process. The physicochemical properties were characterized and evidence from X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) indicated that MnOx exhibited a mixed crystal phase structure of Mn2O3, Mn3O4 and MnO2. The catalytic properties of the three samples prepared with 5%, 10%, and 20% volume fractions of ethanol (denoted as MnOx-5, MnOx-10, and MnOx-20) were found to be related to the valent states of Mn, and MnOx-10 exhibited a better performance with a mole ratio of Mn4+/ Mn2+ close to 1.0. The electrochemical characterization demonstrated that MnOx-10 performed a more effective redox process which may determine the catalytic activity of MnOx. Moreover, quenching tests, electron paramagnetic resonance (EPR) and gas chromatography-mass spectrometry (GC-MS) were used to evaluate the possible activation processes of peroxydisulfate (PDS) and the intermediates of Orange II degradation. This catalyst preparation strategy provides a new idea for the preparation of high-efficiency environmental catalysts.