Synergistic effect between MnO and CeO2 in the physical mixture: Electronic interaction and NO oxidation activity

MnO and CeO2 powders were mechanically mixed by a spatula and by milling to obtain loose-contact and tight-contact mixed oxides, respectively. The monoxides and their physical mixtures were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), Raman, O2 temperature-programmed desorption (O2-TPD), H2 temperature-programmed reduction (H2-TPR) and NO temperature-programmed oxidation (NO-TPO). The MnOx-CeO2 solid solutions did not form without any calcination process. The oxidation state of manganese tended to increase while the ionic valence of cerium decreased in the mixed oxides, accompanied with the formation of oxygen vacancies. This long-ranged electronic interaction occured more significantly in the tight-contact mixture of MnO and CeO2. The formation of more Mn4+ and oxygen vacancies promoted the catalytic oxidation of NO in an oxygen-rich atmosphere.