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,O2temperature-programmed desorption(O2-TPD),H2temperature-programmed reduction(H2-TPR)and NO temperature-programmed oxidation(NO-TPO).The MnOx-CeO2solid 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.
MnOx-CeO2-Al2O3 mixed oxides were prepared by impregnating manganese and cerium precursors on alumina powders via a sol- gel deposition method. The oxide catalyst exhibited a poor resistance to sulfur dioxide after the treatment in 100 ppm SO2/air at 350 °C for 50 h. The formation of manganese sulfate and especially cerium sulfate reduced the availability of surface active metal oxides, blocked the pore structure and decreased the surface area of the catalyst. These changes in chemical and structural and textural properties resulted in a severe loss in the activities of the sulfated catalyst for NO and soot oxidation. The decomposition of sulfates was almost complete during the calcina-tion in air at 800 °C for 30 min, which partially recovered the surface active sites and the catalyst surface area despite the significant sintering of metal oxides. Consequently, the NOx-assisted soot oxidation activity of the catalyst was regenerated to some extent by the oxidation treatment.
Manganese oxide-loaded and -doped ceria as well as the corresponding barium-modified oxide catalysts were prepared for soot oxidation in the presence of NOx, and were characterized by using X-ray diffraction, Brunauer-Emmett-Teller and NO temperature- programmed oxidation measurements. The activity of catalyst depended strongly on the NO 2 production capacity, and the importance of surface nitrates was weakened without heat transfer limitations. The formation of perovskite-type oxides after the high-temperature calcination caused the loss of NOx storage capacity for the Ba-modified catalysts, but did not seem to affect the NO oxidation activity obviously. The addition of barium did not prevent the phase separation of MnOx-CeO 2 solid solutions, whereas it inhibited the sintering of oxide crystallites effectively. This, as well as the relatively high surface area, resulted in a small increase in soot oxidation temperature for the thermally aged Ba/Mn-Ce catalyst.
