Influence of ceria on physicochemical, surface and catalytic properties of alumina supported manganese catalysts

摘要

A series of catalysts containing manganese oxides supported on alumina and ceria-promoted alumina was prepared by wet impregnation method using finely powdered support material and manganese and/or cerium nitrate. The physicochemical, surface and catalytic properties of the thermally treated solids (500, 800 and 900 degrees C) were investigated using XRD, ESR, nitrogen adsorption at -196 degrees C and hydrogen peroxide decomposition in a aqueous solution at 30-50 degrees C. Experimental results show that Mn oxidation state changes from Mn4+ to Mn2+ by increasing the calcination temperature of solids studied. However, there are two unique features associated with CeO2 as a material of interest. The first favors the dispersion of manganese oxides deposited on gamma-Al2O3 catalyst calcined at 500 degrees C. The second enhances the formation of Mn3O4 species from Mn2O3 deposited initially on the alumina support calcined at 800 and 900 degrees C. These conclusions can be discussed in terms of modifications of both the oxygen storage capacity of ceria and structural properties of manganese oxides which are found in the investigated solids. The specific surface area of Mn/Al mixed oxides system calcined at 500 degrees C was found to increase by increasing the amount of dopant added. Opposite effect was observed by increasing the clacination temperature from 500 to 900 degrees C. The doping process followed by calcination at different temperatures brought about an increase in the catalytic activity of mixed oxides system. These treatments did not modify the mechanism of the catalyzed reaction but changed the number of catalytically active sites taking place in it without changing the energetic nature of these sites.