Catalytic combustion of methane over cerium-doped palladium catalysts

Various Pd-supported catalysts have been prepared using three different types of alumina as support material: (a) γ-alumina, (b) Ba-stabilized alumina, and (c) La-stabilized alumina. The presence of La and Ba negatively affects the catalytic activity of the supported palladium particles, despite improving the thermal stability of alumina. The previously mentioned supports have then been doped by addition of cerium, prior to impregnation with Pd. In the case of the La–alumina support, Ce was deposited by incipient wetness (IW) or specific adsorption (SA). The palladium catalysts were prepared by the incipient wetness technique in order to attain a noble metal loading of 2.5 wt%. The catalyst powders, calcined at 1000 °C in air for 4 h, have been coated onto a mullite tube in order to test their activity for the combustion of methane in a single annular channel reactor. Specific attention was given to the intensity and temperature of the PdO–Pd transition, which directly affects the combustion of methane. The presence of Ce enhances the thermal stability of alumina but entails a decrease in combustion activity. Considering the La–Al2O3 and Ba–Al2O3 supported catalysts, the presence of Ce results in an increased combustion activity. In addition, the reoxidation of Pd into PdO occurs at higher temperature than in the case of Ce-free samples. The mode of preparation of the Ce-doped support plays an important role, and the use of specific adsorption results in a catalyst with a higher combustion activity, as well as an improved thermal stability when compared to Al2O3.