Effect of support on the apparent activity of palladium oxide in catalytic methane combustion

The support effect on the low temperature catalytic oxidation of methane over palladium catalysts was studied by comparing a series of metal oxides as the support. Samples of 0.010 g/g Pd catalysts supported on different grades and/or phases of TiO₂, Al₂O₃, and ZrO₂ were prepared via incipient impregnation and their catalytic activity was assessed using a laboratory plug-flow reactor. The specific surface area of the supports determined by nitrogen adsorption varied from about 13-220 m²/g. Initial experiments conducted with titania (anatase) as a support demonstrated a low apparent activity and a poor thermal stability. Focusing on anatase, its thermal stability was boosted by additions of Al₂O₃ or by doping with CeO₂, or La₂O₃. However, contrary to expectations based on previous data, the activity decreased in the sequence of Al₂O₃ > ZrO₂> TiO₂, and was not a direct function of specific surface area. This was especially evident in the case of titania. The surface structure of the support and the nature of its interaction with the active component PdO seemed to play a far more important role in activity than the apparent specific surface area. Also, anatase-supported catalysts showed a very rapid deactivation, while rutile-supported catalysts were relatively stable. The observed phenomena could potentially be related to the interaction between support and the active phase of palladium. Several models have been proposed to describe the strong metal-support interaction, but either charge transfer or encapsulation seems to be the most probable. (author)