Role of the ceria-zirconia support in the reactivity of platinum and palladium catalysts for methane total oxidation under lean conditions

Platinum or palladium catalysts deposited on a Ce0.67Zr0.33O2 solid solution were tested in methane total oxidation. The ceria-zirconia mixed oxide is itself an active catalyst for methane combustion, in the 673-1073 K temperature range. Deposition of platinum or palladium on this support results in a strong increase in activity, which now takes place at low temperature (473-773 K). The ceria-zirconia support is particularly beneficial for the activity of platinum, when compared to a Pt/Al2O3 catalyst. Methane oxidation takes place at the active ceria-zirconia/metal interface, according to a redox mechanism involving the reaction of dissociated methane with lattice oxygen from the support. However, a time-on-stream deactivation is observed at moderate temperatures (573-623 K), which is associated with an oxidized state of the catalysts, whereas a reduction at 573 K strongly activates the solids. In situ electrical conductivity measurements show that the reduction at 573 K of the Pt/Ce0.67Zr0.33O2 and Pd/Ce0.67Zr0.33O2 Catalysts creates a large amount of oxygen vacancies. Although the reduced ceria-zirconia support is largely reoxidized by oxygen at 573 K, some vacancies still remain and require a higher oxidation temperature (773 K) to be filled. The presence of vacancies, even in small amounts, seems to favor activity because of an electron transfer to the noble metals and an improvement of the lattice oxygen mobility. The slow filling of these vacancies is probably an important factor contributing to the observed time-on-stream deactivation. (C) 2001 Academic Press.