Impact of Thermal Aging on the Kinetic Parameters of the NO/H2 Reaction on Pd/LaCoO3

This paper deals with the kinetics of the reduction of NO by H-2 on Pd supported over reducible supports, such as LaCoO3. An extensive kinetic investigation was performed on aged catalysts after exposure to successive reductive and oxidative atmospheres at a high temperature (T = 500 degrees C). Typically, Pd/LaCoO3 was reduced in pure H-2 and then aged under reactive conditions in the presence of an excess of O-2. Both thermal treatments are accompanied by significant bulk and surface reconstructions characterized by X-ray diffraction and X-ray photoelectron spectroscopy. Pd/LaCoO3 is extensively reducef to Pd-0/CoOx/La2O3 after H-2 exposure. It was previously found that the NO/H-2 reaction on Pd-0/CoOx/LaO3 involved the Pd/support interface. This study reports that subsequent thermal aging under oxidative conditions leads to the restoration of the perovskite structure with parallel changes in the chemical environment of oxidic palladium species. The NO reduction by hydrogen was used as a probe reaction to investigate related modifications of the surface properties. It was found that significant changes in the kinetic features of Pd/LaCoO3 occur after thermal aging, compared to the prereduced catalyst, because of surface modifications that might alter the adsorptive properties of Pd and more significantly the nature of the dissociation step of NO. These surface modifications induce a selectivity enhancement to the formation of nitrogen.