REDOX PROCESSES ON PURE CERIA AND ON RH/CEO2 CATALYST MONITORED BY X-RAY-ABSORPTION (FAST ACQUISITION MODE)

Redox processes occurring on ceria and Rh on ceria catalyst have been monitored by X-ray absorption spectroscopy (XAS) at the Ce L(III) edge using a fast collection mode of the data. From changes in the Ce L(III) Shape, a quantitative evaluation of the Ce(III)/Ce(IV) content has been monitored within an error estimated to 5%. This kinetic study is both confirmed and extended to N2O chemisorption and thermogravimetric measurements. The influence of the ceria surface area and the effect of rhodium are investigated in detail. The redox processes are simulated by alternate exposure to H-2 and dry air at atmospheric pressure and moderate temperatures (573-773 K). The initial reduction content is dependent an the ceria surface area, and a kinetic model accounting for this behavior is developed. Between 573 and 673 K the reduction starts on the surface through successive hydrogen dissociation and anionic vacancy formation, followed by a much slower bulk diffusion step. It is stated that the hydrogen dissociation is the limiting step of the surface process on pure ceria. On Rh/CeO2 sample the surface reduction becomes very fast, due to the faster hydrogen dissociation on metallic rhodium. By contrast with reduction, the kinetic behavior for oxidation is very fast on pure ceria, but needs an induction time on Rh/CeO2. These deep differences for the redox properties induced both by the high surface area of ceria and the presence of a transition metal are related to the behavior of ceria for depollution catalysis.