Abstract
X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) were used to probe the electronic and geometric structures of Rh-Pd/CeO2 catalysts before and after ethanol steam reforming (ESR) at 400 degrees C, as well as after heating in CO atmospheres as a function of time and temperatures. The O/Ce ratio determined by XPS for the as-prepared Rh-Pd/CeO2 catalysts (individual metal loadings of 0.5 or 2 wt%) were close to 2, with the Rh and Pd present primarily in oxidized form (as oxide and chloride surface species). After ESR at 400 degrees C the ceria support was heavily reduced to CeO2-x (x = 0.3-0.4), with Rh-0 (64 % of total Rh) and Pd-0 (67 % of total Pd) being the dominant species. Heating the as-prepared Rh-Pd/CeO2 catalysts under a CO atmosphere at 200-400 degrees C while monitoring the Pd and Rh K edges by XAS was also conducted. XAS of the K edge of Pd and Rh indicated, as expected, reduction of Pd before that of Rh from 200 degrees C onward. The FCC order of Pd is however perturbed at 400 degrees C. On the contrary, Rh once reduced maintained a well ordered (FCC) structure. This observation is rationalised by the greater reduction effect of Pd on CeO2 compared to Rh and therefore at the interface Pd structure may be altered at high temperatures.