Structural Analysis of Rh-Pd/CeO₂ Catalysts Under Reductive Conditions: An X-ray Investigation

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 °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 °C the ceria support was heavily reduced to CeO_2-x (x = 0.3-0.4), with Rh⁰ (64 % of total Rh) and Pd⁰ (67 % of total Pd) being the dominant species. Heating the as-prepared Rh-Pd/CeO₂ catalysts under a CO atmosphere at 200-400 °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 °C onward. The FCC order of Pd is however perturbed at 400 °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.