AN X-RAY PHOTOELECTRON-SPECTROSCOPY INVESTIGATION OF PALLADIUM IN AUTOMOTIVE CATALYSTS - BINDING-ENERGIES AND REDUCTION CHARACTERISTICS

Binding energies and reduction properties of palladium in commercial monolithic automotive catalysts were measured using X-ray photoelectron spectroscopy (XPS). It was shown that XPS in conjunction with temperature selective reduction can be used to associate palladium binding energies and reduction characteristic with varying degrees of palladium-support and palladium-support additive interactions. Good correlation was found for the palladium binding energies and reduction properties of the commercial catalysts, and those reported previously for model studies of gamma-alumina supported palladium. In addition, a palladium state was observed consistent with that reported for a gamma-alumina supported Pd-Ce system, suggesting the presence of Pd-Ce interactions in the commercial catalysts. This work indicates that with the combination of these techniques, dispersed and particulate palladium, as well as palladium-support additive interactions can be identified. The effects of thermal aging (sintering) on precious metal interactions were also investigated, with results demonstrating the utility of these techniques for assessing catalyst aging.