SURFACE ENRICHMENT OF PT(10)RH(90)(111) .2. EXPOSURE TO HIGH-TEMPERATURE ENVIRONMENTS AT 760-TORR

The variation in the surface composition of a model catalyst surface, a Pt10Rh90(111) single crystal, has been studied after exposure to oxidizing and reducing gas phase environments at 760 Torr at temperatures typical of automotive catalytic converter operation using ion scattering spectroscopy (ISS) with Ne-20, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Under reducing conditions, the surface composition is enriched in Pt, consisting of 30% Pt in the surface layer. The Pt enrichment occurs only on the top one-two surface layers. This condition is identical to that obtained after annealing in vacuum at high temperature (1000-degrees-C) followed by cooling. Under oxidizing conditions, the surface layer quickly becomes enriched in Rh compared to the buk and is accompanied by oxidation of Rh to Rh2O3, as indicated by AES and XPS. The Rh enrichment in the oxide form extends several layers below the surface (5-6 nm), but the Rh oxide only accounts for part of the oxygen incorporated in the sample during the high temperature treatment in oxidizing conditions. This suggests a significant portion of the oxygen occupies interstitial sites in the alloy. Temperature programmed desorption in vacuum results in the observance of an intense oxygen desorption peak around 700-degrees-C, accounting for both the decomposition of surface Rh2O3 and removal of interstitial oxygen. At this point, the surface is still enriched in Rh, but is converted to a Pt-enriched surface by 950-degrees-C.