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 oxi dizing and reducing gas phase environments at 760 Torr at temperatures typical of automotive catalytic converter operation using ion scatter ing spectroscopy (ISS) with Ne-20, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and temperature programmed deso rption (TPD). Under reducing conditions, the surface composition is en riched in Pt, consisting of 30% Pt in the surface layer. The Pt enrich ment occurs only on the top one-two surface layers. This condition is identical to that obtained after annealing in vacuum at high temperatu re (1000-degrees-C) followed by cooling. Under oxidizing conditions, t he surface layer quickly becomes enriched in Rh compared to the buk an d 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 o xygen incorporated in the sample during the high temperature treatment in oxidizing conditions. This suggests a significant portion of the o xygen occupies interstitial sites in the alloy. Temperature programmed desorption in vacuum results in the observance of an intense oxygen d esorption peak around 700-degrees-C, accounting for both the decomposi tion of surface Rh2O3 and removal of interstitial oxygen. At this poin t, the surface is still enriched in Rh, but is converted to a Pt-enric hed surface by 950-degrees-C.