SURFACE ENRICHMENT OF PT10RH90(111) .1. ANNEALING IN VACUUM AND LOW-PRESSURE ENVIRONMENTS

The variation in the surface composition of a model catalyst surface, a Pt10Rh90(111) single crystal, has been studied as a function of temperature and gas phase composition using ion scattering spectroscopy (ISS) with Ne-20, Auger electron spectroscopy (AES) and temperature programmed desorption (TPD). As the single crystal is heated in vacuum, the surface composition of the first layer of this alloy becomes enriched in Pt beginning at 600-degrees-C, reaching a surface composition of 30% Pt at 1000-degrees-C, in general agreement with previously reported findings. The equilibrated surface compositions measured by ISS and AES were very similar. Depth profiling shows the Pt-enriched layer is of atomic thickness on the surface. A comparison of the ISS and AES data suggests that the sub-surface layer is enriched in Rh, in agreement with the oscillatory behavior in the composition of near-surface layers reported by other researchers. In low pressure reducing conditions (1 X 10(-6) Torr H-2), Pt enrichment measured by ISS on the surface approaches 40%. In low pressure oxidizing conditions (1 x 10(-6) Torr O2), the surface layer equilibrates near, but above the bulk composition in Pt concentration, but no oxidation of the metal was observed.