INFLUENCE OF HYDROGEN CHEMISORPTION ON THE SURFACE-COMPOSITION OF PT-RH AL2O3 CATALYSTS/

The surface compositions of a series of platinum-rhodium bimetallic ca talysts supported on gamma-alumina were determined in the presence of chemisorbed hydrogen by H-1 NMR spectroscopy. The surface compositions of hydrogen-covered Pt-Rh bimetallic catalysts were found to be sligh tly enriched in Rh, significantly different from the surface compositi ons of adsorbate-free Pt-Ph surfaces, which are enriched in Pt. Furthe r, based on selective excitation NMR experiments it was found that bim etallic particles of fairly uniform compositions were formed. Atomisti c simulations of hydrogen-covered Pt-Rh bimetallic catalysts were done using a method that involved coordination-dependent bond energies. Th e simulations indicated that the heat of adsorption of hydrogen on rho dium is about 13 kJ/mol higher than that on platinum. Finally, a gener al, qualitative method for predicting the influence of adsorbates on t he surface segregation behavior of bimetallic systems is described on the basis of knowledge of the surface energies at various sites and th e heats of adsorption. (C) 1996 Academic Press, Inc.