INTERACTION OF ATOMIC-HYDROGEN WITH CLUSTER-MODELS OF PD, RH AND BIMETALLIC PDSN AND RHSN CATALYSTS

Results of LCGTO-MCP-LSD (Linear Combination of Gaussian Type Orbitals-Model Core Potentials-Local Spin Density) calculations are reported for chemisorption and for bulk and surface diffusion of atomic hydrogen on clusters simulating the (111) and (100) surfaces of pure transition metals (Pd and Rh) and bimetallic catalysts (PdSn, RhSn). The replacement of a Pd or Rh atom near the hydrogen atom by Sn always decreases the binding energy. Hydrogen surface diffusion has been calculated with the aim of explaining the nature of the second, high-temperature desorption peak recently reported for RhSn/SiO2 catalysts. Our results, for Rh8Sn3-H, show that hydrogen diffusion into the tin region is clearly unfavored. A consistent pattern has emerged from the calculations; the proximity of tin to hydrogen leads to destabilisation. This should affect the recombination process to form H2 and the implications of this for catalysis are discussed. © 1990.