Reaction of S_2 and SO_2 with Pd/Rh(111) surfaces: Effects of metal-metal bonding on sulfur poisoning

摘要

The surface chemistry of S_2 and SO_2 on Rh(111), Pd/Rh(111) and polycrystalline Pd has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent-field calculations. Pd adatoms lead to an increase in the rate of adsorption of S_2 on Rh(111), but they are less reactive than atoms of pure metallic palladium: Rh(111) < Pd/Rh(111) < Pd. The adsorption of sulfur induces a large reduction in the density of states (DOS) near the Fermi level of Pd/Rh(111) surfaces. The decrease in the DOS is smaller than in S/Pd(111) but bigger than in S/Rh(111). The chemistry of SO_2 on Rh(111), Pd/Rh(111), and Pd is rich. At 100 K, SO_2 adsorbs molecularly on these systems. Above 200 K, the adsorbed SO_2 decomposes (SO_(2, a) -> S_a + 2O_a) or transforms into SO_3/SO_4 species. The molecular SO_x species disappear upon annealing to 450 K and only atomic S and O remain on the surfaces. A Pd monolayer supported on Rh(111) is not very active for the dissociation of SO_2. In this respect, the Pd_(1.0)/Rh(111) system is less chemically active then pure Pd or Rh(111). The electronic perturbations associated with the Pd-Rh bonds reduce the electron donor ability of Pd, weakening the interactions between the Pd 4d orbitals and the lowest unoccupied molecular orbitals of S_2 and SO_2. The behavior of the S_2/Pd/Rh(111) and SO_2/Pd/Rh(111) systems shows that bimetallic bonding can reduce the reactivity of Pd towards sulfur-containing molecules. A very large drop in reactivity can be expected when Pd is bonded to s, p or early transition metals.