Reaction of S2 and SO2 with Pd/Rh(111) surfaces: Effects of metal–metal bonding on sulfur poisoning

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Abstract
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 +2 O 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 than 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.