The bonding of sulfur to Pd surfaces: photoemission and molecular–orbital studies

The adsorption and dissociation of S2 on Pd(111) and polycrystalline Pd were studied using synchrotron-based high-resolution photoemission spectroscopy and ab initio SCF calculations. The photoemission results show several sulfur species with distinct electronic properties. The formation of Pd–S bonds induces large positive binding-energy shifts in the core and valence levels of Pd. After chemisorbing sulfur on the Pd surfaces, there is a substantial decrease in the electron density that the metal exhibits near the Fermi level and a simultaneous drop in the electron population of its 4d band. Pd is more sensitive to sulfur than other metals frequently used in automotive exhaust catalysts (Rh and Pt).