H2S adsorption on chromium, chromia, and gold/chromia surfaces: Photoemission studies
Journal of Chemical Physics
The reaction of H 2 S with chromium, chromia, and Au/chromia films grown on a Pt͑111͒ crystal has been investigated using synchrotron-based high-resolution photoemission spectroscopy. At 300 K, H 2 S completely decomposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features. No evidence was found for the formation of CrS x species. The dissociation of H 2 S on Cr 3 O 4 and Cr 2 O 3 films at room temperature produces a decrease of 0.3-0.8 eV
... ase of 0.3-0.8 eV in the work function of the surface and significant binding-energy shifts ͑0.2-0.6 eV͒ in the Cr 3 p core levels and Cr 3d features in the valence region. The rate of dissociation of H 2 S increases following the sequence: Cr 2 O 3 ϽCr 3 O 4 ϽCr. For chromium, the density of states near the Fermi level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of H 2 S than the valence and conduction bands of the chromium oxides. This leads to a large dissociation probability for H 2 S on the metal, and a low dissociation probability for the molecule on the oxides. In the case of Cr 3 O 4 and Cr 2 O 3 , there is a correlation between the size of the band gap in the oxide and its reactivity toward H 2 S. The uptake of sulfur by the oxides significantly increases when they are "promoted" with gold. The Au/Cr 2 O 3 surfaces exhibit a unique electronic structure in the valence region and a larger ability to dissociate H 2 S than polycrystalline Au or pure Cr 2 O 3 . The results of ab initio SCF calculations for the adsorption of H 2 S on AuCr 4 O 6 and AuCr 10 O 15 clusters show a shift of electrons from the gold toward the oxide unit that enhances the strength of the Au(6s)↔H 2 S(5a 1 ,2b 1 ) bonding interactions and facilitates the decomposition of the molecule.