H2S ADSORPTION ON CHROMIUM, CHROMIA, AND GOLD CHROMIA SURFACES - PHOTOEMISSION-STUDIES/

The reaction of H2S with chromium, chromia, and Au/chromia films grown on a Pt(111) crystal has been investigated using synchrotron-based hi gh-resolution photoemission spectroscopy. At 300 K, H2S completely dec omposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features, No evidence was found for the formation of CrSx species. The dissociation of H2S on Cr3O4 and C r2O3 films al room temperature produces a decrease of 0.3-0.8 eV in th e work function of the surface and significant binding-energy shifts ( 0.2-0.6 eV) in the Cr 3p core levels and Cr 3d features in the valence region. The rate of dissociation of H2S increases following the seque nce: Cr2O3<Cr3O4<Cr. For chromium, the density of states near the Ferm i level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of H2S than the valence and conduction bands of the chromium oxides. This leads to a large dissociation probability for H2S on the metal, and a low dissociation probability for the molecule on the oxides. In the c ase of Cr3O4 and Cr2O3, there is a correlation between the size of the band gap in the oxide and its reactivity toward H2S. The uptake of su lfur by the oxides significantly increases when they are ''promoted'' with gold. The Au/Cr2O3 surfaces exhibit a unique electronic structure in the valence region and a larger ability to dissociate H2S than pol ycrystalline Au or pure Cr2O3 The results of ab initio SCF calculation s for the adsorption of H2S on AuCr4O6, and AuCr10O15 clusters show a shift of electrons from the gold toward the oxide unit that enhances t he strength of the Au(6s)<----> H2S(5a(1),2b(1)) bonding interactions and facilitates the decomposition of the molecule. (C) 1997 American I nstitute of Physics.