Reaction of H2S with MgO(100) and Cu/MgO(100) surfaces:: Band-gap size and chemical reactivity

The interaction of H2S, SH, and S with MgO(100) and Cu/MgO(100) surfaces has been investigated using synchrotron-based high resolution photoemission and density functional calculations. Metallic magnesium reacts vigorously with H2S fully decomposing the molecule at temperatures below 200 K. In contrast, the Mg atoms in MgO exhibit a moderate reactivity. At 80 K, most of the H2S molecules (similar to 80%) chemisorb intact on a MgO(100) surface. Annealing to 200 K induces cleavage of S-H bonds leaving similar amounts of H2S and SH on the surface. The complete disappearance of H2S is observed at 300 K, and the dominant species on the oxide is SH which is coadsorbed with a small amount (similar to 10%) of atomic S. The adsorbed SH fully decomposes upon heating to 400 K producing S adatoms that are stable on the surface at temperatures well above 500 K. The results of density functional calculations indicate that the bonding interactions of SH and S with pentacoordinated Mg sites of a flat MgO(100) surface are strong, but the bonding of the H2S molecule is relatively weak. Defect sites probably play an important role in the dissociation of H2S. Cu adatoms facilitate the decomposition of H2S on MgO(100) by providing electronic states that are very efficient for interactions with the frontier orbitals of the molecule. The rate of H2S decomposition on MgO is substantially lower than those found on Cr3O4, Cr2O3, ZnO, and Cu2O. For these systems, the smaller the band-gap in the oxide, the bigger its reactivity towards H2S. Theoretical calculations indicate that this trend reflects the effects of band-orbital mixing. The electrostatic interactions between the dipole of H2S and the ionic field generated by the charges in an oxide play only a secondary role in the adsorption process. (C) 1999 American Institute of Physics. [S0021-9606(99)71041-X].