Reaction of H_2S with MgO(100) and Cu/MgO(100) surfaces: Band-gap size and chemical reactivity

摘要

The interaction of H_2S, 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 H_2S 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 H_2S molecules (approx 80%) chemisorb intact on a MgO(100) surface. Annealing to 200 K induces cleavage of S-H bonds leaving similar amounts of H_2S and SH on the surface. The complete disappearance of H_2S is observed at 300 K, and the dominant species on the oxide is SH which is coadsorbed with a small amount (approx 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 H_2S molecule is relatively weak. Defect sites probably play an important role in the dissociation of H_2S. Cu adatoms facilitate the decomposition of H_2Sd on MgO(100) by providing electronic states that are very efficient for interactions with the frontier orbitals of the molecule. The rate of H_2S decomposition on MgO is substantially lower than those found on Cr_3O_4, Cr_2O_3, ZnO, and Cu_2O. For these systems, the smaller the band-gap in the oxide, the bigger its reactivity towards H_2S. Theoretical calculations indicate that this trend reflects the effects of band-orbital mixing. The electrostatic interactions between the dipole of H_2S and the ionic field generated by the charges in an oxide play only a secondary role in the adsorption process.