Reaction of H2S and S2 with metal/oxide surfaces:: Band-gap size and chemical reactivity

The adsorption and dissociation of H2S and S-2 on a series of oxide (Al2O3, Cr2O3, Cr3O4, Cu2O, ZnO) and metal/oxide (Cu/Al2O3, Cu/ZnO) surfaces have been studied using synchrotron-based high-resolution photoemission. H2S and S-2 mainly interact with the metal centers of the oxides. At 300 K, H2S undergoes complete decomposition. The rate of decomposition on Al2O3 is much 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 toward S-containing molecules. The results of ab initio SCF calculations for the adsorption of H2S, HS, and S on clusters that resemble the (0001) face of alpha-Al2O3, alpha-Cr2O3, and ZnO show that the S-containing species interact stronger with Cr or Zn than with Al centers. These theoretical results and the trends seen in the experimental data indicate that the reactivity of an oxide mainly depends on how well its bands mix with the orbitals of H2S or HS. The electrostatic interactions between the dipole of H2S and the ionic field generated by the charges in the oxide play only a secondary role in the adsorption process. Photoemission results show that the rate of adsorption of H2S and S-2 On Cu/Al2O3 and Cu/ZnO surfaces is much faster than on the pure oxides. A simple model based on perturbation theory and orbital mixing is able to explain the effects of the band-gap size on the reactivity of an oxide and the behavior of metal/oxide surfaces in the presence of S-containing molecules.