Reaction of H₂S and S₂with Metal/Oxide Surfaces:  Band-Gap Size and Chemical Reactivity

The adsorption and dissociation of H 2 S and S 2 on a series of oxide (Al 2 O 3, Cr 2 O 3, Cr 3 O 4, Cu 2 O, ZnO) and metal/oxide (Cu/Al 2 O 3, Cu/ZnO) surfaces have been studied using synchrotron-based high-resolution photoemission. H 2 S and S 2 mainly interact with the metal centers of the oxides. At 300 K, H 2 S undergoes complete decomposition. The rate of decomposition on Al 2 O 3 is much lower than those found on Cr 3 O 4, Cr 2 O 3, ZnO, and Cu 2 O. 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 H 2 S, HS, and S on clusters that resemble the (0001) face of α-Al 2 O 3, α-Cr 2 O 3, 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 H 2 S or HS. The electrostatic interactions between the dipole of H 2 S 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 H 2 S and S 2 on Cu/Al 2 O 3 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.