Adsorption of Sulfur on Ag/Al2O3and Zn/Al2O3Surfaces: Thermal Desorption, Photoemission, and Molecular Orbital Studies

The interaction of S 2 with Ag/Al 2 O 3 and Zn/Al 2 O 3 surfaces has been investigated using thermal desorption mass spectroscopy, core- and valence-level photoemission, and ab initio self-consistent-field calculations. The sticking coefficient of S 2 on clean alumina is small (<0.1) at temperatures between 300 and 700 K. In the S 2 /Al 2 O 3 system, there is an energy mismatch between the orbitals of the molecule and the bands of the oxide, and the reactivity of S 2 on pure alumina is very low. The adsorption of sulfur on Ag/Al 2 O 3 and Zn/Al 2 O 3 surfaces induces strong perturbations in the electronic properties of the admetals and oxide support. In the Ag/Al 2 O 3 and Zn/Al 2 O 3 systems, the supported metal clusters (or particles) provide a large number of electronic states that are very efficient at donating charge into the S 2 (2π g ) orbitals, inducing in this way the breaking of the S−S bond and the formation of admetal sulfides (AgS x and ZnS y ). The larger the electron transfer from the supported metal into the S 2 (2π g ) orbitals, the bigger the adsorption energy of the molecule and the easier the cleavage of the S−S bond. The AgS x and ZnS y species decompose at temperatures between 750 and 900 K following two different reaction pathways: AgS x,solid → S 2,gas + Ag solid and ZnS y,solid → S 2,gas + Zn gas .