The interaction of Cu and S 2 with aluminum and alumina surfaces: a comparative study

Abstract

The bonding interactions between Cu and Al are much stronger than those between Cu and Al 2O 3. Cu atoms supported on alumina show a narrow 3d band with a centroid shifted ∼0.35 eV with respect to that of the 3d band in bulk metallic Cu. In contrast, Cu atoms deposited on aluminum exhibit shifts of 1.3–1.6 eV in the centroid of the 3d band. Similar differences are observed when comparing the behavior of Ag and Pt overlayers on alumina and aluminum. The d band shifts on the oxide substrate are in the order of 0.3–0.4 eV, whereas on the metal substrate they vary from 0.8 to 2.0 eV. These trends are explained in terms of a simple model that takes into account changes in the energy of the Al(3s, 3p) bands when going from metallic aluminum to alumina. The sticking coefficient of S 2 on alumina surfaces is at least one order of magnitude smaller than on aluminum, a difference that also reflects variations in the position of the Al(3s,3p) bands. Submonolayer coverages of Cu do not produce significant changes in the electronic properties of Al 2O 3. In contrast, the deposition of small amounts of sulfur (∼0.1 ML) induces a substantial reduction (0.4–0.5 eV) in the binding energies of the O KVV, O 1s and Al 2p features of alumina. This is consistent with a transfer of electrons from alumina into the S atoms that produces a transformation similar to a change from n-type to p-type semiconductors. The reactivity of Cu Al 2 O 3 surfaces toward sulfur is much larger than that of pure Al 2O 3 surfaces. Cu clusters supported on alumina react with S 2 to form CuS x compounds that decompose at temperatures between 850 and 1100 K.