THE INTERACTION OF CU AND S-2 WITH ALUMINUM AND ALUMINA SURFACES - A COMPARATIVE-STUDY

The bonding interactions between Cu and Al are much stronger than thos e between Cu and Al2O3. Cu atoms supported on alumina show a narrow 3d band with a centroid shifted similar to 0.35 eV with respectto that o f 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 substr ate 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 sti cking coefficient of S-2 on alumina surfaces is at least one order of magnitude smaller than on aluminum, a difference that also reflects va riations in the position of the Al(3s,3p) bands. Submonolayer coverage s of Cu do not produce significant changes in the electronic propertie s of Al2O3. In contrast, the deposition of small amounts of sulfur (si milar to 0.1 ML) induces a substantial reduction (0.4-0.5 eV) in the b inding energies of the O KW, O Is and Al 2p features of alumina. This is consistent with a transfer of electrons from alumina into the S ato ms that produces a transformation similar to a change from n-type to p -type semiconductors. The reactivity of Cu/(Al2O3)-O-., surfaces towar d sulfur is much larger than that of pure Al2O3, surfaces. Cu clusters supported on alumina react with S-2 to form CuSx compounds that decom pose at temperatures between 850 and 1100 K. (C) 1997 Elsevier Science B.V.