The adsorption of benzene on Pd(111) and ordered Sn/Pd(111) surface alloys

Abstract

The adsorption of benzene on Pd(1 1 1) and ordered Sn/Pd(1 1 1) surface alloys has been studied using ultraviolet photoelectron spectroscopy (UPS), high-resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD) and low energy electron diffraction (LEED). Two ordered surface alloys were prepared by thermal treatment of vapor deposited Sn-films on Pd(1 1 1). Depending on the preparation conditions, the surface exhibited a p (2×2) or a (√3×√3) R 30° LEED pattern, corresponding to surface alloys of composition Pd 3 Sn and Pd 2 Sn, respectively. The measurements reveal that benzene adsorbs molecularly on Pd(1 1 1) at temperatures below 400 K and that the molecule is π-bonded to the surface with the ring plane parallel to the surface. At low initial coverage, heating leads to complete decomposition of the adsorbed benzene, while at higher coverage additional desorption of molecular benzene occurs in two desorption states at ∼400 and ∼500 K, respectively. The evolution of the low temperature state is correlated with variations in the electronic states of the adsorbed benzene and attributed to a change in the adsorption configuration. For the first time a CO-free (√19×√19) R ±23.4° superstructure has been discovered on Pd(1 1 1) at benzene monolayer saturation, allowing for a determination of the adsorption sites. In comparison to the pure Pd(1 1 1) surface, alloying leads to successive weakening of the benzene–surface bond with increasing Sn-content in the topmost layer of the substrate. Decomposition is suppressed on both surface alloys. Finally there was no evidence for a tilted benzene configuration on either of the surfaces investigated.