Chemisorption of ethylene on ordered Sn/Pt(111) surface alloys

Abstract

The adsorption of ethylene on ordered Sn/Pt surface alloys has been investigated using temperature programmed desorption (TPD), ultraviolet photoelectron spectroscopy (UPS), and high resolution electron energy loss spectroscopy (HREELS). The ordered surface alloys are prepared by annealing (to 1000 K) monolayer amounts of Sn vapor deposited onto Pt(111). This produces Sn/Pt(111) surfaces with p(2 × 2) or (√3 × √3)R30 ° LEED patterns, depending on the conditions used. These surfaces are postulated [M.T. Paffett and R.G. Windham, Surface Sci. 208 (1989) 34] to be the (111) face of Pt 3Sn and a substitutional surface alloy of composition Pt 2Sn, respectively. Ethylene is reversibly adsorbed at temperatures below 150 K on both ordered Sn/Pt(111) surface alloys: no ethylene decomposition occurs upon heating either surface to above 600 K. As the Sn concentration in the surface alloy is increased, there is a marked decrease in the ethylene desorption temperature from 285 K on Pt(111) to 240 K on the p(2 × 2) alloy and to 184 K on the (√3 × √3)R30 ° alloy. This indicates a significant electronic effect induced by Sn on the Pt-ethylene chemisorption bond strength. However, UPS and HREELS data are consistent with retention of the di-σ-bonding interaction of ethylene on both surface alloys as on Pt(111). The adsorption/desorption behavior and decomposition of ethylene on these surface alloys is consistent with the ensemble size requirement of 4 Pt atoms for ethylene adsorption and 6 Pt atoms for ethylene decomposition on Pt(111) estimated by us previously [14], from studies of C 2H 4/Bi/ Pt(111). However, electronic effects of alloying Sn with Pt may complicate this simple model and are discussed in describing ethylene chemistry on these surface alloys.