Sulfur adsorption on Ni(100) and its effect on CO chemisorption: I. TDS, AES and work function results

Abstract

The adsorption of H 2S on Ni(100) at 300 K was investigated. Adsorption is dissociative and proceeds via a mobile precursor, with sulfur strongly chemisorbed and hydrogen desorbing both during the dose and upon subsequent heating. Chemisorbed sulfur causes an increase in surface work function due to charge transfer of about 0.04 e from nickel to sulfur. The adsorption of CO at 95 K was studied on the sulfur-predosed surface using AES, TDS and work function change measurements. Evidence is presented for steric and relatively short-range ( ≈ 4 A ̊ ) electronic effects of S on CO. One sulfur blocks two high-temperature ( β 2) CO sites by steric (site-blocking) effects. In the presence of sulfur, CO redistributes into several lower-temperature desorption states, some of which exist on regions of bare Ni, and some within sulfur domains. At a sulfur coverage of 0.40 ML, a new CO states at 140 K was observed which, unlike the other CO states, caused a slight decrease in the surface work function. The behavior of this state is explained in terms of reduced back-donation from the metal due to close association of the CO with sulfur. Several proposed modes for sulfur's action are evaluated on the basis of the data. These include steric effects, a through-space electrostatic potential, and a through-metal electronic effect involving density of state changes.