A TPD, SIMS and Δφ study of the influence of coadsorbed potassium on the adsorption and decomposition of ethylene on Pt(111)

Abstract

The influence of potassium on the adsorption and decomposition of ethylene on Pt(111) has been studied using TPD, positive and negative SIMS and Δφ measurements. Potassium alters the bonding mode of ethylene and, at low coverage, increases the sticking coefficient at 100 K. Potassium inhibits ethane formation, eliminates hydrogen evolution at room temperature and induces a new hydrogen desorption peak at 420 K. Around room temperature, a new species, which we propose is ethylidene (CH 3CH=), is formed in the presence of K. This species becomes dominant for θ K between 0.07 and 0.14 ML and converts into ethylidyne at higher temperature (near 360 K). The species left on the surface after ethylidyne decomposition is characterized as CCH and it is destabilized by potassium. At larger θ K, ethylene rapidly dehydrogenates to CCH without the accumulation of ethylidyne. Decomposition of CCH leaves surface carbon, which starts to polymerize at 460 K. Throughout all these processes, CC bond cleavage is not favored.