Ethylene oxide adsorption on K-promoted Ni(111)

Abstract

Ethylene oxide (Et-O) adsorption on clean and K-modified Ni(111) surfaces was studied with thermal desorption spectroscopy (TDS) and work function change measurements (Δø) in the temperature range of 100–1000 K. On clean Ni(111), in agreement with previous measurements, evidence is found for a molecular adsorption of Et-O. The binding energy towards the Ni(111) surfaces is weak. With TDS desorption maxima are detected at 120 K (Et-O ice layer), 150 K (Et-O from a second adsorption layer) and finally at 195–215 K from fractional Et-O monolayer coverages. On clean Ni(111), Et-O lowers the work function with a maximum change of Δø = −1.6 eV. The negative sign of Δø is consistent with a perpendicular orientation of the Et-O molecular axis with the negative end — the O atom — directed towards the substrate. On K-dosed Ni(111) the Et-O adsorption behavior is significantly changed for low K coverages ( θ(K) < 0.15) and drastically altered for higher coverages, θ(K) ⩾ 0.36. Up to θ(K) = 0.15 the Et-O adsorption is still molecular. Due to the lateral interaction between Et-O and K the effective Et-O dipole moment as judged from Δø measurements and the repulsive interaction between neighboring Et-O (TDS peak narrowing) decreases to zero. For higher K coverages, gq(K) ⩾ 0.36, Et-O is thermally stabilized to temperatures above 400 K. This behavior is believed to be due to a ring breaking of Et-O and the formation of a K-stabilized radical. Above 540 K, evolution of H 2 and other molecular fragments gives evidence for further decomposition processes.