Kinetics and structure of O 2 chemisorption on Ni(1 1 1)

Abstract

The kinetics of O 2 chemisorption at low dose on Ni(1 1 1) and the nature of the chemisorption site have been studied at 300 and 500 K using time-of-flight scattering and recoiling spectrometry (TOF-SARS), low energy electron diffraction, and scattering and recoiling imaging code (SARIC) simulations. Variations in the TOF-SARS spectra with different crystal alignments during O 2 dosing provide direct information on the location of oxygen adatoms on the Ni(1 1 1) surface at very low coverages as well as site-specific occupation rates ( S fcc and S hcp) and occupancies ( θ fcc and θ hcp). A system of equations has been developed that relate the slopes of the scattering and recoiling intensities versus O 2 exposure dose to these probabilities and occupancies. The results identify three chemisorption stages as a function of oxygen exposure, each with its own specific occupation rates and occupancies. The first- stage is observed up to θ 1=θ fcc ∼0.21 ML with θ hcp=0, constant S= S fcc∼0.18±0.01, and coverage ratio w= θ hcp/ θ fcc∼0 for both 300 and 500 K. The second-stage is observed at coverages between θ 1∼0.21 and θ 2∼0.32 ML with constant S fcc=−(0.05±0.01) and S hcp=(0.16±0.02) at 300 K and S fcc=(0.005±0.003) and S hcp=(0.007±0.003) at 500 K, and coverage ratios w= θ hcp/ θ fcc∼1 at 300 K and w= θ hcp/ θ fcc∼0.10 at 500 K. The third-stage, observed for θ>0.32 ML, involves saturation coverage of the adsorption sites. SARIC simulations were used to interpret the spectra and the influence of oxygen chemisorption and vibrational effects. A method for determining the “effective Debye temperature Θ D *” that uses the experimental TOF-SARS intensity variations as a function of temperature and the simulated SARIC signals as a function of the mean square vibrational amplitude 〈 u 2〉 has been developed. The result for this system is Θ D *=314±10 K.