On the viability of single atom abstraction in the dissociative chemisorption of O2 on the Al(111) surface

Abstract

The dissociative chemisorption of O2 on the Al(111) surface is investigated by means of a Monte Carlo simulation that incorporates two mechanisms that have been proposed for this reaction in the literature: single atom abstraction and two-atom adsorption that generates translationally hot atoms on the surface. A comparison is made to the much-debated STM results of Brune et al. [J. Chem. Phys. 99, 2128 (1993)], in which the oxygen island density (number of islands per binding site) was determined as a function of coverage. Since the two-atom channel has been discussed heavily in the literature, we focus primarily on the abstraction mechanism. We show that atom abstraction in its basic form is incompatible with the STM results; however, we propose two simple modifications that enable atom abstraction to reproduce the STM results. In the first modification, the probability of dissociation is higher at sites next to preexisting O adatoms. In essence, we are proposing that the increased Al–O bond strength at sites next to preexisting O adatoms [Jacobsen et al., Phys. Rev. B 52, 14954 (1995)] stabilizes the transition state for dissociation. If atom abstraction is assumed to be the only operative mechanism, and if its probability increases by a factor of ∼10 next to a site that is occupied versus unoccupied, the STM island density data can be approximately reproduced. In the second modification, the abstracted atom is permitted to make a single hop in the direction of a preexisting, nearby O adatom. The allowance of merely a single, directed hop has a dramatic effect on the coverage dependence of the island density.