Dissociative chemisorption of oxygen on Ir(110) as a function of surface coverage

Abstract

We have investigated the probability of dissociative chemisorption of molecular oxygen on the reconstructed Ir(110) surface as a function of surface coverage using supersonic molecular beams over a wide range of surface temperatures (80 to 1000 K) and beam energies (1 to 28 kcal/mol) at different angles of incidence. These results are highly non-Langmuirian, with sudden transitions in the slope of the probability of chemisorption as a function of coverage. These distinctive transitions in the probability of chemisorption are attributed to the kinetic effects of multiple precursors on the surface, as well as effects due to the highly corrugated nature of the surface, and are extant over the entire spectrum of the parameters we investigated. A kinetic model with two precursors to dissociative chemisorption, a molecularly chemisorbed state and a physically adsorbed state, yields an excellent fit to the data at a beam energy of 1.2 kcal/mol for surface temperatures from 150 to 600 K. At a surface temperature of 85 K and beam energies between 1.2 and 8.5 kcal/mol, the probability of chemisorption as a function of coverage can be resolved into two components: a trapping-mediated and a direct component. The trapping-mediated component remains constant until high coverages ( θ > 0.6) and then decreases rapidly, and it decreases exponentially with increasing beam energy. The other component is direct chemisorption into the molecularly chemisorbed well, whose contribution increases with increasing energy and shows an effective activation barrier of 5 kcal/mol. The direct component is fit very well by a model of dissociative chemisorption using second-order Langmuirian adsorption kinetics. We have also determined the difference between the barrier to desorption and the barrier to reaction (dissociation) as measured from the bottom of the molecularly chemisorbed well at non-zero coverage, and E d − E r was determined to be 2.2 ± 0.1 kcal/mol with k (0) d k (0) r = 190 ± 60 . The probability of chemisorption is at most weakly sensitive to the angle of incidence at any surface coverage over the entire range of beam energies.