Initial probability of dissociative chemisorption of oxygen on iridium(110)

Abstract

The dissociative chemisorption of oxygen on Ir(110) has been investigated using supersonic molecular beam techniques. The initial probability of dissociative chemisorption (in the limit of zero surface coverage) as a function of incident kinetic energy between 1 and 28 kcal/mol and surface temperature from 85 to 1000 K is reported. For beam kinetic energies less than approximately 4 kcal/mol, the measured values of the initial probability of dissociative chemisorption are explained by a trapping-mediated adsorption mechanism. In this adsorption regime initial probabilities of dissociative chemisorption decrease with both increasing beam energy and surface temperature. The trapping probability of oxygen into the physically adsorbed state on Ir(110) as a function of incident beam energy is presented. For beam kinetic energies greater than ∼4 kcal/mol, a direct chemisorption mechanism dominates. In the direct adsorption regime, initial probabilities of dissociative chemisorption increase with increasing beam energy, and they are dependent on surface temperature, with the dependence decreasing with increasing surface temperature. This behavior is attributed to direct chemisorption into a molecularly chemisorbed state, from which there is a thermally activated kinetic competition between desorption and dissociation. A pseudo-steady-state kinetic model including physically adsorbed oxygen, molecularly chemisorbed oxygen, and atomically chemisorbed oxygen is applied to find that the activation barrier to desorption from the physically adsorbed molecular state is 1.6±0.1 kcal/mol higher than the barrier to conversion to the molecularly chemisorbed state. The activation barrier for desorption from the molecularly chemisorbed state is 1.5±0.15 kcal/mol greater than the barrier to dissociation from this state.