Direct and trapping-mediated pathways to dissociative chemisorption: CH₄ dissociation on Ir(111) with step defects

The indirect chemisorption of methane on a transition metal, where the incident molecule first traps onto the surface and then reacts from a physisorbed molecular state, has only been observed on Ir(111) and Ir(110) at very low collision energies. We use quantum scattering methods to describe the direct reaction of methane on Ir(111) at high energy and rate theory to examine the indirect pathway at low energy. Overall, we find good agreement with the experiment with respect to the variation of sticking with the incident energy, surface temperature, and vibrational state. Compared with methane dissociation on other metals, vibrational excitation is found to be less effective at promoting the reaction, while coupling to the lattice motion is unusually strong. We show how step defects, even at low concentrations, can contribute significantly to indirect chemisorption. We explore indirect chemisorption on Ni(111) and Pt(111) and find that the indirect path can be important when relaxed lattice barriers are sufficiently low and the temperature is sufficiently high so that the indirect sticking is within detection limits and the incident energy is low enough so that the trapping is large and the direct component is small.