Trapping-mediated and direct dissociative chemisorption of methane on Ir(110): A comparison of molecular beam and bulb experiments

Abstract

Molecular-beam and bulb gas techniques were employed to study dissociative chemisorption and physical adsorption of methane on Ir(110). The initial dissociative chemisorption probability (S0) was measured as a function of incident kinetic energy (Ei), surface temperature, and angle of incidence. With this investigation, we provide the first unambiguous evidence of a trapping-mediated pathway for methane dissociation on any surface. This interpretation is supported by excellent quantitative agreement between our data at low kinetic energies and a simple kinetic model of the trapping-mediated mechanism. Additionally, this is the first molecular-beam study of any gas on any surface that is consistent with a simple trapping-mediated model in which the barrier to dissociation from the physically adsorbed state is greater than the barrier to desorption. At high-incident kinetic energies, the value of S0 increases with Ei indicative of a direct mechanism. The values of the reaction probability determined from the molecular-beam experiments are integrated over a Maxwell–Boltzmann energy distribution to predict the initial chemisorption probability of thermalized methane as a function of gas and surface temperature. These calculations are in excellent agreement with the results obtained from bulb experiments conducted with room-temperature methane gas over Ir(110) and indicate that a trapping-mediated pathway governs dissociation at low gas temperatures. At the high gas temperatures characteristic of catalytic conditions, however, a direct mechanism dominates reactive adsorption of methane over Ir(110).