TRAPPING-MEDIATED AND DIRECT DISSOCIATIVE CHEMISORPTION OF METHANE ONIR(110) - A COMPARISON OF MOLECULAR-BEAM AND BULB EXPERIMENTS

Molecular-beam and bulb gas techniques were employed to study dissocia tive chemisorption and physical adsorption of methane on Ir(110). The initial dissociative chemisorption probability (So) was measured as a function of incident kinetic energy (E-i), surface temperature, and an gle of incidence. With this investigation, we provide the first unambi guous evidence of a trapping-mediated pathway for methane dissociation on any surface. This interpretation is supported by excellent quantit ative agreement between our data at low kinetic energies and a simple kinetic model of the trapping-mediated mechanism. Additionally, this i s the first molecular-beam study of any gas on any surface that is con sistent with a simple trapping-mediated model in which the barrier to dissociation from the physically adsorbed state is greater than the ba rrier to desorption. At high-incident kinetic energies, the value of S o increases with Ei indicative of a direct mechanism. The values of th e 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 fun ction. of gas and surface temperature. These calculations are in excel lent agreement with the results obtained from bulb experiments conduct ed 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). (C) 1997 American Institute of Physics.