DISSOCIATIVE CHEMISORPTION OF METHANE ON IR(111) - EVIDENCE FOR DIRECT AND TRAPPING-MEDIATED MECHANISMS

Molecular beam and bulb gas techniques were employed to study dissocia tive chemisorption of methane on Ir(111). The initial dissociative che misorption probability (S-0) was measured as a function of incident ki netic energy (E-i), surface temperature, and angle of incidence (theta (i)). As the incident kinetic energy increases, the value of S-0 first decreases and then increases with E-i indicating that a trapping-medi ated chemisorption mechanism dominates methane dissociation at low kin etic energy, and a direct mechanism dominates at higher kinetic energi es. The values of the reaction probability determined from molecular b eam experiments of methane on Ir(111) are modeled as a function of E-i , theta(i), and surface temperature. These fits are then integrated ov er a Maxwell-Boltzmann energy distribution to calculate the initial ch emisorption probability of thermalized methane as a function of gas an d surface temperature. The calculations are in excellent agreement wit h results obtained from bulb experiments conducted with room-temperatu re methane gas over Ir(111) and indicate that a trapping-mediated path way governs dissociation at low gas temperatures. At the high gas temp eratures characteristic of catalytic conditions, however, these calcul ations indicate that a direct mechanism dominates methane dissociation over Ir(111). These dynamical results are qualitatively similar to th e results of a previous study of methane dissociation on Ir(110), alth ough the reactivity of thermalized methane is approximately an order o f magnitude higher on the (110) surface of iridium. (C) 1997 American Institute of Physics. [S0021-9606(97)01047-7].