Molecular beam and bulb gas techniques were employed to study dissociative chemisorption of methane on Ir(111). The initial dissociative chemisorption probability (S0) was measured as a function of incident kinetic energy (Ei), surface temperature, and angle of incidence (θi). As the incident kinetic energy increases, the value of S0 first decreases and then increases with Ei indicating that a trapping-mediated chemisorption mechanism dominates methane dissociation at low kinetic energy, and a direct mechanism dominates at higher kinetic energies. The values of the reaction probability determined from molecular beam experiments of methane on Ir(111) are modeled as a function of Ei, θi, and surface temperature. These fits are then integrated over a Maxwell–Boltzmann energy distribution to calculate the initial chemisorption probability of thermalized methane as a function of gas and surface temperature. The calculations are in excellent agreement with results obtained from bulb experiments conducted with room-temperature methane gas over Ir(111) and indicate that a trapping-mediated pathway governs dissociation at low gas temperatures. At the high gas temperatures characteristic of catalytic conditions, however, these calculations indicate that a direct mechanism dominates methane dissociation over Ir(111). These dynamical results are qualitatively similar to the results of a previous study of methane dissociation on Ir(110), although the reactivity of thermalized methane is approximately an order of magnitude higher on the (110) surface of iridium.

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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).

We have used supersonic molecular beam techniques to measure the initial dissociative chemisorption probability S 0 of O 2 on Ru͑001͒ as a function of incident kinetic energy E i , surface temperature T s , and angle of incidence i . We observe different behavior in the adsorption dynamics in two separate kinetic energy regimes: the value of S 0 decreases with incident energy in the low kinetic energy regime, and the value increases with incident energy in a higher kinetic energy regime. In the low energy regime, we observe a large inverse dependence of S 0 on surface temperature which is consistent with a trapping-mediated mechanism. Moreover, adsorption in the low energy regime can be accurately modeled by a trapping-mediated mechanism, with a surface temperature independent trapping probability ␣ into a physically adsorbed state followed by a temperature dependent kinetic competition between desorption and dissociation. The barrier to dissociation from the physically adsorbed state is ϳ28 meV below the barrier to desorption from this state as determined by analysis of kinetic data. In the high kinetic energy regime, values of the initial adsorption probability scale with normal kinetic energy, and S 0 approaches a value of unity for the highest incident energies studied. However, we report an unusual surface temperature dependence of S 0 in the high energy regime that is inconsistent with a simple direct mechanism. Indeed, in this higher energy regime the value of S 0 rises as the surface temperature is increased. We suggest a mechanism involving electron transfer from the ruthenium surface to account for this phenomena.

A density functional theory study of CO oxidation on Ru(0001) at low coverage Low temperature CO oxidation triggered by the gas-phase D atom incident on Pt(111) covered with O 2 and CO Impinging O-atoms react with adsorbed CO on Pt͑111͒, Ir͑111͒, and Ru͑001͒, to form CO 2 at surface temperatures as low as 77 K. The initial reaction probability is measured on these three surfaces using reflectivity techniques and is much lower on Pt͑111͒ than previously supposed. The reaction probability is measured as a function of surface temperature, incident O-atom flux, kinetic energy, and angle. Interestingly, a significant dependence on incident angle is observed on all surfaces ͑the reaction probability is ϳ2.5 times greater at normal incidence than at glancing angles͒, and a kinetic energy effect is noted at the higher incident angles studied. Also, surface temperature is shown to have an effect on the reaction probability in measurements performed on Pt͑111͒ and Ir͑111͒ at normal incidence.