Bulk gold has long been regarded as a noble metal, having very low chemical and catalytic activity. However, metal oxide-supported gold particles, particularly those that are less than 5 nm in diameter, have been found to have remarkable catalytic properties. In this study we show that impinging gas-phase CO molecules react readily with oxygen adatoms preadsorbed on Au/TiO(2)(110) to produce CO(2) even under conditions in which the sample is cryogenically cooled. Gold particle size seems to have little effect on the CO oxidation reaction when oxygen adatoms are preadsorbed. We also show that as the oxygen adatom coverage increases, the rate of CO oxidation decreases on Au/TiO(2) at cryogenic temperatures.

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.

We present results of a reactive scattering study of CO from an oxygen-atom-covered Au/TiO 2 model catalyst. We studied the CO oxidation reaction with preadsorbed atomic oxygen over a surface temperature range from 65 to 250 K and observed CO 2 production at all temperatures studied from the gold-covered titania samples. No reaction is observed on the bare TiO 2 (i.e., no gold) at any of the temperatures studied. The CO oxidation reaction is observed to proceed on all gold coverages studied (0.25-30 ML) as well as on a nominally continuous gold film. Hence, we conclude that the reaction of an oxygen atom with CO is relatively particle size independent and even independent of the TiO 2 support and therefore not exclusive to the gold-titania interface, if oxygen atoms are preadsorbed prior to CO exposure.

We present the results from a molecular beam study of the initial adsorption probability (S0) of O2 on Si(100)-2×1 as a function of surface temperature, incident kinetic energy and angle. The data show two distinct kinetic energy regimes with opposite temperature and energy dependencies, and correspond to two different adsorption mechanisms. For low incident kinetic energies, a trapping-mediated mechanism is dominant, exhibiting a strong increase in S0 with decreasing surface temperature and kinetic energy. Also, adsorption at low kinetic energies is independent of incident angle, indicating total energy scaling. Data in this range are well-described by a simple precursor model, which gives a difference in activation barrier heights of (Ed−Ec)=28 meV, and a ratio of preexponentials νd/νc=24.2. Trapping probabilities can also be estimated from the model, and show a strong falloff with increasing energy, as would be expected. At high incident kinetic energies, a strong increase in S0 with kinetic energy indicates that a direct chemisorption mechanism is active, with the observed energy scaling proportional to cos θi. There is also an unusual increase in S0 with surface temperature, with only a weak increase below 600 K, and a stronger increase above 600 K. The direct mechanism trends are discussed in terms of a possible molecular ion intermediate with thermally activated charge transfer. The molecular beam measurements are also used in calculating the reactivity of a thermalized gas with a clean surface. The precursor model is combined with a two-region fit of the direct adsorption data to predict chemisorption probabilities as a function of the incident conditions. These functions are then weighted by a Maxwell-Boltzmann distribution of incident angles and energies to calculate the adsorption probability for a thermal gas. These calculations indicate that the predominant mechanism depends strongly on temperature, with trapping-mediated chemisorption accounting for all of the adsorption at low temperatures, and direct adsorption slowly taking over at higher temperatures.

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.