An effusive molecular beam technique for studies of polyatomic gas-surface reactivity and energy transfer.

An effusive molecular beam technique is described to measure alkane dissociative sticking coefficients, S(T(g), T(s); ϑ), on metal surfaces for which the impinging gas temperature, T(g), and surface temperature, T(s), can be independently varied, along with the angle of incidence, ϑ, of the impinging gas. Effusive beam experiments with T(g) = T(s) = T allow for determination of angle-resolved dissociative sticking coefficients, S(T; ϑ), which when averaged over the cos (ϑ)/π angular distribution appropriate to the impinging flux from a thermal ambient gas yield the thermal dissociative sticking coefficient, S(T). Nonequilibrium S(T(g), T(s); ϑ) measurements for which T(g) ≠ T(s) provide additional opportunities to characterize the transition state and gas-surface energy transfer at reactive energies. A resistively heated effusive molecular beam doser controls the T(g) of the impinging gas striking the surface. The flux of molecules striking the surface from the effusive beam is determined from knowledge of the dosing geometry, chamber pressure, and pumping speed. Separate experiments with a calibrated leak serve to fix the chamber pumping speed. Postdosing Auger electron spectroscopy is used to measure the carbon of the alkyl radical reaction product that is deposited on the surface as a result of alkane dissociative sticking. As implemented in a typical ultrahigh vacuum chamber for surface analysis, the technique has provided access to a dynamic range of roughly 6 orders of magnitude in the initial dissociative sticking coefficient for small alkanes on Pt(111).

Transient and steady state CO oxidation kinetics on nanolithographically prepared supported Pd model catalysts: experiments and simulations.

Applying molecular-beam methods to a nanolithographically prepared planar PdSiO2 model catalyst, we have performed a detailed study of the kinetics of CO oxidation. The model catalyst was prepared by electron-beam lithography, allowing individual control of particle size and position. The sample was structurally characterized by atomic force microscopy and scanning electron microscopy before and after reaction. In the kinetic experiments, the O-rich and CO-rich regimes were investigated systematically with respect to their transient and steady-state behaviors, both under bistable and monostable reaction conditions. Separate molecular beams were used in order to supply the reactants, allowing individual control over the reactant fluxes. The desorbing CO2 was detected by both angle-resolved and angle-integrated mass spectrometries. The experimental results were analyzed using different types of microkinetic models, including a detailed reaction-diffusion model, which takes into account the structural parameters of the catalyst as well as scattering of the reactants from the support. The model quantitatively reproduces the results as a function of the reactant fluxes and the surface temperature. Various kinetic effects observed are discussed in detail on the basis of the model. Specifically, it is shown that under conditions of limited oxygen mobility, the switching behavior between the kinetic regimes is largely driven by the surface mobility of CO.

Local reaction rates and surface diffusion on nanolithographically prepared model catalysts: experiments and simulations.

Combining molecular beam methods and angular resolved mass spectrometry, we have studied the angular distribution of desorbing products during CO oxidation on a planar Pd/silica supported model catalyst. The model catalyst was prepared by means of electron beam lithography, allowing individual control of particle size, position, and aspect ratio, and was characterized by atomic force microscopy and scanning electron microscopy before and after reaction. In the experiment, both oxygen and CO rich regimes were investigated using separate molecular beams for the two reactants. This allows exploration of diffusion effects of reactants on the particles and of shadowing and backscattering phenomena. A reaction-diffusion model was developed in order to extract information about local reaction rates on the surface of the catalyst nanoparticles. The model takes into account the structural parameters of the catalyst as well as the backscattering of the reactants and products from the support. It allows a quantitative description of the experimental data and provides a detailed understanding of temperature and reactant flux dependent effects. Moreover, information on the surface mobility of oxygen under steady-state reaction conditions could be obtained by comparison with the experimental results.

Fluctuations and bistabilities on catalyst nanoparticles.

We show that coverage fluctuations on catalyst particles can drastically alter their macroscopic catalytic behavior. Scrutinizing the occurrence of kinetic bistabilities, it is demonstrated by molecular beam experiments on model catalysts that macroscopically observable bistabilities vanish completely with decreasing particle size, as previously predicted by theory. The effect is attributed to fluctuation-induced transitions between two kinetic reaction regimes, with a transition rate controlled by both particle size and surface defects. These results suggest that fluctuation-induced effects represent a general phenomenon affecting the reaction kinetics on nanostructured surfaces.