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We have designed a high-pressure reaction cell and sample transfer system as an addition to an existing ultrahigh vacuum chamber. The system enables us to study catalytic reactions on both single crystals and polycrystalline foils over a large range of pressures from 10−4 Torr to 1 atm. The key advantage of the setup is that the thermocouple is in direct contact with the sample, providing exact measurement of the sample temperature, while allowing transfer between two different manipulators. We demonstrate the utility of the experimental setup by monitoring oscillations in the rate of carbon monoxide oxidation over a platinum catalyst.

The initial probabilities of dissociative chemisorption 13 CH 4 and CD 4 have been measured on the close-packed Ir(111) surface at both low and high total pressures. At pressures below ∼ 10 −3 Torr, trapping-mediated dissociative chemisorption is the dominant reaction mechanism since the gas temperature is equal to the wall temperature of 300 K, with activation energies for C H (C D) bond cleavage of 12.6 kcal/mol for 13 CH 4 and 13.7 kcal/mol for CD 4 with respect to the gas phase energy zero of the methane infinitely far from the surface and at rest. For both methane isotopomers the ratio of the pre-exponential factor for desorption relative to that of reaction is ∼ 180 due to the larger phase space available to the molecule for desorption relative to reaction. The measured differences in C H versus C D bond activation for methane are attributed to zero-point energy differences between each isotopomer and point to classical over-the-barrier reaction dynamics as the reaction pathway for trapping-mediated chemisorption. The incident kinetic energy of the impinging methane was increased by diluting the methane in argon at a total pressure of 1 Torr which raised the gas temperature of the methane to the surface temperature. In this case, methane chemisorbs dissociatively via both trapping-mediated and direct C H bond activation. Direct activation of 13 CH 4 is characterized by an activation energy of 17.4 kcal/mol and a pre-exponential factor of ∼ 0.61, while direct activation of CD 4 occurs with an activation energy of 18.1 kcal/mol and the same pre-exponential factor. These activation energies are averages of a distribution of activation barriers which occur because of the nature of the multidimensional potential energy surface describing the interaction of the methane with the surface.

The initial probabilities of precursor-mediated dissociative chemisorption of 13 C 2 H 6 , C 2 D 6 , C 3 H 8 , CH 3 CD 2 CH 3 and C 3 D 8 have been measured on the hexagonally close packed Ru(001) surface. The activation energies for CH bond cleavage are 9310 cal mol −1 for 13 C 2 H 6 and 9900 cal mol −1 for C 2 D 6 with respect to the bottom of the physically adsorbed well of the ethane molecule. For each isotopomer of propane, these measured activation energies are 10 430, 10 620 and 10 910 kcal mol −1 for C 3 H 8 , CH 3 CD 2 CH 3 and C 3 D 8 , respectively. For each alkane investigated the ratio of the preexponential factor for desorption relative to that of reaction is ∼ 100 due to the larger phase space available to the molecule for desorption relative to reaction. An examination of the selective activation of primary (1°) versus secondary (2°) CH bonds in propane reveals that 2° CH bond cleavage is favored energetically by 240 cal mol −1 and entropically by a factor of 2.1 over 1° bond cleavage. However, the formation of 1° Ru-propyl intermediates is favored over the formation of 2° Ru-propyl intermediates at all investigated temperatures due to the greater number of 1° CH bonds in propane compared to 2° CH bonds. The measured differences in CH versus CD bond activation for both ethane and propane are attributed to zero-point energy differences between each isotopomer and point to classical over the barrier reaction dynamics as the reaction pathway.

The initial probabilities of precursor-mediated dissociative chemisorption of c-C3H6 and c-C3D6 have been measured on the hexagonally close-packed Ru(001) surface at surface temperatures from 400 to 1100 K. The measured activation energies for both c-C3H6 and c-C3D6 are 9470±120 cal mol−1 with respect to the bottom of the physically adsorbed well. Trapping-mediated dissociative chemisorption of cyclopropane occurs via CC rather than CH bond cleavage at all temperatures studied, as judged by the lack of a kinetic isotope effect. The preferred reaction pathway of CC bond cleavage is a consequence of the ring strain contained in molecular cyclopropane. For both isotopomers of cyclopropane, the ratio of the pre-exponential factor of desorption to that of reaction is ∼100 due to the larger phase space available to the molecule for desorption relative to reaction.

Both the transient and the isothermal decomposition of formate on Ru(001) have been studied using thermal desorption mass spectrometry. The formate decomposition rate, measured by monitoring the production rate of CO2, exhibits “stabilization” in both thermal desorption and isothermal desorption measurements. At a constant heating rate, the thermal desorption spectrum of CO2 shifts up in peak temperature and becomes progressively more narrow with increasing initial coverage of surface formate. The isothermal decomposition rate reveals a convex parabolic curve with time prior to passing through a maximum. A simple mean-field kinetic model is used to describe both the transient thermal desorption and the isothermal desorption spectra. The stabilization kinetics are attributed both to attractive interactions between formates as well as between a formate and its chemisorbed decomposition product, CO. Calculations using the mean-field model quantitatively reproduce the experimentally observed kinetic behavior.

This article focuses on the technical issues of imaging of dynamic adsorbate pattern formation on platinum surfaces using laser light to illuminate the area of interest. In particular, ellipsomicroscopy for surface imaging (EMSI) is a powerful tool to follow spatio-temporal patterns of adsorbate layers on catalyst surfaces at arbitrary pressures. This novel method is uniquely sensitive to submonolayer coverages of adsorbates. It expands the range of observable pressure conditions by many orders of magnitude, thus bridging the pressure gap in imaging surface reactions. EMSI is a versatile technique that opens new avenues of potential applications to resolve dynamic surface processes, such as adsorbate diffusion or coating formation.