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Scanning Tunneling Microscopy; CO oxidation; Pd(111); Ruthenium dioxide
Abstract:
Scanning Tunneling Microscopy has already been established as a tool for the investigation of simple reaction mechanisms. The aim of this thesis was to apply this technique to study emmore complicated reactions. The oxidation of CO on Pd(111) and on a RuO2 film grown on Ru(0001) was investigated. Structural analyses of the O, CO and (CO+O) adlayers on Pd(111) and on RuO2 reveal the microscopic distributions of the adsorbates on the surfaces. Dynamic and quantitative analyses of the reactions yield the reaction kinetics and the reaction mechanisms in a direct way at the microscopic level.
O atoms on Pd(111) at intermediate coverages (0.10<0.15) show mobility at sample temperatures (T mathrm sample) higher than 135 K. The activation energy for diffusion is E* mathrm diff=0.54 pm 0.08 eV and the pre-exponential factor for hopping is Gamma mathrm o = 10 16 pm 3 s-1. CO molecules on Pd(111) at low coverages (theta mathrm CO sim 0) show very high mobility, even at T mathrm sample = 60 K. Assuming a value for Gamma mathrm o of 10 13 s-1, a value of 0.15 eV was estimated for E* mathrm diff for CO. Adsorption of CO molecules on a (2times2)-O adlayer at T mathrm sample > 130 K causes phase transitions of the adlayer into the (sqrt3 times sqrt3) R30 circ-O structure and finally into the (2times1) structure. During the phase transitions, the mobility of the O atoms increases, reflected by a 10 sim 20 % lower E* mathrm diff (under the assumption that Gamma mathrm o = 10 16 s-1) than in the absence of CO. At the end of the phase transitions, many small patches with a (2times1) superstructure emerge from a disordered (CO+O) co-adlayer, which then agglomerate to form larger (2times1) islands. (2times1) islands are highly reactive even at T mathrm sample = 136 K. The quantitative analysis of the reaction of the (2times1) islands reveals that the reaction rate is proportional to the total area of the islands, rather than to the total length of the boundary of the islands. The reaction order is sim 1 with respect to theta mathrm (2times1). For E* mathrm reac a value of 0.41 eV was estimated under the assumption of a pre-exponential factor k mathrm o = 10 13 s-1. Adsorption of CO molecules on the (2times2)-O adlayer at T mathrm sample < 130 K does not cause a phase transition, but CO adsorbs on the (2times2)-O islands.
The RuO2 film was grown on a Ru(0001) surface between 650 K and 900 K. The morphology of the oxide film is strongly dependent on the sample temperature during the preparation (Tprep). The morphology of the oxide film is predominantly kinetically controlled at Tprep sim 650 K, and thermodynamic effects become more important as Tprep is increased to 900 K. The thickness of the oxide film is independent of Tprep, and it ranges between 7 AA and 15 AA, corresponding to 2 to 5 oxide (Ru-O) monolayers. Partial evaporation of the oxide film by flashing the sample to various temperatures indicates the thermodynamic stability of its morphology. The film does not evaporate layer by layer, but holes emerge in the film, which have a characteristic shape. They form parallelograms or rectangles, and are longer in the [001] direction. The surface free energy gamma 001 of the (vertical) side of such a hole is 2 to 5 times higher than gamma bar1 10. When the oxide film is evaporated, the Ru atoms from the oxide film remain on the substrate and form a complicated morphology of hexagonal or circular adatom islands.
The microscopic observations of the chemical processes on the film confirm the current model based on previous macroscopic studies. In addition, it was found that CO molecules adsorb in a stable form on the Rulf rows at room temperature once the Ruzf rows are filled with CO. The maximum coverage theta mathrm CO1f is 0.5 and the molecules form locally ordered (2times1), c(2times2) and (1times1) superstructures. However, COlf desorbs slowly at theta mathrm CO1f sim 0.5. Under the assumption that k mathrm o = 10 16 s-1, E* mathrm des can be estimated as 1.00 eV. E* mathrm diff for O or CO on the surface can be estimated to range between 0.89 eV and 0.93 eV and E* mathrm reac for the reaction between COlf and Olf is sim 0.87 eV, all under the assumption that Gamma mathrm o or k mathrm o = 10 13 s-1. The reactions between Ozf and COlf, Olf and COzf, and Olf and COlf are mostly statistical. However, a preferential reaction perpendicular to the Rulf and Ruzf rows is occasionally observed. Under steady-state reaction conditions, CO can adsorb on the surface, provided that the partial pressure of CO is sufficiently high. The COlf superstructure is the same under steady-state conditions as that in a pure CO atmosphere or during CO titration. When the surface is exposed to very large doses of Oz and CO (sim 100 L), white dots similar to COlf are observed on the surface. However, they do not react with either Oz or CO. This observation suggests that the chemical properties of the oxide surface in this state are different from those of the original clean RuO2 surface.
