Curved single crystals provide variable, but well-defined surface structures. These curved surfaces have enormous potential for applications in surface chemistry, e.g. to unravel the role of... Show moreCurved single crystals provide variable, but well-defined surface structures. These curved surfaces have enormous potential for applications in surface chemistry, e.g. to unravel the role of specific sites in the reaction mechanism. Measurements can be repeated on the same crystal with incremental changes of the surface structure. Because effectsof sample heterogeneity are avoided, curved crystals make it possible to detect even subtle differences in surface reactivities and assign them to specific surface sites. They were used throughout this thesis to study the effects of surface structure onelemental chemical reaction steps, such as hydrogen dissociation and recombination, or water desorption. Show less
Understanding heterogeneous catalysis at the atomic level requires detailed knowledge of the reactivity of different surface sites toward specific bond breaking and bond making events. We... Show moreUnderstanding heterogeneous catalysis at the atomic level requires detailed knowledge of the reactivity of different surface sites toward specific bond breaking and bond making events. We illustrate a new method in such investigations. We use a macroscopically curved Pt single crystal containing a large variation in density of highly kinked steps of two different chiralities. Scanning tunneling microscopy maps the entire range of surface structures present on the 31 degrees section surrounding the Pt(111) apex. Whereas most of the surface shows the expected characteristic arrays of parallel steps, hexagonally-shaped, single-atom deep pits remain after cleaning procedures near the apex. Their orientation is indicative of the different chiralities present on the two sides of the crystal's apex. These unintended defects locally raise the surface defect concentration, but are of little consequence to subsequent reactivity measurements for D-2 dissociation and H-D exchange as probed by supersonic molecular beam techniques. We quantify absolute elementary dissociation and relative isotopic exchange rates across the surface with high spatial resolution. At low incident energies, elementary dissociation of the homonuclear isotoplogues is dominated by the kinked steps. H-D exchange kinetics depend also mostly linearly on step density. The changing ratio of D(2)dissociation to H-D formation, however, suggests that anisotropic diffusion of H(D) atoms is of influence to the measured HD production rate. Show less
We determine absolute reactivities for dissociation at low coordinated Pt sites. Two curved Pt(111) single-crystal surfaces allow us to probe either straight or highly kinked step edges with... Show moreWe determine absolute reactivities for dissociation at low coordinated Pt sites. Two curved Pt(111) single-crystal surfaces allow us to probe either straight or highly kinked step edges with molecules impinging at a low impact energy. A model extracts the average reactivity of inner and outer kink atoms, which is compared to the reactivity of straight A- and B-type steps. Local surface coordination numbers do not adequately capture reactivity trends for H(2)dissociation. We utilize the increase of reactivity with step density to determine the area over which a step causes increased dissociation. This step-type specific reactive area extends beyond the step edge onto the (111) terrace. It defines the reaction cross-section for H(2)dissociation at the step, bypassing assumptions about contributions of individual types of surface atoms. Our results stress the non-local nature of H(2)interaction with a surface and provide insight into reactivity differences for nearly identical step sites. Show less
