Single-Crystal Palladium-Silver Alloy Model Systems for Heterogeneous Catalysis

Abstract

There is considerable interest among those in academia and in the industry in the use of metal alloys to combine the effect of the high reactivity of one component metal with the selectivity derived from a second, less reactive metal to optimize the performance of heterogeneous catalysts. This thesis investigates interactions between palladium-silver model catalyst surfaces and adsorbates that can affect the function and performance of these heterogeneous catalysts using surface science studies. First, the utilization of X-ray photoelectron spectroscopy, low-energy ion scattering spectroscopy, and temperature-programmed desorption and reaction spectroscopy (Chapter 1) is described. An ambient-pressure gas cell with laser-assisted sample heating was custom built and retrofitted to expand the capabilities of the existing vacuum chamber for ambient-pressure sample treatments (Chapter 2). Second, interactions of small molecules on evaporated thin palladium films on silver, palladium-on-silver surfaces, are explored for a range of systems and reactions, where the overarching principle is for a minority component of active palladium to enable the adsorption and formation of intermediates on a surface and a majority component of selective silver to facilitate reaction pathways with high selectivity. The structure and composition of palladium-silver surfaces is highly dependent on sample preparation and adsorbates interacting with the surface. For example, surface restructuring as a function of reaction conditions favors palladium termination when carbon monoxide is absorbed, or palladium is oxidized. In the absence of adsorbates, silver termination is favored. Significant migration of Ag out of the surface is observed on palladium-on-silver overlayer structures leading to encapsulation of palladium islands by silver (Chapter 3). Oxophilicity drives the oxidation of palladium in the vicinity of silver oxide leading to three-dimensional palladium particles on silver (Chapter 4). The oxygen predominantly is transferred to the edges of the three-dimensional palladium nanoclusters, giving rise to high activity for CO oxidation on palladium-silver surfaces (Chapter 5). Different methods of preparing oxidized palladium-on-silver surfaces lead to variations in dispersion and structure of bimetallic palladium-silver oxides. These differences yield dramatically different reactivity of the oxide surface with both CO (Chapter 6) and dihydrogen (Chapter 7). The reaction of dihydrogen with an oxidized palladium-silver surface necessitates the formation of atomic hydrogen on palladium atoms first. Once generated, hydrogen atoms diffuse through palladium-silver adsorption sites to promote silver oxide reduction. This process involves material transport and leads to a locally intermixed palladium-silver surface after full reduction (Chapter 8). Thermodynamically, continuous intermixing would lead to an eventual and permanent bulk dissolution of active palladium. However, palladium can be kinetically stabilized at the surface during repeated redox cycles (Chapter 9). Third, using bulk alloys instead of evaporated structures presents a complex but increasingly realistic model system for heterogeneous catalysis at the industrial scale. Other advantages include its increased thermodynamic stability and facile sample preparation. Well-defined palladium sites, from single palladium atoms to small palladium ensembles and larger ensembles on the Pd33Ag67 surface were created (Chapter 10, Chapter 11). The dissociation of dideuterium on single Pd atoms is inefficient, at least an order of magnitude less efficient than on small palladium ensembles, establishing the different chemical functionality of single palladium atoms and small palladium ensembles on the surface of this bulk alloy (Chapter 12). Ensembles are required for the activation of the O-H bond of methanol on the Pd33Ag67 surface. Single, isolated Pd atoms have little or minimal ability to activate methanol and form methyl formate, indicating the potential for anhydrous methanol activation under mild conditions for PdAg catalysts (Chapter 13).