Surface Reactivity of Single Atom Alloys: Model Studies Guiding the Design of Atom Efficient Nanoparticle Catalyst

Catalysts are used in the majority of industrial chemical processes including the productions of $450 billion worth of chemicals. Heterogeneous catalysts often use precious metals such as Pt or Pd which have a high susceptibility to deactivation and unselective reactivity. By reducing the catalytic element to the minimum atomic ensemble, the catalytic reactivity, selectivity and stability can be optimized while using a reduced concentration of precious metal. This thesis focuses on developing atom efficient precious metal catalysts that exhibit enhanced reactivity and selectivity compared to monometallic surfaces for industrially important processes. In order to understand the capabilities of isolated atoms in inert Cu and Au hosts, the fundamental surface interactions of adsorbates with isolated reactive atoms were probed with a unique combination of atomic scale microscopy and desorption studies to elucidate the energetics of the catalyzed processes.;The majority of this work focused on Pt-Cu single atoms alloys (SAAs) where isolated Pt atoms, stable in a Cu surface, were identified as superior active sites for selective hydrogenation reactions. The bi-functional nature of Pt-Cu SAAs enables facile hydrogen activation without C-C bond scission and selective partial hydrogenations of butadiene on Cu terraces. In general, the weak adsorption of reactants and products to Pt-Cu SAAs yields improved selectivity and reduced susceptibility to CO poisoning and coke formation. The mechanistic details determined though surface studies on model catalysts guided the design of a new generation of Pt based catalysts for hydrogenation and methanol decomposition reactions under realistic reaction conditions.;The atom efficiency of Au based model catalysts were also studied to determine the atomic Pd ensemble in Au needed for diatomic molecule activation of H2 and O2. By systematically probing H2 activation, isolated Pd atoms in Au were shown to activate H2, a process previously thought to require to two Pd atoms. Unfortunately, the weak adsorption of hydrocarbons to Au impedes further hydrogenation reactivity in ultra-high vacuum. In order to expand the reactivity of SAAs, the potential of isolated Pd atoms for oxidation reactions was probed by examining the energetics of recombinative desorption of O2 where dilute concentrations of Pd atoms alter the morphology of the oxidized Au surface. By understanding the surface structure of model catalysts and the energetics of probe reactions, this thesis highlights the viability of optimizing catalytic activity at the single atom limit.