Surface Reactivity of Single Atom Alloys: Model Studies Guiding the Design of Atom Efficient Nanoparticle Catalysts
Abstract: 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 ... read morestability 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.
Thesis (Ph.D.)--Tufts University, 2016.
Submitted to the Dept. of Chemistry.
Advisor: E. Charles Sykes.
Committee: Cynthia Friend, Jonathan Kenny, and Mary Jane Shultz.
Keyword: Chemistry.read less
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