Bridging the Gap: From Model Surfaces to Nanoparticle Analogs for the Selective Oxidation and Steam Reforming of Methanol and for Selective Hydrogenation Catalysis.
Boucher, Matthew.
2013
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Abstract: Most
industrial catalysts are very complex, comprising of non-uniform materials with varying
structures, impurities, and interaction between the active metal and supporting
substrate. A large portion of the ongoing research in heterogeneous catalysis focuses on
understanding structure-function relationships in catalytic materials. In parallel,
there is a large area of surface science ... read moreresearch focused on studying model catalytic
systems for which structural parameters can be tuned and measured with high precision.
It is commonly argued, however, that these systems are oversimplified, and that
observations made in model systems do not translate to robust catalysts operating in
practical environments; this discontinuity is often referred to as a "gap." The focus of
this thesis is to explore the mutual benefits of surface science and catalysis, or
"bridge the gap," by studying two catalytic systems in both ultra-high vacuum (UHV) and
near ambient-environments. The first reaction is the catalytic steam reforming of
methanol (SRM) to hydrogen and carbon dioxide. The SRM reaction is a promising route for
on-demand hydrogen production. For this catalytic system, the central hypothesis in this
thesis is that a balance between redox capability and weak binding of reaction
intermediates is necessary for high SRM activity and selectivity to carbon dioxide. As
such, a new catalyst for the SRM reaction is developed which incorporates very small
amounts of gold (<1 atomic %) supported on zinc oxide nanoparticles with controlled
crystal structures. The performance of these catalysts was studied in a fixed-bed
micro-reactor system at ambient pressures, and their structure was characterized by
high-resolution microscopic and spectroscopic techniques. Pre-existing oxygen defects in
zinc oxide {0001} surfaces, and those created by a perturbation of the defect
equilibrium by addition of gold, provide an anchoring site for highly dispersed gold
species. By utilizing shape control of zinc oxide supports, it is found that highly
dispersed gold, capable of low-temperature redox behavior is most prominent on zinc
oxide {0001} surfaces and leads to high SRM activity and selectivity to carbon dioxide.
Like other Group IB metal catalysts the SRM over gold-zinc oxide proceeds through the
formation and weak binding of formaldehyde, and subsequent coupling with methoxy to
produce methyl formate. Mechanistic clarification of this point was achieved by studying
the interaction methanol-water mixtures with model catalyst surfaces. Model catalysts
were studied in a UHV chamber where the base pressure was maintained at 10-10 mbar. High
resolutions surface science techniques show that hydrogen-bonded networks of water are
capable of deprotonating methanol to methoxy on low index surfaces in the absence of
atomic oxygen. These UHV studies show that adsorbates, other than oxygen, are capable of
activating methanol on Group IB metal surfaces. The second reaction involves the
selective hydrogenation of alkynes to alkenes. Selective hydrogenations of carbon-carbon
multiple bonds are important for a wide range of industrial processes. The governing
hypothesis for this reaction system is that cooperation between a minority metal with a
low barrier for hydrogen dissociation, and a less-reactive host metal capable of
hydrogen uptake via spillover will lead to high alkene selectivity. A strategy for the
preparation of such a catalyst is developed using model catalyst studied in a UHV
chamber. The model catalyst features isolated palladium atoms in a copper(111) surface,
termed single atom alloy (SAA). Individual, isolated palladium atoms act as sites for
hydrogen uptake, dissociation, and spillover onto an otherwise inert copper(111) host.
Weak binding offered by copper provides a surface where selective hydrogenation
reactions can take place. Palladium-copper SAA model catalysts are highly selective to
the partial hydrogenation of acetylene, whereas surfaces containing larger palladium
ensembles facilitate complete hydrogenation and decomposition. Nanoparticle analogs of
palladium-copper SAAs were prepared to investigate the feasibility of this strategy for
practical application. Very small amounts of palladium (<0.2 atomic %) on the surface
of copper nanoparticles are highly active and selective catalysts for the partial
hydrogenation of phenylacetylene to styrene. The performance of these catalysts was
studied in a liquid-phase, stirred-tank batch reactor under a hydrogen head pressure of
approximately 7 bar. Palladium alloyed into the surface of otherwise inactive copper
nanoparticles shows a marked improvement in selectivity when compared to monometallic
palladium catalysts with the same metal loading. This effect is attributed hydrogen
spillover onto the copper surface. In summary, the development of new, highly active and
selective catalysts for the methanol steam reforming reaction and for the partial
hydrogenation of alkynes to alkenes was accomplished by the use of state-of-the-art
techniques in both surface science and heterogeneous catalysis. The implications of this
work can be extended to a wide variety of catalytic
systems.
Thesis (Ph.D.)--Tufts University, 2013.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Charles Sykes, Howard Saltsburg, Terry Haas, and Gary Haller.
Keywords: Chemical engineering, Chemistry, and Materials Science.read less - ID:
- th83m874n
- Component ID:
- tufts:21878
- To Cite:
- TARC Citation Guide EndNote
