Single atom alloy catalysts for C-H bond activation and propane dehydrogenation
Abstract: Single atom
alloys (SAAs) have shown considerable promise as heterogeneous catalysts for a wide
range of industrially important reactions, by providing atomically dispersed active
species to enable near 100% atom efficiency in catalytic systems. These materials are
explored in this thesis for C-H bond activation reactions, which have become
increasingly relevant in recent years as ... read morecrude oil production in the US has become
lighter due to increases in shale oil production. Single atom platinum, palladium, and
nickel in the surfaces of supported copper nanoparticles were prepared by galvanic
replacement for comparison to their monometallic nanoparticle counterparts in C-H bond
activation reactions in butane and propane. As part of this work, deactivation kinetics
were compared, for the first time, between single atom Pt, Pd, and Ni in Cu
nanoparticles and their monometallic nanoparticle counterparts. The PtCu- and PdCu-SAAs
were shown to have superior coke resistance when compared to Pt and Pd nanoparticles
under realistic propane dehydrogenation conditions in a fixed bed reactor. For our
reactions, we demonstrated that coke production rates are reduced by more than 90% in
PdCu- and PtCu-SAAs. Deactivation mechanisms considered for PtCu- and PdCu-SAAs were
most consistent with particle sintering and subsurface metal atom diffusion, while their
monometallic equivalents were more consistent with deactivation by coking. Similar
results were found for the C-H bond activation in butane, tested by H-D exchange over a
range of temperatures. Additionally, reaction rates were measured for each system, and
the apparent activation energies, Eapp, were calculated. These measurements were
performed approximately at steady-state, but on partially deactivated catalysts. The
extent of deactivation due to carbon deposition was different among the three metals.
The Eapp were similar between single atom alloys and monometallic catalysts in contrast
to DFT calculations performed elsewhere, showing that SAA catalysts should have higher
Eapp . Eapp were as follows: for Pt and PtCu = 77±3 and 68±9 kJ/mol; for Pd
and PdCu = 30±2 and 35±5 kJ/mol; and for NiCu-SAAs 17±5 kJ/mol. The
reason for the different apparent activation energies between Pt and Pd is attributed to
pore closure by carbon deposition in the latter. Excessive coking and very low reaction
rates made calculation of activation energies over monometallic Ni nanoparticles
impractical for our system. This is the first time that ambient pressure catalytic
measurements in propane dehydrogenation were made for this series of bimetallic single
atom catalysts, allowing for comparison between expected DFT trends and actual
experimental data. This type of work will allow rational catalyst design to further
refine the understanding of materials needed to allow access to low cost fine chemicals
produced from olefins.
Thesis (M.S.)--Tufts University, 2017.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Charles Sykes, and Prashant Deshlahra.
Keyword: Chemical engineering.read less
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