Synthesis and Evaluation of Nanostructured Gold-Iron Oxide Catalysts for the Oxidative Dehydrogenation of Cyclohexane.
Wu, Peng.
2013
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Abstract:
Shape-controlled iron oxide and gold-iron oxide catalysts with a cubic inverse spinel
structure were studied in this thesis for the oxidative dehydrogenation of cyclohexane.
The structure of iron oxide and gold-iron oxide catalysts has no major impact on their
oxidative dehydrogenation activity. However, the product selectivity is influenced. Both
cyclohexene and benzene are formed ... read moreon bare iron oxide nanoshapes, while benzene is the
only dehydrogenation product in the presence of gold. The selectivity of benzene over
CO2 depends strongly on the stability of the iron oxide support and the gold-support
interaction. The highest benzene yield has been observed on gold-iron oxide octahedra.
{111}-bound nanooctahedra are highly stable in reaction conditions at 300 °C, while
{100}-bound nanocubes start to sinter above 250 °C. The highest benzene yield has
been observed on gold-iron oxide nanooctahedra, which are likely to have gold atoms, and
few-atom gold clusters strongly-bound on their surface. Cationic gold appears to be the
active site for benzene formation. An all-organic method to prepare Au-FeOx
nano-catalysts is needed due to the inconvenience of the half-organic, half-inorganic
synthesis process discussed above. Several methods from the literature to prepare
gold-iron oxide nanocomposites completely in organic solvents were reviewed and
followed. FeOx@Au synthesis procedures in literatures are initially designed for a Au
content of over 70%. This approach was tried here to prepare composites with a much
lower Au content (2-5 atom. %). Heat treatment is required to bond Au and FeOx NPs in
the organic-phase syntheses. Au-FeOx-4 was obtained as a selective catalyst for the ODH
of cyclohexane. A Auδ+ peak is observed in the UV-Vis spectrum of sample
Au-FeOx-4. This different Auδ+ form may be cationic Au nano-clusters interacting
with the FeOx support. It has been demonstrated that cationic gold is responsible for
dehydrogenation behavior. Furthermore, the yield of benzene in preliminary ODH tests of
Au-FeOx-4 also proved the interaction between Au and FeOx. That explains the
dehydrogenation activity of sample Au-FeOx-4. Therefore, the synthesis parameters of
sample Au-FeOx-4 are recommended in further studies. Thermodynamic equilibria governing
the cyclohexane dehydrogenation reaction and the phase transformation of the iron oxides
used as catalysts have been considered as a reference, in a separate chapter. The
all-organic method by thermolysis described in this thesis work can be used in
preparations of Au/FeOx so that the density of Au cations is maximized. Gold deposition
in an organic phase helps to manipulate the surface dispersion of Au, perhaps more
precisely than doping Au in the inorganic phase. Such materials can be good not only for
the ODH reaction, but also for the WGS reaction. This newly developed synthesis method
is worth exploring in future.
Thesis (M.S.)--Tufts University, 2013.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Hyunmin Yi, and Terry Haas.
Keywords: Chemical engineering, Chemistry, and Materials Science.read less - ID:
- mg74qz475
- Component ID:
- tufts:22044
- To Cite:
- TARC Citation Guide EndNote
