Catalytic Conversion of Short-Chain Alcohols on Atomically Dispersed Au and Pd Supported on Nanoscale Metal Oxides
Wang, Chongyang.
2016
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Abstract: With the
development of technologies for cellulosic biomass conversion to fuels and chemicals,
bio-alcohols are among the main alternative feedstocks to fossil fuels. The research
pursued in my thesis was the investigation of gold and palladium as catalysts for the
application of short aliphatic alcohols to hydrogen generation and value-added chemicals
production. Specifically, ... read moreselective methanol steam reforming and non-oxidative ethanol
dehydrogenation to hydrogen and acetaldehyde were investigated in this thesis work. A
major aim of the thesis was to develop atomically efficient catalysts with tuned surface
chemistry for the desired reactions, using suitable synthesis methods. Methanol steam
reforming (SRM) for hydrogen production has recently been investigated on gold catalysts
to overcome the drawbacks of copper catalysts (deactivation, pyrophoricity). Previous
work at Tufts University has shown that both CeO2 and ZnO are suitable supports for
gold. In this thesis, nanoscale composite oxides ZnZrOx were prepared by a carbon
hard-template method, which resulted in homogeneous distribution of Zn species in the
matrix of ZrO2. Tunable surface chemistry of ZnZrOx was demonstrated by varying the
Zn/Zr ratio to suppress the strong Lewis acidity of ZrO2, which leads to undesired
production of CO through methanol decomposition. With atomic dispersion of gold,
Au/ZnZrOx catalyzes the SRM reaction exclusively via the methanol self-coupling pathway
up to 375˚C. The activity of Au/ZnZrOx catalysts was compared to Au/TiO2, which is
another catalyst system demonstrating atomic dispersion of gold. Similarity in the
apparent activation energy of SRM on all the supported gold catalysts studied in this
thesis and in the literature further confirms the same single-site Au-Ox-MO centers as
active sites for SRM with indirect effects of the supports exploited. With this
fundamental understanding of gold-catalyzed C1 alcohol reforming, the Au/ZnZrOx catalyst
was evaluated for the dehydrogenation of ethanol. Bare ZnZrOx activate ethanol
conversion in the range of 280-300˚C and produce undesired ethylene as product of
ethanol dehydration, whereas, addition of small amount of gold (<1wt.%) was found to
significantly change the product distribution in the low-temperature range
(200˚C-350˚C). As gold passivates the strong Brønsted acid sites of
ZrO2 and selectively facilitates the dehydrogenation of ethanol at low-temperature, a
wide temperature range was found between the production of acetaldehyde (dehydrogenation
products) and ethylene (dehydration product), which can be harnessed for the industrial
application. Interestingly, the steam reforming of ethanol did not take place in the
low-temperature region, thus the selectivity to acetaldehyde and hydrogen was 100% even
in the presence of water. In addition to gold, palladium was also studied in this thesis
work on the ZnZrOx composite oxides, and its activity and selectivity were compared to
Au/ZnZrOx. Monometallic Pd catalyzes the decomposition of methanol and ethanol,
resulting in different product distribution for C1-C2 alcohol reactions. With ZnZrOx
employed as the catalyst support in this thesis work, the finely dispersed ZnO species
in ZrO2 were found to alloy with the supported palladium under reduction treatment.
Alloying with Zn tunes the chemistry of Pd to catalyze the SRM reaction through the
methanol coupling mechanism, shutting off the undesired methanol decomposition pathway.
A preliminary study of the Pd/ZnZrOx system for ethanol dehydrogenation also
demonstrated the modification of Pd when in the PdZn alloy form. Different from the
monometallic Pd catalyst, which primarily catalyzes the C-C bond scission of ethanol,
high selectivity to ethanol dehydrogenation products was found on PdZn, over the
temperature range of 200-400˚C. Formation of the PdZn alloy broadens the
application of Pd and potentially other Group VIII metals for selective alcohol
conversion reactions. In summary, this thesis work has investigated two noble metals Au
and Pd from Group IB and Group VIII, respectively, for methanol and ethanol alcohol
reforming reactions employing a novel ZnZrOx composite oxide as a platform catalyst
support. Comprehensive study of Au catalyst has deepened our understanding of atomically
dispersed Au anchored on various supports through oxygen bonds as the active sites for
alcohol reforming reactions, and showed the support effect to be indirect, serving as
the carrier and stabilizer of the gold species. For Pd, the Zn species of the composite
oxide is necessary to modify the Pd catalyst and the PdZn alloy gives it the desired
Au-like properties. Full characterization of the catalysts used here by ICP, XPS, XRD,
FTIR and STEM imaging was conducted throughout the thesis to identify the stable species
and correlate the catalyst performance with its composition and morphology. Surface
acidity titration by isopropanol temperature-programmed desorption/mass spectrometry
(IPA-TPD/MS) and pyridine-IR adsorption/desorption was conducted in parallel to
temperature-programmed surface reaction (TPSR) studies and products from isothermal
steady-state reactions were monitored online by mass
spectrometry.
Thesis (Ph.D.)--Tufts University, 2016.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Terry Haas, Yuriy Roman, and Matthew Panzer.
Keywords: Chemical engineering, Nanoscience, and Chemistry.read less - ID:
- g732dn50r
- Component ID:
- tufts:21332
- To Cite:
- TARC Citation Guide EndNote
