Platinum and Gold Species on Non-Reducible Oxides as Active and Stable Catalysts for the Low-Temperature Water-Gas Shift Reaction.
Abstract: Hydrogen is
of interest because it is a clean, safe and efficient energy source. For the
low-temperature fuel cell application, the water-gas shift (WGS) reaction is an
important step in upgrading hydrogen streams produced from fuel reforming operations. It
has been challenging to develop low-temperature WGS catalysts that are both highly
active and stable at the operation condition... read mores of proton exchange membrane (PEM) fuel
cell systems. The current catalysts used in industrial WGS reactors, Cu/ZnO/Al2O3, are
unsuitable for fuel cell systems because they are pyrophoric, lack thermal stability,
and require activation procedures. Noble metal (Au, Pt, Pd etc.) on oxide support (CeO2,
TiO2, FeOx etc.) catalysts have emerged as attractive catalyst alternatives possessing
high activity and stability for the low-temperature WGS reaction. These have to be
designed with trace amounts of valuable metals to be economical. In this thesis,
platinum and gold species on non-reducible silica and alumina supports were investigated
as novel catalysts for the low-temperature WGS reaction. In particular, encapsulated
platinum in the form of core-shell Pt@SiO2 material was prepared by a reverse
microemulsion technique, while supported gold was prepared by deposition on modified
γ-alumina supports with OH-rich surfaces through various hydrothermal treatments.
Addition of alkali ions, associating Pt and Au species with the OH-rich oxide support
surfaces, rendered the corresponding Pt-Na@SiO2 and Na-Au/Al2O3-OH samples active and
stable catalysts for the low-temperature WGS reaction, whereas their alkali-free
counterparts showed low activity and poor stability. Samples were characterized by BET
surface area, UV-Visible Spectroscopy (UV-Vis), X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS), high resolution transmission electron microscopy
(HRTEM) and CO chemisorption. Temperature-programmed reduction (TPR) measurements and
cyclic TPR tests were conducted using the reactant CO as the reductant. Catalytic
activities of the samples were evaluated for the WGS reaction under various gas
conditions in temperature-programmed surface reaction (TPSR) and steady-state modes, and
compared with other Pt- and Au-based catalysts prepared on various oxides supports by
different methods. The Pt-Ox(OH)-M or Au-Ox(OH)-M species, where M denotes an alkali
ion, are concluded to be very promising for further development of low-temperature WGS
reaction catalysts. The apparent activation energy, Eapp, for the reaction is the same
for these new catalysts and for other Pt- or Au-based catalysts previously reported. For
the Pt-based catalysts, Eapp = 70 ± 5 kJ/mol, and for Au-based catalysts, Eapp = 45
± 5 kJ/mol. This is true whether a reducible or a non-reducible support is used,
the latter stabilized only with the help of alkali promoters. Hence, the active site
comprises oxygen and hydroxyl groups, but is independent of the type of the support
Thesis (M.S.)--Tufts University, 2012.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Hyunmin Yi, and Terry Haas.
Keyword: Chemical engineering.read less