ChBE in the News
New Catalysts May Provide Path to Low-Cost Production of Future
Gold stabilized by alkali ions on inert supports are promising
new catalysts for low-cost hydrogen production
New catalysts designed by Tufts University School of Engineering
researchers and collaborators from other university and national
laboratories have the potential to greatly reduce processing costs in
future fuels, such as hydrogen. The catalysts, composed of single gold
atoms bound by oxygen to sodium or potassium atoms and supported by a wholly unique structure comprised of non-reactive
silica materials, demonstrate comparable activity and stability with
current catalysts used in producing highly purified hydrogen.
The work, which appears in the November 27, 2014 edition of Science
Express, points to new avenues for producing single-site supported
gold catalysts that could produce high-grade hydrogen for cleaner energy
use in fuel-cell powered devices, including vehicles.
"In the face of precious metals scarcity and exorbitant fuel-processing
costs, these systems are promising in the search for sustainable global
energy solutions," says senior author
Maria Flytzani-Stephanopoulos, the Robert and Marcy
Haber Endowed Professor in Energy Sustainability.
Professor Flytzani-Stephanopoulos' election to the NAE recognizes her as
a leader in the field of clean energy technologies and underscores her
importance to our community as one of our most valued faculty members in
engineering for sustainability research, one of our school's three
strategic cross-disciplinary focus areas.
Flytzani-Stephanopoulos's research group (NanoCEL)
has been active in designing catalysts requiring a lower amount of precious metals to
generate high-grade hydrogen for use in fuel cells. The water-gas shift
reaction, in which carbon monoxide is removed from the fuel gas stream by
reacting with water to produce carbon dioxide and hydrogen, is a key
step in the process. Catalysts, such as metal oxides prepared with
precious metals like platinum and gold, are used to lower the reaction
temperature and increase the production of hydrogen.
The Tufts group was the first to demonstrate that atomically dispersed
gold or platinum on supports, such as cerium oxide, are the active sites
for the water-gas shift reaction. (doi:10.1126/science.1192449). Metal
nanoparticles are "spectator species" for this reaction.
says the new research suggests single precious metal atoms stabilized
with alkali ions may be the only catalyst sites for other catalytic reactions.
"If the other particles are truly 'spectator species', they are
therefore unnecessary. Future catalyst production should then focus on
avoiding particle formation altogether and rather be prepared solely
with single-site atomic dispersion on various supports," says Flytzani-Stephanopoulos.
The just published research describes how single gold atoms dispersed on
non-reactive supports based on silica materials can be stabilized with
alkali ions. As long as the gold atoms, or cations, are stabilized in a
single-site form configuration, irrespective of the type of support, the
precious metal will be stable and operate for many hours at a range of
"This novel atomic-scale catalyst configuration achieves the maximum
efficiency and utilization of the gold," says Flytzani-Stephanopoulos.
"These single-site gold cations were active for the low-temperature
water-gas shift reaction and stable in operation at temperatures as high
Paper co-authors Professor
Manos Mavrikakis at the University of
Wisconsin-Madison and Assistant Professor
Ye Xu at Louisiana State University used quantum mechanics
theoretical calculations to elucidate the likely atomic-scale structure
and bonding with the active site, as well as to evaluate the
thermochemical properties of the Au-Ox(OH)y-Naz
ensembles, which retain the gold atom in cationic state. Weak adsorption
of CO and facile H2O dissociation are key features predicting
good reactivity in the water-gas shift reaction. Researchers
Larry Allard at Oak Ridge National Laboratory and
Sungsik Lee at Argonne National Laboratory used
atomic resolution electron microscopy and x-ray absorption
spectroscopies, respectively, to demonstrate the existence and stability of the
single-site gold species. Co-author
Jun Huang, a lecturer at the University of Sydney,
synthesized and characterized the mesoporous silica materials used as
supports. Several graduate students were involved in all aspects of the
research both at Tufts and Wisconsin.
According to first-author
Yang, a doctoral student working in
Flytzani-Stephanopoulos's group, the preparation of these single-site
gold catalysts does not involve ion exchange in the zeolites. "It cannot
be achieved without due consideration of the support surfaces," says
Yang. For example, solid-state impregnation was used to add the alkaline
component NaOH on these silica-rich supports.
"Armed with this new understanding, practitioners will be able to design
catalysts using just the necessary amount of the precious metals like
gold and platinum, dramatically cutting down the catalyst cost in fuels
and chemicals production processes," Flytzani-Stephanopoulos says.
The paper appears in the November 27 edition of Science Express.
(doi:10.1126/science.1260526). This research is primarily supported by the
U.S. Department of Energy under grant # DE-FG02-05ER15730.