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We are grateful for financial support from the Ministry of Science
and Technology, Taiwan. K.C.H. conceived the idea, designed
the experiments, and wrote the manuscript. A.S. conducted the
experiments. Metrical parameters for the x-ray structures of
2b and 2g are available free of charge from the Cambridge
Crystallographic Data Centre under accession numbers CCDC
1025098 and CCDC 1025099, respectively. A patent application
Materials and Methods
Figs. S1 to S6
Tables S1 to S15
1H and 13C NMR Data
1H and 13C NMR Spectra
6 August 2014; accepted 11 November 2014
Catalytically active Au-O(OH)x-
species stabilized by alkali ions on
zeolites and mesoporous oxides
Ming Yang,1 Sha Li,2 Yuan Wang,1 Jeffrey A. Herron,2 Ye Xu,3 Lawrence F. Allard,4
Sungsik Lee,5 Jun Huang,6 Manos Mavrikakis,2 Maria Flytzani-Stephanopoulos1*
We report that the addition of alkali ions (sodium or potassium) to gold on KLTL-zeolite and
mesoporous MCM-41 silica stabilizes mononuclear gold in Au-O(OH)x-(Na or K) ensembles.
This single-site gold species is active for the low-temperature (<200°C) water-gas shift (WGS)
reaction. Unexpectedly, gold is thus similar to platinum in creating –O linkages with more
than eight alkali ions and establishing an active site on various supports. The intrinsic activity of
the single-site gold species is the same on irreducible supports as on reducible ceria, iron
oxide, and titania supports, apparently all sharing a common, similarly structured gold active
site. This finding paves the way for using earth-abundant supports to disperse and stabilize
precious metal atoms with alkali additives for the WGS and potentially other fuel-processing
The water-gas shift (WGS) reaction (CO + H2O → CO2 + H2) is an important reaction for hydrogen upgrading during fuel gas processing. Emerging applications in fuel cells require active, nonpyrophoric, and cost-
effective catalysts. Along with a new group of
platinum catalysts with atomically dispersed Pt
sites to maximize activity and catalytic efficiency
(1–3), the lower apparent activation energy Ea for
the WGS reaction (~45 kJ/mol) for gold (Au)
versus ~75 kJ/mol for platinum (3–5) can be
exploited for low-temperature WGS and other
reactions (6, 7). Low-temperature activity is im-
portant to avoid multiple-treatment units in prac-
tical low-temperature proton-exchange membrane
(PEM) fuel cell systems, whereby the deleterious
CO should be totally removed for stable, long-
term operation. The active Au species in the WGS
catalysts are atomic species anchored through –O
ligands to different supports such as ceria (3, 8, 9),
iron oxide (10–12), lanthana (13), and titania (4),
and the number of the active Au sites can be
increased through a variety of catalyst prepa-
ration protocols. Gold nanoparticles (Au NPs)
that can form during catalyst preparation are
spectator species in these chemistries (3, 4, 10),
in that most of the Au atoms are not activated
by the support. Thus, the approach of “cage en-
capsulation” of Au NPs in mesoporous supports
is not advantageous for the stability of the active
(atomically dispersed) Au sites.
Other approaches—for example, AuCl3 vapor
produced by sublimation and introduced into
various zeolites (14, 15)—may be used to produce
active Au(I)-Cl species for ambient-temperature
NO reduction to N2O by CO. Mohamed and
Ichikawa (16) have shown that the Au(I) species
are the main active sites for the WGS reaction
at temperatures as low as 50°C. Because these
sites are not chloride-free (Au-Cl bonds exist) and
have weak chemical binding to the zeolites, the
Au(I) sites are easily reduced to inactive Au(0)
and form Au NPs upon increasing the temperature to only 100°C (16). Similarly, low stability
of gold on zeolites was found by Gates and co-workers (17, 18). Careful anchoring of mononuclear Au(III) complexes from organometallic
precursors produced chloride-free single-atom
Au(III)-O-NaY catalytic centers that were active for CO oxidation but unstable at 25°C and
760 torr, losing ~75% of their initial activity after
15 min on stream (17). Finally, attempts to ion
exchange gold in zeolites have been unsuccessful.
Thus, gold ions in zeolites tend to be unstable
toward aggregation in realistic reaction gas environments at temperatures above the ambient, an issue already understood for other inert
supports such as silica or alumina, minimally interacting with gold (19). Hence, it is difficult to
determine if the gold catalysts operate through
similarly structured Au-O(OH)x- species on inert
supports as in the Au-CeOx, Au-FeOx, and Au-TiOx
To study the nature of the active gold sites on
inert supports, it is important to maximize the
number of the atomically dispersed gold sites
and fully eliminate the formation of Au NPs.
Titania is inferior to ceria and iron oxide in that
Au NP growth occurs rapidly on its surfaces (21),
but with special ultraviolet (UV)–assisted preparation methods, mononuclear Au-O(OH)x- species
1Department of Chemical and Biological Engineering, Tufts
University, MA 02155, USA. 2Department of Chemical and
Biological Engineering, University of Wisconsin–Madison, WI
53706, USA. 3Department of Chemical Engineering,
Louisiana State University, Baton Rouge, LA 70803, USA.
4Materials Science and Technology Division, Oak Ridge
National Laboratory, Oak Ridge, TN 37831, USA. 5X-ray
Science Division, Argonne National Laboratory, 9700 South
Cass Avenue, Argonne, IL 60439, USA. 6School of Chemical
and Biomolecular Engineering, University of Sydney, NSW
*Corresponding author. E-mail: maria.flytzani-stephanopoulos@