INSIGHTS | PERSPECTIVES
1564 26 SEPTEMBER 2014 • VOL 345 ISSUE 6204
sciencemag.org SCIENCE
The discovery of highly active gold catalysts for CO oxidation (CO + ½O2 → CO2) about 25 years ago (1) ignited substantial interest in the use of gold as a catalyst. Yet, this seemingly sim- ple reaction has proven to be quite
complicated. No consensus exists regarding
the mechanism by which gold catalyzes CO
oxidation. Confounding the understanding
of this process was the discovery that incorporation of minute quantities of water
in the reactant feed stream can increase
catalytic activity by up to several orders of
magnitude (2). Many conflicting reports
have been proposed for the role of
water in the CO oxidation reaction.
On page 1599 of this issue, Saavedra
et al. (3) present a compelling mech-
anism that ties together the conclu-
sions of many of these reports.
There are two key questions regarding water-enhanced CO oxidation on gold catalysts. Does water
enhance the reaction by promoting the decomposition of surface
intermediates or by assisting in the
activation of reactants? And is the
active site of the catalyst associated
with the gold particle surface or the
gold-support interface?
A few reports have suggested that
incorporation of water into the feed
stream for CO oxidation promotes
the decomposition of surface intermediates (4, 5). Such an effect
could reduce catalyst deactivation,
leading to enhanced activity. For example,
hydroxyl groups located on the supporting
material near the gold-support interface
may abstract hydrogen from a reactive intermediate on the gold surface, resulting in
the formation of water and a more stable
intermediate that blocks the active site (4).
Inclusion of water in the feed stream would
reverse this process by driving equilibrium toward regeneration of the hydroxyl
groups on the support and the less stable
intermediate.
Alternatively, water may assist in the ac-
tivation of reactant species on the catalyst
surface. A number of potential active sites
and mechanisms exist for this type of pro-
cess. Studies have proposed that hydroxyl
species produced by interactions between
water and the gold surface or the gold-sup-
port interface can oxidize CO, enhancing
catalytic activity relative to the hydroxyl-
free surface (6–9). Different authors have
pointed to either cationic gold (6, 7) or me-
tallic gold (8, 9) taking part in this activa-
tion process. Others have proposed that the
generation of a hydroperoxyl-like surface in-
termediate via the interaction of water with
O2 on the surface of the gold particle may
facilitate CO oxidation (10, 11).
The diversity of proposed active sites and
mechanisms has done little to resolve the
debate surrounding water-enhanced CO oxidation. Rather than becoming clearer over
time, many aspects of this process have become more perplexing.
Saavedra et al. explored the water-enhanced CO oxidation reaction on a titania-supported gold catalyst. This system has
been very well studied, but the authors were
nevertheless able to make novel observations
with a relatively simple technique. They controlled the amount of water on the catalyst
surface by gently drying the material while
monitoring adsorbed water and hydroxyl
species with infrared (IR) spectroscopy. Removal of weakly adsorbed water from the
surface resulted in a substantial decrease in
activity for CO oxidation. Furthermore, the
authors observed a large kinetic isotope ef-
Water’s place in Au catalysis
Where water fits in. Many ideas have been proposed for the role
of water in gold-catalyzed CO oxidation. The results reported by
Saavedra et al. indicate that water adsorbed at the gold-support
interface plays a key role in this process.
By Gregory M. Mullen1 and
C. Buddie Mullins1,2
Water plays a key role in gold-catalyzed CO oxidation
CHEMISTRY
1McKetta Department of Chemical Engineering, University
of Texas at Austin, Austin, TX 78712, USA. 2Department of
Chemistry, University of Texas at Austin, Austin, TX 78712,
USA. E-mail: mullins@che.utexas.edu I L
P.
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services to move away from short-term or
emergency responses toward a more coordinated and proactive program of disease
prevention and control. Work is needed to
determine how best to scale up from such
pilot studies to the national and regional
programs that will be needed for eventual
elimination. International human and animal health organizations can play an important support role, for example, establishing
mechanisms to ensure the affordable supply
of human and animal vaccines and their effective cross-sectoral use.
An enduring challenge for global elimination is the ability to work effectively across
national boundaries. This has been achieved
successfully in the Americas through the RE-DIPRA network (Directors of National Rabies Control Programs), with specific budgets
for cross-border control and the transparent
sharing of surveillance and budgetary information resulting in a collective commitment
to a public good, as well as constructive peer
pressure. Newer regional rabies networks
in Asia and Africa could develop along the
same lines. Successful rabies control programs in KwaZulu-Natal, South Africa, supported through pilot funding, have resulted
in transboundary networks and initiatives in
neighboring countries. Initial success provides momentum for further success.
Canine rabies elimination meets all the
criteria for a global health priority: It is epidemiologically and logistically feasible, cost-effective, and socially equitable. Pasteur’s
vision is within our reach—we only need to
move the hand forward to grasp it. ■
REFERENCES AND NOTES
1. G.Geison, The Private Science of Louis Pasteur(Princeton
Univ. Press, Princeton, NJ, 1995).
2. S. Shwiff et al ., Antiviral Res.98, 352 (2013).
3. D. L. Knobel et al., Bull. World Health Organ. 83, 360 (2005).
4. K. Hampson et al ., PLOS Negl. Dis. 2, e339 (2008).
5. T.Lembo et al., Emerg. Infect. Dis. 12,310(2006).
6. W. Suraweera et al. ; Million Death Study Collaborators,
PLOS Negl. Trop. Dis. 6, e1847 (2012).
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10. K. Hampson et al ., PLOS Biol. 7, e53 (2009).
11. A. A. G. Putra et al ., Emerg. Infect. Dis.19, 648 (2013).
12. M. K. Morters et al ., J. Anim. Ecol. 82, 6 (2013).
13. T. Lembo et al., Dogs, Zoonoses, and Public Health, C. N.
L. Macpherson, F.-X. Meslin, A. I. Wandeler, Eds. (CAB
International, Wallingford, UK, ed. 2, 2013), pp. 205–258.
14. S. L. Davlin, H. M. Vonville,Vaccine 30, 3492 (2012).
15. T. Lembo et al ., PLOS Negl. Trop. Dis. 4, e626 (2010).
16. T.Lembo et al., J. Appl. Ecol.45,1246(2008).
17. M.A.Vigilato et al., Philos. Trans. R. Soc. Lond. B Biol. Sci.
368, 20120143 (2013).
18. S.E. Townsend etal., PLOSNegl. Trop.Dis. 7,e2372(2013).
ACKNOWLEDGMENTS
K.H. is supported by the Wellcome Trust (095787/Z/11/Z), and
L.T. by the UBS Optimus Foundation. Opinions, findings, conclusions, and recommendations are those of the authors and do
not necessarily reflect the views of the supporting institutions.
