K. J. Stowers et al.
Table 1 Temperatures for b-
C–H scission of adsorbed
alkoxy on the surface
Surface
Adsorbed RO
Temperature for b C-H scission (K)
npAu
CH3CH2O
CH3CH2O
CH3CH2O
CH3O
300
Ag(110)
275
Au(111)
\235
\260
Ag0.4/Au0.6(111)
production, while higher (although not excess) oxygen
coverage favors coupling. The results observed here sug-
gest that higher selectivities and conversions should be
obtainable under steady state catalytic conditions than
heretofore observed [3, 6]. We are currently investigating
this matter further.
ethanol coupling on the coarsened material indicates that
the activation of O2 on npAu is not strongly dependent on
the ligament size and further suggests that the binding
energy of ethanol is not strongly dependent on the presence
of under coordinated metal atoms.
The work herein provides insight into the ongoing dis-
cussion regarding the roles of Ag and Au in determining
the chemical behavior of npAu. Key to the mechanism for
coupling is b-hydride elimination from adsorbed alkoxy to
form the aldehyde, which then reacts with another adsorbed
alkoxy to yield the ester. The temperature at which b-C–H
bond scission occurs must therefore be less than or equal to
the temperature at which the ester is evolved. The tem-
peratures for b-C–H bond scission for ethoxy on npAu,
5 Conclusions
Oxygen-assisted self-coupling of ethanol and 1-butanol on
npAu ingots readily occurs under the controlled conditions
afforded by ultrahigh vacuum, reflecting the reaction
mechanism observed previously on single crystal gold
surfaces. The selectivity for the esters is characteristic of
reactions on gold, whereas the oxygen dissociation is more
characteristic of metallic silver. The npAu therefore ap-
pears to function as a bifunctional catalytic material, in-
cluding, to a limited extent, competitive reactions on both
the silver and gold at the surface. The facile self-coupling
of ethanol and 1-butanol observed here indicates that their
self-coupling should occur readily over npAu in a flow
reactor under appropriate conditions.
Ag(110), Au(111) and methoxy on a model Ag0.4
/
Au0.6(111) alloy surface [15] are all slightly different
(Table 1). What is most striking about this comparison is
that the temperature for activation of the C–H bond on
npAu is significantly higher than that of both the model
Au(111) surface and the Ag/Au alloy surface and is close
to the temperature observed on metallic silver. This com-
parison suggests that activation of the alkoxy species oc-
curs on sites involving Ag.
Acknowledgments We gratefully acknowledge the support of this
work by the U.S. Department of Energy, Basic Energy Sciences,
Catalysis Science Program (DE-FG-02-84ER13289). Work at LLNL
was performed under the auspices of the U.S. Department of Energy
by LLNL under Contract DE-AC52-07NA27344. Correspondence
and requests for materials should be addressed to C.M.F.
Our investigation also establishes that coarsening of
npAu does not eliminate its activity for selective oxidation
of ethanol, including self-coupling to ethyl acetate.
Although repeated thermal cycling during cleaning and
temperature programmed reaction studies changes the
ligament structure (Fig. 5), the overall pattern of reactivity
remains consistent. The changes in selectivity are at-
tributed to a different catalyst surface composition and
structure as the catalyst surface is dynamic and changes
with high temperature and exposure to organic reagents
[23, 30, 31]. The persistence of activity for ethanol cou-
pling by coarsened npAu is in contrast to the absence of
activity reported for CO oxidation on similarly coarsened
material [32]. We attribute this difference in part to the fact
that there are fewer low coordination sites on coarsened
npAu. The binding of CO to Au and Ag is extremely weak
in general; however, the binding is somewhat stronger on
under coordinated atoms, as established for Au, for ex-
ample [33, 34]. The weak binding of CO would result in a
shorter surface lifetime on coarsened npAu and, therefore,
a reduced reaction rate. The persistence of activity for
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