C O M M U N I C A T I O N S
is reasonably explained in terms of the efficient activation of O2
by small-sized gold NCs.4 Formation of similar complexes between
Au NCs and O2 has also been postulated in the homocoupling of
arylboronic acid catalyzed by Au:PVP NCs.7
In summary, we report herein the first successful application of
colloidal Au NCs toward the aerobic oxidation of benzylic alcohols
in water at ambient temperatures. A size effect is clearly demon-
strated, showing that O2 adsorption onto the gold NCs is the key
factor for the size-specific catalytic activities. The results reported
here will contribute to the development of efficient and environ-
mentally benign gold catalysts for alcohol oxidations, that utilize
ubiquitous air (viz. molecular oxygen) as an oxidant.
Figure 2. (a) Time course of conversion and (b) Arrhenius plots for
oxidation of 1d.
Table 2. Catalytic Activity toward Oxidation of 1d
1
a
a,b
c
1
catalyst
dav (nm)
k (h-
)
k
′
kH/kD
Ea (kJ mol-
)
Acknowledgment. We thank Dr. O. Oishi for performing the
TEM measurements. The present work was financially supported
by a CREST program sponsored by JST, the “Nanotechnology
Support Project” of MEXT, and the “2002 Joint Research Project”
of Sokendai (Soken/K02-1).
Au:PVP-1 1.3 ( 0.3 2.7 × 10-1 1.0
74 ( 6
20
∼25
-
Au:PVP-2 9.5 ( 1.0 9.5 × 10-5 2.6 × 10-3
Pd:PVP-1 1.5 ( 0.3 1.8 × 10-2 7.7 × 10-2
Pd:PVP-2 2.2 ( 0.4 6.2 × 10-2 3.8 × 10-1 23 ( 3
33
a At 300 K. b Ratio of the rate constant normalized by surface area of
the clusters. c At 330 K.
Supporting Information Available: Detailed experimental pro-
cedures of preparation and characterization of catalysts and catalytic
reactions. This material is available free of charge via the Internet at
values are normalized by the surface areas of the corresponding
NCs by assuming spherical shapes with the diameters shown in
Figure 1. The k′ values listed in Table 1 represent the relative rate
constants thus normalized with respect to that of Au:PVP-1. The
k′ value of Au:PVP-2 shows that the smaller (1.3 nm) Au NCs
exhibit higher catalytic activity than the larger (9.5 nm) NCs. The
1.3-nm Au NCs are found to be catalytically more active than the
Pd NCs of similar size at 300 K, as recently observed in the glucose
oxidation catalyzed by their “naked” clusters.3g
To explain these findings, the mechanistic aspects of the
oxidation of 1d were studied in more detail. Kinetic measurements
have shown that Au:PVP-1 NCs cannot catalyze the oxidation in
the absence of either molecular oxygen or base.6 This result
indicates that O2 and the deprotonated form of 1d are intimately
involved in the reaction. The oxidation under air proceeds at a rate
comparable to that under 1 atm of O2, indicating that the reaction
is not retarded by the concentration of O2 dissolved in water.6 A
large kinetic isotope effect (KIE) was observed in the oxidation of
R-deuterated 1d (p-HOC6H4CD2OH); kH/kD ) 74 ( 6 and 23 ( 3
at 330 K for Au:PVP-1 and Pd:PVP-2, respectively (Table 2).11
The primary KIE demonstrates that cleavage of the C-H bond at
the benzylic position is the rate-determining step. The apparent
activation energy, Ea, associated with the C-H bond cleavage is
determined from the Arrhenius plots in the temperature range of
273-345 K (Figure 2b). The least-squares fit analysis yields Ea
values of 20, 25, and 33 kJ mol-1 for Au:PVP-1, Au:PVP-2 and
Pd:PVP-2, respectively (Table 2).
The marked difference in the KIE and Ea values between Au:
PVP-1 and Pd:PVP-2 suggests that different mechanisms are
operating in their catalytic processes. It is accepted that the alcohol
oxidation catalyzed by Pd(0) NCs proceeds via the following
mechanism.12 First, the alcohol is dissociatively adsorbed on the
Pd NC surface, affording the alkoxide and hydride (oxidative
addition). Then the H atom on the â-carbon of the adsorbed alkoxide
is transferred to the Pd NC surface to form the aldehyde and a
Pd-hydride species in the rate-determining step. Finally, the O2
molecule removes the hydride species from the Pd surface to
reactivate it, as well as forming H2O2. In contrast, the rate-
determining step of the Au:PVP-1-catalyzed reaction may involve
H-atom abstraction by a superoxo-like molecular oxygen species,
which is adsorbed on Au:PVP-1. Within the framework of this
model, the size-specific catalytic activity of Au:PVP-1 (Table 2)
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