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cycles may be peroxide-like or hydroperoxide-like species.
25
In addition, a
distinct size dependence of Au
n
(SR)
m
nanocluster catalysts was observed in
the selective oxidation of styrene, with the order of catalytic activity of
Au
25
(SR)
18
4Au
38
(SR)
24
4Au
144
(SR)
60
.
Tsukuda and co-workers
36
investigated a Au
25
(SG)
18
/HAP catalyst for se-
lective oxidation of styrene, where HAP refers to hydroxyapatite
(Ca
10
(PO
4
)
6
(OH)
2
). The ligand-off Au
25
/HAP catalyst was obtained by re-
moving the ligands via 300 1C thermal treatment. The catalyst was explored
in styrene oxidation (with anhydrous tert-butyl hydroperoxide (TBHP) as the
oxidant). The reaction yielded styrene oxide with a selectivity of
d
n
9
r
4
n
g
|
3
92%.
To compare with the catalytic activity of the Au
25
(SR)
18
, Qian et al.
37
fur-
ther synthesized the monoplatinum-doped Pt
1
Au
24
(SR)
18
nanoclusters and
investigated the cluster in the selective oxidation of styrene using PhI(OAc)
2
as the oxidant. The single platinum atom is located in the center of the core,
which implies that the shell (i.e., Au
12
(SR)
18
) of the Pt
1
Au
24
(SR)
18
nanocluster
is the same as that of Au
25
(SR)
18
. The catalytic reaction was carried out at
70 1C for 10 h in acetonitrile under N
2
atmosphere. Pt
1
Au
24
(SR)
18
/TiO
2
catalyst gave much higher catalytic activity (i.e., conversion of styrene) and
also selectivity for benzaldehyde than the Au
25
(SR)
18
/TiO
2
catalyst under the
identical reaction conditions. Therefore, the different catalytic performance
is apparently due to the electronic properties of the nanoclusters. The large
enhancement of catalytic activity through single atom doping is remarkable,
indicating the high sensitivity of nanocluster catalysts to its constituent
atoms, even those not at the surface.
B
.
5.5.2.3 Selective Oxidation of Sulfides
It is known that sulfides can bind to the surface of gold nanoclusters via the
interaction between the sulfur atom (-S-) and the gold surface, while sulf-
oxides (-S(
¼
O)-) only weakly bind to the gold surface; hence the conversion
from sulfide to sulfoxide on the clusters surface should be feasible. Gao
et al.
28
B
1 wt% Au
25
(SR)
18
nanocluster
catalyst for selective oxidation of sulfides to sulfoxides using PhIO as oxidant
(Scheme 5.3).
The TiO
2
-supported Au
25
(SR)
18
nanocluster catalyst gave rise to a high
catalytic activity (e.g.,
investigated the oxide-supported
92% selectivity for
the sulfoxide product). Meanwhile, different types of substrates have been
investigated to test the effects of electronic and steric factors in the selective
oxidation. Sulfides with electron-rich groups were oxidized at higher
B
97% conversion of sulfide with
B
O
O
O
Au
25
(SR)
18
/TiO
2
S
S
S
PhIO
+
+
R
1
R
2
R
1
R
2
R
1
R
2
DCM, 40°C, 12 h
Sulfide
Sulfoxide
Sulfone
Scheme 5.3 Oxidation of sulfides using Au
25
(SR)
18
/TiO
2
catalyst.
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