Colloidal gold nanoparticles: An unexpected catalytic activity in aqueous phase with dioxygen

Article abstract of DOI:10.1007/s10562-014-1255-z

Selective oxidations of alkenes were investigated using molecular oxygen in aqueous solution under mild conditions. Colloidal gold nanoparticles are particularly versatile catalysts for oxidation reaction with exceptionally high efficiency and significant selectivity. Gold nanorods (Au NRs) exhibited a slightly enhanced activity compare to gold nanospheres.

Full text of DOI:10.1007/s10562-014-1255-z

Catal Lett
DOI 10.1007/s10562-014-1255-z
Colloidal Gold Nanoparticles: An Unexpected Catalytic Activity
in Aqueous Phase with Dioxygen
•
Hadi Salari Hossein Robatjazi Mohammad Reza Hormozi-Nezhad
•
•
•
Mohsen Padervand Mohammad Reza Gholami
Received: 7 February 2014 / Accepted: 2 April 2014
Ó Springer Science+Business Media New York 2014
Abstract Selective oxidations of alkenes were investi-
gated using molecular oxygen in aqueous solution under
mild conditions. Colloidal gold nanoparticles are particu-
larly versatile catalysts for oxidation reaction with excep-
tionally high efficiency and significant selectivity. Gold
nanorods (Au NRs) exhibited a slightly enhanced activity
compare to gold nanospheres.
to the rod length axis, in resonance with certain frequencies
of incident light that results in two surface plasmon-reso-
nance modes called transverse and longitudinal plasmon
modes, respectively. These remarkable properties has
motivated their introduction into numerous applications,
such as sensing and imaging [11–14], chemical and bio-
logical diagnostics [15–18], photothermal therapy [14, 19].
In the present study, the catalytic activity of colloidal Au
NRs and gold nanosphere was investigated in ambient
condition for selective oxidation of some alkenes such as
cyclohexene, styrene and 1-hexene. Friend et al. [20]
studied the reversible oxidation of styrene with dioxygen
on Au (111) lattice in high temperature. Accordingly, the
styrene epoxidation reaction should be performed at low
temperature to achieve high activity. Turner at al. [21]
reported the effectiveness of small gold nanoparticles
(*1.4 nm) supported on SiO2, C and BN as catalyst for
selective oxidation of styrene by dioxygen. They repre-
sented *2 nm particle size as a main threshold in catalytic
activity. Hughes and co-workers [22] have explored high
activity of oxygen activated gold nanocrystals in selective
oxidation of cyclohexene and cis-cyclooctene. When gold
is deposited as small nanoparticles on some selected group
of metal oxides, it shows high activity and selectivity [23].
Ample progress has been made in this regard, however,
low efficiency of the system is considered as the main
drawbacks associated with heterogeneous oxidation reac-
tions. Furthermore, the substrates are decomposed in water
and in polar solvents [21–23]. Hence the development of
green catalytic systems in selective oxidation of alkenes is
still in great demand. Nano-gold catalysts might present an
appropriate option to use clean and industrial methods. The
benefit of the present work stems from green active system
by water and oxygen and high activity without any addi-
tives. Rossi and co-workers [24] found that water-dispersed
Keyword Environmental catalysis Á Green chemistry Á
Homogeneous catalysis Á Alkenes Á Surfactants Á Aerobic
oxidation
In past two decades, the possibility of tailoring the physi-
cochemical and optical properties of noble metal nano-
structures by controlling their size, shape, composition and
morphology have been implemented scientists to develop
different morphologies of anisotropic shaped metal nano-
particles [1–7]. As a kind of well-developed nanoscaled
noble metallic material, Au NRs are generating much
enthusiasm, in a few past years, owning to their biocom-
patibility, and unique optical and electronic properties [8–
11] These attributes are accounted by the oscillations of
their conduction free electrons, perpendicular and parallel
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-014-1255-z) contains supplementary
material, which is available to authorized users.
H. Salari Á H. Robatjazi Á M. R. Hormozi-Nezhad Á
M. Padervand Á M. R. Gholami (&)
Department of Chemistry, Sharif University of Technology,
11155-9516, Tehran, Iran
e-mail: gholami@sharif.edu
M. R. Hormozi-Nezhad
Institute for Nanoscience and Nanotechnology, Sharif University
of Technology, Tehran, Iran
123

H. Salari et al.
Fig. 1 Representative TEM
images (a–c) and corresponding
UV–Vis absorption spectra
(d) of prepared gold nanorods
with different average aspect
ratio (a) AR & 1,
(b) AR & 2.0, (c) AR & 3.2.
The average size (length and
width) with standard deviation
of the particles calculated from
TEM images of 100 particles
are listed below: Gold
nanosphere: average size of
45.9 9.1 nm. Gold nanorods:
average AR of 2.0 (length:
38.5 6.1 nm and width:
19.3 4.4 nm). Gold nanorods:
average AR of 3.2 (length:
33.1 5.2 nm and width:
10.2 0.8 nm)
gold sol exhibited a surprising activity when used as
‘‘naked particles’’, that is, in the absence of common pro-
tectors. Aerobic oxidation of glucose to gluconate have
been investigate in this work under mild conditions.
Dioxygen oxidation of 1-phenylethanol with the radical
initiator N-hydroxyphthalimide (NHPI) showed good con-
version and selectivity which can be rationalized through
the radical mechanism in Ionic Liquid matrix [25].
Hexadecyl-trimethyl-ammonium bromide (CTAB)-sta-
bilized Au NRs were synthesized in high purity through
wet chemical seed-mediated method developed by El-
Sayed et al. [10]. To prepare NRs with various average
aspect ratio (AR) ranging from 1.0 (spherical particles) to
3.2, 12 lL freshly prepared CTAB-capped gold seeds were
added to the growth solutions containing CTAB (10 mL,
0.1 M), HAuCl4 (0.2 mL, 0.025 M), different quantities of
AgNO3 (0.01 M), and ascorbic acid (70 lL, 0.1 M). The
mixtures were left undistributed 24 h for completing the
growth process. The resulting NRs were characterized by
UV–Visible spectrometry and transmission electron
microscopy (Fig. 1).
Table 1 Catalytic results of cyclohexene oxidation with molecular
oxygen
Catalysta
Conversion % TOFb Selectivity %
No gold
–
–
Trace
32.4
Trace
67.6
Trace
–
GNR.
80.3
4.42
AR & 3.2
GNR/PAA.
AR & 3.2
40.8
84.9
45
2.24
4.67
2.47
3.35
2.86
45.5
35.6
77.6
93.4
80.1
54.4
61.5
11.2
6.5
–
GNR.
AR & 2.1
2.8
11.0
–
GNR/PAA.
AR & 2.1
GNS.
AR & 1
61
GNS/PAA.
AR & 1
52
11.2
8.6
Reaction conditions: T = 50 °C; P = 0.8 atm; reaction time = 24 h,
[Catalyst] = 0.5 nM, [alkene] = 0.65 M
a
Alkene oxidation reaction over the catalysts was carried
out at 0.88 atm pressure of oxygen gas by contacting 3 ml
of catalyst solution (0.5 nM) with alkene (0.65 M) in a
GNR refers to as-prepared gold nanorod, GNR/PAA refers to PAA
coated gold nanorod, GNS refers to gold nanosphere
b
TOF is regarded as [mol cyclohexene (nmol NPs)-1 h-1
]
123

Colloidal Gold Nanoparticles: An Unexpected Catalytic Activity in Aqueous Phase
Table 2 Catalytic results of 1-hexene oxidation with molecular oxygen
Catalysta
Conversion %
TOFb
Selectivity %
O
O
HO
No gold
–
–
Trace
65.5
61.5
72.1
70.7
78.5
71.2
Trace
34.5
37
Trace
–
GNR. AR & 3.2
GNR/PAA. AR & 3.2
GNR. AR & 2.1
GNR/PAA. AR & 2.1
GNS. AR & 1
70.2
40.3
75.8
42.2
65.8
50.1
3.08
1.77
3.35
1.85
2.89
2.20
–
24.1
25.8
21.5
28.5
3.6
3.5
–
GNS/PAA. AR & 1
–
Reaction conditions: T = 50 °C; P = 0.8 atm; reaction time = 24 h, [Catalyst] = 0.5 nM, [alkene] = 0.65 M
a
GNR refers to as-prepared gold nanorod, GNR/PAA refers to PAA coated gold nanorod, GNS refers to gold nanosphere
TOF is regarded as [mol 1-hexene (nmol NPs)-1 h-1
b
]
stirred batch reactor (capacity: 10 cm3), under reflux (at
50–52 °C) and vigorously stirring for a period of 24 h. The
reaction products and unconverted reactants were analyzed
by GC-FID equipped with HP5 column (30 m) and He was
used as carrier gas.
nanoparticles and support is not necessary in all cases for
performing these catalytic reactions. Many of metallic
elements dissociatively chemisorb O2 to yield O adatoms,
but they usually disturb the electronic structures of co-
adsorbed substrate and finally they burn the organic mol-
ecules to carbon dioxide and water [27, 28]. Colloidal gold
nanoparticles could adsorb O2 and organic molecule
chemically and onset the catalytic reaction. High activity
which observed in polar solvent like water can be related to
the stabilization of polar intermediates via hydrogen
bonding, electrostatic interactions and dipolarity/polariz-
ability effects. Gold nanorods with different size show
equal activity in cyclohexene and 1-hexene cases and
aggregation which is accompanied with polymerization
happened in the presence of styrene substrate [29]. Despite
polymerization, styrene oxidation reaction happened with
5–50 % efficiency in the presence of different colloidal
gold nanoparticles. Investigation of gas phase interactions
of oxygen molecule with gold clusters exhibited a sensi-
tivity to the cluster size and gold charge state [26]. The
adsorption, activation and dissociation of O2 can be done
with electron pushing and donor/acceptor mechanism in
supported gold catalysts [28]. It is supposed that chargeless
colloidal gold particles in liquid phase demonstrate similar
mechanism. To explain how the oxidation occurred, and
how the products were produced, we also estimate the
oxidation mechanism as described in literature (Schemes 1
and 2 in supplementary information) [30–33]. The oxida-
tion of cyclohexene with molecular oxygen initially formed
2-cyclohexene-1-hydroperoxide which was unstable and
easy formed other products. A similar trend has been
observed for 1-hexene.
In Preliminary experiments, various colloidal gold
nanocatalysts were tested for their catalytic activity, the
products were obtained and they are collectively called
P
C6 for cyclohexene and 1-hexene.
Using colloidal gold nanoparticles, high efficiency with
visible selectivity was achieved. Tables 1, 2 show the
conversion of cyclohexene and 1-hexene and selectivity of
products. Compare with the uncatalyzed reaction (Entry1,
Tables 1, 2) both spherical and rod catalysts exhibit high
catalytic performance. The comparison study between the
rod shaped and spherical gold nanoparticles showed that
the Au NRs exhibited a slightly enhanced activity with
comparable selectivity. More than 80 % selectivity for
P
C6 including 2-cyclohexen-1-ol, 2-cyclohexene-1-one
and cyclohexene oxide was obtained. When the reaction
was done with GNS, 61 % conversion was obtained which
increase to 80 % with GNR (Table 1, Entry 2 and 5). This
improved activity might be attributed to the higher active
site at the interface and differences in electronic structures
of gold which led to different activity. This observations
show that colloidal gold nanoparticles could adsorb and
activate O2. With dissociation of O2 to O adatoms the
catalytic oxidation reaction is initiated [25, 26]. Electronic
properties of very small gold particles are differed dra-
matically from those of the bulk materials. This size-
dependent change in electronic structures which lead to
unusual catalytic activity could exist in colloidal gold
nanoparticles.
Surface coating with polyelectrolyte would be one
approach to tailor the properties of gold nanocrystals,
which has been studied in detail by Caruso and co-workers
[33, 34]. For Au NRs, the main breakthrough, until today,
This catalytic system showed that gold intrinsically
catalyze the reaction and dissociation of O2 bond happened
in surface of nanoparticles and common interface between
123

H. Salari et al.
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