P123-stabilized Au-Ag alloy nanoparticles for kinetics of aerobic oxidation of benzyl alcohol in aqueous solution

Article abstract of DOI:10.1016/j.jcat.2013.02.011

Colloidal Au-Ag alloy nanoparticles with various Ag/Au molar ratios were first prepared by a co-reduction method in P123 aqueous solution and characterized by UV-vis, TEM, and XPS. The prepared Au-Ag alloy nanoparticles were substantially stable and had c

Full text of DOI:10.1016/j.jcat.2013.02.011

Journal of Catalysis 301 (2013) 217–226
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
P123-stabilized Au–Ag alloy nanoparticles for kinetics of aerobic oxidation of
benzyl alcohol in aqueous solution
b
a
a
a
Xuemin Huang a, Xueguang Wang a, , Xiaoshu Wang , Xinxing Wang , Mingwu Tan , Weizhong Ding ,
⇑
Xionggang Lu a,
⇑
a Shanghai Key Laboratory of Modern Metallurgy and Material Processing, Shanghai University, Shanghai 200072, China
b Center of Modern Analysis, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Colloidal Au–Ag alloy nanoparticles with various Ag/Au molar ratios were first prepared by a co-reduc-
tion method in P123 aqueous solution and characterized by UV–vis, TEM, and XPS. The prepared
Au–Ag alloy nanoparticles were substantially stable and had comparable particle sizes (3–4 nm) for
erobic oxidation kinetics of benzyl alcohol. The addition of Ag not only significantly enhanced the
reaction rate but also increased the apparent activation energy (Ea) compared to that for monometallic
Au nanocatalysts. The kinetics investigations indicated that on the pure Au sites, the oxidation of benzyl
alcohol followed 1.5-order reaction kinetics, while on Au sites adjacent to Ag atoms, it followed 0.5-order
reaction kinetics. The presence of Na2CO3 greatly improved the catalytic activity of Au–Ag nanocatalysts
but decreased the selectivity to benzaldehyde.
Received 15 January 2013
Revised 11 February 2013
Accepted 11 February 2013
Keywords:
Au–Ag alloy
Colloid
Nanoparticle
Aerobic oxidation
Benzyl alcohol oxidation
Kinetics
Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction
Au–Ag alloy nanocatalysts for aerobic oxidation reactions are still
ambiguous, which are of fundamental importance.
Au nanocatalysts have attracted considerable attention because
of their nontoxic nature and their ability to catalyze many impor-
tant chemical reactions, such as low-temperature oxidation of CO
[1,2], epoxidation of alkenes [3,4], oxidation of alcohols [5–8],
homocoupling of phenylboronic acid [9], and synthesis of H2O2
[10,11]. For practical catalytic applications, bimetallic alloy nano-
particles of Au with a second metal component have been exten-
sively investigated to improve their catalytic properties by
forming new active sites, improving the electronic structure and
inducing synergistic effects. Among these bimetallic nanocatalysts,
Ag-doped Au alloy nanocatalysts have been reported to show cat-
alytic performance for oxygen transfer reactions superior to that of
their monometallic counterparts, and the degree of enhancement
is strongly correlated with the surface silver content [12–20]. How-
ever, these Au–Ag nanoparticles are commonly stabilized with me-
tal oxides or activated carbon as supports, resulting in inherent
heterogeneity of the reaction and inaccessibility of the substrate
to the catalysts. In addition, the catalytic properties are influenced
not only by the individual chemical natures of Au and the support,
but also by complex interactions between them [21–23]. So, the
nature of the active sites and the catalytic roles of Ag species in
Monodisperse colloidal metal nanocatalysts for various reac-
tions combining the advantages of both homogeneous and hetero-
geneous catalysts have received significant interest [24–28]. The
active sites on the surfaces of metal nanoparticles with weak inter-
actions with the organic stabilizers in solution can be fully exposed
to the reactants and exhibit catalytic activities under mild reaction
conditions. Well-dispersed colloidal Au nanoparticles in aqueous
solution have been reported as catalysts for aerobic oxidation of
glucose [29], dehydrogenation of formic acid [30], and aerobic oxi-
dation of alcohols [31–35] at near-ambient temperatures, and the
catalytic properties are governed by the intrinsic chemical proper-
ties of Au nanoparticles. This information will provide an ideal
opportunity for us to better understand fundamental aspects of
the mechanism and kinetics of the reaction and the correlation
between Au and Ag species for aerobic oxidation of alcohols on
Au–Ag alloy nanocatalysts.
Au–Ag alloy nanoparticles are currently synthesized in aqueous
solution or organic solvents mainly through co-reduction of Au and
Ag salts in the presence of a stabilizing agent [36–43]. The synthe-
sis conditions strongly influence the reduction potentials and the
effectiveness of the stabilizing agent, resulting in more complexity
in size tuning for the alloy nanoparticles than for the monometallic
ones. Most of the current preparation methods are unable to
decouple size and composition control, that is, producing alloy
⇑
Corresponding author. Fax: +86 21 56338244.
E-mail address: wxg228@shu.edu.cn (X. Wang).
0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.02.011

218
X. Huang et al. / Journal of Catalysis 301 (2013) 217–226
nanoparticles with the same composition but different sizes, or the
same size with different compositions. In addition, due to the pro-
pensity to form insoluble AgCl(s) (Ksp = 1.6 ꢀ 10–10), it is very hard
to increase the concentration of reactants when using HAuCl4 as
the gold precursor. The catalytic properties of the alloy nanoparti-
cles are also associated with the stabilizing agents. For example,
thiol-stabilized metal nanocatalysts can be poisoned because of
their strong interaction with the particle surface and tendency to
form a monolayer with high packing density [43–45]. Tetraalkyl-
ammonium-protected nanoparticles tend to aggregate, resulting
in unsatisfactory long-term stability and the loss of their original
properties [46]. Up to now, only a few applications of colloidal
Au–Ag alloy nanoparticles in catalytic reactions have been
reported, because of the difficulty of stabilizing the fragile systems
without modification of their intrinsic nature. The synthesis of
monodisperse Au–Ag alloy nanoparticles with controlled particle
sizes and compositions is still of key importance in the study of
catalytic mechanism and kinetics of the reaction. Recently,
colloidal Au–Ag nanoparticles stabilized with poly(N-vinyl-2-
pyrrolidone) (PVP) in aqueous solution have been reported as
catalysts for oxidation of benzyl alcohol using molecular oxygen
[40]. Nevertheless, PVP-stabilized Au–Ag alloy nanoparticles
showed the catalytic activity only in the presence of a strong base
(e.g., KOH or K2CO3).
2.2. Characterization
UV–vis absorption spectra were recorded with a UV-2501
spectrophotometer at ambient temperature, with P123 aqueous
solution as a standard for background correction. Before measure-
ment, 1 mL of the colloidal solution was diluted five times with
deionized water.
Transmission electron microscopy (TEM) images were obtained
with a JEM-2010F transmission electron microscope operating at
200 kV. The specimens for TEM measurements were prepared by
dropping the colloidal dispersion onto carbon-coated copper grids
and dried for 1 day in a desiccator. Size distributions of the parti-
cles were obtained by counting ca. 200–250 particles in TEM
images.
X-ray diffraction (XRD) was carried out on a Rigaku D/MAX-
2500 apparatus using Cu Ka radiation (k = 0.15418 nm) at a voltage
of 40 kV and a current of 40 mA. The nanoparticle colloid was
dropped on the glass substrate and dried to form a uniform film
and finally was washed with deionized water to remove inorganic
salts and parts of the surfactant.
X-ray photoelectron spectra (XPS) of the particles were mea-
sured using a PHI 5000 VersaProbe spectrometer equipped with
monochromatized Al K
a radiation (hm = 1486.6 eV) operated at a
pressure of ca. 1 ꢀ 10–9 Torr. The samples for the XPS measure-
ments were prepared on an aluminum sheet by centrifuging the
colloids and washed to remove some of the excess P123 with
deionized water. The spectra for the nanoparticles were calibrated
using the most intense C1s peak of P123 as an internal standard
[32,40].
In this paper, we first report a facile method for preparing highly
stable monodisperse Au–Ag alloy nanoparticles of varied Ag/Au
molar ratios with a high total metal concentration of 5 ꢀ 10–4
M
by simultaneous reduction of HAuCl4 and AgNO3 mixtures with
NaBH4 in an aqueous solution of a typical amphiphilic triblock
copolymer, poly(ethylene oxide)–poly(propylene oxide)–poly
(ethylene oxide) (P123; EO20PO70EO20). The copolymer-assisted
nanoparticles are known to be stabilized through weak steric
force, so that the reactants can access the particle surface for
catalytic reactions [19,47]. The prepared Au–Ag nanoparticles
have comparable particle sizes of 3–4 nm and show a certain
activity for benzyl alcohol as a model reaction with molecular
oxygen in the absence of base. This is highly desirable as a way to
measure the aerobic oxidation kinetics of benzyl alcohol and to
investigate the roles of Ag in Au–Ag alloy nanocatalysts.
2.3. Oxidation reaction of benzyl alcohol
The oxidation of benzyl alcohol was carried out with O2 in a
three-necked flask of 50 mL with a reflux condenser, and the flask
was placed in a thermostatic bath with a magnetic stirrer. In a typ-
ical experiment, metal nanoparticle colloid (20 mL) was heated at
the set temperature with an accuracy of 0.1 K and bubbled with
pure O2 for 60 min. Then, an appropriate amount of benzyl alcohol
was added into the reaction mixture, and the oxidation reaction
was started under vigorous magnetic stirring by bubbling O2. The
reactions were monitored by sampling the reaction mixture
(0.5 mL) at fixed intervals. The reaction mixture was immediately
quenched with 2 M HCl aqueous solution (0.5 mL) and extracted
three times with ethyl acetate (5 mL). The obtained organic layer
was dried with Na2SO4 and analyzed by a CP-3800 gas chromato-
graph with an FID detector using the external standard method.
2. Experimental
2.1. Chemicals and preparation of Au–Ag nanoparticles
P123 (EO20PO70EO20, Mav = 5800) and NaBH4 were purchased
from Aldrich and other chemicals of reagent grade were from Sin-
opharm Chemical Reagent Co., Ltd. All chemicals and solvents were
used as received without further purification.
3. Results and discussion
The Au1ꢁxAgx alloy nanoparticles (x: Ag mole fraction) with var-
ious Ag/Au molar ratios (0::1–0.50::0.50) were prepared through
the simultaneous reduction of HAuCl4 and AgNO3 in a P123 aque-
ous solution at room temperature (293 K) using NaBH4 as the
reducing agent. Typically, an appropriate amount of AgNO3 aque-
ous solution (1.5 ꢀ 10–3 M) was first introduced into P123 aqueous
solution (25 mL) and stirred vigorously for 5 min. Then, HAuCl4
aqueous solution (1.5 ꢀ 10–3 M) was added to the AgNO3/P123
solution under vigorous stirring. At this time, the solution became
clear and light yellowish, and no flocculation due to the formation
of AgCl was observed. Immediately, 1 g/L NaBH4 aqueous solution
(15 mL) was added very slowly to the mixture of HAuCl4 and
AgNO3 in the P123 aqueous solution under vigorous stirring within
15 min. Finally, the solution was exactly adjusted at 100 mL with
deionized water and stirred for 60 min further. The ratios of molar
concentrations of P123 monomer units to total metal atoms,
[P123]/[Au + Ag], were in the range of 5–20.
3.1. Preparation of Au–Ag nanoparticles
The Au–Ag alloy nanoparticles were prepared through the
simultaneous reduction of HAuCl4 and AgNO3 in a P123 aqueous
solution with NaBH4 as reducing agent. It is well known that in
preparing Au–Ag bimetallic particles in aqueous solution, the most
troublesome problem is the precipitation of AgCl when the two
metallic solutions are mixed [48]. To solve it, AgNO3 was first
introduced into P123 aqueous solution to stabilize Ag+. As a result,
no precipitation or turbidity was observed when HAuCl4 precursor
solution was added into AgNO3/P123 aqueous solution, although
[Ag+][Cl–] was much higher than Ksp of AgCl(s). This result was also
confirmed by UV–vis spectra of the mixture solutions (not shown),
in which no absorption band appeared at about 295 nm [49]. Very
importantly, once HAuCl4 precursor is introduced into the AgNO3/
P123 aqueous solution and appears homogeneous, dropping of

X. Huang et al. / Journal of Catalysis 301 (2013) 217–226
219
NaBH4 aqueous solution should begin within 30 s; otherwise,
Au(III) ions will interact with the surfactant P123 to form a light
blue colloidal solution, failing to produce stable Au–Ag nanoparti-
cle colloids, as reported previously for P123-stabilized pure Au
nanoparticles in aqueous solution [34]. Upon the addition of
NaBH4, the light yellowish color of the solution immediately chan-
ged toward wine red. The final color of the prepared colloids was
dependent on the Ag/Au molar ratio in solution, visually indicating
that Au and Ag salts were reduced to metal nanoparticles. Varying
[P123]/[Au + Ag] molar ratios in the range of 5–20 had little influ-
ence on the sizes of the Au–Ag alloy nanoparticles. However, when
[P123]/[Au + Ag] was 5, the black precipitant could be observed in
the colloidal solution within 2 months at room temperature, indi-
cating unsatisfactory stability. Thus, Au–Ag colloids obtained at
[P123]/[Au + Ag] = 10 were used to characterize the physical and
chemical properties of alloy nanoparticles.
blueshifted between 396 and 518 nm, which are attributed to the
absorptions of pure Ag and Au nanoparticles, respectively, with
increasing Ag/Au molar ratios in solution. These results demon-
strate that the Au–Ag nanoparticles obtained by our method are
Au–Ag alloy with random distribution of Ag atoms rather than
core–shell structured Ag/Au, Au/Ag, or a mixture of Au and Ag
nanoparticles [12,50,51].
3.3. TEM characterization
TEM measurement was used to determine the metal particle
sizes and size distributions of the colloidal Au1ꢁxAgx alloy nanopar-
ticles formed in the P123 aqueous solution. Fig. 2A shows the
images and particle size histograms of pure Au, Ag, and Au–Ag
samples with different Ag/Au molar ratios. For each Au1ꢁxAgx sam-
ple, the characteristic spherical nanoparticles with a uniform elec-
tron density and a relatively narrow particle size (PS) distribution
are observed, also supporting the formation of Au–Ag alloy nano-
particles instead of core–shell [36,52]. The average diameters and
standard deviations are calculated and listed in Table 1 by measur-
ing the sizes of individual particles in TEM images. It can be seen
that average Au1ꢁxAgx particle diameters ranging from 3.1 to
4.0 nm are obtained with relatively high monodispersity, except
that pure Ag nanoparticles have a mean diameter of 10.2 nm. The
mean diameters of the nanoparticles are also estimated by apply-
ing the Scherrer formula to the XRD diffraction peaks (Supplemen-
tary information, Fig. S1) from the Au (Ag) (111) planes [31,53,54]
and listed in Table 1, which are comparable with the results ob-
tained from the statistical analysis of the TEM images.
The HR-TEM image of the representative Au–Ag nanoparticles
in Fig. 2B shows that the individual nanoparticles are single crys-
tals, having clear lattice fringes without lattice defects inside the
particles. Regardless of the Ag/Au molar ratios, the Au–Ag nanopar-
ticles show almost the same interplanar spacings of 0.246 nm and
0.213 nm, with a face angle of 123°, corresponding to the (111)
and (200) planes of the face-centered cubic (fcc) structures of
the Au (Ag) bulk metals, respectively, which are consistent with
the XRD results (Supplementary information, Fig. S1). However,
owing to the small difference in lattice constants of Au and Ag
3.2. UV–vis absorption spectra
UV–vis spectroscopy has been proven to be very useful for dis-
tinguishing two kinds of alloy nanoparticles. In the case of core–
shell alloy nanoparticles, there usually are two surface plasmon
resonance (SPR) bands corresponding to the absorbance of sepa-
rated metal clusters as core or shell. The mixed alloy nanoparticles
have only one absorption peak, whose position is in the region
between the absorption peaks of pure nanoparticles [38,48]. The
UV–vis spectra for the as-prepared colloidal Au–Ag nanoparticles
are presented in Fig. 1a. In each case, the spectrum of the colloid
has only a single SPR peak assigned to the absorbance of the
metal particles. As shown in Fig. 1b, the peak position is linearly
Ag
Au0.50Ag0.50
(a)
Au0.80Ag0.20
Au0.85Ag0.15
Au0.90Ag0.10
Au0.95Ag0.05
Au0.98Ag0.02
Au0.99Ag0.01
Au
(aAu = 0.4078 nm, aAu = 0.4086 nm), it is very hard to identify
whether the bimetallic nanoparticles are formed from the analysis
of the XRD patterns [38,55]. The composition distributions of Au
and Ag across the surfaces of individual particles were further
examined by TEM energy-dispersive X-ray spectroscopy (EDX) to
obtain direct evidence of the formation of Au–Ag alloy nanoparti-
cles, and the results are summarized in Table 1. It can be seen that
the average compositions of the obtained nanoparticles are very
close to those initially used in the P123 aqueous solutions.
400
500
600
700
Wavelength (nm)
525
500
475
450
425
400
3.4. XPS spectra
(b)
To further determine the oxidation states of metallic nanoparti-
cles and analyze the chemical compositions of the nanoparticle
surfaces, XPS measurements were employed to characterize the
prepared Au1ꢁxAgx nanoparticles (x = 0, 0.02, 0.05, 0.10, 0.15, and
1). The XPS results of the Au4f and Ag3d regions are shown in
Fig. 3 and Table 2. Both Au and Ag peaks can be clearly identified,
and the changes in their intensity follow a trend similar to that of
the Ag/Au molar ratios used in P123 aqueous solution. These re-
sults indicate that the prepared nanoparticles are composed of
Au and Ag elements. In the case of pure Au nanoparticles, the bind-
ing energies (BEs) of Au4f7/2 and Au4f5/2 are 83.4 and 87.1 eV,
λ
0.0
0.2
0.4
0.6
0.8
1.0
Au molar fraction
respectively, which are lower than those (84.0 for Au4f7/2
,
87.7 eV for Au4f5/2) of bulk metallic gold. Such negative shifts have
also been reported for micrometer-sized Au/PVP (83.7 eV) [56], Au
clusters stabilized by PVP (82.8 eV) [31,40], and Au clusters
Fig. 1. (a) Normalized UV–vis absorption spectra of monometallic Au, Ag, and
Au1ꢁxAgx colloids and (b) dependence of peak wavelength of the SPR band on the Au
molar fraction.

220
X. Huang et al. / Journal of Catalysis 301 (2013) 217–226
(A) Au0.99Ag0.01
(A) Au
3.2 0.5 nm
3.1 0.5 nm
0 1 2 3 4 5 6 7 8
P.S. (nm)
0 1 2 3 4 5 6 7 8
P.S. (nm)
(A) Au0.95Ag0.05
(A) Au0.98Ag0.02
3.2 0.6 nm
3.5 0.5 nm
0 1 2 3 4 5 6 7 8
P.S. (nm)
0 1 2 3 4 5 6 7 8
P.S. (nm)
(A) Au0.90Ag0.10
(A) Au0.85Ag0.15
4.0 0.7 nm
3.8 0.9 nm
0 1 2 3 4 5 6 7 8
P.S. (nm)
0 1 2 3 4 5 6 7 8
P.S. (nm)
Fig. 2. (A) Typical TEM images and sizes distributions (insets) for Au1ꢁxAgx nanoparticles and (B) HR-TEM image for Au0.90Ag0.10 nanoparticles.
protected by carboxyl-terminated poly(amidoamine) (PAMAM)
dendrimer [48], which were attributed to possible electron transfer
from surfactant to the particle. On the other hand, the size and
shape of metal nanoparticles are also responsible for the binding
energy shift [17,55]. As for pure Ag nanoparticles, the Ag3d5/2
and Ag3d3/2 BEs are 368.0 and 374.0 eV, respectively, which are
consistent with the values of zerovalent silver [57].
For the bimetallic Au1ꢁxAgx nanoparticles, the BEs of both Au4f
and Ag3d show observable increases with increasing Ag/Au molar
ratio from 0::1 to 0.10::0.90. In the case of most transition metals,
upon oxidation, the observed core-level BEs shift toward higher
energies, and the positive BE shifts increase as the oxidation state
increases [55]. However, silver is one of the few examples of low-
ered binding energy in the oxidized state [57]. These results mean
that there exists an interaction between Au atoms and Ag atoms,
and Au atoms transfer electrons toward Ag atoms through alloying
with Ag on the surface of the particles. This transfer has been re-
ported for the Au–Ag alloy particles stabilized by PAMAM [48].
However, for the Au–Ag nanoparticles in PVP aqueous solution,
the inverse relationship between Au4f and Ag3d BEs was observed;
namely, Au atoms accept electrons from Ag atoms [40]. These
results imply that the interaction between Au and Ag atoms may
also be affected by the organic stabilizer. The continuous increase
of the Au4f BEs may be assigned to the increase in the number of
Au atoms adjacent to Ag atoms in the Au–Ag particles, making
the Au more cationic with total effect. In contrast, the gradual
increase of the Ag3d BEs implies that the degree of electron trans-
fer from Au atoms to Ag atoms also becomes increased with the in-
crease in the [P123]/[Au+Ag] molar ratio range of 0::1–0.10::0.90.
Noticeably, for all the Au1ꢁxAgx nanoparticles, the Ag3d BEs display
negative shifts with respect to those for pure Ag particles. This can
be attributed to the big difference in the electron structures of Ag
atoms between pure Ag and Au–Ag particles, which have different
coordination environments on the surface. When the Ag/Au molar
ratio is further increased to 0.15::0.85, both Au4f and Ag3d BEs
start to decline, indicating that the interaction between Au and
Ag atoms may undergo significant change. It is plausible to explain
this phenomenon by the following aspect. As the Ag/Au molar ratio
was further increased, more Ag–Ag structures were formed, in-
stead of Au–Ag structures. This caused a change in the coordination
environments of Ag or Au atoms on the surface of the particles,
affecting the electronic structures of Au and Ag atoms.

X. Huang et al. / Journal of Catalysis 301 (2013) 217–226
221
(A) Au0.50Ag0.50
(A) Au0.80Ag0.20
3.4 0.9 nm
3.2 0.8 nm
0 1 2 3 4 5 6 7 8
P.S. (nm)
0 1 2 3 4 5 6 7 8
P.S. (nm)
(A) Ag
(B) Au0.90Ag0.10
10.2 3.5 nm
0
5
10 15 20
P.S. (nm)
Fig. 2. (continued)
Table 1
The chemical compositions and average particle sizes of Au1ꢁxAgx nanoparticles.
3.5. Stability of colloidal Au–Ag alloy nanoparticles
It is well known that small nanoparticles have a strong self-
aggregation tendency because of their huge specific surface area
and large surface energy, which influence the stability of nanopar-
ticles in solution for catalytic application. The colloidal Au–Ag alloy
nanoparticles in the [P123]/[Au+Ag] molar ratio range of 5–20 are
found to be surprisingly stable by the present method. They
could be stored at room temperature for months and treated at
353 K for 8 h without a sign of changes in color and particle size,
which is also confirmed by almost unchanged UV–vis spectra
(Fig. 4a and c) and TEM images (Fig. 5a and b) of representative
colloidal Au0.90Ag0.10 nanoparticles after treatment. The copoly-
mer-assisted colloidal system is believed to stabilize nanoparticles
mainly by steric stabilization, and the surfaces of nanoparticles can
be exposed for catalytic reactions due to the weak interactions
[19]. Thus, the current Au–Ag alloy colloidal system in P123
aqueous solution should be the most desirable for studying the
oxidation kinetics of alcohols with molecular oxygen and the
catalytic effect of Ag in the Au–Ag alloy nanocatalysts.
Catalyst
Diameter by TEM
(nm)
Diameter by XRD
(nm)
Au/Ag ratio by
EDX
Au
3.2 0.5
3.1 0.5
3.2 0.6
3.5 0.5
3.8 0.9
4.0 0.7
3.2 0.8
3.4 0.9
10.2 3.5
3.6
4.3
4.4
4.3
4.3
4.4
4.1
4.0
9.2
1::0
–
Au0.99Ag0.01
Au0.98Ag0.02
Au0.95Ag0.05
Au0.90Ag0.10
Au0.85Ag0.15
Au0.80Ag0.20
Au0.50Ag0.50
Ag
0.98::0.02
0.94::0.06
0.89::0.11
0.84::0.16
0.78::0.22
0.47::0.53
0::1
For bimetallic core–shell particles, the surface layer should con-
sist of exclusively one kind of metal atoms, with the core part con-
sisting of the other kind of metal atoms. As illustrated in Table 2,
the Ag/Au molar compositions analyzed by XPS increase with the
increase in the Ag/Au molar ratio, further indicating the formation
of Au–Ag alloy nanoparticles rather than the core–shell structure.
However, the Ag/Au molar ratios are always higher than that ex-
pected from the nominal compositions in the P123 solutions, indi-
cating the surface segregation of Ag atoms on the Au–Ag alloy
particles. For example, the Au–Ag alloy particles with the total
Ag/Au molar ratio of 0.10::0.90 reach a surface Ag/Au molar ratio
of 0.18::0.82, also meaning that all Au and Ag atoms can be in inti-
mate contact with each other, and every Ag atom is formed by the
most compact arrangement with the highest possible coordination
number of 6 Au atoms on the surface of Au–Ag alloy particles. This
is in agreement with the changes of Au4f and Ag3d BEs. The surface
segregation is a natural phenomenon in bimetallic alloys just like
Pd–Cu [58], Pt–Au [59], and Pd–Ag [60], which may have a signif-
icant influence on the surface properties of metal alloys.
3.6. Oxidation reaction of benzyl alcohol
3.6.1. The activity of the Au–Ag nanocatalysts
For the purpose of investigating the catalytic effect of Ag in
Au–Ag alloy nanocatalysts for alcohol oxidation, aerobic oxidation
of benzyl alcohol was used as a model reaction. It was reported
that Au nanocatalysts in P123 aqueous solution could show
excellent activity without added base [34]. In order to accurately
measure the oxidation kinetics and to understand the intrinsic
chemical properties of the active sites, the pH value of the colloids
was adjusted to 7.0 to avoid the effect of aqueous solution basicity
before the reaction.

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X. Huang et al. / Journal of Catalysis 301 (2013) 217–226
Oxidation of benzyl alcohol was first tested in P123 aqueous
(a)
Au 4f7/2
solution without metal particles. In all cases, no conversion of ben-
zyl alcohol was observed. In contrast, both Au and Au–Ag nanopar-
ticles showed catalytic activity for oxidation of benzyl alcohol in
P123 aqueous solutions with molecular oxygen as oxidizing agent,
indicating that Au and Au–Ag nanoparticles were active compo-
nents. Whether in air or under 1 atm of pure O2, when the gas flow
rate in the reaction procedure was above 3 mL/min, the activity of
the nanocatalysts did not change with the increase of the flow rate,
demonstrating that the reaction was not retarded by the concen-
tration of O2 dissolved in water. In addition, [P123]/[metal] molar
ratios in the range of 5–20 were found to have little effect on the
activity of the catalysts (Supplementary information, Fig. S2).
Therefore, the flow rate of O2 and the [metal]/[P123] molar ratio
were kept at 30 mL/min and 1::10, respectively, in the following
experiments.
Au 4f5/2
0.15 : 0.85
0.10 : 0.90
0.05 : 0.95
0.02 : 0.98
0 : 1
82
84
86
88
Binding energy (eV)
(b)Ag 3d
5/2
Ag 3d3/2
We investigated the effect of Ag content on the catalytic activity
of the Au–Ag alloy nanocatalysts with various Ag/Au molar ratios
for the oxidation of benzyl alcohol with O2. The conversion–time
curves of pure Au, Ag, and representative Au0.90Ag0.10 alloy nano-
particles in P123 aqueous solution at 313 K are presented in
Fig. 6. The colloidal Ag nanoparticles having an average diameter
of 10.2 nm show negligible catalytic activity. In contrast, pure Au
and Au–Ag alloy nanoparticles with comparable particle sizes of
3–4 nm are active in the initial 30 min. During this period, the
selectivity to benzaldehyde reaches 100%, and no other oxidation
products such as benzoic acid and benzyl benzoate are detected
within our detection limit. As the reaction time is prolonged, trace
amounts of benzoic acid are detected in the products, resulting in
gradual deactivation due to its strong adsorption onto the surfaces
of metal particles [35]. Table 3 lists the initial conversions of benzyl
alcohol (<3 mol%) within the first 2-min reaction, where the cata-
lytic reactions are governed by the chemical kinetics and the corre-
sponding turnover frequency (TOF, number of converted molecules
per surface metal atom and hour) (see the Supporting informa-
tion). The presence of Ag can significantly enhance the oxidation
rate of benzyl alcohol. When the Ag/Au molar ratio is 0.10::0.90,
TOF reaches the maximal value (216 h–1), which is ca. four times
higher than that of pure Au nanoparticles (53 h–1). As the Ag/Au
molar ratio is further increased, the oxidation rate of benzyl alco-
hol starts to decline, likely due to the decrease in Au active sites
on the surface of the alloy particles. On the other hand, as shown
in Tables 2 and 3, the decrease in the electron density of Ag atoms
due to the coordination environments of Ag atoms may also affect
the catalytic activity of nanoparticles when the Ag/Ag molar ratio is
higher than 0.10::0.90 in the feed solution. The reproducibility of
the reactions was performed on three-group colloidal nanocata-
lysts that had been independently prepared. So, the present mea-
surements are focused on the oxidation of benzyl alcohol on the
Au–Ag nanoparticles with varying Ag/Au molar ratio from 0::1 to
0.15::0.85. After the reaction ran for 8 h, the used colloids showed
no visible change in color, and the particle sizes were still kept
unchanged, as shown in Fig. 4d for the UV–vis spectrum and in
Fig. 5c for the TEM image and the particle size distribution of the
Au0.90Ag0.10 alloy nanoparticles.
1 : 0
0.15 : 0.85
0.10 : 0.90
0.05 : 0.95
0.02 : 0.98
366
368
370
372
374
376
Binding energy (eV)
Fig. 3. XPS spectra of (a) Au4f and (b) Ag3d regions for the Au1ꢁxAgx nanoparticles
with different Ag/Au molar ratios.
Table 2
Positions of XPS peaks and surface compositions for Au1ꢁxAgx nanoparticles.
Catalyst
Au/Ag ratio by XPS
Binding energy (eV)
Au
Ag
4f7/2
4f5/2
3d5/2
3d3/2
Au
1::0
83.4
83.6
83.8
84.0
83.8
–
87.1
87.2
87.4
87.7
87.3
–
–
–
Au0.98Ag0.02
Au0.95Ag0.05
Au0.90Ag0.10
Au0.85Ag0.15
Ag
0.95::0.05
0.90::0.10
0.82::0.18
0.77::0.23
0::1
367.4
367.6
367.9
367.6
368.0
373.3
373.6
373.8
373.6
374.0
(e)
(d)
(c)
3.6.2. Kinetics of aerobic oxidation of benzyl alcohol
(b)
(a)
To better measure the kinetics of oxidation of benzyl alcohol
with molecular oxygen on the Au–Ag nanocatalysts, Au0.90Ag0.10
alloy colloids with different metal concentrations in the range of
10–4–5 ꢀ 10–4 M were used for reactions at the same concentration
of benzyl alcohol (5 ꢀ 10–2 M). A linear correlation between the
metal concentration and the initial catalytic activity is observed
(Fig. 7). This result indicates that the initial oxidation rate is
controlled only by chemical reactions rather than by a diffusive
mass transport process.
300
400
500
600
700
Wavelength (nm)
Fig. 4. UV–vis absorption spectra of the Au0.90Ag0.10 nanoparticles: (a) freshly
prepared, (b) stored at room temperature for 6 months, (c) treated at 353 K for 8 h,
(d) after reaction at 313 K for 8 h in the absence of Na2CO3, and (e) after reaction at
313 K for 8 h in the presence of Na2CO3.

X. Huang et al. / Journal of Catalysis 301 (2013) 217–226
223
(a)
(b)
4.0 0.8 nm
3.9 0.6 nm
0 1 2 3 4 5 6 7 8
P.S. (nm)
0 1 2 3 4 5 6 7 8
P.S. (nm)
(c)
(d)
3.9 0.8 nm
3.8 0.6 nm
0 1 2 3 4 5 6 7 8
P.S. (nm)
0 1 2 3 4 5 6 7 8
P.S. (nm)
Fig. 5. TEM images and particle size distributions of the Au0.90Ag0.10 nanoparticles: (a) stored at room temperature for 6 months, (b) treated at 353 K for 8 h, (c) after reaction
at 313 K for 8 h in the absence of Na2CO3, and (d) after reaction at 313 K for 8 h in the presence of Na2CO3.
30
25
20
15
10
5
3
2
1
0
Ag0.90Ag0.10
Au
Ag
0
0
60 120 180 240 300 360
Time (min)
0
1
2
3
4
5
[Metal] × 10−4 M
Fig. 6. The conversion–time curves for aerobic oxidation of benzyl alcohol over pure
Au, Ag, and Au0.90Ag0.10 nanoparticles. Reaction conditions: [metal], 5 ꢀ 10–4 M;
[benzyl alcohol], 5 ꢀ 10–2 M; reaction temperature, 313 K.
Fig. 7. Dependence of the metal molar concentration on the initial conversion of
benzyl alcohol. Reaction conditions: [benzyl alcohol], 5 ꢀ 10–2 M; reaction temper-
ature, 313 K; reaction time, 2.0 min.
Table 3
initial reaction rates as a function of temperature in the range
303–323 K, shown in Fig. 8. Table 3 lists the Ea values obtained
by least-squares analysis. Surprisingly, pure Au nanoparticles have
the lowest Ea of 45 kJ/mol, while Au–Ag alloy nanoparticles with
low Ag content, Au0.99Ag0.01 and Au0.98Ag0.02, exhibit higher Ea of
55 kJ/mol and 68 kJ/mol, respectively. When the Ag/Au molar ratio
gets to 0.05:0.95 or higher, the Ea values are kept at 74 kJ/mol. That
is, Ea shows an obvious increasing trend with the increase in the
oxidation rate. This strongly suggests that there are different reac-
tion mechanisms operating in their catalytic processes.
To elucidate this finding, further kinetics measurements were
performed for aerobic oxidation of benzyl alcohol. Taking into ac-
count that a large excess of molecular oxygen was provided in
the colloidal solutions, it was assumed that the concentration of
molecular oxygen dissolved in solutions was kept constant in the
reaction process; namely, the initial oxidation rate of benzyl
The catalytic activity of Au1ꢁxAgx for aerobic benzyl alcohol oxidation.
Catalyst
Conversion (%)a
TOF (h–1
)
Ea (kJ/mol)
n
Au
0.8
1.3
1.5
1.9
2.8
2.4
0.6
53
84
45
55
68
74
74
74
–
1.5
0.9
0.8
0.5
0.5
0.5
–
Au0.99Ag0.01
Au0.98Ag0.02
Au0.95Ag0.05
Au0.90Ag0.10
Au0.85Ag0.15
Au0.80Ag0.20
100
137
216
193
40
a
Reaction conditions: [metal], 5 ꢀ 10ꢁ4 M; [benzyl alcohol], 5 ꢀ 10–2 M; reaction
temperature, 313 K; reaction time, 2.0 min.
The apparent activation energies (Ea) of the Au–Ag alloy nano-
catalysts having comparable particle sizes for aerobic oxidation
of benzyl alcohol were determined from the Arrhenius plot of the

224
X. Huang et al. / Journal of Catalysis 301 (2013) 217–226
Ag atoms produces a synergistic effect, resulting in different reac-
Au0.85Ag0.15
Au0.90Ag0.10
Au0.95Ag0.05
Au0.98Ag0.02
Au0.99Ag0.01
Au
tion mechanisms from pure Au nanoparticles. The apparent orders
of reaction kinetics decrease with the increase in the Ag/Au molar
ratio as shown in Table 3. From the data determined by XPS in Ta-
ble 2, when the Ag/Au molar ratio in the feeding solution is in-
creased to 0.10:0.90, so that almost every Ag atom is coordinated
by six Au atoms with the most compact arrangement of metal
atoms on the surface of Au–Ag alloy particles, the reaction rate
reaches the maximum, and at this time, the oxidation of benzyl
alcohol obeys a 0.5-order reaction mechanism. We suppose that
there is a chemical reaction kinetic process of simultaneous 1.5-or-
der and 0.5-order reactions with respect to benzyl alcohol on the
surface of the Au–Ag alloy nanoparticles. As illustrated in Scheme 1,
the doping of Ag only affects the catalytic properties of Au active
sites adjacent to Ag atoms on the surface, on which the oxidation
of benzyl alcohol can be described by means of an apparent 0.5-or-
der reaction, while on other Au active sites not next to Ag atoms,
the same 1.5-order reaction mechanism is still kept as on the pure
Au nanoparticles.
Model studies on single-crystal Au showed that atomic oxygen
was the key species that promoted a range of selective oxidation
transformations. Its presence activated Au surfaces for reactions
with organic species [18,61]. The previous study showed that the
activity of Au nanoparticles in P123 aqueous solution could be
improved in the presence of base. It is reasonable to assume that
both the deprotonation of the alcohol and the formation of atomic
oxygen may be involved in the rate-determining step of the
Au-catalyzed reaction for aerobic oxidation of alcohols in P123
aqueous solution. As shown in Tables 2 and 3, there is a correlation
between the catalytic activity and charge state of Au atoms in the
Au–Ag alloy particles. The electron transfer toward Ag atoms
makes Au atoms electron-poorer, enhancing the electron-accepting
ability from the adsorbed alcohol species. As a result, the deproto-
nation rate of organic species is increased.
-6
-7
-8
-9
3.10
3.15
3.20
1000/T (K-1)
3.25
3.30
Fig. 8. The initial reaction rates as a function of temperature for oxidation of benzyl
alcohol over the Au1ꢁxAgx nanoparticles. Reaction conditions: [metal], 5 ꢀ 10–4 M;
[benzyl alcohol], 5 ꢀ 10–2 M; reaction time, 2.0 min.
alcohol could be expressed as ꢁdC0=dt ¼ kCn, where k is the
0
pseudo rate constant, C0 is the initial concentration of benzyl alco-
hol in M, and n is the reaction order with respect to benzyl alcohol.
We determined that the correlation between ln(ꢁdC0/dt) and lnC0
was in the C0 range of 0.25 ꢀ 10–1–1.25 ꢀ 10–1 M, as shown in
Fig. 9. The slopes (n) obtained from the ln(ꢁdC0/dt) ꢂ ln C0 plots
are summarized in Table 3. It can be seen that different reaction
orders are observed for the Au–Ag alloy nanoparticles with respect
to benzyl alcohol: 1.5 for pure Au nanoparticles, 0.9 and 0.8 for
Au0.99Ag0.01 and Au0.98Ag0.02, respectively, and 0.5 for Au0.95Ag0.05
,
Au0.90Ag0.10, and Au0.85Ag0.05, which match well with the changes
in Ea. The reactions for each catalyst were repeated more than five
times and confirmed the reproducibility of the above results. It is
reasonable to propose that there are two kinds of active sites on
the surface of the Au–Ag nanoparticles (Scheme 1), namely Au
atoms and Au atoms in combination with Ag, which exhibit
reaction orders of 1.5 and 0.5, respectively. When Ag content is
lower in the Au–Ag particles, two reaction mechanisms are
separately performed on the respective active sites.
It is known that O2 dissociation is considerably more facile at Ag
atoms than at Au atoms [62]. This means that even a single Ag
atom can enhance the reactivity of gold atoms in proportion to
the Ag/Au atomic ratio. For the P123-stabilized Au–Ag particles,
Au atoms transfer electrons toward the neighboring Ag atoms, fur-
ther activating molecular oxygen to be dissociated into atomic
oxygen species by donating an excess electronic charge to the anti-
bonding orbital for the oxidation of organic species on Au sites.
This synergistic effect between Au atoms and neighboring Ag
atoms results in the change of the oxidation mechanism of benzyl
alcohol.
3.6.3. Roles of Ag in Au–Ag nanoparticles
From the experimental results, oxidation of benzyl alcohol can
be performed on pure Au nanoparticles in neutral P123 aqueous
solution with molecular oxygen. Although Ag nanoparticles show
no activity for oxidation of benzyl alcohol, the proper doping of
Ag in Au nanoparticles can significantly enhance the oxidation rate
of benzyl alcohol. The alloying of Au with Ag is the main cause of
the enhanced activities for the aerobic oxidation of benzyl alcohol
on the Au–Ag nanoparticles. The charge transfer between Au and
3.6.4. The performance of Au–Ag nanoparticles in the presence of
Na2CO3
Although the Au–Ag nanocatalysts in P123 aqueous solution
show suitable activity for the kinetic study of the aerobic oxidation
of benzyl alcohol under neutral conditions, the quick deactivation
of the Au–Ag nanoparticles is still a big challenge for practical
applications, due to adsorption of benzoic acid. It has been re-
ported that the presence of Na2CO3 could significantly improve
the catalytic activity of Au nanoparticles in P123 aqueous solution
for oxidation of alcohols with O2 [34]. In order to achieve a satisfac-
torily high conversion rate and excellent stability, we also exam-
ined the catalytic performance of colloidal Au–Ag nanocatalysts
at 313 K in the presence of Na2CO3, and the reaction results of
the representative Au0.90Ag0.10 alloy catalysts are displayed in
Fig. 10. It is found that the addition of Na2CO3 in aqueous solution
can also significantly enhance the catalytic activity of the Au–Ag
alloy nanocatalyst for aerobic oxidation of alcohols. The initial
TOF is calculated as 543 h–1, which is more than 2.5 times that
(216 h–1) under neutral conditions (pH 7.0). To the best of
our knowledge, such a high level of catalytic activity for aerobic
-3.0
-3.5
Au0.85Ag0.15
Au0.90Ag0.10
Au0.95Ag0.05
Au0.98Ag0.02
Au0.99Ag0.01
Au
-4.0
-4.5
-1.6
-1.4
-1.2
-1.0
-0.8
lg C0
Fig. 9. The initial reaction rates versus initial benzyl alcohol concentration plots for
oxidation of benzyl alcohol over the Au1ꢁxAgx nanoparticles. Reaction conditions:
[metal], 5 ꢀ 10–4 M; reaction temperature, 313 K; reaction time, 2.0 min.

X. Huang et al. / Journal of Catalysis 301 (2013) 217–226
225
Scheme 1. Proposed aerobic oxidation kinetics of benzyl alcohol on the surface of Au–Ag alloy nanoparticles.
oxidation has never been reported previously over Au or Au–Ag
nanocatalysts stabilized in solution. More importantly, the conver-
sion of benzyl alcohol increases with increasing reaction period,
and finally almost all benzyl alcohol is converted to benzaldehyde,
benzoic acid, and benzyl benzoate, without deactivation from sur-
face poisoning by the resulting carboxylic acid [35]. However, the
presence of Na2CO3 also promotes the formed benzaldehyde to
be further oxidized to benzoic acid and benzyl benzoate, resulting
in a decrease in the selectivity to benzaldehyde, which is similar to
that for pure Au nanoparticles in P123 aqueous solution for aerobic
oxidation of alcohols [34]. After the reaction, the colloidal Au–Ag
nanoparticles showed no visible change in color and had no obser-
vable precipitation. The used Au0.90Ag0.10 alloy nanoparticles for
the aerobic oxidation of alcohols in the presence of Na2CO3 showed
almost the same UV–vis spectrum (Fig. 4e) and TEM image (Fig. 5d)
as freshly prepared nanoparticles. This indicates that the colloidal
Au–Ag alloy nanoparticles are highly stable for the aerobic oxida-
tion of alcohols in the presence of base. It was reported that pure
Au nanoparticles in P123 solution during the reaction procedure
had aggregation [34]. This demonstrates that the alloying of Au
with Ag not only greatly enhances the catalytic activity but also
improve the stability of Au nanoparticles. Organic products can
be separated easily from the acidified Au–Ag nanoparticle colloidal
in P123 aqueous solution using ethyl acetate or diethyl ether. This
makes Au–Ag nanoparticles in P123 aqueous solution promising
for practical catalytic applications.
4. Conclusions
In summary, we successfully prepared a series of Au–Ag alloy
nanoparticles with comparable particle sizes (3–4 nm) in P123
aqueous solution. The prepared alloy nanoparticles show catalytic
activity for aerobic oxidation of benzyl alcohol in the absence of
base, which is highly desirable for investigating the kinetics of oxi-
dation of benzyl alcohol with molecular oxygen. The presence of Ag
(Ag/Au molar ratio 60.10:0.90) significantly enhances the catalytic
activity of Au–Ag nanoparticles in P123 aqueous solution. XPS
analyses reveal that Au atoms transfer electrons toward neighbor-
ing Ag atoms, resulting in different reaction mechanisms and
different kinetics for aerobic oxidation of benzyl alcohol from those
for pure Au nanocatalyst. On the active sites with Au alone the
oxidation of benzyl alcohol follows 1.5-order reaction kinetics,
while on Au active sites adjacent to Ag atoms a 0.5-order reaction
is observed with respect to benzyl alcohol. Besides, the presence of
Na2CO3 significantly improves the catalytic activity and stability of
the Au–Ag alloy nanocatalyst for aerobic oxidation of alcohols. We
believe that this research should be useful for providing new
insights into the nature of Au nanocatalysts and the interaction
between Au and Ag atoms for aerobic oxidation of benzyl alcohols.
Acknowledgment
100
80
60
40
20
0
80
60
40
20
0
This research was supported by the Specialized Research Fund
for the Doctoral Program of Higher Education of China and National
Natural Science Foundation of China (No. 51225401).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2013.02.011.
0
100
200
300
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