Monodispersed AuPd nanoalloy: Composition control synthesis and catalytic properties in the oxidative dehydrogenative coupling of aniline

Article abstract of DOI:10.1007/s11426-015-5358-1

A series of AuPd@C nanoalloy catalysts with tunable compositions were successfully prepared by a co-reduction method. The use of borane-tert-butylamine complex as reductant and oleylamine as both solvent and reductant was very effective for the preparation of the monodispersed nanoalloy. We evaluated the catalytic activity of these AuPd@C nanoalloys for oxidative dehydrogenative coupling of aniline, which showed better catalytic activity than equal amounts of sole Au@C or Pd@C catalyst. The Au₁Pd₃@C catalyst exhibited the best performance, indicating that the conversion and selectivity were improved along with the increase of Pd composition. However if the Pd composition was too high in the AuPd alloy, Au₁Pd₇@C achieved only 81% conversion in this reaction.

Full text of DOI:10.1007/s11426-015-5358-1

SCIENCE CHINA
Chemistry
January 2015 Vol.58 No.1: 1–5
doi: 10.1007/s11426-015-5358-1
• ARTICLES •
doi: 10.1007/s11426-015-5358-1
Monodispersed AuPd nanoalloy: composition control synthesis and
catalytic properties in the oxidative dehydrogenative coupling of
aniline
Fangyu Fu, Sen He, Sha Yang, Chen Wang, Xun Zhang, Peng Li,
Hongting Sheng* & Manzhou Zhu*
Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui University, Hefei 230601, China
Received November 14, 2014; accepted December 15, 2014
A series of AuPd@C nanoalloy catalysts with tunable compositions were successfully prepared by a co-reduction method. The
use of borane-tert-butylamine complex as reductant and oleylamine as both solvent and reductant was very effective for the
preparation of the monodispersed nanoalloy. We evaluated the catalytic activity of these AuPd@C nanoalloys for oxidative
dehydrogenative coupling of aniline, which showed better catalytic activity than equal amounts of sole Au@C or Pd@C cata-
lyst. The Au1Pd3@C catalyst exhibited the best performance, indicating that the conversion and selectivity were improved
along with the increase of Pd composition. However if the Pd composition was too high in the AuPd alloy, Au1Pd7@C
achieved only 81% conversion in this reaction.
AuPd, alloy, catalyst, oxidative coupling
than NPs of the parent metals [16–19]. Of these, the AuPd
1 Introduction
bimetallic system, including core-shell, alloy, etc., has
drawn wide attention in the fields of surface-enhanced Ra-
man scattering (SERS), DAFCs electrocatalysis, organic
catalysis, and so forth [20–26]. For example, Sakurai and
coworkers [27] demonstrated a simple and versatile method
for the preparation of bimetallic AuPd nanoclusters with
controlled atomic gold distributions and stabilization by
poly(vinyl-2-pyrrolidone), which are highly active and se-
lective catalysts for the dehydrogenative aromatization of
tetralin into naphthalene. In addition, Chen et al. [28] re-
ported geometrically controlled nanoporous AuPd bimetal-
lic catalysts with higher catalytic activity for direct ethanol
fuel cells.
Bimetallic nanoparticles (NPs) have received great attention
for their magnetic, optical, and catalytic properties [1–8].
Compared to monometallic nanocrystals, bimetallic NPs
ranging from core-shell structures to alloy have excellent
physical and chemical properties because of the different
geometric and electronic structures caused by the introduc-
tion of foreign metals [9–13]. The bimetallic NPs have un-
usual functionality due to the synergistic effects between
two different metals. Notable breakthroughs have been
made toward synthesizing well-defined NPs to elucidate the
underlying relationship between catalytic activity and cata-
lyst properties such as size, shape and composition [14,15].
Pt-based or Pd-based bimetallic nanostructures used as
catalysts often show better selectivity, activity, and stability
In this work, we employed a co-reduction method to
synthesize a series of AuPd alloy NPs with tunable compo-
sitions by using borane-tert-butylamine as reductant and
oleylamine (OA) as both solvent and reductant. These alloys
were supported on the activated carbon, which is notable
*Corresponding authors (email: sht_anda@126.com; zmz@ahu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2015
chem.scichina.com link.springer.com

2
Fu et al. Sci China Chem January (2015) Vol.58 No.1
because it showed that these AuPd@C nanoalloy catalysts
are highly active as catalysts for the oxidative dehydrogena-
tive coupling of aniline.
mmol), KOH (0.1229 g, 2.19 mmol), catalyst (1% based on
initial aniline) and 5 mL DMSO. After stirring at 333 K for
12 h under the O2 balloon, the reaction was completed. The
reaction was tracked by TLC analysis. The mixture was
rotaevaporated to dryness under vacuum and purified by
silica-gel column chromatography. The product was ana-
lyzed by GC.
2 Experimental
2.1 Materials and instruments
3 Results and discussion
All of the chemicals and reagents are commercially availa-
ble and were used as received. Thin-plate chromatography
(TLC) used commercial Merck Silica Gel 60 F254 (USA)
for analytical TLC Merck Kieselgel 200-300 (USA) was
used in preparative column chromatography. 1H NMR
spectra were acquired on a Brucker AM 400 (Switzerland)
operating at 400 MHz (solvent: CDCl3). 13C NMR spectra
were acquired on a Bruker AM 400 (Switzerland) operating
at 100 MHz (solvent: CDCl3). The yield and selectivity of
product were performed on GC 2010 Plus from Shimadu
(Japan). Transmission electron microscopy (TEM) images
were obtained using a JEM 2100 microscope (Japan) and
high-resolution transmission electron microscopy (HRTEM)
was assessed using a JEOL-2010F instrument (Japan).
Powder X-ray diffraction (XRD) patterns were collected on
an MXP18AHF diffractometer (China) with Cu-K radia-
tion (=1.54178 Å). Elemental analysis was carried out by
inductively coupled plasma-atomic emission spectrometry
(ICP-AES) using an Atomscan Advantage instrument made
by Thormo Jarrell Ash Corporation (USA).
3.1 Synthesis and characterization of AuPd alloy NPs
A co-reduction method was employed to prepare AuPd al-
loy NPs. It is known that the use of amine-borane complex
as reductant is very effective for the preparation of mono-
dispersable NPs. For example, Li and coworkers [29] re-
ported that uniform PdAg NPs could be synthesized by
borane-tert-butylamine complex. Sun et al. [30] reported
that monodisperse Pd NPs could be synthesized by tribu-
tylamine-borane complex. The compositions of AuPd alloy
NPs were controlled by varying the molar ratios of the pre-
cursors. Therefore, thanks to the use of TBAB as an effec-
tive reductant, monodisperse NPs could be prepared under
our experimental conditions. Figure 1 shows the representa-
tive TEM image of Au1Pd3 with a spherical shape, which is
monodispersed particle of narrow size distribution (average
diameter around 4.75 nm). Moreover, the broader applica-
bility of this strategy for ultrasmall and uniform alloy was
validated by the analogous preparation of AuPd alloy NPs
with different compositions. By adjusting the Au-to-Pd mo-
lar ratios, a series of ultrasmall and uniform AuPd alloy NPs
were achieved: these were also characterized by TEM (Fig-
ure 2). Detailed characterization of the as-obtained AuPd
alloy NPs shows that they are uniform in size with an aver-
age diameter of ca. 4 nm. The HRTEM images inserted in
Figures 1 and 2 exhibit the well-defined crystalline struc-
tures of the AuPd alloy NPs.
Evidence of the formation of AuPd alloy NPs is shown in
the XRD patterns (Figure 3). The diffraction peaks are
composed of the standard peaks between Au (JCPDS No.
65-8601) and Pd (JCPDS No. 65-6174). In other words, it is
noticeable that the 2 values of AuPd NPs lie between those
of pure Au and Pd, which demonstrates the formation of
single-phase AuPd alloy NPs. The peaks are noted to con-
tinuously shift from the Au standard peak to those of Pd
when the content of Pd was increased. Importantly, the ex-
act atomic ratios in the as-prepared AuPd alloy NPs were
further determined by ICP-AES, which confirmed that the
exact Au/Pd ratios in the as-obtained bimetallic NPs fol-
lowed the trend of the designed compositions.
2.2 Preparation of AuPd NPs
PdCl2 (8.7 mg, 0.049 mmol) and HAuCl4 (0.098 mL, 0.049
mmol) with 1:1 molar ratio were dissolved in oleylamine (5
mL) and heated to 333 K. Next, a solution of borane-tert-
butylamine (100 mg, 1.15 mmol) in oleylamine (1 mL) was
added quickly when the solution color turned to black, after
which it was heated to 363 K and kept for 30 min before
being cooled down to room temperature. The crude prod-
ucts were washed by ethanol followed by centrifugation. A
similar synthetic process was used for a different Pd/Au
ratio.
2.3 Preparation of immobilized catalyst
The AuPd NPs and activated carbon with mass ratio 5:100
were dispersed in hexane (5 mL), ethanol (15 mL) as well
as glacial acetic acid (1 mL) and stirred for 1 h, then filtrat-
ed with washing by deionized water and dried at 353 K to
prepare the AuPd@C catalyst.
2.4 Typical procedure for the oxidative dehydrogena-
tive coupling of aniline
3.2 Characterization of immobilized AuPd@C catalyst
The sol-immobilisation method permitted the synthesis of
A Schlenk flask was charged with aniline (0.2 mL, 2.19

Fu et al. Sci China Chem January (2015) Vol.58 No.1
3
Figure 1 TEM image of the as-prepared Au1Pd3 nanoalloy.
Figure 2 TEM images of the as-prepared samples. (a) Au; (b) Au3Pd1; (c) Au2Pd1; (d) Au1Pd1; (e) Au1Pd2; (f) Pd; (g) Au1Pd7.
NPs that could subsequently be supported on activated car-
bon. AuPd alloy NPs were prepared and immobilized onto
carbon and dried at 353 K. The evaluation of the particle-
size distributions of the colloidal metal NPs, after immobi-
lisation on the support, is displayed in Figure 4, the typical
Au3Pd1 and Au1Pd3 were examined by TEM. The particle
sizes of the Au3Pd1 and Au1Pd3 were about 4 nm on the
carbon supports, and the range of particle diameters was
between 3 and 6 nm in diameter. It is interesting to note that
the AuPd particles supported on carbon retained the same
spherical shape and particle size as found in the starting
AuPd alloy NPs (Figure 1). Furthermore, those are uni-
formly distributed on the carbon.
However, when O2 was used as the oxidant and reacted at
333 K, its conversion reached 55%. It is worth mentioning
that the selectivity was very high when the reaction was
catalyzed by 5% Au1Pd1@C, but low when the reaction was
catalyzed by unsupported Au1Pd1 nanoalloy. This result
indicates that the activated carbon plays a significant role in
this reaction.
We chose this optimized reaction condition to study the
synergistic effects between Au and Pd. As shown in Table 2,
compared to the Au1Pd1@C catalyst, with the increase of
Au composition, the conversion rose from 55% to 83%
(Au3Pd1@C). However, the selectivity dramatically decrea-
sed to 36% (the byproduct is (Z)-1,2-diphenyldiazene oxide).
Au@C represented high selectivity (95%), which shows
that Au is an effect catalyst for this reaction [32]. Although
there was a dramatic rise in conversion, from 55% to 98%
(Au1Pd3@C), with the increase of Pd composition the selec-
tivity remained at a high level (99%). This phenomenon
indicated that in this alloy system, when Au was dominant,
a small mount of Pd triggered the reduction of selectivity. In
stark contrast, when the Pd was occupied the dominant sta-
tus in the alloy, the Pd would be conducted to catalyzing
this reaction. To our great delight, the obtained Au1Pd3@C
catalyst exhibited the best catalytic activity. However if the
Pd composition was too high in the AuPd alloy, Au1Pd7@C
achieved only 81% conversion in this reaction. Au1Pd3@C
was also better than the supported pure Au or Pd catalysis,
which indicated that Au1Pd3 alloy NPs present excellent
synergistic effects between two different metals.
3.3 Catalytic tests for oxidative dehydrogenative cou-
pling of aniline
The supported AuPd NPs were investigated for the oxida-
tive dehydrogenative coupling of aniline to produce the az-
obenzene. Li et al. [31] reported that various monodispersed
metal NPs could efficiently activate molecular oxygen un-
der mild conditions, as illustrated by the aerobic oxidation
of anilines to form either symmetric or asymmetric aromatic
azo compounds. The effects of reaction temperature and O2
were optimized by the experiment catalyzed by Au1Pd1@C,
in terms of catalyst activity and selectivity (Table 1). The
reaction temperature and O2 affected their catalytic perfor-
mance. If the reaction was conducted at room temperature
and used air as the oxidant, the catalytic activity was low.

4
Fu et al. Sci China Chem January (2015) Vol.58 No.1
Table 1 Catalytic activity of the oxidative dehydrogenative coupling of
aniline with 5% Au1Pd1@C a)
Condition
air, r.t.
Conversion (%)
Selectivity (%)
trace
99
O2, r.t.
20
96
air, 333 K
O2, 333 K
O2, 333 K b)
O2, 333 K c)
17
55
98
98
18
95
no reaction
no reaction
a) Reaction conditions: 0.2 mL aniline, catalyst loading (1% based on
initial aniline), 0.1229 g KOH and 5 mL DMSO. The conversion and se-
lectivity are performed by GC; b) catalyzed by unsupported Au1Pd1
nanoalloy; c) without KOH.
Table 2 Catalytic activity of oxidative dehydrogenative coupling of
aniline with different catalysts a)
Catalyst
Au@C
ICP

Conversion (%)
Selectivity (%)
75
83
72
55
91
98
81
80
95
36
80
98
95
99
90
90
Au3Pd1@C
Au2Pd1@C
Au1Pd1@C
Au1Pd2@C
Au1Pd3@C
Au1Pd7@C
Pd@C
6.8:1
4.2:1
1.3:1
1:2.0
1:3.1
1:10.2

a) Reaction conditions: 0.2 mL aniline, catalyst loading (1% based on
initial aniline), 0.1229 g KOH, and 5 mL DMSO. The conversion and
selectivity were performed by GC.
3.4.2 (Z)-1,2-Diphenyldiazene oxide
Figure 3 XRD patterns of the as-prepared samples. (a) Au; (b) Au3Pd1;
(c) Au2Pd1; (d) Au1Pd1; (e) Au1Pd2; (f) Au1Pd3; (g) Au1Pd7; (h) Pd.
1H NMR (400 MHz, CDCl3) δ (ppm): 7.38 (t, 3J=7.36 Hz,
1H, CH), 7.52 (m, 5H, CH), 8.17 (m, 2H, CH), 8.30 (m, 2H,
CH); 13C NMR (100 MHz, CDCl3) δ (ppm): 144.04, 131.58,
129.60, 128.80, 128.70, 125.53, 122.36.
4 Conclusions
We demonstrated a co-reduction method for the synthesis of
monodispersed AuPd NPs with tunable compositions. The
well-defined crystalline structures and XRD patterns prove
that these AuPd NPs are alloy. Significantly, thanks to the
effect of palladium substitution, the as-prepared AuPd@C
catalysts demonstrated superior catalytic properties for the
oxidative dehydrogenative coupling of aniline to produce
the azobenzene. For the as-prepared AuPd NPs, the ob-
tained heteroatom bonds changed the electronic environ-
ment of the metal surface and the modification of its elec-
tronic structure contributed the excellent catalytic activity.
This method could be extended to the synthesis of other
metal-alloy catalysts in the foreseeable future.
Figure 4 TEM images of 5% Au3Pd1@C catalyst (a) and 5% Au1Pd3@C
catalyst (b).
3.4 Spectroscopic characterization of products
3.4.1 Azobenzene
1H NMR (400 MHz, CDCl3) δ (ppm): 7.93 (d, J=7.3 Hz,
4H), 7.58 (m, 6H); 13C NMR (101 MHz, CDCl3) δ (ppm):
152.66, 131.31, 129.16, 122.86.

Fu et al. Sci China Chem January (2015) Vol.58 No.1
5
This work was supported by the National Natural Science Foundation of
China (21072001, 21372006), the Ministry of Education, the Ministry of
Human Resources and Social Security, the Education Department of Anhui
Province, the Anhui Province International Scientific and Technological
Cooperation Project, the Natural Science Foundation of Education De-
partment of Anhui Province (KJ2014A013), and the 211 Project of Anhui
University.
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