Monodisperse noble metal nanoparticles stabilized in SBA-15: Synthesis, characterization and application in microwave-assisted Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1016/j.jcat.2010.01.004

A series of Pd and Pd/Au alloy nanoparticles (NPs) with narrow size distribution were encapsulated or stabilized by G4-poly (amido-amine) (G4-PAMAM, G represents the generation) planted in SBA-15. Monodisperse NPs are uniformly distributed in channels of SBA-15 as observed by TEM images. HRTEM, ICP and EDS results confirm the alloy structure of bimetallic NPs. A combination of N₂ adsorption-desorption analysis, ICP and GC was used to optimize the best amount of metal precursors to form catalysts. The as-prepared catalyst manifests outstanding activity and recyclability in microwave-assisted Suzuki-Miyaura coupling reaction when compared with commercial Pd/C. Meanwhile, Pd/Au alloy NPs exhibit higher activity than monometallic ones in catalyzing the coupling between aryl bromide/aryl chloride and arylboronic acid. The mechanism of the improvement of catalytic performance is discussed.

Full text of DOI:10.1016/j.jcat.2010.01.004

Journal of Catalysis 270 (2010) 268–274
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Monodisperse noble metal nanoparticles stabilized in SBA-15: Synthesis,
characterization and application in microwave-assisted Suzuki–Miyaura
coupling reaction
Zhaoliang Zheng a,b, Hongfang Li a, Tianfu Liu a,b, Rong Cao a,
*
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou 350002, China
b Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Pd and Pd/Au alloy nanoparticles (NPs) with narrow size distribution were encapsulated or
stabilized by G4-poly (amido-amine) (G4-PAMAM, G represents the generation) planted in SBA-15.
Monodisperse NPs are uniformly distributed in channels of SBA-15 as observed by TEM images. HRTEM,
ICP and EDS results confirm the alloy structure of bimetallic NPs. A combination of N2 adsorption–desorp-
tion analysis, ICP and GC was used to optimize the best amount of metal precursors to form catalysts. The
as-prepared catalyst manifests outstanding activity and recyclability in microwave-assisted Suzuki–
Miyaura coupling reaction when compared with commercial Pd/C. Meanwhile, Pd/Au alloy NPs exhibit
higher activity than monometallic ones in catalyzing the coupling between aryl bromide/aryl chloride
and arylboronic acid. The mechanism of the improvement of catalytic performance is discussed.
Ó 2010 Elsevier Inc. All rights reserved.
Received 18 October 2009
Revised 2 January 2010
Accepted 4 January 2010
Available online 19 February 2010
Keywords:
Pd/Au alloy
Nanoparticles
Microwave-assisted
Suzuki–Miyaura coupling
1. Introduction
homo-coupling of aryl bromides, the same as the finding of Pittel-
kow et al. [19].
In the field of catalytic nanoparticles (NPs), the size dispersity,
morphology and composition of active metals are of importance
for the activity and selectivity of the catalysts [1–3]. The previously
reported capping agents and protective polymers, which were used
to stabilize NPs [4–6] have drawbacks in protecting metals under
harsh catalytic reaction conditions. Dendrimers can act as hosts
or stabilizers for either monometallic [7–11] or bimetallic [12–
15] NPs, interestingly, such NPs are monodisperse and unlikely to
aggregate after a long time, and consequently are competent to be-
have as catalysts. Scott et al. investigated catalytic activity of palla-
dium [13] dendrimer-encapsulated nanoparticles (DENs) in
hydrogenation reactions and found dendritic branches could adjust
and control the access of substrates into the dendrimers [16]. Li
and El-Sayed [17,18] also investigated the influence of dendrimer’s
generation on Pd colloidal NPs in Suzuki–Miyaura coupling reac-
tions. They concluded that the encapsulation of Pd particles pro-
vided by G4-poly (amido-amine) (G4-PAMAM, G represents the
generation) dendrimers led to a loss of catalytic activity, while
G3 did a better job both on catalysis and on stabilizing NPs. But
these types of catalysts had poor performance in catalyzing the
coupling between arylboronic acid and aryl bromides because of
Introducing another metal into pure Pd NPs is a promising
method to improve catalytic activity. In recent years, Pd/Au bime-
tallic system has been widely explored. For example, the bimetallic
NPs have shown excellent catalytic performance for the direct syn-
thesis of H2O2 from H2 and O2 [20–22]. In Hutchings’ group, Pd/Au
catalysts manifested a dramatic increase in activity for the oxida-
tion of alcohols to aldehydes when compared to pure Au or Pd cat-
alysts [23]. Additionally, the catalysts have been used in the
hydrochlorination of acetylene [24], hydrodesulfurization of thio-
phene [25] and other reactions [26–29]. To the best of our knowl-
edge, although palladium NPs have been studied intensively in
Suzuki–Miyaura coupling reaction, the employment of Pd/Au alloy
in the same reaction has not been reported in literatures. For this
reason, it is desirable to fabricate heterogeneous Pd/Au alloy cata-
lysts (stabilized in SBA-15) and investigate their catalytic perfor-
mance in Suzuki–Miyaura coupling reaction.
Moreover, in Suzuki–Miyaura coupling reaction, Glasspoole
et al. found aqueous base was highly detrimental to the structure
of silica-related catalysts, which can be avoided by using boric acid
or aluminum-doped material [30]. In this paper, microwave-
assisted technique is another way to optimize catalysts and im-
prove catalytic activity. In recent literatures, microwave-assisted
Suzuki coupling was almost confined in homogeneous reaction in
solutions; few examples were reported involving catalyst existing
in solid phase, except the research of Baxendale et al. [31].
* Corresponding author. Fax: +86 59183796710.
E-mail address: rcao@fjirsm.ac.cn (R. Cao).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.01.004

Z. Zheng et al. / Journal of Catalysis 270 (2010) 268–274
269
Herein, we will describe a facile method to synthesize size-con-
trollably (1–3 nm) monodisperse Pd and Pd/Au bimetallic NPs sta-
bilized in SBA-15, which can be easily used in microwave-assisted
Suzuki–Miyaura coupling reactions. To optimize the as-prepared
catalysts, microwave-assisted Suzuki coupling reactions between
iodobenzene and arylboronic acid were employed to test catalytic
activity and stabilization. Moreover, Pd/Au alloy nanoparticles
with similar metal loading exhibit higher activity than monometal-
lic ones in catalyzing the coupling between aryl bromide/aryl chlo-
ride and arylboronic acid. The mechanism of the improvement in
catalytic performance of this kind of materials was studied by
XPS measurement.
vial. After sealed up, the reaction was irradiated in a microwave
apparatus at 100 °C for 30 min.
2.4. Characterizations
The nitrogen contents of the G4-PAMAM-SBA-15 were deter-
mined by elemental analyses carried out on a Vario EL III analyzer.
Nitrogen adsorption–desorption isotherms at 77 K were obtained
with a Micromeritics ASAP 2000 apparatus. The samples were pre-
viously degassed under vacuum at 50 °C for 24 h. Surface areas,
pore volume and pore sizes were determined by the BJH method
applied to the adsorption branch of the isotherms. The shape and
structure of catalyst were analyzed on a JEOL 2010 transmission
electron microscope (TEM). The samples were prepared by placing
a drop of catalyst sol in ethanol onto a continuous carbon-coated
copper TEM grid. Inductively coupled plasma (ICP) analysis of cat-
alyst and residual aqueous phase after extraction was performed
on an Ultima 2 analyzer (Jobin Yvon). X-ray photoelectron spec-
troscopy (XPS) measurements were analyzed on coated alumina
using a Physical Electronics PHI- Quantum 2000 Scanning ESCA
Microprobe. UV–vis spectra were obtained using a Lambda 35
UV–visible spectrophotometer. 1H and 13C NMR spectra were re-
corded on an AVANCE III Nuclear Magnetic Resonance Spectrome-
ter (BRUKER BIOSPIN) with deuterated chloroform as the internal
reference. Yields of cross-coupling reactions were determined on
VARIAN 430 Gas Chromatograph.
2. Experimental
2.1. Synthesis of catalyst
Mesoporous silica SBA-15 hard templates were prepared under
hydrothermal conditions of 120 °C for 48 h according to the estab-
lished procedures [32]. Then, four consecutive repetition of a Mi-
chael-type addition reaction and amidation of ester moieties
produced G4-PAMAM planted in the channels of SBA-15 [33,34].
The resulting powder solids were tested to roughly determine
the number of dendrimers before complexation with metal ions.
Briefly, 0.1 g of white powder was dispersed under stirring in
50 ml of HCl aqueous solution (pH = 2). Then, 5, 10, 20, 30 and
40 equiv. (compared with calculated number of dendrimers) of a
freshly prepared K2PdCl4 (99.99%, Alfa Aesar) aqueous solutions
were added dropwise. After stirring for 12 h, the yellow solid
was centrifugally separated and sonic oscillated for no less than
three times before dried at 50 °C. The resulting powder was de-
noted as Pd2+ x-G4-PAMAM-SBA-15, where x referred to the metal
precursors/dendrimer molar ratio. The metal complexation was re-
duced by at least 10-fold molar excess of NaBH4 from a freshly pre-
pared aqueous stock solution. Finally, the catalyst was obtained by
centrifugal separation and dried in vacuum. The obtained sample
was denoted as Pdx-G4-PAMAM-SBA-15. Pd5Au5-G4-PAMAM-
SBA-15 was prepared through above-mentioned procedure but
with the addition of chloroauric acid tetrahydrate (purchased from
Sinopharm Chemical Reagent Co., Ltd.) later than Pd by 11.5 h, fol-
lowed by stirring for 30 min.
3. Results and discussion
3.1. Structural characteristics of the Pd and Pd/Au alloy NPs
To obtain monodisperse NPs, the previous method described by
Gao and coworker [33] was modified. To roughly fix the number of
dendrimers, we suppose that they are all integrated as a half-G4-
PAMAM. Because G4-PAMAM has a globular configuration [35],
dendrimers immobilized in channels are deduced to be hemi-
sphere (as shown in Scheme 1). Consequently, the number of den-
drimers can be calculated based on a certain molar ratio of N atoms
to half-G4-PAMAM (61:1) [36]. Figs. 1 and S1 display the TEM
images of Pd5-G4-PAMAM-SBA-15, the other Pdn-G4-PAMAM-
2.2. Microwave-assisted Suzuki reaction between iodobenzene and
arylboronic acids
L-L'=dendrimer shrinking
v
In a typical procedure (e.g., reaction catalyzed by Pdx-G4-PA-
MAM-SBA-15), 0.5 atom% Pd catalyst and 3 equiv. (3 mmol) of
K3PO4 and 1.2 mmol of arylboronic acid (purchased from Alfa Ae-
sar) were added to an N2 bubbling solution (3 ml) of H2O/ethanol
(v/v = 3:2) with magnetic stirring. Then, 1 mmol of iodobenzene
(98%, Alfa Aesar) was dropped into the reaction vial. After sealed
up, the reaction was irradiated in a microwave apparatus (Initiator
8 Exp, from Biotage) at 100 °C for 30 min. After cooling down to
ambient temperature, the reaction mixture was purified by column
chromatography (silica gel/hexane) to give the desired products.
i
L
u
q
e
L'<L
n
o
i
t
5
.
a
c
u
d
e
R
.
b
a. 10 equiv
b. Reduction
a
.
>
1
0
e
q
u
b
G4-PAMAM
.
nanoparticle
R
i
v
e
d
u
2.3. Microwave-assisted Suzuki reaction between bromoarenes and
arylboronic acids
c
t
i
o
n
The co-solvent was the same as above-mentioned. Briefly, 1%
molar equiv. of Pd or Pd/Au alloy catalyst and 3 equiv. (3 mmol)
of K3PO4 and 1.05 mmol of arylboronic acids were added to an
N2 bubbling solution (3 ml) with magnetic stirring. Then, 1 mmol
of bromoarene (purchased from TCI) was added into the reaction
Scheme 1. The proposed procedure for preparation of catalysts (a, the number
represents the molar ratio of Pd2+/dendrimers and b, the reductant is NaBH4).

270
Z. Zheng et al. / Journal of Catalysis 270 (2010) 268–274
Fig. 1. TEM images of (A) Pd5-G4-PAMAM-SBA-15, (B) Pd5Au5-G4-PAMAM-SBA-15 (the left insert is the HRTEM image displaying the lattice fringes of the randomly selected
two NPs, and Fourier diffractogram shown in the right insert discloses spots corresponding to Pd/Au alloy NPs), and (C) EDS spectra of the two selected NPs.
SBA-15 (n = 10, 20, 30 and 40) and the corresponding size distribu-
tion. Evidently, highly dispersive NPs are indeed stabilized in pores
of SBA-15, with average size and size distribution in 1.6 0.4 nm,
1.9 0.4 nm, 2.3 0.4 nm, 2.4 0.4 nm and 2.5 0.4 nm, respec-
tively. The average diameter of palladium particle is smaller than
the pore size (about 7 nm) of SBA-15, indicating the accessible me-
tal sites in the pores.
3.2. Analysis of different palladium catalysts
Previous studies have shown that the diameter distribution of
Pd particles (G4-OH (Pd10)) is only 1.4 0.4 nm [17]. So we suspect
that the confinement of cavities for our dendrimers may not be as
effective as the one in G4-PAMAM reported previously [13,16,17].
Probably, the number of our dendrimers in SBA-15 is less than ex-
pected; or, the dendrimers are not as integrated as half-G4-PA-
MAM. The elemental analysis (EA) of Gn-SBA-15 (Table S1)
confirms the earlier two conclusions in more detailed way, indicat-
ing the formation of dendrimers at every step is imperfect. Inter-
estingly, although the dendrimers planted in channels may fall
short of expectations in quantity or quality, they show good ability
to cap metal particles when the amount of precursor was properly
controlled.
For simplification, we suppose the dendrimers planted in SBA-
15 are hemispheric in shape. It was suggested that, since the diam-
eter of G4-PAMAM is only 4.5 nm [40], the hemisphere radius is
about 2.2 nm. We deduce the NPs with diameter larger than
2.2 nm may not be effectively encapsulated in dendritic cavity; in-
stead, they may be just stabilized by dendrimers with peripheral
groups. The semiquantitative analysis of pore volume (Fig. 2) pro-
vided by nitrogen adsorption–desorption with the BJH method
proves the conclusion in another way.
It was reported that by judicious controlling of pH value
(pH = 2), the terminal amino groups of NH2-terminated PAMAM
dendrimers were selectively protonated [41,42], and the noble me-
tal ions were inclined to complex with interior tertiary amine
groups of the dendrimers to provide a mean for encapsulation of
noble metal NPs through reduction [43]. Our G4 dendrimers are
not integrated as expected, and some of the terminal groups may
not be amino, when the pH is adjusted to 2, the non-amino termi-
nal groups may capture Pd ions through the interaction between
Attracted by the superior catalytic performance of bimetallic
NPs compared with monometallic ones, great efforts have been
put into the formation of the former. Although TiO2 [23], alumina
sols [25] and hydroxyapatite [37] have been applied to stabilize
bimetallic NPs, the works using regular-structured silica as stabi-
lizer are few. In this work, stable Pd/Au bimetallic NPs were ob-
tained in SBA-15 planted by dendrimer (see below). Controlling
the contact time between [AuCl4]À and dendrimers within
30 min is of importance for the formation of highly dispersive
Pd/Au NPs, otherwise Au3+ ions are firstly reduced into Au particles
before reduction with NaBH4 to lead a wine red system [38] (see
Fig. S2). Similar to palladium NPs, the bimetallic NPs are clearly
distributed in the channels, with average diameter in about
1.5 nm. To understand the structure of NPs, we randomly selected
two particles as shown in the top left inset in Fig. 1B. The HRTEM
image shows spherical structure with distinct lattice fringes. The
Fourier diffractogram obtained from these two NPs as marked in
the top right inset of Fig. 1B clearly shows two sets of spots char-
acteristics of Pd and Au, which are Pd(1 1 1) lattice spacing of
0.225 nm (JCPDS card 46-1043) and Au(1 1 1) plane with
0.235 nm (JCPDS card 04-0784) [39], respectively. In addition, the
EDS analysis (Fig. 1C) of the two particles obviously gives the peaks
of palladium and gold, and the spectra superimpose with those ob-
tained for all NPs in Fig. 1B. These results confirm that the Pd/Au
alloy NPs were obtained and stabilized in the channels of SBA-15
planted by dendrimer.

Z. Zheng et al. / Journal of Catalysis 270 (2010) 268–274
271
formed. The iodobenzene was coupled with 4-methylbenzenebo-
ronic acid in the presence of catalysts (0.5 mol% of metal),
following the condition in Scheme 2. It can be seen from Fig. 3 that
the yields of Suzuki coupling for iodobenzene in the presence of
the five catalysts generally decreases as the Pd2+/dendrimer ratio
increases: 98% for Pd5-G4-PAMAM-SBA-15, 97% for Pd10-G4-
PAMAM-SBA-15, 92% for Pd20-G4-PAMAM-SBA-15, 78% for Pd30
-
G4-PAMAM-SBA-15 and 80% for Pd40-G4-PAMAM-SBA-15, respec-
tively. On the other hand, the curve for residual content of Pd in
0.5% Pd catalyst
Me
Me
B(OH)2
I +
3mmol K3PO4, 100°C
microwave radiation
H2O/ethanol=3/2
Scheme 2. Cross-coupling between iodobenzene and 4-methylbenzeneboronic
acid.
Fig. 2. Molar ratio of Pd precursor/dendrimers vs. pore volume (average value of
three repeated tests).
Pd ions and such groups to lead to the existence of Pd NPs at the
periphery of the dendrimers after reduction [36], and the efficient
pore volume of Pd5-G4 (Fig. 2) is smaller than that of G4 (0.02 cm3/
g). When increasing the ratio of Pd2+/dendrimer to 10, the interac-
tions between Pd2+ and the non-amino terminated group may be
saturated and more Pd2+ ions penetrate into the dendrimer
through the complexation between Pd2+ and interior tertiary
amine groups to lead to ‘‘dendrimer shrinking” after reduction,
and the efficient pore volume of Pd10-G4 (ꢀ0.03 cm3/g) is larger
than that of G4. Further increasing the ratio of Pd2+/dendrimer to
20 or more will lead to the saturation of the complexation between
Pd2+ ions and the interior tertiary amine groups, and more Pd NPs
will exist at the periphery of the dendrimers to give rise to smaller
pore volumes than that of G4. Meanwhile, from TEM observations
(see Fig. 1A and B), no palladium aggregations are formed outside
the channels, indicating there are no free metal ions existing before
reduction; no NPs can be observed except in SBA-15, indicating al-
most all the dendrimers are firmly planted in channel instead of
dissociated into aqueous phase in the procedure of formation of
catalysts. The formation of the NPs catalysts is demonstrated in
Scheme 1.
Fig. 3. Variation of yields (determined by GC) and residual content of Pd in reaction
medium (determined by ICP-AES) vs. molar ratio of metal precursor/dendrimer.
3.3. Microwave-assisted Suzuki–Miyaura reactions
In recent years, thousands of organic reactions including Suzuki
coupling are improved both in catalytic activity and in time reduc-
tion due to microwave-assisted technique [31]. To evaluate the
microwave-assisted reaction, we carried out comparative synthe-
sis involving Pd10-G4-PAMAM-SBA-15 catalyzed coupling between
iodobenzene and benzeneboronic acid, heated by conventional
method and microwave radiation. The solvent (water/etha-
nol = 3:2) was heated with 1 mmol of iodobenzene, 1.2 mmol of
benzeneboronic acid, 0.5% mol of catalyst and 3 equiv. of K3PO4
at 100 °C. In conventional condition, reaction after 8 h gave prod-
uct in 97% yield for 1st cycle and 31% for the 2nd cycle. But the
microwave radiation afforded 97% yield for only 10 min, which
indicated 48-fold rate increase, and the reactions were completed
after 30 min. To clarify the influence of catalyst in microwave-as-
sisted Suzuki reaction, reaction without catalyst under radiation
was carried out under the same condition and almost no product
was obtained, demonstrating that microwave can only promote
the reactions in the presence of the catalyst.
To investigate the catalytic activity and stability of the five as-
prepared Pd NPs catalysts, the following experiments were per-
Fig. 4. Yields (determined by GC) vs. the number of catalytic cycle for Pd10-G4-
PAMAM-SBA-15 and Pd/C.

272
Z. Zheng et al. / Journal of Catalysis 270 (2010) 268–274
Table 1
Product and yields for Suzuki coupling reactions between arylboronic acids and iodobenzene catalyzed by Pd10-G4-PAMAM-SBA-15 (0.5 mol% of metal) in 3:2 H2O/ethanol (base,
K3PO4; temp., 100 °C; time, 30 min).
Entry
1
Arylboronic acid
Product
Yield (%)a
97
Me
B(OH)2
B(OH)2
B(OH)2
Me
2
3
4
>99
90
MeO
OMe
NO2
O2N
Me
83
B(OH)2
OMe
Me
5
6
68
31
B(OH)2
MeO
Me
Me
Me
B(OH)2
Me
a
Determined by GC analysis.
reaction medium (determined by ICP) after reaction goes in an
opposite direction, except for Pd5-G4-PAMAM-SBA-15: 1.8 ppm
for Pd5-G4-PAMAM-SBA-15, 0.4 ppm for Pd10-G4-PAMAM-
zene halide–iodobenzene, to focus on the access of substrates. The
results (Table 1) show that para-substituted arylboronic acids
(regardless of activated or passivated) facilitate coupling to afford
high yields, whereas the ortho-substituted molecules have diffi-
culty in penetrating through dendrimers to interact with NPs to
give poor yield.
It was reported that the coupling between arylboronic acids and
aryl bromides is much slower than the reactions between arylbo-
ronic acids and aryl iodides. Low yields were always due to
byproducts resulting from homo-coupling of aryl bromides
[18,19]. Unfortunately, in spite of microwave-assisted technique
the same poor performance was observed in the presence of
Pd10-G4-PAMAM-SBA-15, as shown in Table 2 (entries 1, 3, 5, 8,
10 and 12). To improve the coupling reactivity, the Pd/Au alloy
NPs catalyst was applied in the reaction. It can be seen from Table
2 (entries 2, 4, 6, 9, 11 and 13), Pd/Au NPs with similar metal load-
ing display superior activity than pure palladium NPs, with higher
yields in general. The bimetallic NPs can also catalyze coupling be-
tween aryl chloride and arylboronic acid with relatively high yields
(entries 14 and 15).
SBA-15, 0.8 ppm for Pd20-G4-PAMAM-SBA-15, 1.4 ppm for Pd30
-
G4-PAMAM-SBA-15 and 4.3 ppm for Pd40-G4-PAMAM-SBA-15,
respectively. Although the activity for Pd5-G4-PAMAM-SBA-15 is
the highest, its stability is poor (also seen in Fig. S3). Combining
the activity and stability of the five catalysts, the ratio of 10 for
Pd2+/dendrimer may be the optimized ratio for the fabrication of
our Pd NPs catalyst.
As shown in Fig. 4, when Pd10-G4-PAMAM-SBA-15 was ap-
plied to coupling between iodobenzene and benzeneboronic acid,
the yield in the 5th cycle is 95%, indicating only a slight loss of
the catalytic activity. However, when the commercial Pd/C is ap-
plied in the reaction, dramatic loss of catalytic activity is ob-
served in the 5th cycle (the yield of coupling product is only
69%). Furthermore, it is found that air has little influence on
the reaction because the procedure without N2 bubbling also
gives 96% yield, which is as high as that using N2. Note: a mild
base is needed for this reaction otherwise the SBA-15 holding den-
drimers disappears in solvent. The results indicate that Pd10-G4-
PAMAM-SBA-15 catalyst is very stable and can be recycled and
reused easily.
3.4. XPS characterization of palladium and Pd/Au alloy NPs
The substrate selectivity is also a function of dendrimers [40],
because higher generation dendrimers are less porous, which lim-
its access of substrate to the encapsulated NPs. To test and verify
this conclusion reported by others, we designed a series of cou-
pling reaction between iodobenzene and several arylboronic acids.
Oxidative addition between iodobenzene and noble metal is rate-
determining step during Suzuki reaction [44]. Furthermore, the
step involving arylbenzene acids is supposed to occur at surface
of NPs [45]. So we fix up a variation by using only one kind of ben-
XPS was employed to investigate the mechanism of the
improvement of catalytic performance of Pd/Au alloy NPs. XPS pro-
vides information about the Pd oxidation state in palladium and
Pd/Au alloy NPs because photoelectrons with energies of 337 eV
(Pd5/2) and 342 eV (Pd3/2) correspond to Pd2+, and those at
335 eV (Pd5/2) and 340 eV (Pd3/2) correspond to metallic Pd0 [46].
In Fig. 5, the peaks for Pd0 are observed for both Pd and Pd/Au alloy
catalysts, while those for Pd2+ is observed only in pure Pd catalyst,
illustrating the presence of Pd2+ ions in pure Pd catalyst but not in

Z. Zheng et al. / Journal of Catalysis 270 (2010) 268–274
273
Table 2
Products and yields for Suzuki coupling reactions between arylboronic acids and aryl bromides catalyzed by Pd NPs and Pd/Au NPs (0.5 mol% of metal) in 3:2 H2O/ethanol (base,
K3PO4; temp., 100 °C; time, 30 min).
Entry
Aryl halides
Arylboronic acid
Catalyst
Product
Yield (%)a
1
2
Pd10
21
78
Pd5Au5
Br
B(OH)2
3
4
Pd10
33
90
Pd5Au5
O2N
Me
Br
B(OH)2
B(OH)2
NO2
Me
5
Pd10
13
6
7
Pd5Au5
Au5
58
Trace
Br
8
9
Pd10
32
92
Pd5Au5
Br
Br
Br
Me
B(OH)2
Me
10
11
Pd10
8
Pd5Au5
10
O2N
MeO
B(OH)2
B(OH)2
NO2
OMe
Me
12
13
Pd10
56
64
Pd5Au5
14
15
Pd5Au5
Pd5Au5
55
43
B(OH)2
B(OH)2
Me
Cl
Cl
MeO
MeO
a
Determined by GC analysis.
than that of Pd catalyst, because Pd0 is the active state for Suzuki
cross-coupling reaction.
4. Summary and conclusions
Pd and Pd/Au alloy NPs catalysts have been successfully pre-
pared by controlling pH value, amount of precursors and contact
time between metal ions and SBA-15 supported dendrimers. The
TEM images indicate that the NPs are nearly monodisperse in size
and stabilized in channels of SBA-15 planted by dendrimers. The Pd
NPs catalysts manifest outstanding activity and recyclability in Su-
zuki–Miyaura coupling reaction when compared with commercial
Pd/C, and the Pd10-G4-PAMAM-SBA-15 catalyst is the best one.
Meanwhile, Pd/Au alloy NPs exhibit higher activity than monome-
tallic ones.
Acknowledgments
Fig. 5. XPS spectra of pure palladium and Pd/Au alloy catalysts.
We acknowledge the financial support from 973 Program
(2006CB932903, 2007CB815303), NSFC (20731005, 20821061,
20873151), Fujian Key Laboratory of Nanomaterials (2006L2005),
CAS and FJIRSM (SZD07002). We also acknowledge the advice pro-
vided by Prof. Ruihu Wang.
Pd/Au alloy catalyst. Previous reports [16,24] found that only par-
tial of metal ions were reduced by NaBH4 because metal ions
would form complexes with ClÀ, H2O and the internal amine
groups of the dendrimer. In Pd/Au alloy system, although pure gold
has no catalytic activity in Suzuki cross-coupling reaction (entry 7,
Table 2), the presence of Au3+ in dendrimer before reduction may
lower the complexation between Pd2+ and internal amine groups
or other groups, leading to the complete reduction of Pd2+ to Pd0.
This induces the higher catalytic activity of Pd/Au alloy catalyst
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2010.01.004.

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