190
J.-J. Lv et al. / Applied Catalysis A: General 522 (2016) 188–193
Table 1
promptly into black. After 5 min, the black mixture was allowed
to stand overnight without stirring. The resulting black precipi-
tates were collected by centrifugation and washed with water and
ethanol. The final products were dried in an oven (50 ◦C).
Optimization of the reaction conditions.a
2.4. Catalytic performance for suzuki cross coupling reactions
Entry
Solvents
Bases Temp.
Time (◦C)
Time (h)
Yield (%)b
In the Suzuki cross coupling reactions, a mixture of phenyl-
boronic acid (0.6 mmol, 1.2 equiv), aryl halide (0.5 mmol, 1 equiv),
K2CO3 (1 mmol, 2 equiv), and Pd-Cu NWs (4 mg) were placed
1
2
3
4
5
6
7
8
EtOH
CH3CN
DMSO
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Na2CO3
Et3N
NaOAc
NaOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
80
80
80
80
80
80
80
80
80
80
80
80
80
80
20
40
60
80
80
80
2
2
2
2
2
2
2
2
2
2
2
0.5
1
1.5
2
2
2
2
40
24
62
76
37
77
86
79
56
69
73
21
44
83
trace
36
57
38
67
99
in
a Schlenk tube (10 mL), which contained 2 mL of N,N-
H2O
dimethylformamide (DMF) and water (H2O) (v/v = 1/1). The
mixture was then stirred for a desired period of time at a selected
temperature, and the reaction was monitored by thin layer chro-
matography (TLC). Afterward, the reaction mixture was cooled
down to room temperature. And the catalyst was recovered by
filtration, followed by washing thoroughly with ethyl acetate and
water. The combined organic layer was dried by Na2SO4, and the
filtered residue was purified by flash column chromatography on
silica gel. As for the recycling experiments, we conducted several
parallel experiments under identical conditions and recycled the
catalyst for next run test. The target reactions would get supple-
ment of catalysts from the other parallel experiments to make sure
the scale of catalyst with 4 mg.
Ethanol/H2Oc
DMF/H2Od
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2O
DMF/H2Oe
DMF/H2Of
DMF/H2O
9
10
11
12
13
14
15
16
17
18
19
20
2
2
a
Reaction conditions: bromobenzene (0.5 mmol), phenylboronic acid (0.6 mmol),
catalyst (Pd-Cu NWs, 4 mg).
b
3. Results and discussion
Solated yields.
EtOH/H2O = 1:1.
DMF/H2O = 1:1.
Catalyst (Pd-Cu NWs, 1 mg).
c
d
3.1. Characterization
e
f
Catalyst (Pd-Cu NWs, 2 mg).
As shown in Fig. 1, we have successfully developed a facile and
efficient synthetic procedure to fabricate Pd-Cu NWs. Fig. 1A–D are
TEM image (Fig. 1A), one can see high-quality intertwining
nanowires with average width of about 4 nm (Fig. 1B), and there
is no detection of any nanoparticles throughout the entire image.
As illustrated in the HR-TEM images (Fig. 1C–D), there is the con-
firmation of crystalline nature and display of well-resolved lattice
fringes with adjacent spacings of 0.224 and 0.191 nm (marked), cor-
responding to the (111) and (200) planes of face-centered cubic
(fcc) metal [32,35], respectively. Furthermore, the lattice spacing
distances are between that of Pd (0.225 nm, 0.195 nm) and Cu
Pd-Cu alloy [36].
In order to determine the distribution of Pd and Cu in Pd-Cu
NWs, we collected the HAADF-STEM-EDS mapping images and EDS
line scanning profiles (Fig. 1E–G). Noticeably, Pd and Cu are homo-
Furthermore, the compositional line scanning profiles confirm their
homogeneous distribution once again (Inset in Fig. 1E), providing
alloy.
The EDS spectrum in Fig. 2A further confirms the coexistence
of Pd and Cu elements. And the atomic ratio of Pd to Cu in Pd-Cu
of line scanning profiles (Inset in Fig. 1E), indicating the effective
reduction of the Pd and Cu precursors. The XRD spectra of the as-
prepared nanocrystals provide information related to composition
and crystal structure (Fig. 2B). The (111) peak of Pd-Cu NWs shows
up at 2 (d-values) of 41.15◦, which is between that of pure Pd
alloy.
thesis [37]. The Cu 2p XPS spectra (Fig. 2D) reveal the co-existence
of Cu0 and Cu2+ species in Pd-Cu NWs [38], while the content of the
former is much higher than that of the latter [39].
To elucidate the role of NP-40 in the formation of Pd-Cu NWs,
we investigated the effects of NP-40 in the synthetic system. The
absence of NP-40 yields agglomerated nanoparticles (Supporting
Information, Fig. S1B,C) improves the dispersity of the result-
ing products, along with the emergence of wire-like structures
at 0.15% NP-40 (Supporting information, Fig. S1C). At NP-40 of
0.2%, the products contain mainly well-defined nanowires with
high dispersity (Fig. 1). It indicates the pivotal role of NP-40 as a
structure-directing and stabilizing agent to induce the formation of
nanowires and prevent the nanoparticles agglomeration [25,40].
3.2. Catalytic activities in suzuki cross-coupling reactions
We tested the catalytic performance of Pd-Cu NWs in the Suzuki
cross coupling reaction, which is a powerful strategy for the syn-
thesis of C C bonds in the production of medicine, agrochemicals,
fragrances, and engineering materials [4,41,42]. As well known,
there are various influencing factors in a Suzuki cross coupling reac-
tion, such as base, solvent, temperature, and reaction time [43].
Accordingly, we tested the catalytic activity of Pd-Cu NWs and
compared the results with those of Pd NPs commonly used for the
provided in ESI.
Phenylboronic acid and bromobenzene were employed for
screening the reaction parameters (Table 1). First, various solvents
were tested (Table 1, entries 1–7). Since water is safe, readily avail-
able, and environment-benign [44,45], we used water to reduce
the amount of organic solvent used in the reaction. It is observed
The XPS measurements were used to probe into the surface
states of Pd-Cu NWs. It is observed that the Pd0 3d5/2 and 3d3/2
peaks are predominant in the high-resolution Pd 3d spectrum
(Fig. 2C), revealing the efficient reduction of H2PdCl4 during syn-