Pd-ZnO nanowire arrays as recyclable catalysts for 4-nitrophenol reduction and Suzuki coupling reactions

Article abstract of DOI:10.1039/c6ra28467a

In this work, we report a facile approach for the preparation of Pd nanoparticle-ZnO nanowire arrays followed by demonstrating their use as recyclable catalyst for reactions under different conditions. The facile synthesis largely relies on a spontaneous reduction of PdCl₄^2? at the surface of oriented ZnO nanowires grown on a Zn foil. This is due to a combination of the strongly reducing ability of Zn foil and the semiconducting nature of ZnO nanowires. At room temperature and under a weak alkaline condition, the as-prepared Pd nanoparticle-ZnO nanowire arrays show a catalytic activity factor up to 76.6 s^?1 g^?1 in the reduction of 4-nitrophenol and no obvious decreases in the catalytic activity even after use of 10 times. Meanwhile, the as-prepared Pd nanoparticle-ZnO nanowire arrays exhibit extraordinary catalytic activity toward Suzuki and carbonylative Suzuki reactions at a higher temperature under a stronger alkaline condition. The outstanding performance of the hybrid nanowire arrays is mainly originated from the small size of the Pd nanoparticles (～2-4 nm) with clean surfaces, as well as a strong affinity between the Pd nanoparticles and ZnO nanowires, leading to marginable catalyst loss and aggregation. Considering the multiple choices of both noble metal nanocatalyst and transition metal oxide nanowire array, we expect noble metal nanoparticle-transition metal oxide nanowire arrays to be emerged as a new class of recyclable catalysts attractive for diverse organic reactions to develop pharmaceuticals, natural products and advanced functional materials.

Full text of DOI:10.1039/c6ra28467a

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Pd–ZnO nanowire arrays as recyclable catalysts for
4-nitrophenol reduction and Suzuki coupling
reactions†
Cite this: RSC Adv., 2017, 7, 7964
a
a
a
Qiyan Hu,ab Xiaowang Liu, a Lin Tang,ab Dewen Min, Tianchao Shi and Wu Zhang
*
*
In this work, we report a facile approach for the preparation of Pd nanoparticle–ZnO nanowire arrays
followed by demonstrating their use as recyclable catalyst for reactions under different conditions. The
2ꢀ
facile synthesis largely relies on a spontaneous reduction of PdCl4 at the surface of oriented ZnO
nanowires grown on a Zn foil. This is due to a combination of the strongly reducing ability of Zn foil and
the semiconducting nature of ZnO nanowires. At room temperature and under a weak alkaline
condition, the as-prepared Pd nanoparticle–ZnO nanowire arrays show a catalytic activity factor up to
76.6 sꢀ1 gꢀ1 in the reduction of 4-nitrophenol and no obvious decreases in the catalytic activity even
after use of 10 times. Meanwhile, the as-prepared Pd nanoparticle–ZnO nanowire arrays exhibit
extraordinary catalytic activity toward Suzuki and carbonylative Suzuki reactions at a higher temperature
under a stronger alkaline condition. The outstanding performance of the hybrid nanowire arrays is mainly
originated from the small size of the Pd nanoparticles (ꢁ2–4 nm) with clean surfaces, as well as a strong
affinity between the Pd nanoparticles and ZnO nanowires, leading to marginable catalyst loss and
aggregation. Considering the multiple choices of both noble metal nanocatalyst and transition metal
oxide nanowire array, we expect noble metal nanoparticle-transition metal oxide nanowire arrays to be
emerged as a new class of recyclable catalysts attractive for diverse organic reactions to develop
pharmaceuticals, natural products and advanced functional materials.
Received 20th December 2016
Accepted 13th January 2017
DOI: 10.1039/c6ra28467a
www.rsc.org/advances
1- or 2-dimensional (D) nanostructure supports, including TiO2
rods, Al2O3 NPs, carbon-based nanomaterials, metal–organic
frameworks and magnetic NPs.5 In fact, these studies have
Introduction
Noble metal nanoparticles (NPs), such as Ag, Au and Pd, have
garnered signicant attention in organic synthesis as viable
catalysts owing to their extraordinary catalytic nature.1 These
nanocatalysts have proven effective for a diverse array of reac-
tions, including C–C coupling, alkene hydrogenation and C–H
arylation.2 However, direct utilization of noble metal NPs as
catalysts usually suffers from high cost due to the lack of effi-
cient strategies for their recollection.3 In addition, the use of
pure small-sized noble metal NPs as catalysts usually leads to
another issue, that is, gradual decrease in their catalytic activity
as a result of nanocatalyst coalesce in the course of catalytic
reactions.4
shown additional benets of supported nanocatalysts besides
facilitating the recollection of catalytically active noble NPs
from the reaction mixture. For example, immobilization of
noble metal NPs on support matrices allows the nanocatalysts
to be well-separated, and effectively prevents the interparticle
communication, giving rise to the persistence of the catalytic
activity. Additionally, the introduction of noble metal–support
interfaces in the catalysts provides new possibility for
improving their catalytic activity through interface engi-
neering.6 Despite the enormous advances, there is still room for
enhancing the performance of supported nanocatalysts in
terms of boosting their catalytic activity and recyclability. One
promising pathway is to optimize the morphology of nano-
structured support to simultaneously increase the loading of
noble metal NPs and improve the interaction between the
matrices and the loaded noble metal NPs.
To enhancing the recyclability while preserving the highly
catalytic activity of nanocatalyst, considerable efforts have been
devoted to growing catalytically active NPs onto the surfaces of
aCollege of Chemistry and Materials Science, Anhui Normal University, Key Laboratory
of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of
Molecule-Based Materials, Wuhu 241000, P. R. China. E-mail: zhangwu@mail.
ahnu.edu.cn; xwliu601@mail.ahnu.edu.cn
bSchool of Pharmacy, Wannan Medical College, Wuhu 241002, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra28467a
Recent studies suggest that nanowire arrays are attractive as
support for the development of highly recyclable catalysts for
their outstanding features: (i) the building blocks of the nano-
wires in the arrays providing large surfaces for the loading of
catalytically active NPs;7 (ii) the sufficient inter-nanowire space
beneting the access of reactants to the catalytic sites and the
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release of catalytic products from the supported catalysts;8 and
(iii) the large dimensions of the array substrate up to square
centimetres favouring efficient catalyst collection.9 More
importantly, reliable synthesis of a variety of nanowire arrays,
such as ZnO, TiO2, SnO2 and ZnMn2O4 via wet chemistry
methods, promises to afford multiple choices to control of NP–
nanowire interfaces for enhancing the catalytic performance of
supported catalysts.10
On the basis of easy access to different types of noble metal–
ZnO nanowire arrays by a interfacial reduction reaction between
noble metal precursors and ZnO nanowire arrays in an aqueous
solution and the excellent catalytic nature of Pd NPs,11 here we
rst demonstrate the preparation of Pd NP–ZnO nanowire
arrays and then reveal their potential as recyclable catalysts for
reactions under different conditions. Our results suggest that
the as-prepared Pd NP–ZnO hybrid nanowire arrays exhibit
highly recyclable and active catalysts toward 4-nitrophenol
reduction at room temperature under weak alkaline conditions.
More interestingly, at a higher temperature (100 ꢂC) and under
a stronger alkaline condition, the as-prepared Pd NP–ZnO
nanowire arrays are also found to be suitable for both Suzuki
and carbonylative Suzuki reactions. We demonstrate the crucial
role of the size of the in situ formed Pd NPs in the determination
of the catalytic performance of the supported nanoarray cata-
lysts. Meanwhile, we reveal that the strong affinity between Pd
NP and ZnO nanowire effectively prevents the aggregation and
loss of the nanocatalysts, permitting the preservation of highly
catalytic activity of the catalysts.
Fig. 1 (a) SEM image of the as-prepared Pd NP–ZnO hybrid nanowire
arrays by immersing ZnO@Zn nanowire arrays into an aqueous solu-
tion of Na2PdCl4 (5 mM) for 5 s. (b and c) SEM and TEM images of
a single hybrid Pd NP–ZnO nanowire in the array. (d–f) Corresponding
elemental mapping of the single hybrid Pd NP–ZnO nanowire as
indicated in (c), (d) Pd, (e) Zn, and (f) O. (g) EDS and (h) XPS profiles of
the as-prepared Pd NP–ZnO nanowire arrays. Inset in (h) shows the
curve fit of Pd 3d spectrum, inset in (g) showing an HRTEM image of
the as-prepared Pd NP–ZnO hybrid nanowire.
Results and discussion
We at rst synthesized Pd NP–ZnO nanowire arrays by putting
the ZnO@Zn nanowire arrays into an aqueous solution of
Na2PdCl4 (5 mM) for different periods of time at room
temperature (5, 15 and 25 s). We observed an immediate color
change from white to black of the nanowire arrays (ESI, Fig. S1†)
once they were exposed to the Pd precursor solution. Aer being affinity between the Pd NPs and the ZnO nanowires (Fig. 1h).12
immersed into the solution for 5 s, ZnO nanowire arrays were The HRTEM image suggests the good crystalline nature of the in
modied with a large number of secondary NPs (Fig. 1a). A situ formed Pd NPs due to the observation of a measured
closer scanning electron microscopy inspection (Fig. 1b) interplanar distance of approximately 0.22 nm in individual
revealed that both size and density of the in situ formed NPs nanosized grains, which is in accordance with the d-spacing in
were decreased gradually from the tip of the ZnO nanowire. the (111) planes of cubic-phased Pd. As expected, average size of
Transmission electron microscopy (TEM) suggested that the the in situ formed Pd NPs was conrmed to be highly dependent
secondary NPs were mainly localized at the rst 2.5 mm of the on the reaction times. For example, extension of the reaction
ZnO nanowire (Fig. 1c) and the average size of the secondary NP times to 15 and 25 s offered Pd NPs with average sizes of ꢁ7 and
was determined to be in the range of 2–4 nm (Fig. S2a†). ꢁ9 nm, respectively (Fig. S2b and c†). The in situ reduction of
Noticeably, the secondary NPs at the tip of the ZnO nanowire are Na2PdCl4 can be attributed to electron transfer from the
coalesced in some cases to form a big mesoporous one with an surfaces of ZnO nanowire to interfacial PdCl42ꢀ as a result of the
average diameter of 20–30 nm. The elemental mapping analysis occurrence of Zn substrate-induced electron transfer.11
showed that Pd is localized at the outer layer of the as-prepared
We next accessed the catalytic activity of the as-prepared Pd
ZnO hybrid nanowire (Fig. 1d–f). The outer layer NPs was veri- NP–ZnO nanowire arrays toward the reduction of 4-nitrophenol
ed to be Pd by energy-dispersive spectroscopy (Fig. 1g), X-ray at room temperature. We rst prepared a stock solution by
photoelectron spectroscopy (XPS) and X-ray diffraction anal- addition of NaBH4 into 4-nitrophenol. Note that the addition of
ysis of ZnO nanowire arrays before and aer the reaction NaBH4 into 4-nitrophenol led to an obvious increase in the pH
(Fig. S3†). Note that the XPS spectrum of Pd 3d5/2 and 3d3/2
s
of the resulting solution and thus gave rise to a red shi in the
were estimated to be at 335.1 and 340.5 eV, which are higher maximum absorption from 317 to 400 nm due to the formation
than the corresponding values for Pd(0), implying the strong of 4-nitrophenolate ions (Fig. S4†).13 The resulting bright yellow
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solution exhibited superior stability at room temperature conrmed by the decrease in the reaction rate constant of other
because long-term aging (one week) did not resulted in notable Pd NP–ZnO nanowire arrays which were prepared with reaction
color changes. In contrast, a quick and complete decline (ꢁ12 times of 15 and 25 s (Fig. S5†). Notably, ICP-OES analysis
min) in the maximum absorption was observed aer adding the showed that the loading of Pd NPs for the two catalysts was 0.13
as-prepared Pd NP–ZnO nanowire arrays into the stock solution. and 0.16 mg, respectively. Consequently, we can conclude that
The absorption decline was accompanied with the generation of the amount of Pd NP also come into play in the reduction
a new absorption peak at 295 nm due to the formation of 4- procedure when the particle size becomes larger. Of note, the
aminophenol (Fig. 2a and b). By comparison, the use of clean surfaces of the Pd NPs prepared in the absence of both
ZnO@Zn nanowire arrays alone under the same conditions led surfactants and reducing agents may also partially contribute to
to a negligible change in the absorption spectrum (Fig. S5†). the highly catalytic behaviour.
Taken together, these results suggest that the catalytic ability of
the hybrid nanowire arrays is originated from the loaded Pd the recycled Pd NP–ZnO nanowire arrays toward catalytic
NPs. reduction of 4-nitrophenol. The experimental results (Fig. 2c)
In a further set of experiments, we probed the reusability of
As the concentration of NaBH4 in the catalytic mixture is showed negligible catalytic decreases of the recollected Pd NP–
largely higher than that of 4-nitrophenol, the reduction process ZnO nanowire arrays even aer use for 10 times. The persis-
can be quantitatively evaluated by pseudo-rst-order kinetic tence of the highly catalytic performance of the collected cata-
model according to eqn (1):
lysts is due to a combination of marginable catalyst loss and Pd
NP coalescence in the course of catalytic reaction. Indeed, we
did not detect Pd leaching from the reaction mixture aer the
removal of Pd NP–ZnO nanowire arrays by ICP-OES analysis.
Also, we observed no obvious changes in the morphology of the
used hybrid nanowire arrays and in the valence state of the Pd
NPs (Fig. S6a and b†). These ndings suggest that the as-
prepared Pd NP–ZnO nanowire arrays rival or even exceed
other types of recyclable Pd NPs toward catalytic reduction of 4-
nitrophenol at room temperature.14
The successful use of the as-prepared Pd NP–ZnO nanowire
arrays as recyclable catalysts under mild reaction conditions
stimulates us to check their promising catalytic utility in much
harsher conditions. In this case, we selected Suzuki reaction as
a model system due to the need for higher reaction tempera-
tures and stronger alkaline conditions. Furthermore, the reac-
tion is known for its robustness and versatility in the
construction of diverse molecules with exceptionally broad
functional group tolerance.15 As shown in Table 1, the as-
prepared Pd NP–ZnO nanowire arrays (5 s) showed excellent
ln(C/C0) ¼ ꢀkt
(1)
where C is the concentrations of 4-nitrophenol at different
times in the course of the reduction reaction, C0 is the initial
concentration of 4-nitrophenol, and k is the apparent rst-order
rate constant (minꢀ1). Our reaction rate constant (k) was esti-
mated to be ꢁ0.278 minꢀ1 (inset, Fig. 2b). The inductively
coupled plasma optical emission spectrometry (ICP-OES) anal-
ysis showed that the loading of Pd on the surface of the ZnO
nanowire array was about 0.0126 mg cmꢀ2, and thus the total
amount of Pd NPs participated in the catalytic reduction was
estimated to be 0.0605 mg. As a result, the catalytic activity
factor (j) was calculated to be 76.6 sꢀ1 gꢀ1 by using eqn (2):
j ¼ k/m
(2)
where m is the weight of Pd NPs. We attribute the impressive
catalytic performance of the as-prepared Pd NP–ZnO nanowire
arrays mainly to the small size of the Pd NPs. This hypothesis is
Fig. 2 (a) Schematic representation of chemical transformation of 4-nitrophenol into 4-aminophenol in the presence of Pd NP–ZnO nanowire
arrays. (b) Time-dependent UV-vis absorption spectra of a stock solution containing 4-nitrophenol and NaBH4 after the addition of Pd NP–
ZnO@Zn nanowires (reaction time: 5 s). Inset: plot of ln(C/C0) at 400 nm as a function of reaction time; (c) plots of C/C0 at 400 nm as a function
of reaction time obtained at different runs of the as-prepared Pd NP–ZnO@Zn in the reduction of 4-nitrophenol.
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Table 1 Scope of the Pd NP–ZnO nanowire array-catalysed Suzuki coupling reactions of aryl halides with aryl boronic acidsa
Entry
Ar1–X
Ar2
T (h)
Biaryl
GC yieldb (%)
1
2
6
8
99
99
3
12
6
94
99
99
99
96
99
93
99
99
99
4
5
8
6
6
7
12
6
8
9
12
5
10
11
12
5
6
13
14
8
99
94
12
15
12
92c
16
17
24
24
91
82
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Table 1 (Contd.)
Entry
Ar1–X
Ar2
T (h)
Biaryl
GC yieldb (%)
18
24
85
a
Reaction conditions: aryl halide (1.0 mmol), aryl boronic acid (1.1 mmol), K2CO3 (1.5 mmol), Pd NP–ZnO@Zn (0.01 mol% Pd) and DMF (3.0 mL),
100 ꢂC for 5 to 24 h. b Yields were determined by GC using dodecane as an internal standard upon the completion of the reactions. c 2.2 mmol of aryl
boronic acid was added into the reaction.
catalytic ability in the coupling reactions of aromatic iodides (or catalytic activity. Notably, the size of the in situ formed Pd NPs
bromides) with aryl boronic acids. Of note, aryl iodides with also exerted profound impact on their catalytic reactivity toward
electron withdrawing groups, including COOC2H5, CH3CO Suzuki reaction. For example, the use of supported Pd NP with
(Table 1, entries 10 and 11), displayed a higher reactivity an average size of 9 nm (25 s for Pd NP growth) led to a yield of
compared with aryl iodide substrates bearing electron donating <40% for the reaction of iodobenzene with phenylboronic acid
groups (such as CH3 and OCH3) as starting reactants (Table 1, under the same conditions.
entries 4 and 6). In contradiction to aryl iodides, aryl bromi-
As an added benet, the as-prepared Pd NP–ZnO nanowire
nated equivalents showed a lower reactivity, and thus longer arrays were found to be applicable to carbonylative Suzuki
reaction times (24 h) were required for producing respectful coupling reactions. Such reactions are of great importance for
yields (>80%) (Table 1, entries 16–18). As expected, sterically the development of bioactive natural products but are largely
hindered or doubly functionalized aryl iodinated substrates untapped.17 The limited reported methods mainly relied on the
(Table 1, entries 13–15) and heterocyclic bromide (Table 1, entry use CO as a carbonylating agent, causing practical handling
18) could react smoothly with aryl boronic acids in the presence difficulties.18 Instead, here we used Mo(CO)6 as an efficient solid
of the as-prepared Pd NP–ZnO nanowire arrays to afford corre- carbonylating agent to replace CO. Reaction of aryl iodide, aryl
sponding biaryls in good yields. Notably, the nature of the boronic acid and Mo(CO)6 in the presence of the as-prepared Pd
functional groups on aryl boronic acids also remarkably NP–ZnO@Zn nanowire arrays (5 s for Pd NP growth) led to
impacts their reactivity. In general, electron donating group- a TON up to 775 and a TOF of approximately 129 (hꢀ1) (Table 2,
substituted substrates generally showed much higher reac- entry 1), respectively. This result shows the better performance
tivity than those bearing electron withdrawing groups. On of Pd NP–ZnO@Zn nanowire arrays relative to other catalysts,
a separate note, the isolated yield of the coupling reaction of such as Pd/SiC and Pd–Fe3O4.19 We then examined the substrate
iodobenzene with phenylboronic acid by column chromatog- scope and found the excellent functional group tolerance of the
raphy was determined to be 96%, which is in accordance with reaction. Similar to the ndings demonstrated in Suzuki
the GC yield (Table 1, entry 1).
coupling reactions, aryl iodides containing electron with-
Based on the evaluation of Pd loading of the supported drawing groups, such as CN, COOC2H5, CH3CO, and CHO
catalyst (5 s for Pd NP growth) by ICP-OES analysis (0.0126 mg (Table 2, entries 7–11) exhibited higher reactivity as well
cmꢀ2), we calculated the turnover number (TON) and turnover compared with reactants bearing electron donating groups
frequency (TOF) up to be 9900 and 1980 hꢀ1 (Table 1, entries 10 including –CH3 and –OCH3 (Table 2, entries 4–6). Also, we
and 11), respectively. It is worth noting that scale-up reaction discovered that functional groups on aryl boronic acids exerted
(10 times) of iodobenzene with phenylboronic acid afforded an important inuence on the reactivity of the substrates. In
a yield of 99%, showing the potential utility of the as-prepared general, the substrates with an electron donating group
catalyst in practical chemical industry. ICP-OES analysis exhibited much higher reactivity than those having an electron
showed no obvious Pd leaching in the reaction system by withdrawing group (Table 2, entries 1–3). In addition to yielding
removal of the nanoarray catalysts (<1 ppm). Taken together, a variety of carbocyclic ketones, carbonylative Suzuki coupling
these results suggest the advantage of the as-prepared Pd NP– reactions have the ability to offer heterocyclic ketone with
ZnO nanowire arrays with respect to reactivity when compared a good yield using heterocyclic iodides such as 3-iodopyridine
with previously reported work.16
as the starting substrates (Table 2, entry 12).
We also studied the recyclability of the Pd NP–ZnO nanowire
arrays in the Suzuki reactions. The results showed catalytic
decrease from 99 to 80% aer the use of 3 times for the coupling
Conclusions
reaction of iodobenzene with phenylboronic acid. This may be In summary, we have examined the catalytic utility of Pd NP–
due to the gradual dissolution of ZnO nanowires under strong ZnO nanowire arrays under different catalytic condition on the
alkaline and high-temperature conditions, leading to slight “fall basis of their easy synthesis by immersing ZnO@Zn nanowire
off” of Pd NPs (Fig. S7a†). XPS analysis of the used Pd NP–ZnO arrays into an aqueous solution of Na2PdCl4 for varied periods
nanowire arrays showed no obvious changes in the valence state of time. The as-prepared Pd NP–ZnO nanowire arrays with an
of the Pd NPs (Fig. S7b†), accounting for the maintenance of the immersion time of 5 s have been found to be applicable as
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Table 2 Scope of the Pd NP–ZnO nanowire array-catalyzed carbonylative Suzuki coupling reaction of aryl iodides with aryl boronic acidsa
Entry
Ar1–X
Ar2
T (h)
Product
GC yieldb (%)
1
6
93
2
3
4
5
12
24
6
85
81
86
82
12
6
7
8
6
87
92
90
6
12
9
6
6
6
89
88
86
10
11
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Table 2 (Contd.)
Entry
Ar1–X
Ar2
T (h)
Product
GC yieldb (%)
12
12
82
a
Reaction conditions: aryl halide (0.5 mmol), aryl boronic acid (0.6 mmol), K2CO3 (1.0 mmol), Mo(CO)6 (0.3 mmol), DMF (3.0 mL), and Pd NP–
b
ZnO@Zn (0.12 mol%), 100 ꢂC for 6–24 h. Yields were determined by GC using dodecane as an internal standard upon the completion of the
reactions.
recyclable catalysts for both 4-nitrophenol reduction and (car-
bonylative) Suzuki reaction, which were performed under mild
and relatively harsh conditions, respectively. These ndings
imply the feasibility of the development of highly recyclable
catalysts through a combination of transition metal oxide
nanowire array supports and viable catalytically active noble
metal NPs.
Synthesis of ZnO@Zn nanowire arrays
ZnO@Zn nanowire arrays were prepared by a hydrothermal
method.20 In a typical synthesis, ammonia (6.0 mL) and distilled
water (34.0 mL) were rst added into a 60 mL Teon-lined
autoclave to form a homogeneous solution. Aer the addition
of four pieces of clean Zn foil (1.5 cm ꢃ 1 cm) into the resulting
ꢂ
solution, the autoclave was heated at 110 C for 14 h.
Synthesis of Pd NP–ZnO nanowire arrays
Experimental section
General
The as-prepared ZnO@Zn nanowire arrays were rst washed
with distilled water, and then dried under a nitrogen atmo-
Zn foil ($99.99%), ammonium hydroxide solution (28.0– sphere. The dried ZnO@Zn NW arrays were then immersed into
30.0%), Na2PdCl4 ($99.99%), dimethylformamide (DMF, an aqueous solution of Na2PdCl4 (5.0 mM) for different periods
analytic grade), 4-nitrophenol ($98%) and NaBH4 (98%) were of time (5, 15 and 25 s) to control the size distribution of Pd NPs.
purchased from Shanghai Chemical Reagents Company. The resulting ZnO@Zn NW arrays were nally washed with
Aromatic iodides and bromides ($98%), aryl boronic acid distilled water and dried under a nitrogen atmosphere before
($98%) and Mo(CO)6 (98%) were purchased from Alfa Aesar. further use.
Unless otherwise stated, all chemicals were used as received
without further purication.
Determination of Pd loading on the surface of ZnO nanowire
arrays
Two pieces of as-prepared Pd–ZnO nanowire arrays (1.5 cm ꢃ 1
cm) were dissolved with aqua regia. The content of Pd in the
sample was determined by ICP-OES. On the basis of ICP-OES
analysis and the total size of the hybrid nanowire arrays, Pd
loading per square centimetre was obtained.
Physical measurements
Transmission electron microscopy (TEM) characterization was
performed on a FEI Tecnai G2 20 operated at 200 kV. The
energy-dispersive X-ray (EDX) spectroscopic analysis was
recorded with an Oxford INCA energy system operated at 200
kV. Scanning electron microscopy (SEM) measurements were
carried out by using a eld-emission scanning electron micro-
scope (Hitachi S-4800). X-ray photoelectron spectroscopy (XPS)
analysis was conducted on a Thermo ESCALAB 250 electron
spectrometer with the use of a monochromated Al Ka X-ray
source. ICP-OES measurement was performed on a Dual-view
Optima 5300 DV ICP-OES system (PerkinElmer). UV-visible
absorption proles were obtained on a Shimadzu UV-2450
spectrometer. The organic products were analyzed by
GC7890F equipped with a ame ionization detector and a GC
(Thermo Trace 1300)-MS (Thermo ISQ QD) system. Note that
during the analysis, temperatures at the column, injector and
ame ionization detector were respectively set at 300, 250 and
300 ꢂC. 1H and 13C NMR spectra were recorded at 25 ꢂC on
Pd NP–ZnO nanowire arrays for 4-nitrophenol reduction
A stock solution was rst prepared by mixing NaBH4 (2.0 mL,
0.2 M) and 4-nitrophenol (2.0 mL, 2 ꢃ 10ꢀ4 M) in a vial (5.0 mL).
Two pieces (ꢁ4.8 cm2 in total) of Pd NP–ZnO were then added
into the resulting solution to initialize the reduction of 4-
nitrophenol. The kinetics of the catalytic reduction of 4-nitro-
phenol was in situ monitored by UV-visible absorption spec-
troscopy. At the end of the reaction, Pd NP–ZnO nanowire arrays
were separated from the mixed solution by tweezers, washed
three times with ethanol, and nally dried at room temperature
for recyclably catalytic studies.
Pd NP–ZnO nanowire arrays for Suzuki reaction
a Bruker Avance-300 at 300 and 75 MHz by using tetrame- Five small pieces (ꢁ1.0 cm2 in total) of Pd NP–ZnO were added
thylsilane as an internal standard, respectively.
into a 25 mL tube reactor containing aryl halide (1.0 mmol), aryl
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boronic acid (1.1 mmol), K2CO3 (1.5 mmol) and DMF (3.0 mL),
and then the mixture was sealed and heated at 100 ꢂC for 5–24 h
under gentle stirring. At the end of the reactions, Pd NP–ZnO
nanowire arrays were recollected from the catalytic mixture by
centrifugation. The organic phase was subsequently obtained
by extraction through the use of ethyl acetate as solvent. The
obtained solution was further washed with brine (3 ꢃ 10.0 mL),
dried over anhydrous magnesium sulphate, and concentrated
under reduced pressure. The residue was puried by column
chromatography on silica gel (EtOAc/petroleum ether ¼ 1/20) to
afford a pure product.
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Recycling test for Suzuki reaction
The supported Pd catalyst was recollected by centrifugation at
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Typically, several small pieces (ꢁ5.0 cm2 in total) of freshly
prepared Pd NP–ZnO NW arrays were added into a mixture of
aryl halide (0.5 mmol), aryl boronic acid (0.6 mmol), K2CO3 (1.0
mmol), Mo(CO)6 (0.3 mmol) and DMF (3.0 mL), and the
ꢂ
resulting mixture was then sealed and heated at 100 C for 6–
12 h under gentle stirring. The process for catalyst collection
and product purication was identical to that described in
catalytic study of the hybrid nanowire arrays in Suzuki
reactions.
Determine the Pd leaching by ICP-OES analysis
´
´
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R. N. Grass, G. Vile, J. Perez-Ramırez, W. J. Stark and
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Aer a run of Suzuki reaction, the reaction mixture was washed
with H2O aer the removal of Pd NP–ZnO nanowire arrays. The
resulting aqueous phase was separated and further diluted for
ICP-OES measurement.
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Acknowledgements
This study was supported by the National Natural Science
Foundation of PR China (No. 21272006, 21471007), and the
research grant for innovative experiments of graduate students
(No. 2016yks053).
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