Direct Synthesis of Palladium Catalyst on Supporting WS2 Nanotubes and its Reactivity in Cross-Coupling Reactions

Article abstract of DOI:10.1002/asia.201500271

Palladium nanoparticles were deposited on multiwall WS₂ nanotubes. The composite nanoparticles were characterized by a variety of techniques. The Pd nanoparticles were deposited uniformly on the surface of WS₂ nanotubes. An epitaxial relationship between the (111) plane of Pd and the (013) plane of WS₂ was mostly observed. The composite nanoparticles were found to perform efficiently as catalysts for cross-coupling (Heck and Suzuki) reactions. The role of the nanotubes′ support in the catalytic process is briefly discussed.

Full text of DOI:10.1002/asia.201500271

DOI: 10.1002/asia.201500271
Full Paper
Nanotubes
Direct Synthesis of Palladium Catalyst on Supporting WS2
Nanotubes and its Reactivity in Cross-Coupling Reactions
ˇ ´ [a]
[a]
[a]
[a]
[b]
Bojana Visic, Hagai Cohen, Ronit Popovitz-Biro, Reshef Tenne, Viacheslav I. Sokolov,*
Natalya V. Abramova,[b] Anastasiya G. Buyanovskaya,[b] Sergei L. Dzvonkovskii,[b] and
Olga L. Lependina[b]
Abstract: Palladium nanoparticles were deposited on multi-
wall WS2 nanotubes. The composite nanoparticles were char-
acterized by a variety of techniques. The Pd nanoparticles
were deposited uniformly on the surface of WS2 nanotubes.
An epitaxial relationship between the (111) plane of Pd and
the (013) plane of WS2 was mostly observed. The composite
nanoparticles were found to perform efficiently as catalysts
for cross-coupling (Heck and Suzuki) reactions. The role of
the nanotubes’ support in the catalytic process is briefly dis-
cussed.
Introduction
porated into porous carbon for Suzuki reactions, owing to the
large surface areas of the pores.[5] Thus, in all the previous
studies different forms of nanocarbon were used as a support
for the Pd-based catalyst.
Supported palladium catalysts are widely used in many organic
reactions including the formation of CÀC bonds, cross-coupling
reactions, etc. Until recently, they were always prepared by the
reduction of PdII compounds, mostly salts, in aqueous solutions
under alkaline pH. The alternative method used the stable Pd0
complex, tris(dibenzylideneacetone)dipalladium(0) or Pd2(dba)3,
as a reagent. This complex is easy to prepare and handle as
a solvate with benzene or chloroform. This process has been
initially elaborated for carbon nanotubes (CNTs). CNTs provide
strained C=C bonds which were the place of the attack by Pd0,
thus promoting the growth of Pd nanoclusters.[1] The reaction
between Pd2(dba3) and carbon nanotubes proceeds smoothly
in an inert solvent under neutral conditions. Later it was found
that this approach was also applicable to promote the growth
of Pd nanoclusters atop of nanodiamonds[2] and graphene.[3]
All these materials have been shown to be effective catalysts
in the hydrogenation of unsaturated hydrocarbons and CÀC
cross-coupling reactions. The synthetic route to form chiral
heterogeneous Pd catalysts on the basis of hydroxyl-CNTs
modified by optically active a-amino acids is under study.[4]
These catalysts require no additional ligands; they are reusable
and able to process in air and water, meeting the requirements
of ’green chemistry’. Additionally, Pd species have been incor-
Very recently, this approach has been used for the first time
outside the realm of carbon chemistry. Here superparamagnet-
ic and magnetically-recoverable nano-Pd clusters were immo-
bilized on layered double hydroxide–iron oxide composite cat-
alysts active in the cross-coupling reactions.[6] Among them,
the usual investigations are Heck and Suzuki reactions.
Layered compounds of MoS2 and WS2 are known to be very
reactive in catalytic reactions at the layer edges, that is, the
prismatic (hk0) planes.[7] These sites provide for the dangling
bonds, while at the basal planes (001) the bonds are fully satu-
rated and therefore chemically inactive. In order to improve
the catalytic behavior, there should be an increase in the
amount of edge sites. The discovery of WS2 nanotubes (INT-
WS2)[8] and scaling-up of the their synthesis[9] made it possible
to explore this material in catalytic reactions in the nanoscale
dimensions and tubular geometry. Both WS2 nanotubes and
nanoparticles have already been tested for their catalytic activi-
ty as the sole catalysts or as a supporting material for metallic
nanoparticles.[10] Additionally, the nanotubes were broken into
nanoflakes using sonochemistry in order to break SÀW bonds
and provide for more active edge sites.[11] The proposed cata-
lytic reactions were hydrodesulfurization,[10a] photocatalysis,[10b]
and electrocatalysis[11]
.
ˇ ´
[a] Dr. B. Visic, Dr. H. Cohen, Dr. R. Popovitz-Biro, Prof. R. Tenne
In the present article, Pd nanoparticles were deposited onto
the INT-WS2 surface and used as catalyst for cross-coupling re-
actions.
Weizmann Institute of Science
Rehovot (Israel)
[b] Prof. V. I. Sokolov, Dr. N. V. Abramova, Prof. A. G. Buyanovskaya,
S. L. Dzvonkovskii, Dr. O. L. Lependina
Nesmeyanov Institute of Organoelement Compounds RAS
28 Vavilov Street, 119991, Moscow (Russian Federation)
E-mail: sokol@ineos.ac.ru
Results and Discussion
In this work, the objective was to demonstrate the catalytic ac-
tivity of as-prepared Pd/INT-WS2 in the representative reactions
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500271.
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Table 1. Atomic concentrations determined from direct combustion
measurements.
Atomic concentration [%]
Concentration ratio
Sample
INT-WS2
H
0
N
0
S
W
Pd
0
W/S
1:2.06
67.32 32.68
Pd-INT-WS2 30.08 0.42 43.08 22.26 4.15 1:1.93
Scheme 1. a) Heck and b) Suzuki reactions.
usually used as standards of cross-coupling. In the experiments
with pristine INT-WS2, under the same conditions, no reaction
products were found. Consequently, the pure nanotubes were
found to be inactive in the catalytic process.
Figure 1. SEM images of a) a larger area of the sample showing a bundle of
Pd-coated WS2 nanotubes and b) an individual Pd-coated nanotube showing
uniform coverage together with an agglomerate of Pd nanoparticles.
The concentrations of the elements in the Pd/INT-WS2
hybrid nanoparticles, as determined by classical elemental
analysis by combustion, are summarized in Table 1. This data
correspond to one Pd atom for approximately 5 WS2 units.
inset of Figure 2. This result is in fair agreement with the ratio
observed through the aforementioned direct combustion
measurements. The C and Cu peaks arise from the TEM grid.
Nevertheless, the composition obtained by EDS should be re-
garded as semi-quantitative only.
½WS2Šn þ Pd2ðdbaÞ3 ! ½ðWS2Þ5Pd1Š
m
To reveal the fine structure of the NP-INT interface, HRTEM
was also performed. The images show that the Pd nanoparti-
cles seem to grow uniformly on the outermost layer of the
WS2 nanotubes with a quasi-epitaxy relationship between the
two surfaces. The analysis of the interlayer distances reveals
lattice fringe continuations. They correspond to Pd(111) and
WS2(013) lattice planes with spacings of 0.224 nm and
0.227 nm, respectively. These observed interplanar distances
are an indication of epitaxial growth. Some examples of these
lattice continuations are depicted in Figure 3. While most of
the deposited Pd nanoparticles showed the above crystalline
orientation, some exhibited another epitaxial relationship with
the INT-WS2 substrate. The inset image shows a lattice spacing
Reactions used for catalytic tests were Heck and Suzuki cou-
plings, with yields of 91% after 3 h at 1208C and 86% after 3 h
at 558C, respectively. Schematics are depicted in Scheme 1.
As revealed by the SEM images shown in Figure 1, the com-
posite material is predominantly uniform and consists mainly
of multiwall WS2 nanotubes (INT-WS2) coated with Pd nanopar-
ticles (Pd NPs). The INTs are up to several micrometers long
with diameters in the range of 40–150 nm. While the majority
of Pd NPs uniformly cover the surface of INTs, in few locations
some free-standing Pd NP agglomerates are discernible.
In order to provide for a better understanding of the Pd-WS2
interface, TEM analysis was performed, as shown in Figure 2. It
can be seen that Pd NPs that are
about 3–8 nm in size decorate
the surface of the nanotubes
quite uniformly and densely. This
observation is in agreement with
the SEM analysis. The nanoparti-
cles seem to be tightly attached
to the nanotube surface, sug-
gesting an intimate contact be-
tween the two materials at the
interface. This contact suggests
a form of an epitaxial relation-
ship and lack of any impurities
such as oxide or carbon contam-
ination at the interface.
The EDS analysis performed
within the TEM shows that the
Pd makes about 10 at% of the
Figure 2. TEM images of a) individual decorated nanotubes demonstrating uniform surface coverage and b) a
amount of W, as shown in the closer look at a decorated nanotube, showing WS2 layers, with the corresponding EDS analysis (inset).
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Figure 4. HRTEM images showing the growth of the Pd nanoparticles on
structural defects of WS2 nanotubes, namely a) a nanoparticle influencing
the topmost nanotube layer and b) a nanoparticle attached to a step.
Figure 3. HRTEM images providing a closer view of the nanotube–nanoparti-
cle interface, with visible fringe continuations. a) Palladium NPs lattice spac-
ing of 0.23 nm and b) with the inset of the continuation of 0.146 nm.
of 0.146 nm. This value corresponds to 5% contraction of the
Pd(220) lattice plane, and 5% expansion of the WS2(112) lattice
plane (with spacings of 0.138 nm and 0.154 nm, respectively).
Furthermore, Pd nanoparticles
and the valence band spectrum in Figure 5d, which are both
typical for INT-WS2. The C signal is attributed to surface con-
taminants (small organic moieties), while the O signal (not
preferably grow on nanotube
steps and defects, and they in-
fluence the topmost WS2 layers,
as shown in Figure 4a. A Pd
nanoparticle situated on the
step is shown in Figure 4b. No
indication for an interfacial reac-
tion between the Pd NPs and
the WS2 nanotubes was ob-
tained from this analysis.
The XPS measurements pro-
vide an insight into the chemis-
try of the outermost surface (ca.
10 nm). The spectra of pristine
INT-WS2 and Pd-coated INT-WS2
are shown in Figure 5. The
atomic concentrations are sum-
marized in Table 2 and the repre-
sentative peak positions are
listed in Table 3.
The pristine INT signals shown
in Figure 5a, b and d are in
good agreement with those in
the literature, with S 2p3/2 line
peaks at 162.04 eV and the W
4f7/2 peaks at 32.40 eV. Their in-
Figure 5. XPS spectra of Pd-coated WS2 nanotubes revealing the core levels of a) Mo, b) W, and c) Pd, together
with d) the valence band of both pristine and decorated nanotubes. The black lines correspond to the spectra of
pristine nanotubes, while red lines correspond to those of decorated nanotubes.
tensity ratio is close to 2:1, with
a deviation (see Table 2) that
arises from the W-oxide filling of
these INTs a result of incomplete
sulfidization in the core of the
nanotubes.[9,12] Traces of this fill-
ing are clearly seen in Figure 2b,
evidenced by the 4f7/2 peak at
35.68 eV corresponding to the
binding of W with O. Note also
the couple of shake-up satellites
around 168–174 eV in Figure 5a,
Table 2. Surface atomic concentrations as determined from the XPS analysis.[a]
Atomic concentration [%]
Concentration ratio
Sample
O
C
S
W
Pd
W/S
INT-WS2
Pd-INT-WS2
19.93 (5)
16.91 (15)
15.18 (7)
53.01 (5)
42.41 (1)
12.22 (5)
22.48 (<1)
5.40 (2)
0
1:1.9
1:2.3
12.46
[a] For the coated sample, evaluation of the O concentration involved its O(KLL) Auger signal, due to the over-
lap between O 1s and Pd 3d lines. Relative experimental errors (in%) are given in brackets, while extracted
concentration values are kept unrounded.
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longstanding question on the role of the support in catalytic
reactions and in the case of the nano-sized substrate has been
investigated in several specific cases only. The catalytic activity
is governed not only by the structure of NPs but also by the
preparation methods and the catalyst–support interactions.
These interactions have already been confirmed to play
a major role in the catalytic properties of Au NPs, which are
not highly active until they are deposited on several metal
oxides.[16] Therefore, the supporting substrate has an important
role in this case. Since the XPS analysis indicates that there is
flow of electrons from the WS2 INT to the Pd NP interface, the
influence that INT have on the catalytic activity of the Pd nano-
particles may not be negligible.
Table 3. Representative binding energies (in eV), as determined by XPS.
No energy calibration is applied.
Element
Orbital
INT-WS2
Pd-INT-WS2
S2À
2p 3/2
4f 7/2
4f 7/2
3d 5/2
162.04
32.40
35.68
162.20
32.56
35.84
W4+
W6+
Pd
335.53
shown) contains a resolved component of the W-oxide filling,
plus a significant amount of water and OH groups. Part of the
latter is probably due to adsorption on the surface of INTs.
Upon Pd-coating, all the relevant INT signals are suppressed,
as expected for a surface-sensitive technique like XPS; yet,
their spectral signatures are generally retained. In addition to
the Pd signal, an increased amount of carbon is observed (see
Table 2). Again, the carbon signal is associated mainly with sur-
face contamination. Such contamination is probably not detri-
mental to the catalytic reactivity, since the carbon is only
loosely attached to the surface and is removed during the cat-
alytic reactions. It is also noted that the S/W ratio in the
coated sample is larger than the pristine stoichiometry. This
latter fact probably originates from the emergence of some S-
contamination that accompanies the much larger C-contami-
nation. The possibility of sulfur out-diffusion from the INTs is
somewhat unlikely, although it cannot be fully ruled out.
Conclusions
Since the precious metals are important materials in catalytic
research, their uniform and efficient attachment to an appro-
priate supporting material can be of great importance for their
catalytic reactivity. It has been shown here that the present
method of depositing Pd nanoparticles on WS2 nanotubes pro-
vides for a uniform coverage of Pd on the nanotubes’ surface.
Furthermore, a closer look at the Pd-WS2 interface by HRTEM
shows fringe continuity. This observation strongly suggests an
epitaxial relationship at this interface. The XPS analysis sug-
gests a barrier-free flow of electron across the WS2 nanotube–
Pd NP interface. This observation suggests that the nanotubes
could play an active role in the catalytic reaction. The obtained
Pd NP–INT-WS2 catalyst gave 91% conversion in the Heck reac-
tion and 86% yield in the Suzuki reaction. In particular, the
charge transfer from the nanotubes to the Pd nanoparticles
could be detrimentally important for the first step of the cou-
pling reactions, that is, the oxidative addition of the reactant
to the metal nanoparticle surface.
The Pd line shape, as shown in Figure 5c, exhibits a clear
shoulder, which does not fit pure Pd composition. This feature
can possibly be attributed to the hydrogen inclusion in the
palladium nanocrystals. Importantly, the metallic conductivity
of the Pd-coating was tested and confirmed in situ by applica-
tion of an electron-flood-gun.[13] Therefore, this line shape is
verified to be free of differential charging artifacts. A rough es-
timate of the Pd-coating thickness can be made, based on the
XPS-derived atomic concentrations. Assuming about 50% sur-
face coverage of the cylindrical tubes, one obtains from the
values in Table 2 an average thickness of Pd coating of about
3.8 nm (a topofactor of 0.79 for cylinders was used here[14]).
The size of the Pd nanoparticles is found to be in reasonable
agreement with the electron microscopy data.
Experimental Section
Material Preparation
Notably, close examination of the XPS lines does not show
any direct indication for the formation of chemical bonds, for
example, Pd-S, at the Pd-INT interface. These findings are con-
sistent with the results of the TEM analysis. On the other hand,
an indirect observation regarding the nature of this interface
arises from the change in W and S binding energies upon coat-
ing with both lines shifting by 160 meV to higher binding en-
ergies. For this powder-like sample, one may interpret this shift
as reflecting electron transfer from the INTs to the Pd particles.
The high yield obtained in both reaction mechanisms may
suggest that the INT-WS2 play an active role rather than being
just a passive substrate for the Pd nanoparticles. It was already
shown that INT decorated with Ni NPs have an improved cata-
lytic activity for hydrodesulfurization compared with pristine
nanotubes.[10a] While there are studies regarding the influence
of size and shape of Pd NPs on the catalytic activity,[15] the
The WS2 nanotubes were synthesized according to a previously de-
scribed procedure[9,12] provided by “NanoMaterials”. Briefly, WO3
nanoparticles were fluidized in quartz reactor at 8608C and were
reacted in H2S atmopshere for 6 h. The multiwall WS2 nanotubes
were found to have diameters between 30–150 nm and length of
1–20 microns.
Preparation of the catalyst by the direct reaction between
Pd2(dba)3 and WS2 nanotubes and the study of its catalytic
properties
The synthesis of the catalyst and the Heck and Suzuki reactions
were performed under the standard conditions elaborated earli-
er.[1–3] The resultant catalyst appeared to be active in both reac-
tions. However, the supported catalyst was found to be mechani-
cally unstable during the reaction and could not be easily reused.
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Preparation of Pd/(WS2)
Suzuki reaction
Pd2(dba)3 (50 mg) was dissolved in toluene (20 mL). The wine-red
solution obtained was filtered through a paper filter to remove
some insoluble material. Afterwards it was stirred magnetically
under argon gas during 40 min at 558C in the presence of sus-
pended INT (100 mg). The color of the solution turned to straw-
yellow (owing to liberated dba ligand). The reaction mixture was
cooled to room temperature, the black precipitate was filtered,
washed with toluene (4”20 mL), then with pentane (4”20 mL)
and dried. The weight of the product Pd/INT-WS2 was 104.2 mg.
A mixture of bromobenzene (2 mm), m-tolylboronic acid (3 mm)
and K2CO3 (4 mm) in methanol (12 mL) and water (4 mL) was
stirred for 5 min, then catalyst Pd/(WS2) containing 7.5% Pd
(10 mg) was added and stirring continued for 3 h at 55–608C. The
reaction mixture was diluted with water, extracted with ethyl ace-
tate (2”50 mL), and dried over Na2SO4. The solvent was evaporat-
ed, the residue was taken up with hexane and purified through
a short SiO2 layer. Evaporation gave the product, 3-methylbiphenyl,
1
in 86% yield, identified by H NMR spectroscopy.
Starting INT: Found (wt%) W 72.80, S 26.16.
3-Methylbiphenyl: 1H NMR (CDCl3): d=2.55 (s, 3H, CH3), 7.29 (m,
1H, Ph), 7.48 (m, 2H, Ph); 7.55 (m, 4H, Ph), 7.70 ppm (m, 2H, Ph).
Pd/(WS2): Found (wt%) C, 0; H, 0.51; N, 0.1; Pd, 7.5; W, 69.4; S,
23.42.
The sample of catalyst containing 3% Pd gave a yield of 85% in
the Suzuki reaction, the repeated use of the catalyst gave a yield
of 58%.
When the INT powder (100 mg) was reacted with Pd2(dba)3
(27.2 mg) at 688C, the Pd/(WS2) formed contained 3% Pd.
It should be stressed that when the reactions were carried out in
the presence of the starting INT (without Pd) the products of
cross-coupling were not found.
Found (wt%): C, 1.05; H, 0; N, 0.1; Pd, 3.0; W, 71.2; S, 24.62.
Characterization
Acknowledgements
Elemental analysis of the samples in question was performed using
two devices. The contents (wt%) of C, H, N, S were determined
(after combustion in oxygen) by using an automatic VarioMicro-
Cube (Elementar, Germany). The W and Pd analysis was done by
non-descructive X-ray fluorescence (XRF) analysis using a VRA-30
(Carl Zeiss Jena, Germany) set-up.
This work has been supported by the Program 01 from Divi-
sion of Chemistry and Materials, Russian Academy of Sciences.
R.T. acknowledges the support of the oFTA-INNI initiative of
VATAT (Israel); Israel Science Foundation; H. Perlman Founda-
tion; the Irving and Azelle Waltcher Foundations in honor of
Prof. M. Levy and the Irving and Cherna Moskowitz Center for
Nano and Bio-Nano Imaging. B. V. acknowledges the support
of EU ITN 317451 grant. We are grateful to Dr. Yishai Feldman
for the XRD measurements.
Electron microscopy was performed using a scanning electron mi-
croscope (SEM) (Zeiss Ultra model V55 SEM and Zeiss LEO Supra
55VP FEG SEM equipped with energy-dispersive X-ray spectroscopy
(EDS) with a probe size of about 1 mm). The transmission electron
microscopy (TEM) was carried out with a Philips CM-120 instru-
ment operating at 120 kV equipped with an energy-dispersive X-
ray analyzer (EDS). High-resolution TEM (HRTEM) was performed
with a FEI Technai F30-UT microscope operating at 300 kV. All pow-
ders that were analyzed by electron microscopy were dispersed in
ethanol, using an ultrasonic bath, and then placed onto carbon/
collodion/Cu grids (for TEM) or on lacey carbon/Cu grids (for
HRTEM).
Keywords: cross-coupling
nanotubes · Pd catalysis · Suzuki reaction
·
Heck reaction
·
inorganic
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Manuscript received: March 23, 2015
Final Article published: June 12, 2015
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