3
0
J.Y. Kim et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 28–32
◦
washed with distilled water, and dried in an oven at 110 C. The
acid-treated MWNTs were dispersed in THF and then an NaSH
aqueous solution was added to produce thiol groups on their sur-
faces, and the resulting CNTs are denoted as CNT-SH. The thiolation
was confirmed using the XPS spectrum in the sulfur 2p region.
Finally, the thiolated MWNTs were dispersed in THF and then a
Pd(dba) /THF solution was added. The mixture was stirred for 20 h
2
until all of the Pd(dba) precursors were anchored onto the MWNTs,
2
and the resulting CNTs are denoted as CNT-Pd. These samples were
separated from the mixture by filtration, washed several times with
◦
pure ethanol and DI water, and dried in a vacuum oven at 50 C for
4
h. To verify the effect of the support, a mixture of Pd(dba) /CNTs
2
was also prepared by the following method. 2.0 mg of Pd(dba)2
and 100 mg of pristine CNTs were added to THF and the reaction
mixture was stirred at room temperature for 3 h. The solvent was
evaporated and the residue was dried in a vacuum for 12 h.
The transmission electron microscopy observations were car-
ried out in a JEM-2200FS microscope at 200 kV. Samples for the
TEM analysis were prepared by extensive sonication of the CNTs in
ethanol. A drop of the solution was deposited on a gold grid and the
solvent was allowed to evaporate in air. XPS analysis was performed
using a VG multilab 2000 spectrometer (ThermoVG scientific) in an
ultra high vacuum. This system uses an unmonochromatized Mg
K␣ (1253.6 eV) source and a spherical section analyzer. Survey scan
data was collected using a pass energy of 50 eV. The content of Pd in
the Pd-CNT nanocomposite was determined by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) with an OPTIMA
Fig. 3. Raman spectra of pristine CNTs and CNT-Pd nanocomposites.
band arises from a tangential shear mode of the C atoms that cor-
ꢀ
responds to the stretching mode in the graphite plane. The D band,
which is a shoulder of the G band at a higher frequency, corre-
sponds to second-order Raman scattering from the variation of the
D-band. The intensity ratio of the D band to G band (ID/I ) has a lin-
G
ear relation with the inverse of the in-plane crystallite dimension.
The value of ID/IG is about 1.38 for the pristine CNTs and about 2.56
for the CNT-Pd. These results suggest that the functionalization of
the CNTs results in a decrease of their crystallinity.
4
300 DV (PerkinElmer). Prior to the measurement, the sample was
treated with a mixture of HNO , HF and HBO3 in order to dissolve
3
it completely. Raman spectra were obtained at room temperature
using an inVia Reflex (Renishaw 1000) micro-Raman spectrometer
with 632.8 nm laser line.
Fig. 4 shows a series of XPS survey spectra from the pris-
tine CNTs, CNT-SH and CNT-Pd. For the pristine CNTs, the XPS
data shows distinct C and O 1s peaks and no other elements are
detected. However, after their thiolation, the presence of S ele-
ment is detected from the CNT-SH. The relative surface atomic ratio
was estimated from the corresponding peak areas, corrected with
the tabulated sensitivity factors. The estimated value of the S con-
tent is about 2.7 at.%. Since the XPS signal is obtained by collection
of photoemitted electrons, this technique is very sensitive to the
properties of the surface. For the Pd-CNT, the photoemitted elec-
trons from S atoms are screened by deposited Pd nanoparticles,
which resulted in the featureless S 2p profile. The XPS data also
confirmed the presence of Pd in the nanocomposites. The Pd con-
tent is estimated to be 2.0 at.%. According to ICP experiments, the
The catalytic activity of the prepared CNT-Pd for the Stille and
Hiyama coupling reaction was examined. These reactions were
used as a standard test reaction to probe the reactivity of Pd-
catalyzed carbon–carbon bond forming reactions. The coupling
reaction was performed in a round-bottom flask fitted with a water-
cooled condenser. 4-Iodotoluene was employed as a standard
substrate for the coupling reactions. The reactions were carried out
with 0.3 mol.% of palladium under the previously reported condi-
tions [21,22].
3
. Results and discussion
The synthetic procedures of the CNT-Pd nanocomposites routes
are shown in Fig. 1. Thiol (–SH) groups were utilized as linkers
between the Pd nanoparticles and CNTs. The Pd nanoparticles are
anchored to the surface of the CNTs due to their interaction with
the free electron pairs of the S atoms. The detailed morphology
of the CNT-Pd nanocomposites was examined by TEM. Fig. 2(a)
shows a typical TEM image of a pristine CNT with a diameter in
the range of 10–20 nm. All of the tubes had a clean surface. For the
CNT-Pd nanocomposites, the TEM image shows that smaller and
highly dispersed nanoparticles were much more abundant than
larger aggregated ones [Fig. 2(b)]. The magnified image reveals that
the nanoparticles are strongly adhered on the sidewalls of the CNTs
[
Fig. 2(c)]. The average particle size is estimated to be ∼6.0 nm. The
EDS analysis shows that the species supported on the CNT was Pd
[Fig. 2(d)].
Raman spectroscopy was used to obtain information about the
average crystallinity of the CNT-Pd compared with that of the pris-
tine CNTs. As shown in Fig. 3, the spectrum consists of three bands
−
1
−1
−1
ꢀ
at ∼1330 cm (D band), 1570 cm (G band), and ∼1610 cm (D
band) [23,24]. The D band is a disorder induced feature originat-
ing from the vibrations of C atoms with dangling bonds. The G
Fig. 4. XPS survey spectra of pristine CNTs, CNT-SH, and CNT-Pd nanocomposites.