The two-step chemical vapor deposition of Pd(allyl)Cp as an atom-efficient route to synthesize highly dispersed palladium nanoparticles on carbon nanofibers

Article abstract of DOI:10.1039/b412150c

Highly dispersed palladium nanoparticles supported on carbon nanoflbers, which show high catalytic activity and stability in the hydrogenation of cyclooctene, were synthesized by the two-step metal organic chemical vapor deposition (MOCVD) of allylcyclopentadienylpalladium (Pd(allyl)Cp) as precursor at atmospheric pressure.

Full text of DOI:10.1039/b412150c

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
The two-step chemical vapor deposition of Pd(allyl)Cp as an atom-
efficient route to synthesize highly dispersed palladium nanoparticles on
carbon nanofibers
a
Changhai Liang, Wei Xia, Hamideh Soltani-Ahmadi, Oliver Schl u¨ ter, Roland A. Fischer and
a
a
a
b
a
Martin Muhler*
Received (in Cambridge, UK) 6th August 2004, Accepted 15th October 2004
First published as an Advance Article on the web 23rd November 2004
DOI: 10.1039/b412150c
Highly dispersed palladium nanoparticles supported on carbon
nanofibers, which show high catalytic activity and stability in
the hydrogenation of cyclooctene, were synthesized by the two-
step metal organic chemical vapor deposition (MOCVD) of
allylcyclopentadienylpalladium (Pd(allyl)Cp) as precursor at
atmospheric pressure.
5 M HNO
sublimed at 50 uC and adsorbed at 80 uC in He as carrier gas. The
adsorbed precursor was reduced at 80 uC in diluted H at
atmospheric pressure. The level of Pd loading was found to depend
on the degree of functionalization of the carbon nanofibers and on
the amount of precursor provided in the sublimation chamber.
Transmission electron microscopy (TEM) images did not reveal
the presence of any Pd particles when the support was not
3
prior to palladium deposition. The precursor was
2
1
Since their discovery in 1991, carbon nanotubes and nanofibers
have attracted great scientific interest from a fundamental point of
3
functionalized by HNO (Fig. 2, left). Instead, the deposition of Pd
view and because of their potential applications such as structure
2
reinforcement, nanoelectronic devices, and field emitters. The
black was observed on the glass walls of the reactor due to the low
amount of anchoring sites on the support in agreement with
phenomena observed during the deposition of platinum onto
novel nanostructured carbon materials are considered promising
heterogeneous catalysts or catalyst supports due to their unique
3
structural, electronic and mechanical properties. Several studies
10
carbon nanospheres, multi-walled carbon nanotubes or graphite.
Samples with Pd loadings of 2.04 wt% and 4.30 wt% (from ICP-
OES) were obtained, when the designed loadings were 2.5 wt%
and 5.0 wt%, respectively. Their X-ray diffraction (XRD) patterns
shown in Fig. 1 only reveal a broad Pd(111) diffraction peak at
about 40.2u for the 4.30 wt% Pd sample, indicating rather small Pd
particles in addition to the diffraction peaks originating from the
carbon nanofibers. The right part of Fig. 2 shows a TEM image of
the functionalized sample with 2.04 wt% Pd. Highly dispersed and
evenly distributed Pd particles with diameters of 2–4 nm are
observed. The high degree of dispersion points to a strong
interaction between the metal precursor and the carbon nano-
fibers, resulting in a large number of Pd nuclei. Similar
report that the use of carbon nanofibers as catalyst supports can
4
improve activity and selectivity in hydrogenation, hydroformyla-
5
6
tion and electrocatalysis. The supported catalysts were prepared
by various conventional methods such as impregnation, co-
precipitation, and ion exchange. However, there are difficulties in
controlling the preparation steps and in obtaining highly dispersed
metal particles due to the various effects of the solvent on the
adsorption process. MOCVD has been shown to be a powerful
method for generating highly dispersed metal catalysts in a
7
controlled and reproducible manner. Recently, palladium was
deposited onto mesoporous MCM materials by the grafting route,
and the as-prepared catalysts showed remarkable activity in Heck
8
reactions. Also, palladium was deposited onto silica supports by
MOCVD in a fluidized-bed reactor resulting in highly dispersed Pd
9
nanoparticles. However, the synthesis of well-dispersed metal
particles on carbon nanofibers is challenging as only a few sites are
10
available for the anchoring of the organometallic precursors.
Here, we report the synthesis of evenly distributed Pd nanopar-
ticles supported on carbon nanofibers by MOCVD. The materials
exhibit excellent activity for the hydrogenation of cyclooctene and
may find additional applications in the synthesis of fine chemicals
and in electrocatalysis.
The palladium deposition onto carbon nanofibers was achieved
by the two-step chemical vapor deposition of Pd(allyl)Cp as a
precursor with high vapor pressure in a fixed-bed reactor. Carbon
nanofibers were supplied by Applied Sciences Inc. The outer
diameter of the carbon nanofibers was 70–200 nm, the length was
2
21
2
0–100 mm, and the BET surface area amounted to 50–60 m g
The carbon nanofibers were functionalized for 3 h at reflux with
.
Fig. 1 XRD patterns of the functionalized carbon nanofibers prior and
*muhler@techem.ruhr-uni-bochum.de
subsequent to Pd MOCVD.
2
82 | Chem. Commun., 2005, 282–284
This journal is ß The Royal Society of Chemistry 2005

View Article Online
adsorbed dissociatively on the surface of the carbon nanofibers.
We simultaneously observed the formation of cyclopentadiene
(m/z 5 66) and propene (m/z 5 41). However, only cyclopenta-
diene was detected during the adsorption when silica was used as
9
support. It is known that the functionalization of carbon
nanofibers under strongly acidic conditions results in the
formation of carboxylic, hydroxylic and carbonyl groups at defect
1
1
sites in the approximate ratio of 4 : 2 : 1. The carboxylic groups
are assumed to be mainly responsible for the dissociative
adsorption of the precursor replacing the roughly equally strongly
bound allyl and Cp ligands. Obviously, it depends on the reactivity
of the surface groups as to which ligand is dissociated off. On silica
hydroxyl groups are considered as the anchoring sites of
Fig. 2 TEM images of untreated (left) and functionalized carbon
nanofibers (right, 2.04 wt% Pd) subsequent to Pd MOCVD.
8,9
observations were reported by Pham-Huu et al. for carbon fiber-
Pd(allyl)Cp.
During the reduction step (Fig. 3, bottom),
supported Pd catalysts synthesized by incipient wetness impreg-
cyclopentane (m/z 5 70) and propane (m/z 5 29) were obtained
simultaneously. It is not possible to decide whether the alkanes
originate from hydrogen-assisted elimination or from subsequent
olefin hydrogenation over metallic Pd particles within the fixed
bed.
4b
nation. However, the distribution of the Pd particles achieved in
this way was in the range from 1 to 9 nm compared with the
distribution in the range from 2 nm to 4 nm (Fig. 2), clearly
demonstrating the advantages of MOCVD.
The two-step chemical vapor deposition was also studied by on-
line mass spectroscopy taking the fragmentation patterns into
account. During the first step (Fig. 3, top), Pd(allyl)Cp was
The hydrogenation of cyclooctene was carried out in a fixed-bed
reactor at 40 uC under atmospheric pressure using N as carrier
2
gas. The molar ratio of N to H to cyclooctene was 100 : 1 : 1. The
2
2
catalyst weight was 25 mg to avoid full conversion. The total flow
3
21
rate amounted to 300 cm min (STP). The composition of the
reactor effluent was analyzed by on-line gas chromatography using
a Chrompack CP-WAX 52 CB column and a flame ionization
detector. The catalytic activities of the catalysts with Pd loadings of
2.04 wt% and 4.30 wt% are shown in Fig. 4. Both catalysts are
found to be highly active and stable, which is a significant
improvement upon the catalysts produced by the impregnation
12a
methods. Jackson et al. observed rapid deactivation and a low
hydrogenation rate for the catalysts produced by impregnation in
12b
agreement with results obtained by Okhlopkova et al. These
results can presumably be attributed both to strong adsorption of
cyclooctene on the catalysts and to the presence of micropores in
the support. For carbon nanofibers, the high activity and stability
can be attributed to the special interaction between the palladium
nanoparticles and the carbon nanofibers, which has also been
4b
4c
observed for Ni/CNFs and Ru/CNFs. In addition, the open
structure of a fixed bed consisting of non-porous carbon
Fig. 3 Mass spectroscopy data obtained during the adsorption of the
Fig. 4 Hydrogenation of cyclooctene over Pd supported on carbon
nanofibers (atmospheric pressure and 40 uC).
2
precursor in He (top) followed by the reduction in 30% H (bottom).
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 282–284 | 283

View Article Online
3
(a) K. P. de Jong and J. W. Geus, Catal. Rev. Sci. Eng., 2000, 42, 481;
nanofibers is beneficial to avoid mass transfer limitations. These
effects are consistent with recent results reported by Pham-Huu
(b) P. Serp, M. Corrias and P. Kalck, Appl. Catal., A, 2003, 253, 337; (c)
G. Mestl, N. I. Maksimova, N. Keller, V. V. Roddatis and R. Schl o¨ gl,
Angew. Chem., Int. Ed., 2002, 40, 2066.
4b
et al. for the hydrogenation of cinnamaldehyde.
In conclusion, the two-step MOCVD can be efficiently used to
synthesize highly dispersed Pd nanoparticles supported on carbon
nanofibers under mild conditions. The functionalization of the
carbon nanofibers is a necessary step to provide anchoring sites.
The catalysts show high activity and stability in the hydrogenation
of cyclooctene, presumably due to special metal–support interac-
tions and the absence of micropores. Thus, the MOCVD route is
of great potential in the controlled synthesis of carbon fiber-
supported catalysts.
4 (a) J. M. Planeix, N. Coustel, B. Coq, V. Brotons, P. S. Kumbhar,
R. Dutartre, P. Geneste, P. Bernier and P. M. Aiayan, J. Am.
Chem. Soc., 1994, 116, 7935; (b) C. Pham-Huu, N. Keller, M. J.
Ledoux, L. Charbonni e` re and R. Ziessel, Chem. Commun., 2000, 19,
1871; (c) C. Park and R. T. K. Baker, J. Phys. Chem. B, 1998, 102,
5
2
168; (d) C. H. Liang, Z. L. Li, J. S. Qiu and C. Li, J. Catal., 2001, 211,
78.
(a) R. Gao, C. D. Tan and R. T. K. Baker, Catal. Today, 1999, 65, 385;
b) R. Giordano, P. Serp, P. Kalck, Y. Kihn, J. Schreiber, C. Marhic
5
6
(
and J. L. Duvail, Eur. J. Inorg. Chem., 2003, 610.
(a) C. A. Bessel, K. Laubernds, N. M. Rodriguez and R. T. K. Baker,
J. Phys. Chem. B, 2001, 105, 1115; (b) E. S. Steigerwalt, G. A. Deluga,
D. E. Cliffel and C. M. Lukehart, J. Phys. Chem. B, 2001, 105,
8097; (c) W. Z. Li, C. H. Liang, J. S. Qiu, W. J. Zhou, H. M. Han,
Z. B. Wei, G. Q. Sun and Q. Xin, Carbon, 2002, 40, 787.
P. Serp and P. Kalck, Chem. Rev., 2002, 102, 3085.
We acknowledge financial support from the Deutsche
Forschungsgemeinschaft. C. H. Liang thanks the Alexander von
Humboldt Stiftung for financial support.
7
a
a
Changhai Liang, Wei Xia, Hamideh Soltani-Ahmadi, Oliver Schl u¨ ter,
a
a
8 C. P. Mehnert, D. W. Weaver and J. Y. Ying, J. Am. Chem. Soc., 1998,
120, 12289.
b
Roland A. Fischer and Martin Muhler*
Laboratory of Industrial Chemistry, Ruhr-University Bochum,
D-44780, Bochum, Germany.
E-mail: muhler@techem.ruhr-uni-bochum.de; Fax: +49-234-32-14115;
Tel: +49-234-32-28754
Laboratory of Inorganic Chemistry II, Ruhr-University Bochum,
a
a
9 X. Mu, U. Bartmann, M. Guraya, G. W. Busser, U. Weckenmann,
R. Fischer and M. Muhler, Appl. Catal., A, 2003, 248, 85.
0 (a) P. Serp, R. Feurer, Y. Kihn, P. Kalck, J. L. Faria and
J. L. Figueiredo, J. Phys. IV, 2002, 12, 29; (b) P. Serp, R. Feurer,
Y. Kihn, P. Kalck, J. L. Faria and J. L. Figueiredo, J. Mater. Chem.,
1
b
2001, 11, 1980.
D-44780, Bochum, Germany
1
1 (a) T. Sainsbury and D. Fitzmaurice, Chem. Mater., 2004, 16, 2174; (b)
T. G. Ros, A. J. van Dillen, J. W. Geus and D. C. Koningsberger,
Chem. Eur. J., 2002, 8, 1151.
Notes and references
1
2 (a) S. D. Jackson, G. J. Kelly, S. R. Watson and R. Gulickx, Appl.
Catal., A, 1999, 187, 161; (b) L. B. Okhlopkova, A. S. Lisitsyn,
V. A. Likholobov, M. Gurrath and H. P. Boehm, Appl. Catal., A,
2003, 204, 229.
1
2
S. Iijima, Nature, 1991, 354, 56.
(a) R. H. Baughman, A. A. Zakhidov and W. A. de Heer, Science, 2002,
97, 787; (b) H. J. Dai, Acc. Chem. Res., 2002, 35, 1035.
2
2
84 | Chem. Commun., 2005, 282–284
This journal is ß The Royal Society of Chemistry 2005