Demonstration of a magnetic and catalytic Co@Pt nanoparticle as a dual-function nanoplatform

Article abstract of DOI:10.1039/b600147e

Co@Pt nanoparticles as a bifunctional nanoplatform system for the hydrogenation of various unsaturated organic molecules under mild conditions and also for magnetic separation and recycling are demonstrated. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b600147e

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Demonstration of a magnetic and catalytic Co@Pt nanoparticle as a
dual-function nanoplatform{
a
a
a
b
b
b
Chul-Ho Jun,* Young Jun Park, Ye-Rim Yeon, Joon-rak Choi, Woo-ram Lee, Seung-jin Ko and
b
Jinwoo Cheon*
Received (in Cambridge, UK) 6th January 2006, Accepted 20th February 2006
First published as an Advance Article on the web 7th March 2006
DOI: 10.1039/b600147e
Co@Pt nanoparticles as a bifunctional nanoplatform system
for the hydrogenation of various unsaturated organic molecules
under mild conditions and also for magnetic separation and
recycling are demonstrated.
Nanoparticles (NPs) with extremely large surface areas can possess
various functions, such as catalytic, magnetic, electronic and
optical properties. In particular, with the emerging interest in
developing versatile NPs for nanoplatform technologies, NPs
exhibiting more than two functions are highly desirable for
simultaneous and efficient technological applications. NPs with a
core–shell structure are one of the promising candidates for
1
integrating multiple functionalities into a single NP system. Such
Fig. 1 The dual functionality of Co@Pt core–shell NPs. Insert: The
core–shell nanostructures have been shown to be very useful for
HRTEM image of a single NP. 2.27 s represents the lattice constant of the
Pt shell.
enhancing or modifying the properties of NPs, such as the surface
plasmonic effect observed in the SiO @Au nanoshell and
2
2
bimetallic nanocatalysts, for example the Co–Rh system.
9
inexpensive core metals. Secondly, the additional functionality of
Although the large surface area of NP-based catalysts is the key
10
the magnetic cobalt core plays a critical role in the separation
3–5
component of their catalytic activity, as the size of the particle
decreases to a nanometer scale, its separation step becomes harder
and particle agglomeration during the reaction or separation
and recycling of the catalyst.
The core–shell type-cobalt–platinum NPs were prepared by a
redox transmetalation reaction between Pt(hfac) and cobalt
2
11
NPs. The platinum forms a shell around the cobalt core and
5
process becomes a troublesome issue. To circumvent such
problems in nanocatalyst synthesis, various possibilities have been
examined. For example, catalytic Pt NPs can be stabilized by large
organic polymers or encapsulated within porous materials such as
the shell surface is stabilized by dodecyl isocyanide capping
molecules (Fig. 1). After the formation of the core–shell structure,
the NP retains its magnetic properties, with a blocking temperature
6
zeolites. Herein, we present the core–shell Co@Pt NP and
(TB) of 15 K and a coercivity of 660 Oe at 5 K, indicating single
demonstrate both the catalytic activity (hydrogenation) of its Pt
shell and the superparamagnetism of its Co core, which allows for
the convenient separation and recycling of the nanocatalyst
domain superparamagnetism at room temperature. The particle
size averages 6.4 nm with a cobalt core diameter of 4.6 nm; its
overall stoichiometry being Co0.45Pt0.55. According to elemental
analysis and high resolution transmission electron microscopy
(Fig. 1). To our knowledge, our study is one of the first
demonstrations of magnetically recovering heterogeneous catalytic
7
NPs, although examples of homogenous organometallic catalysts
(
HRTEM) data, the thickness of the Pt layer is about y0.9 nm,
11
which corresponds to y4 layers of Pt. A particle size of 6.4 nm
was chosen because it gives a high colloidal stability while keeping
a high surface area of Pt and strong magnetism. It is desirable to
minimize the use of Pt as much as possible for economic reasons.
In contrast, if the particle becomes too small (e.g. , 5 nm), the
cobalt core also becomes smaller and does not have a strong
enough magnetism for magnetic recycling processes. In short, our
rationale for selecting the NP composition and size was to have
8
ligated with magnetic NPs have been reported previously.
Our core–shell-type platinum NP catalyst (1) can provide
advantages over single Pt component-based systems: Firstly, it is
‘‘atomically economical’’ because precious Pt metal can be
conserved by replacing the interior of the NPs with other,
a
Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
E-mail: junch@yonsei.ac.kr; Fax: (+82) 2 3147 2644;
Tel: (+82) 2 2123 2644
Department of Chemistry and Nanomedical National Core Research
1
: a high surface area for the Pt shell with minimum use of it,
b
2: retention of high magnetism for recycling purposes and 3: high
colloidal stability for the dispersion of the nanocatalysts into the
reaction solution. Since our core–shell cobalt–platinum NPs are
well dispersed in a typical organic medium, we investigated the
catalytic activity and recyclability of 1 towards hydrogenation,
using 1-decene as a model substrate (Table 1).
Center, Yonsei University, Seoul 120-749, Korea.
E-mail: jcheon@yonsei.ac.kr; Fax: (+82) 2 364 7050;
Tel: (+82) 2 2123 5631
{
Electronic Supplementary Information (ESI) available: Catalytic
procedures, GC–MS data and elemental analysis. See DOI: 10.1039/
b600147e
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1619–1621 | 1619

View Article Online
Table 1 Catalytic hydrogenation of 1-decene by 1 with catalyst
recycling
a
Run
1
2
3
4
5
6
7
b
Yields(%)
100
100
100
100
100
100
100
a
Reaction conditions: Substrate 0.25 mmol, 1-decene/Pt atom = 50
2 mol%), toluene 0.1 mL, 1 atm H (using toy balloon), room
temperature. Determined by GC and GC–MS.
(
2
b
Fig. 3 TEM image of 1 showing the conservation of core–shell structure
and monodispersity without deformation: (a) Before reaction. (b) After
recycling.
When the hydrogenation of 1-decene in the presence of a
catalytic amount of 1 under a H
2
atmosphere was carried out for
2
–7). For comparison, we performed the catalyst recycling
4
h at room temperature (see ESI for details{), a complete
experiments by using platinum on activated carbon (Pt/C) as the
catalyst under the same hydrogenation conditions applied to 1,
except the reaction time (1 h per cycle). The catalytic activity of Pt/
C started to diminish from 100 to 76% conversion after the second
cycle, even though the total exposure period of Pt/C in the catalytic
conversion of 1-decene to n-decane was observed (Table 1, run 1).
Another functional feature of our core–shell NPs is the manner
in which the catalyst recycles itself by magnetic auto-separation.
Initially, all the black-colored Co@Pt nanocatalysts were collected
on the central part of a stirring bar due to magnetic attraction
12
process was much shorter than that of 1 (3 h vs. 28 h). TEM
(Fig. 2(a)1). However, when 1 was stirred in solution, it was fully
analysis clearly shows that the particle size and composition of 1
was unchanged, and that no aggregation occurred during or after
the recycling procedure (Fig. 3 and ESI{).
We further investigated the scope of the substrates as well as the
chemoselectivities of the Co@Pt NP catalytic hydrogenation
system. The results are summarized in Table 2.{
Catalyst 1 smoothly promotes the hydrogenation of various
olefins and alkynes under very mild conditions. The position and
structure of the olefins did not affect the reactivity of 1 towards
them: terminal, internal, conjugated or isolated (Table 2, entries
dispersed by the centrifugal force (Fig. 2(a)2), giving rise to the
desired nanocatalytic functions.
When stirring ceased, 1 spontaneously returned to the stirring
11
bar through magnetic interaction (Fig. 2(a)3–5). After the
catalytic reaction and catalyst isolation process, the recovered 1
can be directly reused in subsequent hydrogenation reactions
(Fig. 2(b)).
In fact, the reused NPs retained the same catalytic activity as
that of freshly prepared NPs after up to seven cycles (Table 1, runs
1–4). The catalytic hydrogenation of alkynes into their correspond-
ing saturated alkanes was facile, regardless of whether internal or
terminal alkynes were used (Table 2, entries 5–8). However, the
reaction of diphenylacetylene did not proceed as rapidly as
expected, probably due to the unfavorable steric interaction
between the two phenyl groups of the diphenylacetylene and the
capping molecules on the surface of the NPs (Table 2, entry 9).
One interesting thing to note is that this catalytic system exhibited
near perfect selectivity when reducing the carbonyl group of
aldehydes to the corresponding alcohol upon hydrogenation in the
presence of ketone. For example, when benzaldehyde and
cyclohexanone were applied to this hydrogenation, only benzalde-
hyde was converted to benzyl alcohol (Table 2, entries 10 and 11).
Hydrogenation of 2-cyclohexenone and cinnamaldehyde was
carried out using this catalytic system, and cyclohexanone and
hydrocinnamalcohol were obtained, respectively, which also
confirms the selectivity of the process (Table 2, entries 12 and
5
b
1
3). This type of dramatic selectivity can be explained as follows:
The sterically less-hindered carbonyl group of aldehydes, com-
pared to ketones, might more readily approach to the catalytic
surface of 1 by minimizing its possible unfavorable steric
interaction with the capping molecules. Interestingly, the nitro
group could also be reduced to an amine group without any
13
noticeable catalytic deactivation (Table 2, entries 14 and 15).
Fig. 2 (a) Visualization of the nanocatalyst collection process using a
magnetic bar (details of the stirring bar are graphically presented): (1)
Nanocatalyst on the magnetic bar. (2) During stirring. (3–5) Collection of
NPs on the magnet after reaction. (b) Schemes for the procedures of
catalyst reaction and the recycling.
In summary, we have demonstrated the efficacy of Co@Pt NPs
as a bifunctional nanoplatform system for the hydrogenation of
various unsaturated organic molecules under mild conditions, and
also demonstrated their magnetic separation and recycling
1
620 | Chem. Commun., 2006, 1619–1621
This journal is ß The Royal Society of Chemistry 2006

View Article Online
a
Table 2 Hydrogenation of various substrates
b
Entry
Substrate
Product
Time/h
Conversion (%)
1
2
3
4
5
6
7
8
9
1-Decene
n-Decane
n-Decane
Cyclododecane
Ethylbenzene
n-Octane
4
4
4
6
8
10
4
4
14
16
12
10
7
100
100
100
100
100
100
100
100
17
1,5,9-Decatriene
Cyclododecene
Styrene
1-Octyne
4-Octyne
3-Phenylpropyne
1-Phenylpropyne
Diphenylacetylene
Benzaldehyde
Cyclohexanone
n-Octane
Propylbenzene
Propylbenzene
1,2-Diphenylethane
Benzylalcohol
—
Hydrocinnamylalcohol
Cyclohexanone
Aniline
10
11
12
13
14
15
100
0
c
c
Cinnamaldehyde
100
100
100
2-Cyclohexenone
Nitrobenzene
2-Bromonitrobenzene
4
16
d
2-Bromoaniline
100
a
Reaction conditions: Substrate 0.25 mmol, substrate/Pt atom = 50 (2 mol%), toluene 0.1 mL, 1 atm H
2
(using toy balloon), room
b
c
d
temperature. Determined by GC and GC–MS. Mixture of partially and fully hydrogenated products. Containing less than 1% of
debrominated product.
6
(a) J. W. Yoo, D. Hathcock and M. A. El-Sayed, J. Phys. Chem. A,
002, 106, 2049; (b) P. Concepci o´ n, A. Corma, J. Silvestre-Albero,
V. Franco and J. Y. Chane-Ching, J. Am. Chem. Soc., 2004, 126, 5523;
(c) M. Arai, Y. Takada and Y. Nishiyama, J. Phys. Chem. B, 1998, 102,
capabilities. This kind of bifunctional core–shell NP may provide
useful applications in various types of transition metal-based
catalytic reaction.
2
1
968; (d) M. Zhao and R. M. Crooks, Angew. Chem., Int. Ed., 1999, 38,
We thank K. T. Son for TEM (KBSI-Chuncheon) and Dr. Y. J.
Kim (KBSI) for high voltage TEM (JEM-ARM 1300S) analyses.
This work was supported by a Korean Research Foundation
Grant funded by the Korean Government (MOEHRD) (KRF-
364; (e) S. Mandal, D. Roy, R. V. Chaudhari and M. Sastry, Chem.
Mater., 2004, 16, 3714; (f) N. Guillon, Q. Gao, P. M. Forster,
J.-S. Chang, M. Nogues, S.-E. Park, G. Ferey and A. K. Cheetham,
Angew. Chem., Int. Ed., 2001, 40, 2831.
7
8
Z. Chang, C.-X. Guo, F. Li, X. Duan and M.-L. Zhang, Acta Chim.
Sin., 2002, 60, 298.
2
005-201-c00024) (C.-H. J.), NCRC (R15-2004-024-02002-0),
National R&D for Cancer Control (0320250-2), AOARD
FA520905P0285), Korea Health R&D (02-PJ1-Pg10-20599-
004) and the R&D Program of Fusion Strategies for Advanced
(a) T.-J. Yoon, W. Lee, Y.-S. Oh and J.-K. Lee, New J. Chem., 2003, 27,
227; (b) P. D. Stevens, J. Fan, H. M. R. Gardimalla, M. Yen and
Y. Gao, Org. Lett., 2005, 7, 2085; (c) A. Hu, G. T. Yee and W. Lin,
J. Am. Chem. Soc., 2005, 127, 12486; (d) S. C. Tsang, V. Caps,
I. Paraskevas, D. Chadwick and D. Thompsett, Angew. Chem., Int. Ed.,
(
0
Technologies (J. C.).
2004, 43, 5645.
S. U. Son, Y. Jang, J. Park, H. B. Na, H. M. Park, H. J. Yun, J. Lee
and T. Hyeon, J. Am. Chem. Soc., 2004, 126, 5026.
0 (a) H. Srikanth, E. E. Carpenter, L. Spinu, J. Wiggins, W. Zhou and
C. O’Connor, Mater. Sci. Eng., A, 2001, 304, 901; (b) W. L. Zhou,
E. E. Carpenter, J. Lin, A. Kumbhar, J. Sims and C. J. O’Connor,
Eur. Phys. J. D, 2001, 16, 289.
1 For the preparation of 1, its magnetic properties and full characteriza-
tion data, see: J.-I. Park and J. Cheon, J. Am. Chem. Soc., 2001, 123,
9
Notes and references
1
{ It is noteworthy that we have not observed any hydrogenation product
from the capping molecule in any of these catalytic reactions.
1
(a) S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser, Science,
000, 287, 1989; (b) V. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko,
1
2
J. A. Hollingsworth, C. A. Leatherdale, H.-J. Eisler and M. G. Bawendi,
Science, 2000, 290, 314; (c) X. Zhong, M. Han, Z. Dong, T. J. White
and W. J. Knoll, J. Am. Chem. Soc., 2003, 125, 8589; (d) X. Peng,
M. C. Schlamp, A. V. Kadavanich and A. P. Alivisatos, J. Am. Chem.
Soc., 1997, 119, 7019.
(a) K. H. Park and Y. K. Chung, Adv. Synth. Catal., 2005, 347, 854; (b)
C. Radloff and N. J. Halas, Nano Lett., 2004, 4, 1323; (c) T. Pham,
J. B. Jackson, N. J. Halas and T. R. Lee, Langmuir, 2002, 18, 4915.
(a) L. N. Lewis, Chem. Rev., 1993, 93, 2693; (b) A. P. Alivisatos, Science,
5
743.
1
2 In terms of the recoverability, it is noteworthy that the turnover
21
frequencies (TOF) of 1 were constant (2 h ) during the seven cycles.
However, in the control experiment using Pt/C (2 mol%) as a
hydrogenation catalyst, the TOFs of Pt/C rapidly reached half the
2
21
21
value (4.16 h ) of the initial TOF (8 h ) at the fifth cycle: 100% (1st
cycle), 100% (2nd cycle), 76% (3rd cycle), 67% (4th cycle), 52% (5th
cycle). Although the TOFs of Pt/C are apparently higher than those of
1, it seems to be difficult to compare the catalytic activity of 1 with that
of Pt/C directly, because there is a significant difference in their nature:
the existence or non-existence of the capping ligand in each system.
For a similar discussion, see: (a) C. A. Stowell and B. A. Korgel,
Nano Lett., 2005, 5, 1203. Special issue on recoverable catalysts and
reagents; (b) J. A. Gladysz, Chem. Rev., 2002, 102, 3215.
3
4
1
996, 271, 933; (c) A. Henglein, Chem. Rev., 1989, 89, 1861.
(a) C.-W. Chen, T. Serizawa and M. Akashi, Chem. Mater., 1999, 11,
381; (b) H. Weller, Angew. Chem., Int. Ed. Engl., 1993, 32, 41; (c)
1
A. Henglein, J. Phys. Chem., 1993, 97, 5457.
(a) J. A. Widegren and R. G. Finke, J. Mol. Catal. A: Chem., 2003, 191,
5
1
87; (b) A. Roucoux, J. Schulz and H. Patin, Chem. Rev., 2002, 102,
13 S. Ichikawa, M. Tada, Y. Iwasawa and T. Ikariya, Chem. Commun.,
2005, 924.
3757.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1619–1621 | 1621