Near-monodisperse tetrahedral rhodium nanoparticles on charcoal: The shape-dependent catalytic hydrogenation of arenes

Article abstract of DOI:10.1002/anie.200603961

The shape of things to come? Monodisperse (4.9±0.4)-nm tetrahedral rhodium nanoparticles on charcoal (/C) are compared to (4.8±0.4)-nm spherical rhodium nanoparticles on charcoal(?/C) and commercial Rh/C as a catalyst for the hydrogenation of anthracene (see picture). The former is 5.8- and 109-times more active than the latter two, respectively. It also shows a higher selectivity and excellent activity in the hydrogenation of several other arenes. (Graph Presented).

Full text of DOI:10.1002/anie.200603961

Communications
DOI: 10.1002/anie.200603961
Shape-Dependent Catalysis
Near-Monodisperse Tetrahedral Rhodium Nanoparticles on Charcoal:
The Shape-Dependent Catalytic Hydrogenation of Arenes**
Kang Hyun Park, Kwonho Jang, Hae Jin Kim, and Seung Uk Son*
Transition metals loaded on charcoal are useful reagents for a
wide variety of organic transformations. Moreover, these
heterogeneous systems are promising industrial catalysts. For
example, commercially available Pd/C is frequently used in
nanocrystals are more active than spherical or cubic ones
because they contain more catalytically active atoms on the
surface.[10] The groups of Somorjai and Tilley have reported
the synthesis of cubic and flower-like rhodium nanoparti-
cles.[11] However, to our knowledge, there are no reports on
tetrahedral rhodium nanoparticles. Herein we report the
preparation of tetrahedral Rh nanoparticles loaded on
charcoal by an organometallic approach along with their
excellent catalytic activity in the hydrogenation of arenes.
Low-temperature, kinetically controlled synthesis is one
of the most common strategies for the shape evolution of
nanoparticles because the reactivity of each crystal plane can
be differentiated efficiently under these conditions.[12] The
key strategy in this study is based on the use of relatively
unstable compounds 1–3 as precursors for the formation of
nanocrystals at low temperature.
À
debenzylation, hydrogenation, and C C bond-formation
reactions in the laboratory and is also used in industry,[1]
and Ni/C,[2] Cu/C,[3] and Co/C[4] have recently been developed
as catalysts for coupling, hydrosilylation, and cycloaddition
reactions, respectively.
Commercially available Rh/C shows excellent catalytic
activity in a broad range of organic reactions, including
hydrogenation, hydroformylation, hydrocarbonylation, and
reductive coupling of aryl halides,[5] and considerable effort
has been aimed at enhancing the efficiency of this reagent.
For instance, better solid supports, such as nanoporous carbon
and layered clays, have been developed.[6] Recently, the Wai
and Koningsberger groups have shown that carbon nanotubes
and nanofibers are better supports owing to their enhanced
dispersion ability.[7] An ionic liquid has also been used as a
promising support for rhodium; the resulting complex showed
good stability and catalytic activity in the hydrogenation of
benzene.[8]
Other research has focused on the size- and shape-control
of rhodium-metal particles. There has been significant
progress in reducing their size to obtain a larger surface
area, and a wide variety of synthetic routes that lead to
spherical rhodium nanoparticles are known.[9] However,
shape-control to form other types of rhodium nanoparticles
has been little explored.
Recently, there has been a great deal of interest in the
shape-dependent physical and chemical properties of nano-
crystals. The El-Sayed group, for example, described the
shape-dependent catalytic activity of platinum nanocrystals in
electron-transfer reactions. They reported that tetrahedral Pt
In a typical synthesis, compound 1 was added to well-dried
oleylamine and the temperature was increased gradually to
1708C. The color of the solution changed from red to black
during this process. The reaction mixture was stirred for one
hour at this temperature and then poured into methanol to
form a black precipitate, which was separated by centrifuga-
tion (see Supporting Information for details). The TEM
images of these black powders show the presence of (4.9 Æ
0.4)-nm-sized, near-monodisperse tetrahedral nanoparticles
(Figure 1a and Supporting Information). The catalytically
active (111) lattice planes are mainly observed in the HR-
TEM images, which also indicate the high crystallinity of the
synthesized nanoparticles (inset of Figure 1a) When the
reaction temperature was decreased to 1308C, (2.8 Æ 0.3)-nm-
sized tetrahedral nanoparticles were obtained, which suggests
the incomplete consumption of the precursors. The powder X-
ray diffraction (XRD) and electron diffraction (ED) patterns
of the tetrahedral nanoparticles were indexed to the (111),
(200), (220), and (311) lattice planes of cubic rhodium (JSPDS
no. 5-685; see Supporting Information).
[*] Dr. K. H. Park, K. Jang, Prof. S. U. Son
Department of Chemistry
Sungkyunkwan University
Suwon 440-746 (Korea)
Fax: (+82)31-299-4572
E-mail: sson@skku.edu
Homepage: http://chem.skku.ac.kr/~sson
Dr. H. J. Kim
Korea Basic Science Institute
Daejeon 350-333 (Korea)
[**] This work was supported by the Hydrogen Energy R&D Center, a
21st century Frontier R&D Program funded by the Ministry of
Science and Technology of Korea. K.H.P. is grateful for a grant from
the Korea Research Foundation (MOEHRD) (KRF-2005-005J11901).
We also prepared spherical nanoparticles to compare the
shape-dependent catalytic activity of both types of nano-
particles. The surfactants surrounding the surface of nano-
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
1152
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 1152 –1155

Angewandte
Chemie
fore turned our attention to other precursors that are more
stable than 1.
A mixture of tetrahedral and spherical nanoparticles is
formed when precursor 2 is heated at 1908C. The proportion
of spherical nanoparticles is higher than with precursor 1
(Figure 2e). However, there is still a significant proportion of
tetrahedral nanoparticles present. Moreover, the size distri-
bution, (5.1 Æ 0.9) nm, becomes broad at a temperature of
2358C, therefore the more stable precursor 3 was used
instead. When the reaction temperature is slowly increased to
2358C, irregularly shaped nanoparticles with a large size
distribution, (4.1 Æ 0.8) nm, are obtained. However, when a
solution of precursor 3 is rapidly injected into hot oleylamine
at 2208C, rectangular nanoparticles with a size distribution of
(9.3 Æ 2.6) nm are mainly observed (Figure 2 f). Interestingly,
no tetrahedral nanoparticles are found. The HR-TEM image
of these rectangular nanoparticles mainly shows the less
catalytically active (200) lattice plane (Figure 2g). Spherical
nanoparticles with a somewhat broad size distribution, (5.0 Æ
0.6) nm, begin to form at 2358C (Figure 2h). This observation
suggests that the rectangle is a metastable shape in the process
Figure 1. Low-magnification TEM and HR-TEM images (insets) of
(4.9Æ0.4)-nm tetrahedral (a) and (4.8Æ0.4)-nm spherical (b) Rh nano-
particles prepared from precursors 1 and 3, respectively (see Support-
ing Information for size-distribution diagrams).
particles need to be considered as it was reported recently
that surfactants have a significant influence on the catalytic
activity of nanoparticles by changing the chemical environ-
ment of the surface atoms.[13] Therefore, only oleylamine was
used as a surfactant so as to
normalize the effect of the sur-
factant on the catalysis. It is
believed that spherical nanopar-
ticles are thermodynamically
favorable for synthesis,[10b] there-
fore harsher conditions were
used with precursor 1. As the
reaction
temperature
is
increased to 1908C, the tetrahe-
dral nanoparticles appear to coa-
lesce to form larger, sphere-like
nanoparticles
0.6) nm; Figure 2d). Figure 2a
shows the dimerization of the
(size
(6.4 Æ
tetrahedral
nanoparticles
through the (111) lattice plane.
Polycrystalline
and
almost
single-crystalline
sphere-like
nanoparticles can be observed
in the HR-TEM images shown
in Figures 2b and c. When the
reaction
temperature
is
increased to 2358C, approxi-
mately half of the nanoparticles
present are spherical with
a
diameter of (9.5 Æ 1.0) nm. How-
ever, there is still a significant
portion of tetrahedral nanoparti-
cles. When precursor 1 is rapidly
injected into hot oleylamine at
2358C, the tetrahedral nanopar-
ticles disappear completely and
irregularly shaped polydisperse
nanoparticles are formed. These
observations imply that spherical
nanoparticles are thermodynam-
ically more favorable. We there-
Figure 2. Selected HR-TEM images (a–c) of the particles in (d) and normal TEM images of the mixture
of tetrahedral and spherical Rh nanoparticles (d) formed at 1908C from precursor 1; e) the mixture of
tetrahedral and spherical particles formed at 1908C from precursor 2; f) a TEM image of rectangular
rhodium nanoparticles formed at 2208C by injection of precursor 3 into the hot solution and g) an HR-
TEM image for one of the nanoparticles in (f); h) a TEM image of polydisperse spherical nanoparticles
formed at 2358C from precursor 3 and i) (7.1Æ0.7)-nm spherical nanoparticles formed at 2508C from
the double concentration of precursor 3.
Angew. Chem. Int. Ed. 2007, 46, 1152 –1155
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1153

Communications
Table 1: Comparison of catalytic activities and selectivities of Rh on
of forming spherical particles. Highly crystalline and mono-
charcoal for the hydrogenation of anthracene.[a]
disperse spherical nanoparticles with a size distribution of
(4.8 Æ 0.4) nm are obtained at 2508C (Figure 1b). Similar to
the tetrahedral nanoparticles, the HR-TEM images of these
spherical nanoparticles mainly show the catalytically active
(111) lattice plane. The size of these spherical nanoparticles
can be controlled by changing the amount of precursor 3. For
example, particles with a size distribution of (7.1 Æ 0.7) nm are
formed when the precursor concentration was doubled
(Figure 2i).
The chemical environment of the surface atoms is a key
factor that determines the catalytic activity and selectivity of
these nanoparticles. Coordinatively unsaturated metal atoms
on the corners or edges in nanocrystals are crucial for the
catalytic activity of particles. The (111) faces are the most
catalytically active in cubic phase nanoparticles and the (100)
faces are the least active.[10] It has been reported that
tetrahedral Pt nanoparticles have entirely (111) faces on the
surface and spherical nanoparticles have both (111) and (100)
faces.[10] Moreover, the atoms on the corners or edges of 4.8-
nm, tetrahedral Pt nanoparticles comprise 28% of the total
number of atoms and 35% of the surface atoms. In contrast,
the atoms on the corner or edges of 4.9-nm, almost-spherical
Pt nanoparticles comprise 3% of the total atoms and 13% of
the surface atoms. There is a recent report on the synthesis of
rhodium nanocubes.[11] From the viewpoint of catalytic
applications, the surface of a cube is known to show the
catalytically less active (100) faces.[10a,11] As Rh nanoparticles
have the same space group (Fm3m) as Pt metal, it is expected
that tetrahedral rhodium nanoparticles should be the most
promising for use as a heterogeneous catalyst.
Entry
Cat.[b]
P
[atm]
t
[h]
Conv.[c]
[%]
Selectivity [%]
B
C
D
~
1
2
3
4
5
10
1
1
10
1
0.5
2
2
0.5
2
100
100
99.5
57.3
4.8
0.7
2.0
6.7
6.4
35.4
0.0
0.0
76.3
81.0
45.8
99.3
98.0
17.0
12.6
18.8
/C
~
/C
*
/C
Rh/C
Rh/C
[a] Reaction conditions: 1.00 mmol of anthracene and 1 mol% of Rh/C
(based on ICP analysis) in methanol at room temperature. [b] (4.9Æ0.4)-
~
nm tetrahedral rhodium nanoparticles on charcoal ( /C), (4.8Æ0.4)-nm
*
spherical rhodium nanoparticles on charcoal ( /C), and commercial Rh/
C. [c] Conversion determined by GC-MS.
~
Figure 3. A comparison of the catalytic activities of tetrahedral ( ) and
The catalytic activities of tetrahedral and spherical nano-
particles were compared by immobilizing colloidal nano-
particles on activated charcoal. The nanoparticles were
deposited by refluxing dispersed colloidal nanoparticles with
dried activated charcoal in methanol. The absolute amount of
rhodium metal on the charcoal was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES).
The hydrogenation of arenes was chosen as a model reaction
because it is a simple but industrially very important
reaction.[14] Initially, the catalytic activity for the hydrogena-
tion of anthracene was tested because both the activity and
selectivity can be determined. Three main products are
formed in the hydrogenation of anthracenes:[7a] those pro-
duced by hydrogenation of the central ring (B), the two side
rings (D), or only one side ring (C). Table 1 and Figure 3
summarize the results. As expected, tetrahedral Rh nano-
particles on charcoal show excellent activity and selectivity in
this reaction, catalyzing hydrogenation far more efficiently
than either spherical or commercial Rh/C catalysts.
Monitoring the reaction by gas chromatography revealed
that the first hydrogenation is complete to form products B
and C within 10 min under 1 atm of H2 at room temperature,
whereas it takes two hours to convert C into D. It appears that
tetrahedral nanoparticles are 5.8-times (Figure 3 and Table 1,
entry 2 vs. entry 3) and 109-times (Figure 3 and Table 1,
entry 2 vs. entry 5) more active than spherical nanoparticles
and commercial Rh/C, respectively. Interestingly, the selec-
tivity (hydrogenation of the central ring vs. side rings (B:-
*
spherical Rh nanoparticles on charcoal ( ) and commercial Rh/C
catalysts for the formation of D. See Supporting Information for
selectivity data for D and conversion of substrate.
(C+D)) of tetrahedral rhodium nanoparticles (1:49; entry 2,
Table 1) is much higher than those of spherical nanoparticles
(1:14; entry 3, Table 1) and commercial Rh/C (1:1.8; entry 5,
Table 1). The selectivity of tetrahedral nanoparticles is higher
(up to 1:142; entry 1, Table 1) under a pressure of 10 atm of
H2, and that of commercial Rh/C increases to 1:15 (entry 4,
Table 1). Tetrahedral rhodium nanoparticles on charcoal are
therefore more active and selective in the hydrogenation of
anthracene.
A recent review has highlighted that a high activity is not
necessarily indicative of a low selectivity in many reactions.[15]
Many other factors related to the activity and selectivity of
heterogeneous catalysts have been discussed. In particular,
the fact that the atoms on different faces of nanocrystals have
different reaction kinetics and preferences in chemical
reactions has been studied experimentally by molecular-
beam-based methods.[16]
An iodine poisoning experiment showed that the reaction
occurs in a heterogeneous manner (see Supporting Informa-
tion).
The next set of experiments was aimed at screening a
range of substrates. The reactions shown in Table 2 were
1154
www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 1152 –1155

Angewandte
Chemie
Table 2: Hydrogenation of arenes in the presence of tetrahedral rhodium
Keywords: heterogeneous catalysis · hydrogenation ·
nanoparticles · rhodium · supported catalysts
.
nanoparticles on charcoal.[a]
Entry Substrate
Product
t
Conv.[b] TOF[c]
[h]
[%] [hÀ1
]
[1] For recent examples see: a) C. R. LeBlond, A. T. Andrews, J. R.
Sowa, Jr., Y. Sun, Org. Lett. 2001, 3, 1555; b) H. Sakurai, T.
Tsukuda, T. Hirao, J. Org. Chem. 2002, 67, 2721; c) Y. Mori, M.
Seki, J. Org. Chem. 2003, 68, 1571.
1
anisole
methyl cyclohexyl ether
methyl cyclohexyl ether
cyclohexane
methylcyclohexane
cyclohexanol
1
1
0.5
0.5
1
100 300
100 300
100 600
100 600
100 300
100 600
2[d] anisole
3
4
5
6
benzene
toluene
phenol
[2] B. H. Lipshutz, Adv. Synth. Catal. 2001, 343, 313.
[3] B. H. Lipshutz, B. A. Frieman, A. E. Tomaso, Jr., Angew. Chem.
2006, 118, 1281; Angew. Chem. Int. Ed. 2006, 45, 1259.
[4] S. U. Son, S. I. Lee, Y. K. Chung, Angew. Chem. 2000, 112, 4318;
Angew. Chem. Int. Ed. 2000, 39, 4158.
[5] a) P. N. Rylander, Hydrogenation Methods, Academic Press,
New York, 1985; b) W. M. Pearlman, Org. Synth. Coll. 1973, 5,
670; c) M. Fujita, T. Hiyama, Org. Synth. Coll. 1993, 8, 16; d) A.
Akao, K. Sato, N. Nonoyama, T. Mase, N. Yasuda, Tetrahedron
Lett. 2006, 47, 969.
[6] a) G. Vitulli, C. Evangelisti, P. Pertici, A. M. Caporusso, N.
Panziera, P. Salvadori, M. G. Faga, C. Manfredotti, G. Martra, S.
Coluccia, A. Balerna, S. Colonna, S. Mobilio, J. Organomet.
Chem. 2003, 681, 37; b) M. Fuchs, B. Jenewein, S. Penner, K.
Hayek, G. Rupprechter, D. Wang, R. Schlögl, J. J. Calvino, S.
Bernal, Appl. Catal. A 2005, 294, 279.
methyl benzoate methyl cyclohexane-
carboxylate
naphthalene
diphenylmethane dicyclohexylmethane
diphenylmethane dicyclohexylmethane
diphenyl ether
0.5
7
8
9
octahydronaphthalene
2
2
3
1.5
2.5
100 250
83 249
100 200
50 200
100 224
10
oxydicyclohexane
oxydicyclohexane
11[e] diphenyl ether
[a] Reaction conditions: 1.00 mmol of substrate and 1 mol% of
tetrahedral rhodium nanoparticles on charcoal (based on ICP analysis)
in methanol at room temperature under 1 atm of H2. [b] Determined by
GC-MS. [c] TOF=turnover frequency defined as mol of H2 consumed
per mol of total metal per hour. [d] The catalyst recovered from entry 1
was used. [e] Cyclohexanol (13%) was detected as a by-product. The
formation of cyclohexane could not be ascertained by GC-MS.
[7] a) B. Yoon, C. M. Wai, J. Am. Chem. Soc. 2005, 127, 17174;
b) T. G. Ros, D. E. Keller, A. J. van Dillen, J. W. Geus, D. C.
Koningsberger, J. Catal. 2002, 211, 85.
monitored by GC-MS. As expected, the tetrahedral rhodium
nanoparticles on charcoal show excellent activity with a wide
range of arenes. The hydrogenation of monoarenes, for
example, is complete within one hour (entries 1, 3–6 in
Table 2). ICP-AES analysis of the reaction mixture after the
hydrogenation of anisole showed no loss of rhodium metal
and the recovered catalysts showed the same catalytic activity,
as shown by comparing entries 1 and 2 in Table 2. Naphtha-
lene and diphenylmethane were hydrogenated cleanly after
two and three hours, respectively. In the case of diphenyl
ether the reactant had been consumed after 2.5 h; in this
reaction, cyclohexanol was formed as a by-product (13%).
In conclusion, near-monodisperse tetrahedral rhodium
nanoparticles have been successfully prepared by an organ-
ometallic approach. High-quality spherical rhodium nano-
particles have also been synthesized for comparison by a hot-
injection method. The (4.9 Æ 0.4)-nm tetrahedral rhodium
nanoparticles on charcoal show 5.8- and 109-times higher
activity for the hydrogenation of anthracene than spherical
nanoparticles and commercial Rh/C, respectively. Moreover,
the tetrahedral nanoparticles show excellent activity with a
range of arenes. We believe that the development of more
active Rh/C catalysts by taking advantage of the shape-
control of metal particles will have a large impact on organic
synthesis and industry.
[8] a) M. A. Gelesky, A. P. Umpierre, G. Machado, R. R. B. Correia,
W. C. Magno, J. Morais, G. Ebeling, J. Dupont, J. Am. Chem. Soc.
2005, 127, 4588; b) X. Mu, J. Meng, Z. Li, Y. Kou, J. Am. Chem.
Soc. 2005, 127, 9694; c) G. S. Fonseca, A. P. Umpierre, P. F. P.
Fichtner, S. R. Teixeira, J. Dupont, Chem. Eur. J. 2003, 9, 3263.
[9] a) J. Schulz, S. Levigne, A. Roucoux, H. Patin, Adv. Synth. Catal.
2002, 344, 266; b) J.-L. Pellegatta, C. Blandy, V. Colliꢀre, R.
Choukroun, B. Chaudret, P. Cheng, K. Philippot, J. Mol. Catal. A
2002, 178, 55; c) M. Harada, D. Abe, Y. Kimura, J. Colloid
Interface Sci. 2005, 292, 113; d) A. J. Bruss, M. A. Gelesky, G.
Machado, J. Dupont, J. Mol. Catal. A 2006, 252, 212.
[10] a) R. Narayanan, M. A. El-Sayed, Nano Lett. 2004, 4, 1343; b) R.
Narayanan, M. A. El-Sayed, J. Am. Chem. Soc. 2004, 126, 7194.
[11] J. D. Hoefelmeyer, K. Niesz, G. A. Somorjai, T. D. Tilley, Nano
Lett. 2005, 5, 435.
[12] N. Pradhan, H. Xu, X. Peng, Nano Lett. 2006, 6, 720.
[13] a) C. A. Stowell, B. A. Korgel, Nano Lett. 2005, 5, 1203; b) S.
Jansat, M. Gꢁmez, K. Philippot, G. Muller, E. Guiu, C. Claver, S.
Castillꢁn, B. Chaudret, J. Am. Chem. Soc. 2004, 126, 1592; c) M.
Tamura, H. Fujihara, J. Am. Chem. Soc. 2003, 125, 15742.
[14] a) V. Mꢂvellec, A. Nowicki, A. Roucoux, C. Dujardin, P.
Granger, E. Payen, K. Philippot, New J. Chem. 2006, 30, 1214;
b) G. S. Fonseca, E. T. Silveira, M. A. Gelesky, J. Dupont, Adv.
Synth. Catal. 2005, 347, 847; c) I. S. Park, M. S. Kwon, N. Kim,
J. S. Lee, K. Y. Kang, J. Park, Chem. Commun. 2005, 5667; d) A.
Roucoux, J. Schulz, H. Patin, Adv. Synth. Catal. 2003, 345, 222.
[15] H. Mayr, A. R. Ofial, Angew. Chem. 2006, 118, 1876; Angew.
Chem. Int. Ed. 2006, 45, 1844.
[16] J. Libuda, ChemPhysChem. 2004, 5, 625.
Received: September 27, 2006
Revised: October 23, 2006
Published online: December 21, 2006
Angew. Chem. Int. Ed. 2007, 46, 1152 –1155
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1155