Ligand-free platinum nanoparticles encapsulated in a hollow porous carbon shell as a highly active heterogeneous hydrogenation catalyst

Article abstract of DOI:10.1002/anie.200602700

(Figure Presented) Wrapped up but still active: A platinum nanoparticle that is 2.2 nm in diameter and not coated by any stabilizing agent can be encapsulated in a hollow carbon shell 30-40 nm in diameter with a micro-to nanoporous wall structure (see picture). This encapsulated nanoparticle acts as an efficient and recyclable heterogeneous catalyst for various liquid-phase hydrogenation reactions.

Full text of DOI:10.1002/anie.200602700

Angewandte
Chemie
Encapsulated Catalysts
DOI: 10.1002/anie.200602700
Ligand-Free Platinum Nanoparticles
Encapsulated in a Hollow Porous Carbon Shell as
a Highly Active Heterogeneous Hydrogenation
Catalyst**
Shigeru Ikeda,* Satoru Ishino, Takashi Harada,
Natsumi Okamoto, Takao Sakata, Hirotaro Mori,
Susumu Kuwabata, Tsukasa Torimoto, and
Michio Matsumura
Studies on the synthesis of metal nanoparticles (MNPs) and
their catalytic activity have become increasingly common
because their specific properties, which are often distinct from
those of bulk metals, lead to the induction of unique organic
reactions.[1–7] These MNP-based catalysts are usually pre-
served by organic ligands, such as polymers,[2–4] surfactants,[5]
and phosphines[6] to prevent coalescence and are used in
homogeneous systems similar to those in which metal-
complex catalysts are used. Current interest in the design of
high-performance practical catalysts is directed towards the
heterogenization of MNPs by fixing them on organic and
inorganic solid surfaces. Thus, numerous supported MNPs
have been studied as catalysts for a variety of reactions.[8]
However, since these supported MNPs still have a propensity
to coalesce, they are used in the presence of organic
stabilizing and/or capping agents in most cases. Our research
interest is therefore focused on the evaluation of the catalytic
properties of ligand-free MNPs heterogenized on a solid
support.
The creation of a core–shell structure is one technique to
stabilize MNP cores in a solid shell without the use of organic
[*] Prof. Dr. S. Ikeda, S. Ishino, T. Harada, N. Okamoto,
Prof. Dr. M. Matsumura
Research Center for Solar Energy Chemistry
Osaka University
1-3 Machikaneyama, Toyonaka 560-8531 (Japan)
Fax: (+81)6-6850-6699
E-mail: sikeda@chem.es.osaka-u.ac.jp
Dr. T. Sakata, Prof. Dr. H. Mori
Research Center for Ultra-High Voltage Electron Microscopy
Osaka University, 7-1, Mihogaoka, Ibaraki 567-0047 (Japan)
Prof. Dr. S. Kuwabata
Division of Applied Chemistry
Graduate School of Engineering, Osaka University
2-1, Yamadaoka, Suita 565-0871 (Japan)
Prof. Dr. T. Torimoto
Department of Crystalline Materials Science
Graduate School of Engineering, Nagoya University
Furo-cho, Chikusa-ku, Nagoya, 464-8603 (Japan)
[**] This research was partially supported by a Grant-in-Aid for Young
Scientists (A) (grant no. 16685020) fromthe Ministry of Education,
Culture, Sports, Science and Technology, and the Iketani Science
and Technology Foundation.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
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ligands.[9] A promising structure for a catalyst in such a core–
shell approach is a core MNP encapsulated by a hollow
porous shell whose size is larger than that of the core particle.
This “rattle-type” nanostructure enables stabilization of the
core MNP without any ligands even under severe reaction
conditions in a catalytic system. Although various examples
of rattle-type nanostructures have been reported,[10] their
catalytic properties have not been demonstrated owing to the
difficulty of obtaining a core particle of sufficiently small size
for use as a catalyst. Herein we fabricate a ligand-free
platinum nanoparticle 2.2 nm in diameter as a typical catalyst
which is encapsulated in a hollow porous carbon shell 30–
40 nm in diameter (Pt@hmC). Since the carbon shell acts as a
barrier that prevents the coalescence of Pt nanoparticles and
also provides a void space where organic transformation
occurs on the surface of the ligand-free Pt nanoparticle, the
Pt@hmC particle was found to work as a robust and reusable
heterogeneous catalyst for hydrogenation reactions.
Fabrication of Pt@hmC was performed using the proce-
dure for fabrication of
a hollow-core/mesoporous-shell
carbon containing a gold nanoparticle about 20 nm in
diameter.[10d] This procedure is based on the successive
coating of a silica layer and a mesoporous silica layer on the
surface of an MNP, subsequent growth of a carbon shell inside
the mesoporous silica shell, and removal of the siliceous
components by treatment with aqueous HF. When applying
the procedure to much smaller Pt nanoparticles, we modified
both the silica-coating and carbon-grafting steps of the above
procedure (see Experimental Section). We employed Pt
nanoparticles stabilized by poly(N-vinyl-2-pyrrolidone) (Pt–
PVP) with an average particle size of 1.8 nm as a starting
material (Figure 1a). As confirmed by the TEM image shown
in Figure 1b, the Pt nanoparticles retain the size and size
distribution of the original Pt–PVP after being covered with a
double layer of silica and mesoporous silica (Pt@
SiO2–mSiO2), while some Pt@SiO2–mSiO2 particles contain
more than one Pt nanoparticle in each silica shell.
Figure 1. TEM images and Pt particle size distributions of a) Pt–PVP,
b) Pt@SiO2–mSiO2, and c) Pt@hmC.
isotherm at a relative pressure (P/P0) of 0.42–0.46. Since this
phenomenon is typically observed in materials having a large
mesopore encapsulated by a pore system of relatively small
pore size,[12] the carbon shell of Pt@hmC is likely to consist of
a porous wall structure. In practice, micropore and mesopore
analyses of the adsorption branch by an as-plot and the BJH
method revealed that the carbon shell has both micro-
(< 0.9 nm) and nanosized (1–4 nm) pore systems.[11] It is also
notable that the BET surface area calculated from the
Figure 1c shows a TEM image of Pt@hmC obtained from
the Pt@SiO2–mSiO2 and the size distribution of the Pt
nanoparticles in this material. The Pt@hmC particle consists
of a hollow carbon cage (30–40 nm in diameter with a shell
thickness of 15 nm) and a central Pt nanoparticle with an
average particle size of 2.2 nm. This value is consistent with
the crystallite size determined by X-ray diffraction (XRD)
analysis and implies the single crystalline nature of each Pt
nanoparticle.[11] The increase (0.3 nm) of the Pt particle size
relative to that in Pt@SiO2–mSiO2 is probably due to the
coalescence of the Pt particles originally included in the same
silica shell. The XP spectrum of Pt@hmC (Pt 4f region) is
typical for Pt0 with a small contribution from Pt2+. The fact
that almost no peak for expected impurities, such as Cl and F,
is observed within the sensitivity of this measurement
indicates that the surface of the Pt nanoparticle is free from
any ligand but is partially oxidized.[11] The content of Pt in
Pt@hmC, as measured by inductively coupled plasma (ICP)
analysis, is 2.1 wt% under these reaction conditions.[11]
isotherm at a relative pressure of 0.05–0.09 reaches
1830 m2 gÀ1, which is an extremely high value that is com-
parable to representative activated carbons.
Figure 2 shows the catalytic performance of Pt@hmC for
hydrogenation of nitrobenzene compared with that of the
original Pt–PVP, a commercial Pt catalyst supported on
activated carbon (Pt/AC),[13] and Pt@SiO2–mSiO2. It is clear
that remarkable activity is achieved with Pt@hmC: all of the
The N2 adsorption–desorption isotherm of Pt@hmC
measured at 77 K shows a significant hysteresis loop enclosed
by a sudden drop in the adsorbed volume for the desorption
Figure 2. Liquid-phase hydrogenation of nitrobenzene into aniline by
various Pt catalysts at 303 K.
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Chemie
nitrobenzene is converted into aniline by Pt@hmC, whereas
the reaction does not go to completion with Pt–PVP and Pt/
AC.[14] The Pt@SiO2–mSiO2 particle shows no activity, thus
indicating that the presence of voids in Pt@hmC is indispen-
sable for induction of the reaction. Although the original Pt–
PVP exhibits a higher level of activity than Pt/AC, it cannot
be recovered and reused efficiently. However, the Pt@hmC
catalyst can be recovered simply by centrifugation and
recycled for further reaction. A TEM image of Pt@hmC
taken after the reaction revealed that there is no change in the
structure of the catalyst.[11]
Experimental Section
Poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Pt (Pt–PVP) was syn-
thesized as reported elsewhere.[4b] Typically, PVP (66 mg) was added
to a solution of hexachloroplatinic acid (H2PtCl6·6H2O; 0.03 mmol,
15.5 mg) in water (5 mL) and ethanol (45 mL) and the mixture was
heated under reflux for 3 h. After adding acetone (160 mL) to 16 mL
of the solution, the mixture was centrifuged (23000 g, 20 min) to
collect the Pt–PVP precipitate. A mixture of ethanol (30.7 mL),
aqueous NH3 (0.28%, 1.28 mL), and tetraethylorthosilicate (TEOS,
0.1 mL, 0.45 mmol) was then added to the Pt–PVP precipitate and the
mixture was stirred at room temperature for 16 h. The resulting
precipitate of silica-covered Pt nanoparticles (Pt@SiO2) was isolated
by centrifugation (23000 g, 20 min). A mixture of ethanol (30.7 mL),
aqueous NH3 (0.28%, 1.38 mL), TEOS (0.1 mL, 0.45 mmol), and n-
octadecyltrichlorosilane (ODTS, 0.04 mL, 0.09 mmol) was then
added to this Pt@SiO2 precipitate. After stirring the suspension at
room temperature for 2 h, the precipitate was isolated by centrifu-
gation (23000 g, 20 min), washed with ethanol, and calcined at 823 K
for 6 h in air to produce Pt@SiO2 coated with a mesoporous silica shell
(Pt@SiO2–mSiO2).
As summarized in Table 1, Pt@hmC also shows a high
level of catalytic activity for the hydrogenation of primary,
secondary, and cyclic olefins in comparison with Pt–PVP and
Table 1: Hydrogenation of various olefins by Pt@hmC, Pt/AC, and Pt-
PVP catalysts.[a]
The typical procedure for obtaining Pt@hmC: The carbon source
used was resol-type phenol-formaldehyde resin (PF), which was
prepared by the reaction of phenol (90 mmol, 8.46 g) and formalde-
hyde (110 mmol, 4 mL) in water (5.6 mL) in the presence of aqueous
NH3 (28%, 0.75 mL) at 363 K for 2 h. Pt@SiO2–mSiO2 (60 mg) was
added to an ethanolic solution (2 mL) of PF (60 mg) and the solution
was stirred overnight at room temperature to yield a PF–Pt@
SiO2–mSiO2 composite. This composite was heated at 1173 K under
vacuum for 2 h to carbonize the PF. Dissolution of the SiO2–mSiO2
shell with 10% aqueous HF generated Pt@hmC, which was charac-
terized by TEM, XRD, XPS, N2 adsorption–desorption measure-
ments, and ICP analysis (see Supporting Information).
Catalytic hydrogenation of nitrobenzene was performed at 303 K
whilst bubbling H2 through the reaction vessel. Thus, 0.1 mmol of Pt
catalyst (1.0 mg of Pt@hmC) and ethanol (5 mL) were placed in a
cylindrical Schlenk flask (30 mL) and, after addition of substrate
(0.5 mmol) under argon, the mixture was vigorously stirred
(1200 rpm) at 303 K under H2 bubbling (20 mLminÀ1) for 1.5 h. For
the hydrogenation of olefins, the reactions were conducted in a
stainless-steel autoclave equipped with a glass reactor (50 mL).
Typically, Pt@hmC (1.0 mg) and acetone (5 mL) were put into the
glass vessel under N2. After addition of substrate (0.5 mmol), the
reactor was purged with H2, pressurized with H2 at 0.2 MPa (in
absolute pressure), and heated to 348 K whilst stirring (1200 rpm).
Catalytic activities were evaluated by determining the conversion of
substrate with a Shimadzu GC-2010 gas chromatograph equipped
with a flame ionization detector and a TC-FFAP capillary column.
Substrate
Catalyst
Product
t [h][b]
Conv. [%][c]
Pt@hmC
Pt–PVP
Pt/AC
2
2
2
>99
91
7
Pt@hmC
Pt–PVP
Pt/AC
2
2
2
96
70
16
Pt@hmC
Pt–PVP
Pt/AC
1
1
1
91
42
3
Pt@hmC
Pt–PVP
Pt/AC
15
15
15
72
46
16
[a] All reactions were carried out with 0.1 mmol of catalyst (Pt) and
0.5 mmol of substrate under H2 (0.2 MPa in absolute pressure) at 348 K.
[b] Reaction time. [c] Conversion of substrate.
Pt/AC catalysts. The corresponding turnover frequency TOF
(hÀ1) for Pt was calculated to be more than 20000, which is
much larger than that obtained with the platinum-nano-
particle-based system reported recently.[7] It is notable that
the Pt@hmC catalyst also gives higher conversion for the
hydrogenation of trans-stilbene (trans-1,2-diphenylethylene)
into 1,2-diphenylethane than Pt–PVP or Pt/AC, which
suggests that there is no significant effect of the porous
carbon shell on the mass transfer of such a bulky substrate.
Thus, we have demonstrated that Pt@hmC has high potential
as a heterogeneous hydrogenation catalyst.
Received: July 7, 2006
Published online: September 28, 2006
Keywords: heterogeneous catalysis · hydrogenation ·
.
materials science · nanostructures · platinum
All of the results described above are consistent with our
prediction that a metal nanoparticle that is not bound to
anything should be able to catalyze a chemical reaction
efficiently. We have employed only Pt nanoparticles as
hydrogenation catalysts in the present study. However, the
basic concept of the system should be applicable to the
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Communications
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[13] The Pt/AC sample was supplied by N. E. Chemcat Corporation.
[14] Since these catalysts gave aniline as the sole product, we used the
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