8
76
Chemistry Letters Vol.35, No.8 (2006)
Highly-dispersed and Size-controlled Ruthenium Nanoparticles on Carbon Nanofibers:
Preparation, Characterization, and Catalysis
ꢀ
1;2
2
2
1
Yukihiro Motoyama,
Isao Mochida, and Hideo Nagashima
Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580
Mikihiro Takasaki, Kenji Higashi, Seong-Ho Yoon,
1
1;2
1
2
Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580
(Received May 16, 2006; CL-060575; E-mail: motoyama@cm.kyushu-u.ac.jp)
Facile synthesis of ruthenium nanoparticles supported on the
(1.1–3.8 wt % for Ru/CNF-T and 1.1–1.6 wt % for Ru/CNF-H).
The TEM images of these carbon materials showed the size dis-
tribution and the location of nanoparticles are highly dependent
on the structure of carbon materials. The photo of the Ru/CNF-P
is especially of interest, revealing that small and spheroidal
species (dav ¼ 2:5 nm) are homogeneously dispersed selectively
carbon nanofibers (CNFs) is accomplished by thermal decompo-
sition of Ru3(CO)12; ruthenium species on the platelet-type CNF
are dispersed homogeneously and selectively on the edge of the
graphite layers with narrow size distributions and behaves as an
excellent catalyst for arene hydrogenation.
8
on the edge of the graphite layers (Figure 1). This is in sharp
contrast to the TEM images of the Ru/CNF-T and Ru/CNF-
H, in which coexistence of some large Ru masses (10 < d <
50 nm for Ru/CNF-T and 50 < d < 150 nm for Ru/CNF-H,
respectively) and small particles (<4:5 nm). The small species
on CNF-T are located both in the tubes and on the surface,
whereas nano particles were found between the graphite layers
Recent progress of transition metal-immobilized materials
for heterogeneous catalysis has provided a variety of interesting
aspects for nano-sized metal particles on the nano-level-control-
1
led solid supports. Activated carbon (AC) has been the support
1
a
of choice for metal-anchoring, however, there still remain
drawbacks which should be improved; e.g. less-reproducibility
for the catalyst performances and the deactivation of the catalyst
by sintering and/or leaching of the metallic species. These draw-
backs can be attributed to ill-controlled structures of AC having
a wide variety of surface and pore structures. In this context, car-
bon nanofibers (CNFs) having well-controlled nano-structures
could be attractive catalyst supports, if metal nanoparticles are
highly dispersed on the surface of CNFs. The CNFs are classified
into three types: Graphite layers are parpendicular (platelet:
9
and on the surface for Ru/CNF-H (see ESI). Existence of
dispersed nanoparticles is observed in the photo of Ru/AC; how-
ever, a majority of the Ru particles is located inside the pores
(d ¼ 1:5{4:0 nm). The formation of the large ruthenium masses
in Ru/CNF-T and Ru/CNF-H may relate to non-reproducibility
of the ruthenium content in their production as described above.
The XPS survey spectra of these Ru catalysts displayed close
3
two Ru d
and oxidized RuO ; a characteristic feature of the Ru/CNF-P
5=2
signals assignable to the zero-valent ruthenium
2
0
CNF-P), parallel (tubular: CNF-T), and stacked obliquely
2
is that the high contents of the Ru species existed on the surface
0
IV
10
(
synthesis of these three types of CNFs in large scales. The
herringbone: CNF-H). We have recently reported selective
3
(Ru /Ru = 74:26). Thus, the present method allowed an
important discovery that the CNF-P is the most effective for
anchoring highly-dispersed ruthenium nanoparticles of narrow
4
CNF-supported catalysts are reportedly prepared by the incipi-
0
ent wetness method to immobilize metal salts on the surface;
however, it is necessary to perform subsequent reduction of
the metal salts with H2 at high temperature, and the process
size on the carbon surface with high contents of Ru species.
Since the hydrogenation is known to proceed efficiently by an
active zero-valent metal catalyst with large surface area (i.e.,
small size metal particles) located on the surface, the Ru/
CNF-P is expected to act as efficient hydrogenation catalyst.
Indeed, high catalytic activity of the Ru/CNF-P was verified
in the hydrogenation of toluene (Table 1). First, the Ru/CNF-P
smoothly catalyzed the reduction of toluene to afford methylcy-
clohexane as the sole product in quantitative yield without any
induction period (Entry 1). The chemical yield of the product
was reproducible over ten different experiments. In sharp con-
trast, the conversion of toluene was different in each experiment
5
sometimes causes aggregation of the metallic species. This
means that effective methods for anchoring size-controlled
metal particles on the CNFs is an important research project to
be explored. We wish to report here a method for the Ru nano-
particles on the CNFs (Ru/CNFs) using Ru3(CO)12 as a zero-
6
valent organometallic precursor; the reaction with CNF-P is
particularly important leading to highly-dispersed and size-con-
trolled Ru nanoparticles on the surface. Although Ru3(CO)12 is
well known to react facilely with polyaromatic hydrocarbons
7
a
7b
such as acenaphthylene derivatives, fullerenes, and graphite,
there has been no precedence for the reaction with CNFs.
dav = 2.45 nm
The Ru/CNFs catalysts were readily prepared by thermal
decomposition of Ru3(CO)12 in refluxing toluene in the presence
of CNFs under an argon atmosphere (see Electronic Supporting
Information; ESI). For comparison, we also immobilized Ru
species on AC (Ru/AC) by the same method. The inductively
coupled plasma-mass (ICP–MS) analysis revealed that the Ru
content was constant in several samples of Ru/CNF-P (1.6–
10 nm
1
2
3 4 / nm
1.7 wt %) and Ru/AC (1.3–1.4 wt %). In contrast, that of the
others was not reproducible, and was dependent on each sample
Figure 1. TEM image and histogram of the Ru particles on
CNF-P.
Copyright Ó 2006 The Chemical Society of Japan
Motoyama, Yukihiro
