Nanosize-induced hydrogen storage and capacity control in a non-hydride-forming element: Rhodium

Article abstract of DOI:10.1021/ja2027772

We report the first example of nanosize-induced hydrogen storage in a metal that does not absorb hydrogen in its bulk form. Rhodium particles with diameters of <10 nm were found to exhibit hydrogen-storage capability, while bulk Rh does not absorb hydroge

Full text of DOI:10.1021/ja2027772

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Nanosize-Induced Hydrogen Storage and Capacity Control in a
Non-Hydride-Forming Element: Rhodium
,
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Hirokazu Kobayashi,*
Hitoshi Morita, Miho Yamauchi,
Ryuichi Ikeda, Hiroshi Kitagawa,*
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Yoshiki Kubota, Kenichi Kato, and Masaki Takata
†
Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
‡
§
Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
Catalysis Research Center, Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan
^
#
PRESTO and CREST, Japan Science and Technology Agency (JST), Sanbancho 5, Chiyoda-ku, Tokyo 102-0075, Japan
r
INAMORI Frontier Research Center, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-3095, Japan
O
Department of Physical Science, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
[
RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
z
Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
S Supporting Information
b
possibility that nanosize-induced hydrogen storage can be exhibited
bynon-hydride-formingmetalssuchasFe, Ru, Pt, Ir, andRh, asthese
ABSTRACT: We report the first example of nanosize-
induced hydrogen storage in a metal that does not absorb
hydrogen in its bulk form. Rhodium particles with diameters
of <10 nm were found to exhibit hydrogen-storage cap-
ability, while bulk Rh does not absorb hydrogen. Hydrogen
storage was confirmed by in situ powder X-ray diffraction,
1,6a,7
metals have a strong affinity toward hydrogen molecules.
Here
we report the first example of nanosize-induced hydrogen storage by
a metal that does not absorb hydrogen in its bulk form.
Rhodium is a major catalyst in many types of reactions because
of its unique properties as a catalyst in hydrogenation, hydroformyla-
2
solid-state H NMR, and hydrogen pressureꢀcomposition
6
tion, and carbonylation reactions and in emission gas purification.
isotherm measurements. The hydrogen absorption capacity
could be tuned by controlling the particle size.
Although it is well-known that Rh reacts strongly with hydrogen
molecules and can dissociate hydrogen atoms on its surface, bulk Rh
does not form a stable hydride. Therefore, Rh has been considered
not to have hydride-forming or hydrogen-storage capability. This
paper discusses the hydrogen-storage capability of Rh particles with
diameters of <10 nm, even though bulk Rh does not absorb hydro-
gen. Moreover, the concentration of absorbed hydrogen in the Rh
nanoparticles can be tuned by controlling the particle size.
esearch into the reaction of metals with hydrogen is very
R
important for the development of potential applications,
such as effective catalysts for hydrogenation or advanced materi-
als for hydrogen storage and purification. Many investigations
into hydrogen storage using bulk metals or alloys have been
The Rh nanoparticles were synthesized from the polyol reduc-
1
carried out over the past half-century. On the other hand, metal
tion of rhodium chloride (RhCl ) in the presence of poly(N-vinyl-
3
8
nanoparticles as hydrogen-storage materials have received scant
2-pyrrolidone) (PVP). Transmission electron microscopy (TEM)
2
attention. Previous studies of the hydrogen-storage properties
images of the Rh nanoparticles showed that the mean diameters of
the nanoparticles used were 2.4 ( 0.5, 4.0 ( 0.7, and 7.1 ( 1.2 nm
(Figure 1). Control of the nanoparticle size was performed by
of metal nanoparticles have been confined to hydride-forming
elements, and there have been no reports concerning nanosize
effects on non-hydride-forming elements.
9
changing the concentration of Rh precursor and/or PVP. All of the
It is well-known that metal nanoparticles show chemical and
physical properties that differ from those of bulk metals because
of their unique properties based on their high surface-to-volume
Rh nanoparticles showed powder X-ray diffraction (PXRD) pat-
terns originating from a single face-centered-cubic (fcc) lattice. The
9
diffraction peak broadened for the smaller Rh nanoparticles, con-
3
9
ratio and quantum size effects. In particular, nanosized metals
sistent with the Scherrer equation.
show characteristic phase behaviors, such as lowering of the
It is known that hydrogen permeates the interior of a metal or
alloy lattice as individual atoms and that these interstitial hydro-
4
5
melting point and spontaneous alloying. As the hydrogen-
1
,2a
storage properties of a metal are strongly related to its electronic
gen atoms induce an expansion in the metal lattice.
To
1
state, metal nanoparticles are expected to show specific hydro-
elucidate the lattice expansion following hydrogen absorption
gen-storage properties according to their structure or size. We
have shown that in comparison with bulk Pd, hydrogen atoms are
strongly trapped inside Pd nanoparticles because of the forma-
tion of stable PdꢀH bonds. Even non-hydride-forming ele-
ments in the bulk are expected to exhibit a hydrogen-storage
capability as a result of nanosize effects. In particular, there is the
in Rh nanoparticles, we performed in situ PXRD measurements
on the BL02B2 beamline at SPring-8. The position of the
diffraction peaks of the Rh nanoparticles shifted continuously to
10
2a
Received: March 28, 2011
Published: June 27, 2011
r 2011 American Chemical Society
11034
dx.doi.org/10.1021/ja2027772
|J. Am. Chem. Soc. 2011, 133, 11034–11037

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Figure 1. TEM images of Rh nanoparticles with mean diameters of (A)
2.4 ( 0.5, (B) 4.0 ( 0.7, and (C) 7.1 ( 1.2 nm.
2
Figure 3. Solid-state H NMR spectra of Rh nanoparticles with diameters
of (a) 2.4, (b) 4.0, and (c) 7.1 nm. Spectra for (d) a bulk Rh sample (d) and
2
(
e) H
2
gas were also measured for reference. The inset shows expansions of
the spectra near 0 ppm. All of the spectra were measured at a pressure of
2
8
6.7 kPa of H gas at 303 K.
2
(
Figure 3). Sharp lines at ca. 0 ppm and broad absorption lines at
ca. ꢀ100 ppm were observed for all of the Rh nanoparticles. The
NMR spectra expanded around 0 ppm are shown in the Figure 3
2
inset. In the spectra for H gas and bulk Rh, only a peak at 3.4
2
ppm was observed. In addition to this peak, a sharp component
appeared at ꢀ1.2 ppm in the spectra of the three nanoparticle
samples. On the basis of a comparison of these spectra, it is
reasonable to attribute the spectral component observed at 3.4
ppm to free deuterium gas. Because the component observed
2
at ꢀ1.2 ppm was very sharp and located near the signal from H
2
2
gas, it can be assigned to deuterium atoms ( H) diffusing and/or
2
a
exchanging with free deuterium on the nanoparticles' surfaces.
The broad component in the spectra was observed only for the
nanoparticles. This component is attributed to deuterium atoms
2
(
H) whose motions are restricted to within the Rh lattice and
experience an insufficiently averaged electric field gradient on the
nucleus. The peak position of the broad component for the Rh
nanoparticles was located at high field (ca. ꢀ100 ppm). This
2
implies that the nuclear spins of the H atoms located inside the
Figure 2. (a) In-situ PXRD patterns of 2.4 nm diameter Rh nanopar-
ticles during the process of hydrogen absorption. Black lines show curves
obtained from Le Bail fitting. (b) Increase in the lattice constant (Δa) of
the Rh samples estimated from the Le Bail fits of the diffraction patterns
during hydrogen absorption.
Rh nanoparticles are polarized from the interaction with the
polarized electron spins of the s and d conduction electrons of
the Rh nanoparticles arising from the formation of hydride
(RhꢀH). This observation contrasts with the low-field shift of
2
11
the H atoms absorbed in Pd hydride. The d band in Pd hydride
lower angles in response to increasing hydrogen pressure (see
Figure 2a for 2.4 nm Rh nanoparticles and the Supporting Infor-
mation for 4.0 nm, 7.1 nm, and bulk samples). The hydrogen-
pressure dependence of the lattice constants was determined
from Le Bail fitting of the diffraction patterns and the change in
lattice constant (Δa) with the absorption of hydrogen. As shown
in Figure 2b, the lattice constants increased with increasing
hydrogen pressure for all of the Rh nanoparticles, indicating that
the Rh atoms within the lattice were forced apart. This behavior
can be the consequence of hydrogen atoms penetrating the
is almost filled, so the spin correlation is weakened and the
2
Knight shift is the dominant factor in the NMR shift of H in Pd
12
hydride. On the other hand, as a large number of holes exist in
the d band of Rh, the spin correlation between the conduction
electrons of the Rh hydride is large enough to cause spin
2
polarization in the H atoms in the Rh hydride. This high-field
2
shift of H in Rh hydride is the first such observation to be
reported. Our NMR experimental results clearly indicate the
formation of Rh hydride.
Pressureꢀcomposition (PC) isotherms of the Rh samples
were measured at 303 K to quantify how much hydrogen was
absorbed by the Rh nanoparticles. As shown in Figure 4, the
concentration of hydrogen in the Rh nanoparticles with dia-
meters of <10 nm increased with increasing hydrogen pressure,
while the bulk Rh sample did not absorb hydrogen at all. This
result provides the first example of nanosize-induced hydrogen
inside of the Rh lattice and expanding it. On the other hand, the
9
lattice constant of bulk Rh exposed to H does not change at all.
2
These results strongly suggest that the nanosized Rh particles
absorb hydrogen gas.
2
Solid-state H NMR measurements were carried out to
2
investigate the state of the H atoms in the Rh nanoparticles
1
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dx.doi.org/10.1021/ja2027772 |J. Am. Chem. Soc. 2011, 133, 11034–11037

Journal of the American Chemical Society
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’
ACKNOWLEDGMENT
This work was partially supported by Grants-in-Aid for the
Global COE Program “Science for Future Molecular Systems”
and the Elements Science and Technology Project from MEXT,
Japan, and by the Asahi Glass Foundation.
’
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hkobayashi@icems.kyoto-u.ac.jp; kitagawa@kuchem.kyoto-u.ac.jp
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