Synthesis and characterization of nickel nanoparticles dispersed in imidazolium ionic liquids

Article abstract of DOI:10.1039/b703979d

The diameter and size-distribution of Ni nanoparticles prepared by the decomposition of [bis(1,5-cyclooctadiene)nickel(0)] organometallic precursor dissolved in 1-alkyl-3-methylimidazolium N-bis(trifluoromethanesulfonyl) amide ionic liquids depend on the length of the alkyl side-chain of the imidazolium ring. The increase of the organization range order of the ionic liquid that increases with that of the alkyl side-chain (from n-butyl to n-hexadecyl) induces the formation of nanoparticles with a smaller diameter and size-distribution. The cubic fcc Ni nanoparticles with 4.9 ± 0.9 to 5.9 ± 1.4 nm in mean diameter and monomodal size-distribution thus prepared are probably composed of a small cap layer of NiO around a core of Ni metal. The contribution of the oxide layer also depends on the medium i.e. the metal oxide ratio increases in salts containing four to eight carbons on their side-chains and then decreases as the number of carbons increases. The Ni nanoparticles dispersed in the ionic liquids are active catalysts for the hydrogenation of olefins under relatively mild reaction conditions. the Owner Societies.

Full text of DOI:10.1039/b703979d

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Synthesis and characterization of nickel nanoparticles dispersed in
imidazolium ionic liquids
a
a
b
a
Pedro Migowski, Giovanna Machado, Sergio R. Texeira, Maria C. M. Alves,
c
b
a
Jonder Morais, Agnes Traverse and Jairton Dupont*
`
Received 15th March 2007, Accepted 27th June 2007
First published as an Advance Article on the web 9th July 2007
DOI: 10.1039/b703979d
The diameter and size-distribution of Ni nanoparticles prepared by the decomposition of
bis(1,5-cyclooctadiene)nickel(0)] organometallic precursor dissolved in 1-alkyl-3-methylimidazolium
[
N-bis(trifluoromethanesulfonyl) amide ionic liquids depend on the length of the alkyl side-chain
of the imidazolium ring. The increase of the organization range order of the ionic liquid that
increases with that of the alkyl side-chain (from n-butyl to n-hexadecyl) induces the formation of
nanoparticles with a smaller diameter and size-distribution. The cubic fcc Ni nanoparticles with
4
.9 Æ 0.9 to 5.9 Æ 1.4 nm in mean diameter and monomodal size-distribution thus prepared are
probably composed of a small cap layer of NiO around a core of Ni metal. The contribution of
the oxide layer also depends on the medium i.e. the metal oxide ratio increases in salts containing
four to eight carbons on their side-chains and then decreases as the number of carbons increases.
The Ni nanoparticles dispersed in the ionic liquids are active catalysts for the hydrogenation of
olefins under relatively mild reaction conditions.
3
3–36
Introduction
aggregates display charge-ordering structures.
This struc-
tural organization of ILs may be used as ‘‘entropic drivers’’ for
spontaneous, well-defined, and extended ordering of nanoscale
37
Metal nanoparticles with a characteristic high surface-to-
volume ratio (consequently a large fraction of the metal atoms
are at the surface) often exhibit very interesting electronic,
optical, magnetic, and chemical properties, which are usually
structures. The unique combination of adaptability towards
other molecules and phases plus the strong H-bond-driven
structure makes ILs potential key tools in the preparation of a
new generation of chemical nanostructures. It is reasonable to
assume that by increasing the range of the organization of
these salts it will be possible to improve the control of
diameter, size-distribution and shape of the nanomaterials
prepared in these fluids. Indeed, it has recently been reported
that the size of stable colloid solutions containing nanoparti-
cles of cyano-bridged molecule-based magnets prepared in
1
not observed with their bulk counterparts. These properties
for colloidal nanoparticles of transition metals depend not
only on their diameter, shape and size-distribution but also on
their concentration, type of stabilizers present in the medium
and on the nature of the interaction between the metal-surface
2
–4
and the stabilizer(s).
The synthetic methods available for
the control of diameter, shape and size-distribution of transi-
tion-metal nanoparticles in a colloidal solution usually depend
on the type of stabilizer in conjunction with or without
1
-alkyl-3-methylimidazolium tetrafluoroborate ILs can be
controlled by varying the length of the N-alkyl chain on the
38
imidazolium cation. Moreover, the type of the IL cation has
5
,6
modifiers. These methods allowed for the preparation of
various types of water or organic solvent-soluble nanomater-
a dramatic effect on the grain size of electrochemically made
39–41
nanoparticles.
7
ials such as uniform nanometer sized particles. Ionic liquids
–9
1
ILs), in particular those based on the 1,3-dialkylimidazo-
0
(
The range order of organization of 1-alkyl-3-methyl imida-
lium cation, have recently been used as both ‘‘solvents’’ and
1
zolium ionic liquids increases with the augmentation of the
42–44
1–29
stabilizers for the preparation of metal nanoparticles
but
number of carbons of the 1-alkyl side-chain.
Therefore,
the diameter, shape and size-distribution could only be
1
achieved in these fluids with the aid of modifiers.
these materials can be used as a probe to check whether or not
they exert a template control on the preparation of ionic
liquid soluble transition metal nanoparticles. With this pur-
3,30–32
Imidazolium ILs possess pre-organized structures through
mainly hydrogen bonds that induce structural directionality,
contrary to classical quaternary ammonium salts, in which the
pose we have investigated the diameter, shape and size-
45–51
distribution of nickel nanoparticles
prepared by the ther-
mal decomposition of [bis(1,5-cyclooctadiene)nickel(0)] orga-
nometallic precursor dispersed in 1-alkyl-3-methylimidazolium
N-bis(trifluoromethane sulfonyl) amide ILs (Fig. 1).
a
Laboratory of Molecular Catalysis, Instituto de Quı´mica—UFRGS,
P.O. Box 15003, Porto Alegre, 91501-970, RS Brazil. E-mail:
dupont@iq.ufrgs.br; Fax: +55 5133087204; Tel: +55 5133086321
Instituto de Fı ´s ica-UFRGS, P.O. Box 15051, Porto Alegre, 91501-
The 1-alkyl-3-methylimidazolium N-bis(trifluoromethane
sulfonyl) amide salts (Fig. 1) have been chosen for this study
since they possess one of the least coordinating anions and are
b
970, RS Brazil
c
Centre Laser Infrarouge d’Orsay, Baˆt 201 P2 Universite´ Paris-Sud,
1405 Orsay Cedex, France
5
2
9
one of the least water- and air-sensitive ionic liquids.
4
814 | Phys. Chem. Chem. Phys., 2007, 9, 4814–4821
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was observed). The fall in the hydrogen pressure in the tank
was monitored with a pressure transducer interfaced through a
Novus converter to a PC and the data workup via Microcal
Origin 5.0.
5
3,54
The ILs (Table 1) are known compounds
and have been
prepared by alkylation of methylimidazole using n-alkyl
methane sulfonates followed by anion metathesis with lithium
N-bis(trifluoromethane sulfonyl) amide. The purity (499.4%)
Fig. 1 1-Alkyl-3-methylimidazolium N-bis(trifluoromethanesulfonyl)
amide salts employed for the preparation of the Ni nanoparticles.
1
of these salts was checked by H NMR spectra using the
1
intensity of the C satellites of the imidazolium N-methyl
3
Table 1
preparation of the Ni nanoparticles
T
m
(1C) and T
g
(1C) of the imidazolium salts employed in the
55
group as an internal standard. A representative example of
the preparation and characterization of [C
presented below.
1
C
8
Im] Á NTf
2
is
Salt
T
m
/1C
T
g
/1C
Reference
[C
[C
[C
[C
[C
1
C
1
C
1
C
1
C
1
C
4
8
Im] Á NTf
2
2
À6
À87
À84
À83
53
53
53
54
54
Im] Á NTf
n.o.
À29
34.7
43.1
10Im] Á NTf
14Im] Á NTf
16Im] Á NTf
2
2
2
1
-Methyl-3-n-octylimidazolium methanesulfonate
a
n.o.
n.o.
a
.
1 8 3
[C C Im] MeSO
a
n.o. not observed.
Methanesulfonyl chloride (18.3 g; 0.16 mol) was added, with
vigorous stirring, to an ice bath cooled solution of n-octanol
(
20.5 g; 0.16 mol) and triethylamine (16.2 g; 0.16 mol) in 150
mL of CH Cl . After addition, stirring was continued for
2
2
Moreover, most of them have already been synthesized and
most of their physico-chemical properties determined (Table
further 2 h at room temperature. Water (30 mL) was added,
the aqueous layer containing the triethylammonium chloride
by-product was separated; the organic layer was washed with
water (20 mL) and dried over magnesium sulfate. The solvent
evaporation afforded the desired product as a colorless liquid
5
3,54
1
).
The nanoparticles dispersed in the ionic liquids have
been analyzed by TEM, SAXS, and EXAFS.
(
30.6 g, 92% yield). A mixture of n-octyl methanesulfonate
Experimental
(15.1 g; 0.07 mol) with 1-methylimidazole (6.0 g; 0.07 mol) was
stirred over one week at 60 1C affording 20.3 g of a very
hygroscopic amber liquid that was used without further
General
All reactions were performed under an argon atmosphere
using Schlenk techniques. The substrates (Acros or Aldrich)
were obtained from commercial sources and used as received.
Infrared spectra were obtained on a Bomen B-102 spectro-
purification for the metathesis with lithium N-bis(trifluoro-
1
methane sulfonyl)amide. H NMR (300 MHz, CDCl
3
) d ppm
9.80 (s, 1H, C–H imidazolium); 7.60 (s, 1H, C–H imidazo-
3
lium); 7.44 (s, 1H, C–H imidazolium); 4.26 (t, JHH = 7.39
1
13
meter. H- and C-NMR spectra were recorded on a Varian
Gemini 300 MHz spectrometer. The chemical shifts were
Hz, 2H, NCH ); 4.05 (s, 3H, NCH ); 2.77 (s, 3H, CH SO );
3
2
3
3
2.00–1.77 (m, 2H, CH ); 1.44–1.13 (m, 10H, CH ); 0.87
2
2
3
13
measured in ppm relative to TMS or acetone-d as external
6
(t, J = 6.72 Hz, 3H, CH3). C NMR (75 MHz, CDCl )dppm
HH
3
standard. The ESI-HRMS spectra were recorded in a Waters
Micromass Q-Tof micro Mass Spectrometer YB 320. Mass
spectra were obtained using a GC/MS Shimatzu QP-5050
137.5, 123.6 and 121.8 (C–H imidazolium); 49.6 (NCH ); 39.5
2
(CH3S
(6 C
O
3
); 36.1 (NCH
3
); 31.4, 30.0, 28.8, 28.7, 26.00 and 22.30
3
H
); 13.80 (CH ). Mp: n.o. HRMS-ESI-Q-TOF: ESI-(+)
2 3
(
EI, 70 eV). Gas chromatography analysis was performed
calculated 195.1861 found 195.1864 ESI-(À) calculated
À1
with a Agilent 6820 gas chromatograph with an FID detector
and a 30 m capillary column with a diphenyldimethylpolysi-
loxane stationary phase.
94.9803 found 94.9800 IR: (film, KBr, wavenumber in cm
)
3428 (n OH, water); 3147, 3085 and 3018 (n C–H imidazo-
lium); 2956, 2928 and 2857 (n C–H alkyl side-chain), 1630 and
DSC measurements were carried out on a Thermal Analyst
1571 (n CQC), 1465 (ds CH
2 3
), 1378 (ds CH ); 1206 and 1056
À
2
100 (TA instruments), calibrated using an indium primary
(n SO
3
anion).
standard. Ionic liquid samples were sealed in Al pans (in the
nitrogen-filled glove box) with an empty Al pan used as
reference. DSC measurements were carried out in nitrogen
atmosphere on samples of different sizes and the samples
initially examined using different rates of heating and cooling.
All of the DSC data discussed in the manuscript were mea-
sured on 7–12 mg samples. The nanoparticle formation and
hydrogenation reactions were carried out in a modified
Fischer-Porter bottle immersed in a silicone oil bath and
connected to a hydrogen tank. The temperature was main-
tained at 75 or 100 1C by a hot-stirring plate connected to a
digital controller (ETS-D4 IKA). A deliberated stirring of
1
-Methyl-3-n-octylimidazolium N-bis(trifluoromethanesulfonyl)
5
6
.
1 8 2
amide [C C Im] NTf .
A mixture of lithium N-bis(trifluoromethanesulfonyl) amide
(13.4 g; 46 mmol) dissolved in water (7 mL) and 1-methyl-
3-octyl imidazolium methanesulfonate (12.4 g; 43 mmol) also
dissolved in water (17 mL) was mixed and vigorously stirred
for 30 min. Methylene chloride (50 mL) was then added, the
organic phase was separated and dried with magnesium
sulfate. Solvent evaporation afforded the desired 1-methyl-
3-octyl imidazolium N-bis(trifluoromethanesulfonyl) amide
(21.5 g, 96% yield).
1
200 rpm was used (no ionic catalytic solution projection
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Formation of Ni nanoparticles
done as per the guidelines set by the International XAFS
5
Society Standards and Criteria Committee.
8
2
In a typical experiment a solution of [Ni(cod) ] (69 mg,
5
An IFEFFIT analysis package was used for the EXAFS
9
0
.25 mmol) in 6 mL of anhydrous benzene was charged in a
À1
˚
data analysis. The EXAFS signal between 2.5 and 12.0 A
Fischer-Porter bottle containing 2 mL of ionic liquid. The
volatiles were then removed at reduced pressure (0.1 bar) at
room temperature, except for the ionic liquids with the cation
1
was Fourier-transformed with a k weighting and a Hanning
window. Details of the fitting are described in the Results and
Discussion section. From these analyses structural parameters
[
C C Im] and [C C Im] that are solid at room temperature
1 14 1 16
0
including N, R, s and E were calculated for the different
(
40 and 45 1C, respectively). The system was kept at 75 1C and
atomic shells. N corresponds to the number of atoms in the
shell, R to the interatomic distance, s is the full width of the
half maximum of each peak that accounts for the disorder in
hydrogen (4 bar) was admitted. After 30 min the reactor was
cooled in vacuum and the sample stored in a flask under an
argon atmosphere.
the shell and E
0
is the energy shift.
Catalytic hydrogenations of cyclohexene
SAXS Analysis
For the catalytic experiments, the solutions of nanoparticles
were prepared with the same procedure used for the formation
of the nanoparticles, but the amounts of ionic liquid and
organometallic precursor were reduced 4 times. Degassed
cyclohexene (1.06 g, 13 mmol) was added into the Fischer-
Porter reactor containing a freshly prepared solution of Ni
nanoparticles and then the system was kept at 100 1C and
charged with 4 bar of hydrogen. After the hydrogen consump-
tion had stopped, the organic phase (upper phase) was ana-
lyzed by CG and CG-MS.
The samples of nickel nanoparticles were analyzed using
SAXS at room temperature, utilizing the beam line of the
Laboratorio Nacional de Luz Sıncrotron, LNLS, Brazil. The
´ ´
samples containing the dispersion of the nanoparticles and ILs
were injected with a needle syringe into the sample chamber.
The sample chamber was sealed with mica sheets on both sides
for incident and scattered X-ray. These measurements were
˚
undertaken in a transmission geometry with l = 1.743 A to
avoid fluorescence from Ni nanoparticles. All samples were
manipulated in the same way. To avoid porosities the solid
ones were melted at 40 1C before being injected in the sample
chamber. During the experiment they were at solid state. The
time exposure for each sample was around 1200 s, and the
distance between sample and SAXS detection plane was
TEM analysis
The morphologies and the electron diffraction (ED) of the
obtained particles were carried out on a JEOL JEM À 2010
equipped with an energy dispersive X-ray spectroscopy (EDS)
system and JEOL JEM À 1200 EXII electron microscope
operating at an accelerating voltage of 200 and 120 kV,
respectively. The samples for TEM were prepared by deposi-
tion of the nickel nanoparticles dispersed in the ILs at room
temperature on a carbon-coated copper grid. The histograms
of the nanoparticle size-distribution were obtained from mea-
surement of around 300–400 particles, and were reproduced in
different regions of the Cu grid, assuming spherical shape,
found in an arbitrary chosen area of enlarged micrographs.
4
31.2 mm. The SAXS spectra were obtained using linear
position sensitive proportional counter as a detector. The
SAXS data were corrected following the standard procedures.
Results and discussion
The nanoparticles were prepared by the addition of 2 mL of
6
0
the ionic liquid to a yellow solution of [Ni(cod)2] (0.25 mmol)
in benzene (6 mL) at room temperature, followed by removal
of the volatiles under reduced pressure. The use of benzene is
essential for the complete dispersion of the Ni precursor in the
IL. Note that highly pure reagents (benzene and ILs) should
XAS (X-ray absorption spectroscopy) analysis
2
be used in order to avoid the decomposition of the [Ni(cod) ]
X-Ray absorption measurements at the Ni K edge were
performed at the LNLS (Campinas, Brazil), using the
during its dispersion in the ionic liquids (the solution becomes
green).
5
7
XAS1 beam line, with a Si(111) channel-cut monochromator
and 0.2 mm vertical slits. The intensity of the incoming X-ray
beam was measured using an ionization chamber filled with
air. A 15 element germanium solid state detector (Ge-15
Camberra) measured the fluorescence photons emitted by
the sample. A syringe was used to introduce the nickel
particles dispersed in the ILs in the Teflon sample holders,
The yellow suspension was then submitted to 4 bar of
molecular hydrogen at 75 1C affording a black mixture after
30 min. The hydrogen was then substituted with argon and the
black suspension was analyzed by transmission electron
microscopy (TEM), small angle X-ray scattering (SAXS),
extended X-ray absorption fine structure (EXAFS). All mea-
surements were performed at RT (25 Æ 3 1C).
s
which were sealed with Kapton tape in both sides. The
Representative TEM micrographs with their corresponding
histograms showing the size-distribution of the black suspen-
sion in the five ionic liquids are presented in Fig. 2. The
histograms of the nanoparticles size-distribution were ob-
tained from measurement of around 300–400 particles
(600–800 counts).
spectra were taken at room temperature with the samples
oriented at a 451 angle with respect to the incoming beam.
The EXAFS spectra were acquired from 8210 up to 9000 eV
with a 0.8 eV step from 8210 to 8400 eV and a 2 eV step from
8
400 up to 9000 eV. Several scans were averaged in order to
achieve a good signal to noise ratio. The sample preparation,
control of parameters for extended X-ray absorption fine
structure measurements and data collection modes were all
It is clear that there are slight decreases in both the
nanoparticle diameter and the size-distribution with the in-
crease of the numbers of carbons of the alkyl side-chain up to
4
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Fig. 2 Parts of TEM micrographs and respective histograms showing the size-distribution of Ni nanoparticles prepared and dispersed in
[
C
1
C
4
Im] Á NTf
2
[a], [C
1
C
8
Im] Á NTf
2
[b], [C
1
C
10Im] Á NTf
2
[c], [C
1
C
14Im] Á NTf
2
[d] and [C
1
C
2
16Im] Á NTf [e] ILs.
1
4 carbons (from 5.9 Æ 1.4 nm for [C
1
C
4
Im] Á NTf
2
to 4.9 Æ
due to the increase of the electronic contrast when the nano-
particles are dispersed inside the ILs—the scattering intensity
grows. It is clear by comparing the Bragg reflections of both
0
.9 nm for [C ) and then an increase in diameter
1
C
14Im] Á NTf
2
and size-distribution as observed for the material in
[
C
1
C
16Im] Á NTf
2
ionic liquid (5.5 Æ 1.1 nm) as depicted in
sides of Fig. 3 for pure [C
1
C
14Im] Á NTf
2
and [C
1
C
16Im] Á NTf
2
Fig. 2. Although these differences in diameter and size-
distribution are relatively small and could be a TEM artefact
and Ni(0) dispersed in the same liquids, respectively. Note that
the nanoparticles have been prepared under the same reaction
conditions for all salts at temperatures in which these salts are
liquid.
(
sample preparation and/or particle counting, for instance)
they do suggest the existence of a trend: increasing the ionic
liquid organization range order induces the formation of
nanoparticles with smaller diameter and size-distribution,
SAXS analysis of these salts clearly shows an increase in the
range order with the increase of the number of carbons of the
alkyl side-chain (from isotropic liquid to crystalline material)
(
see Fig. 2). Indeed, [C
1
C
4
Im] Á NTf
are liquids at room temperature, while
and [C are solids at RT (Table
16Im] Á NTf
2
, [C
1
C
8
Im] Á NTf
2
and
[
C
1
C
C
10Im] Á NTf
14Im] Á NTf
2
up to [C
for the [C
1
C
14Im] Á NTf
16Im] Á NTf
2
and a slight decrease in the order
[
C
1
2
1
C
2
1
C
2
. In particular, a broad peak at q =
À1
˚
1
). Typical SAXS data collected at room temperature for the
0.28 A corresponding to 2y = 4.341 for [C
1
C
10Im] Á NTf
2
pure ionic liquids and for Ni(0) nanoparticles plus ionic liquids
are shown in Fig. 3.
and at least one narrow peak in the low-angle region q = 0.23
À1
˚
À1
˚
A
, 2y = 3.681 and q = 0.22 A , 2y = 3.51 for [C
1
C
14Im] Á
The SAXS data clear show a great increase in the scattered
intensities of the samples that contain Ni nanoparticles. This is
NTf and [C , respectively. Interestingly, the
2
1
C
16Im] Á NTf
2
SAXS data collected for the colloidal suspension shows an
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Fig. 3 Typical SAXS spectra for pure ILs (left) and Ni(0) nanoparticles in colloidal suspension in the ILs (right).
increase in the range order of the salts indicating a synergic
organization effect, i.e., the more regular with smaller diameter
and size-distribution nanoparticles were obtained in the more
[C
1
C
16Im] Á NTf
2
is less organized and the correlation distance
among particles in this IL is worse than for [C C Im] Á NTf
1
10
2
and [C C Im] Á NTf , respectively. The comparison of the
1
14
2
‘
‘pre-organized’’ ionic liquids, see Fig. 3, right side. For longer
EXAFS signals, Fig. 4a, and the corresponding Fourier trans-
forms, Fig. 4b enables one to distinguish two groups of
side-chain lengths the correlation among the particles in-
creases, i.e., the distance distribution from one particle to
samples with closer environments. The [C
[C and [C 10Im] Á NTf ILs display similar
Im] Á NTf
EXAFS signals extending up to 12 A . However, for
[C and [C , a more dumped signal
1
C
4
Im] Á NTf
2
,
the other is much sharper for [C 10Im] Á NTf
1
C
2
and
1
C
8
2
1
C
2
À1
˚
[
C
1
C
14Im] Á NTf
2
than for [C ILs, see arrows in
1
C
16Im] Á NTf
2
Fig. 3. It is important to note that the distance between
nanoparticles decreases progressively for Ni nanoparticles in
1
C14Im] Á NTf
2
1
C
16Im] Á NTf
2
with a different beat pattern from the previous samples is
observed. The FT’s display several peaks that suggest the
existence of medium range order around the Ni particles.
[C C Im] Á NTf and [C C Im] Á NTf have two broad
[
C
1
C
4
Im] Á NTf
2
up to [C
1
C
14Im] Á NTf
2
, increasing again for
Ni nanoparticles in [C
1
C
16Im] Á NTf
2
. The calculated mean
distances between two adjacent nanoparticles are 10.7, 7.7
1
14
2
1
16
2
˚
and 9.5 nm for Ni nanoparticles in [C C Im] Á NTf ,
peaks pointing at about 1.5 and 2.5 A. In the case of
1
10
2
[
C C Im] Á NTf and [C C Im] Á NTf , respectively. This
[C C Im] Á NTf , [C C Im] Á NTf and [C C Im] Á NTf , the
1
14
2
1
16
2
1
4
2
1
8
2
1
10
2
effect can be ascribed to the indentation of the chains as the
number of carbons increases. However, for longer side-chain
lengths such as 16 C, the chains start to fold and thus render
difficult the indentation among the chains and, as a conse-
quence, increase the mean distance between adjacent particles
intensity of the first peak decreases and there is a narrowing
as well as a contraction in the distances. The peaks at about
˚
3.9 and 4.6 A are clearly defined. The first two peaks corre-
spond to the Ni–O and Ni–Ni bonds, respectively (Fig. 5).
˚
The first two shells of the NiO (6 O at 2.09 A and 12 Ni at
˚ ˚
2.95 A) and the first shell of Ni metal (12 Ni at 2.49 A) were
as in [C
1
C16Im] Á NTf
2
. This folding may also explain why
Fig. 4 EXAFS signals (a) and corresponding Fourier transforms (b) for Ni(0) colloidal suspensions in the various ILs.
818 | Phys. Chem. Chem. Phys., 2007, 9, 4814–4821
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4

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Fig. 5 Fitting of the EXAFS signal (left) and Fourier transform (right) for [C
1
C
16Im] Á NTf
2
.
included. For the NiO phase the variables obtained from the
the heterogeneity of the samples and to the presence of Ni in
different forms (metallic, oxide as well as other soluble species)
and the EXAFS measures average coordination numbers that
result in much lower values than the expected ones for single
2
2
fit were N1, E01, R1, s1 and N2, R2, s2 that correspond,
respectively, to the coordination number (N), energy shift,
change in distance relative to the reference and Debye Waller
factor for Ni–O and Ni–Ni contributions. For the Ni metal
6
2
phases. It is possible to have two distinct situations: (i)
coexisting Ni metal and Ni oxide nanoparticles or (ii) a core/
shell type nanoparticles formed by a Ni metal core surrounded
by a NiO shell. The formation of these NiO species is to a great
extent probably due to the exposure of the sample to air, as
already observed earlier for Ru nanoparticles under similar
phase the variables obtained from the fit were N3, E03, R3
2
and s
3
.
In the fitting of the two first peaks of the Fourier trans-
forms, the contributions of three pairs must be included, the
Ni–O and the Ni–Ni from the NiO shell and the Ni–Ni from
6
1
20
Ni metal core. Attempts to use cumulants expansion which
accounts for disordered phases, yielded low quality fits sup-
porting the existence of two Ni phases in the samples.
conditions.
However, the Ni nanoparticles dispersed in [C C Im] Á NTf
1
4
2
are an effective catalyst for the hydrogenation of olefins under
relatively mild reaction conditions. For example, the hydro-
genation of cyclohexene by Ni nanoparticles (cyclohexene/
The quantitative results are displayed in Table 2. The results
suggest that the contribution of oxide and metal depends on
the medium. Pronounced oxide contribution is observed for
Ni = 250) dispersed in [C
1
C
4
Im] Á NTf
2
(0.5 mL) at 100 1C
[
C
1
C
4
Im] Á NTf
2
, [C
1
C
14Im] Á NTf
2
and [C
1
C
16Im] Á NTf
2
, as
under 4 bar of hydrogen (constant pressure) is completed after
À1
observed by the higher number of oxygen atoms (N1). The
Ni–O, Ni–Ni distances and the Debye Waller do not change
significantly in the different ILs. However, very low values of
coordination numbers are found for Ni in all ILs (N1, N2 and
N3) as compared to the bulk values. This is probably related to
14 h (Fig. 6) with a turnover frequency of 91 h
(mol
of cyclohexene converted by exposed Ni atoms per hour).
Assuming that the cyclohexene hydrogenation by the Ni
nanoparticles dispersed in the ionic liquid follows the classical
1
6
monomolecular surface reaction mechanism the catalytic
À4 À1
activity expressed as the kinetic constant is 9.2 Â 10
s .
This value is two orders of magnitude greater than those
Table 2 Structural parameters obtained from the fitting: N is the
0
coordination number, E is the energy shift, R is the distance, s is the
Debye Waller factor, R-factor is the fit quality. The indices 1, 2 and 3
correspond to Ni–O, Ni–Ni (oxide layer) and Ni–Ni (metal) distances,
respectively. The uncertainties in the variables are approximately 10%
2
in N, 1% in R, 20 % in s
[
C
1
a
C
4
Im] Á [C
1 8
C
Im] Á [C
1
C
10Im] Á [C
1
C
14Im] Á [C
1
C
16Im] Á
Bond Sample
X
X
X
X
X
N1
2.3
1.9
1.9
2.3
3.1
Ni–O
E
0
1
À5.4
À2.3
À4.9
À7.0
À3.7
R1
2.02
2.03
2.01
2.03
2.03
2
s
N2
0.001
4.7
0.003
3.7
0.003
3.9
0.002
4.7
0.004
6.2
Ni–Ni R2
2.99
0.009
2.8
2.97
0.009
3.7
3.00
0.011
3.7
3.01
0.013
2.0
2.99
0.012
2.0
2
Oxide
s
2
N3
Ni–Ni
E
0
3
À6.6
À5.6
À6.4
À5.0
À4.3
Metal R3
2
2.48
2.47
2.47
2.51
0.005
0.01
2.48
s
R Fac . 0.006
3
0.002
0.003
0.004
0.004
0.003
0.004
0.002
b
Fig. 6 Hydrogenation of cyclohexane by Ni nanoparticles dispersed
a
b
anion. R Fac = R-Factor.
X = NTf
2
in [C
1
C
4
2
Im] Á NTf (100 1C, 4 bar of hydrogen).
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View Article Online
observed in the cyclohexene hydrogenation by classical sup-
6
14 D. Zhao, Z. Fei, J. Geldbach Tilmann, R. Scopelliti and J. Dyson
Paul, J. Am. Chem. Soc., 2004, 126, 15876–15882.
3
ported Ni catalysts at 60 1C and 5 atm.
1
1
1
1
5 A. J. Bruss, M. A. Gelesky, G. Machado and J. Dupont, J. Mol.
Catal. A: Chem., 2006, 252, 212–218.
6 G. S. Fonseca, J. B. Domingos, F. Nome and J. Dupont, J. Mol.
Catal. A: Chem., 2006, 248, 10–16.
7 A. P. Umpierre, G. Machado, G. H. Fecher, J. Morais and
J. Dupont, Adv. Synth. Catal., 2005, 347, 1404–1412.
8 M. A. Gelesky, A. P. Umpierre, G. Machado, R. R. B. Correia, W.
C. Magno, J. Morais, G. Ebeling and J. Dupont, J. Am. Chem.
Soc., 2005, 127, 4588–4589.
Conclusions
The simple decomposition of [bis(1,5-cyclooctadiene) nickel(0)]
organometallic precursor dissolved in 1-alkyl-3-methylimida-
zolium N-bis(trifluoromethane sulfonyl) amide ionic liquids
yields Ni nanoparticles probably composed of Ni metal core
with a cap of NiO. The diameter, size-distribution and the
metal/metal oxide ratio depend on the structural arrangement
of the salt. There are slight decreases in both the nanoparticle
diameter and the size-distribution with an increase of the
carbon numbers of the alkyl side-chain of the imidazolium
1
9 L. M. Rossi, J. Dupont, G. Machado, P. F. P. Fichtner, C. Radtke,
I. J. R. Baumvol and S. R. Teixeira, J. Braz. Chem. Soc., 2004, 15,
9
04–910.
20 E. T. Silveira, A. P. Umpierre, L. M. Rossi, G. Machado,
J. Morais, G. V. Soares, I. L. R. Baumvol, S. R. Teixeira, P. F.
P. Fichtner and J. Dupont, Chem.–Eur. J., 2004, 10, 3734–3740.
2
1 G. S. Fonseca, J. D. Scholten and J. Dupont, Synlett, 2004,
1525–1528.
cation up to 14 carbons (from 5.9
Æ
1.4 nm for
[
C C Im] Á NTf to 5.1 Æ 0.9 nm for [C C Im] Á NTf and then
22 L. M. Rossi, G. Machado, P. F. P. Fichtner, S. R. Teixeira and
J. Dupont, Catal. Lett., 2004, 92, 149–155.
1
4
2
1
14
2
an increase in diameter and size-distribution as observed for
the material in [C
2
3 G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner, S. R. Teixeira
and J. Dupont, Chem.–Eur. J., 2003, 9, 3263–3269.
1
C
16Im] Á NTf ionic liquid (5.5 Æ 1.1 nm).
2
SAXS data show an increase in the range order of the salts
indicating a synergic organization effect, i.e., more regular
with a smaller diameter and size-distribution nanoparticles
were obtained in the more ‘‘pre-organized’’ ionic liquids. The
FT’s of the EXAFS signals display several peaks suggesting
the existence of a medium range order around the Ni particles.
24 C. W. Scheeren, G. Machado, J. Dupont, P. F. P. Fichtner and
S. R. Texeira, Inorg. Chem., 2003, 42, 4738–4742.
2
5 K. Anderson, S. Cortinas Fernandez, C. Hardacre and P. C. Marr,
Inorg. Chem. Commun., 2003, 7, 73–76.
26 S. M. Chen, Y. D. Liu and G. Z. Wu, Nanotechnology, 2005, 16,
2360–2364.
2
7 S. Y. Gao, H. J. Zhang, X. M. Wang, W. P. Mai, C. Y. Peng and
L. H. Ge, Nanotechnology, 2005, 16, 1234–1237.
[
C C Im] Á NTf and [C C Im] Á NTf have two broad peaks
1
14
2
1
16
2
28 J. Huang, T. Jiang, B. X. Han, H. X. Gao, Y. H. Chang, G. Y.
Zhao and W. Z. Wu, Chem. Commun., 2003, 1654–1655.
˚
pointing at ca.1.5 and 2.5 A, whereas in the case of
2
9 L. S. Ott, M. L. Cline, M. Deetlefs, K. R. Seddon and R. G. Finke,
J. Am. Chem. Soc., 2005, 127, 5758–5759.
0 Y. Wang and H. Yang, Chem. Commun., 2006, 2545–2547.
1 R. Marcilla, M. L. Curri, P. D. Cozzoli, M. T. Martinez, I. Loinaz,
H. Grande, J. A. Pomposo and D. Mecerreyes, Small, 2006, 2,
[
C C Im] Á NTf , [C C Im] Á NTf and [C C Im] Á NTf the
1
4
2
1
8
2
1
10
2
intensity of the first peak decreases and there is a narrowing
as well as a contraction in the distances. The IL–Ni colloidal
dispersions can be used as catalysts for the biphasic hydro-
genation of olefins.
3
3
5
07–512.
2 B. S. Lee, Y. S. Chi, J. K. Lee, I. S. Choi, C. E. Song, S. K.
3
3
Namgoong and S. G. Lee, J. Am. Chem. Soc., 2004, 126,
4
80–481.
3 J. Dupont and P. A. Z. Suarez, Phys. Chem. Chem. Phys., 2006, 8,
441–2452.
Acknowledgements
2
We are indebted to the staff of LNLS for operating the storage
ring and for assistance during the experiments at the XAS
beam line. The beam time was supported by LNLS under the
proposals XAFS1 4638 and SAXS1 4618.
3
3
4 J. Dupont, J. Braz. Chem. Soc., 2004, 15, 341–350.
5 C. S. Consorti, P. A. Z. Suarez, R. F. de Souza, R. A. Burrow, D.
H. Farrar, A. J. Lough, W. Loh, L. H. M. da Silva and J. Dupont,
J. Phys. Chem. B, 2005, 109, 4341–4349.
6 F. C. Gozzo, L. S. Santos, R. Augusti, C. S. Consorti, J. Dupont
and M. N. Eberlin, Chem.–Eur. J., 2004, 10, 6187–6193.
7 M. Antonietti, D. B. Kuang, B. Smarsly and Z. Yong, Angew.
Chem., Int. Ed., 2004, 43, 4988–4992.
3
3
References
1
2
3
G. Schmid, Endeavour, 1990, 14, 172–178.
C. A. Stowell and B. A. Korgel, Nano Lett., 2005, 5, 1203–1207.
A. Roucoux, J. Schulz and H. Patin, Adv. Synth. Catal., 2003, 345,
38 G. Clavel, J. Larionova, Y. Guari and C. Guerin, Chem.–Eur. J.,
2006, 12, 3798–3804.
39 S. A. Meiss, M. Rohnke, L. Kienle, S. Z. El Abedin, F. Endres and
J. Janek, ChemPhysChem, 2007, 8, 50–53.
40 S. Z. El Abedin, E. M. Moustafa, R. Hempelmann, H. Natter and
F. Endres, ChemPhysChem, 2006, 7, 1535–1543.
41 S. Z. El Abedin, E. M. Moustafa, R. Hempelmann, H. Natter and
F. Endres, Electrochem. Commun., 2005, 7, 1111–1116.
42 C. M. Gordon, J. D. Holbrey, A. R. Kennedy and K. R. Seddon,
J. Mater. Chem., 1998, 8, 2627–2636.
43 S. M. Urahata and M. C. C. Ribeiro, J. Chem. Phys., 2004, 120,
1855–1863.
44 S. M. Urahata and M. C. C. Ribeiro, J. Chem. Phys., 2005, 122,
024511.
45 Y. H. Leng, Y. H. Zhang, T. Liu, M. Suzuki and X. G. Li,
Nanotechnology, 2006, 17, 1797–1800.
46 G. J. Cheng, V. F. Puntes and T. Guo, J. Colloid Interface Sci.,
2006, 293, 430–436.
2
22–229.
L. S. Ott and R. G. Finke, Coord. Chem. Rev., 2007, 251,
075–1100.
A. Roucoux, J. Schulz and H. Patin, Chem. Rev., 2002, 102,
757–3778.
D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., Int. Ed., 2005,
4, 7852–7872.
4
5
6
7
8
9
1
3
4
C. Burda, X. B. Chen, R. Narayanan and M. A. El-Sayed, Chem.
Rev., 2005, 105, 1025–1102.
S. U. Son, Y. Jang, K. Y. Yoon, E. Kang and T. Hyeon, Nano
Lett., 2004, 4, 1147–1151.
S. W. Kim, J. Park, Y. Jang, Y. Chung, S. Hwang, T. Hyeon and
Y. W. Kim, Nano Lett., 2003, 3, 1289–1291.
0 F. Endres and S. Z. El Abedin, Phys. Chem. Chem. Phys., 2006, 8,
1
2101–2116.
1
1
1 J. Dupont and P. Migowski, Chem.–Eur. J., 2007, 13, 32–39.
2 J. Dupont, G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner and S.
R. Teixeira, J. Am. Chem. Soc., 2002, 124, 4228–4229.
47 M. Paillet, V. Jourdain, P. Poncharal, J. L. Sauvajol, A. Zahab,
J. C. Meyer, S. Roth, N. Cordente, C. Amiens and B. Chaudret,
J. Phys. Chem. B, 2004, 108, 17112–17118.
1
3 D. B. Zhao, Z. F. Fei, W. H. Ang and P. J. Dyson, Small, 2006, 2,
48 N. Cordente, C. Amiens, B. Chaudret, M. Respaud, F. Senocq and
M. J. Casanove, J. Appl. Phys., 2003, 94, 6358–6365.
879–883.
4
820 | Phys. Chem. Chem. Phys., 2007, 9, 4814–4821
This journal is ꢀc the Owner Societies 2007

View Article Online
4
5
5
9 N. Cordente, M. Respaud, F. Senocq, M. J. Casanove, C. Amiens
and B. Chaudret, Nano Lett., 2001, 1, 565–568.
0 J. S. Bradley, B. Tesche, W. Busser, M. Masse and R. T. Reetz,
J. Am. Chem. Soc., 2000, 122, 4631–4636.
1 T. Ould-Ely, C. Amiens, B. Chaudret, E. Snoeck, M. Verelst, M.
Respaud and J. M. Broto, Chem. Mater., 1999, 11, 526–529.
2 T. Welton, Coord. Chem. Rev., 2004, 248, 2459–2477.
3 S. V. Dzyuba and R. A. Bartsch, ChemPhysChem, 2002, 3,
57 H. C. N. Tolentino, A. Y. Ramos, M. C. M. Alves, R. A. Barrea,
E. Tamura, J. C. Cezar and N. Watanabe, J. Synchrotron Radiat.,
2001, 8, 1040–1046.
58 (a)
http://ixs.iit.edu/subcommittee_reports/sc/SC00report.pdf;
(b) http://ixs.iit.edu/subcommittee_reports/sc/err-rep.pdf.
59 M. Newville, J. Synchrotron Radiat., 2001, 8, 322–324.
60 T. M. G. Carneiro, J. Dupont, M. Luke and D. Matt, Quim. Nova,
1988, 11, 215–217.
5
5
1
61–166.
61 P. Fornasini, F. Monti and A. Sanson, J. Synchrotron Radiat.,
2001, 8, 1214–1220.
62 Y. Sun, A. Frenkel, R. Isseroff, C. Shonbrun, M. Forman, K. Shin,
T. Koga, H. White, L. Zhang, Y. Zhu, M. H. Rafailovich and J. C.
Sokolov, Langmuir, 2006, 22, 807–816.
63 P. C. L’Argentiere, E. A. Cagnola, M. G. Canon, D. A. Liprandi
and D. V. Marconetti, J. Chem. Technol. Biotechnol., 1998, 71,
285–290.
5
4 A. E. Bradley, C. Hardacre, J. D. Holbrey, S. Johnston, S. E. J.
McMath and M. Nieuwenhuyzen, Chem. Mater., 2002, 14,
629–635.
5
5 C. C. Cassol, G. Ebeling, B. Ferrera and J. Dupont, Adv. Synth.
Catal., 2006, 348, 243–248.
6 P. Wasserscheid, M. Sesing and W. Korth, Green Chem., 2002, 4,
5
134–138.
This journal is ꢀc the Owner Societies 2007
Phys. Chem. Chem. Phys., 2007, 9, 4814–4821 | 4821