Synthesis and characterization of Pt(0) nanoparticles in imidazolium ionic liquids

Article abstract of DOI:10.1021/jp0623037

The controlled decomposition of Pt₂(dba)₃ (dba = dibenzylideneacetone) dispersed in 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF₄) and hexafluorophosphate (BMI.PF ₆) ionic liquids in the presence of cyclohexene by molecular hydrogen produces Pt(0) nanoparticles. The formation of these nanoparticles follows the two-step [A → B, A + B → 2B (k₁, k ₂)] autocatalytic mechanism. The catalytic activity in the hydrogenation of cyclohexene is influenced by the nature of the anion rather than the mean-diameter of the nanoparticles. Thus, higher catalytic activity was obtained with Pt(0) dispersed in BMI.BF₄ containing the less coordinating anion although these nanoparticles possess a larger mean diameter (3.4 nm) than those obtained in BMI.PF₆ (2.3 nm). Similar mean diameter values were estimated from in situ XRD and SAXS. XPS analyses clearly show the interactions of the ionic liquid with the metal surface demonstrating the formation of an ionic liquid protective layer surrounding the platinum nanoparticles. SAXS analysis indicated the formation of a semi-organized ionic liquid layer surrounding the metal particles with an extended molecular length of around 2.8 nm in BMI.BF₄ and 3.3 nm in BMI.PF₆.

Full text of DOI:10.1021/jp0623037

J. Phys. Chem. B 2006, 110, 13011-13020
13011
Synthesis and Characterization of Pt(0) Nanoparticles in Imidazolium Ionic Liquids
Carla W. Scheeren,† Giovanna Machado,† Sergio R.Teixeira,‡ Jonder Morais,‡
Josiel B. Domingos,† and Jairton Dupont*,†
Laboratory of Molecular Catalysis, Instituto de Qu´ımica-UFRGS, 9500 P.O. Box 15003, 91501-970,
Porto Alegre, Brazil, and Instituto de F´ısica-UFRGS, P.O. Box 15051, 91501-970, Porto Alegre, Brazil
ReceiVed: April 13, 2006; In Final Form: May 13, 2006
The controlled decomposition of Pt2(dba)3 (dba ) dibenzylideneacetone) dispersed in 1-n-butyl-3-
methylimidazolium tetrafluoroborate (BMI.BF4) and hexafluorophosphate (BMI.PF6) ionic liquids in the
presence of cyclohexene by molecular hydrogen produces Pt(0) nanoparticles. The formation of these
nanoparticles follows the two-step [A f B, A + B f 2B (k1, k2)] autocatalytic mechanism. The catalytic
activity in the hydrogenation of cyclohexene is influenced by the nature of the anion rather than the mean-
diameter of the nanoparticles. Thus, higher catalytic activity was obtained with Pt(0) dispersed in BMI.BF4
containing the less coordinating anion although these nanoparticles possess a larger mean diameter (3.4 nm)
than those obtained in BMI.PF6 (2.3 nm). Similar mean diameter values were estimated from in situ XRD
and SAXS. XPS analyses clearly show the interactions of the ionic liquid with the metal surface demonstrating
the formation of an ionic liquid protective layer surrounding the platinum nanoparticles. SAXS analysis indicated
the formation of a semi-organized ionic liquid layer surrounding the metal particles with an extended molecular
length of around 2.8 nm in BMI.BF4 and 3.3 nm in BMI.PF6.
Introduction
We9-20 and others21-29 have recently reported that imidazo-
lium ionic liquids (ILs) are an interesting medium for the
The use of water or organic solvent stabilizing agents such
as surfactants, polymers, quaternary ammonium salts, and
polyoxoanions is one of the most current and used approaches
for the stabilization of soluble transition-metal nanoparticles.1-3
Indeed, the advent of these stable metallic particles, which are
finely dispersed in organic solvents or in water has enabled the
development of one-phase and two-phase catalytic systems for
various reactions that operate under relatively mild reaction
conditions.4 The catalytic activity and selectivity of soluble-
metal-particle catalysts depends not only on the relative
abundance of different types of active sites but also on the
concentration and type of stabilizers present in the medium.5
For example, the catalytic activity of iridium nanoparticles is
extremely sensitive to the nature of the capping ligand and ligand
coverage. Thus, good capping ligandssthat stabilize robust
nanocrystals with very narrow size distributionssare almost
inactive catalysts for the hydrogenation of olefins.6 Quaternary
ammonium salts are one of the most popular and investigated
classes of stabilizing agents for soluble transition-metal nano-
particle catalysts. It is assumed that the stabilization in these
cases is due to the positive charge on the metal surface, which
is ultimately induced by the adsorption of the anions onto the
coordinative unsaturated, electron-deficient, and initially neutral
metal surface.7 Not surprisingly, the stability and catalytic
properties of these soluble nanoparticles are strongly dependent
on the nature of the anion. For example, it was recently
demonstrated that the catalytic properties of Rh nanoparticles
are strongly influenced by the nature of the N-containing
surfactant counteranion.8
formation and stabilization of catalytically active transition-metal
nanoparticles. The high catalytic activity probably results from
the presence of highly undercoordinated atoms on the metal
surface due to the loosely supported nanoparticles by the IL.
The intrinsic high charge of imidazolium salts, which creates
an electrostatic colloid-type protection (DLVO-type stabiliza-
tion)30 for the transition-metal nanoparticles similar to those
proposed for quaternary ammonium salts,31-33 may be, as the
first approximation,34 adequate for the description of the
stabilizing effect. However, imidazolium salts differ in various
aspects from classical quaternary ammonium salts or even
molten salts. Imidazolium ILs possess preorganized structures
through mainly hydrogen bonds35,36 that induce structural
directionality in opposition to classical salts in which the
aggregates display mainly charge-ordering structures.37 Imida-
zolium ILs form an extended hydrogen bond and π-π stacking
(involving the imidazolium rings) network in the liquid state
and are consequently highly structured, that is, they can be
described as “supramolecular” fluids.35,38 This is one of the
special qualities of imidazolium ILs that differentiates them from
the classical ion aggregates of which ion pairs and ion triplets
are widely recognized examples.39 This structural organization
of imidazolium ILs functions as an “entropic driver” for
spontaneous, well-defined, and extended ordering of nanoscale
structures.40 Therefore, the unique combination of adaptability
toward other molecules and phases associated with the strong
H-bond-driven fluid structure makes imidazolium ILs “tem-
plates” for the formation of nanostructures. Moreover, it is also
possible that imidazolium-based ILs may form surface-attached
carbenes,41 at least as transient species, and may also be
responsible for the stabilization of zero-valent transition-metal
nanoparticles in these fluids.42 It is evident that a knowledge of
the mechanism of formation, the interaction of the ionic liquid
* To whom correspondence should be addressed. E-mail: dupont@
iq.ufrgs.br.
† Instituto de Qu´ımica-UFRGS.
‡ Instituto de F´ısica-UFRGS.
10.1021/jp0623037 CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/15/2006

13012 J. Phys. Chem. B, Vol. 110, No. 26, 2006
Scheeren et al.
with the nanoparticles, and how this leads to the stabilization
of the nanoparticles without losing their catalytic properties can
yield important information for the design and synthesis of stable
nanoparticles. In this respect, we have reported that the
multiplying k2obs by the substrate/catalyst molar ratio, to obtain
the reported rate constants, k2 (for further details on these fitting
procedures see elsewhere).51-54
Preparation and Isolation of Pt(0) Nanoparticles. In a
typical experiment, to a Fischer-Porter bottle containing
BMI.PF6 (1 mL) or BMI.BF4 (1 mL) or BMI.CF3SO3 (1 mL)
was added Pt2(dba)3 (30 mg, 0.02 mmol), which was stirred at
room temperature for 15 min resulting in a violet solution. The
system was heated to 75 °C and hydrogen (4 bar) was admitted.
After stirring for 1.5 h, a black “solution” was obtained. This
solution was washed with dichloromethane (3 × 15 mL), and
the Pt(0) nanoparticles were isolated by centrifugation (3500
rpm) for 3 min and washed with acetone (3 × 15 mL) and dried
under reduced pressure. The Pt(0) samples thus obtained were
prepared for TEM, XRD, XPS, and SAXS analysis and for
catalytic experiments.
In situ Preparation of Nanoscale Pt(0) Particles and
Cyclohexene Hydrogenation. In a typical experiment, to a
Fischer-Porter bottle containing BMI.PF6 (1 mL) or BMI.BF4
(1 mL) or BMI.CF3SO3 (1 mL) was added Pt2(dba)3 (30 mg,
0.02 mmol) and cyclohexene (11 mg, 0.02 mmol). The system
was heated to 75 °C under hydrogen (6 bar). A drop of these
Pt(0) samples in the ILs were diluted in 1 mL of the ionic liquid,
placed in carbon-copper grid, and analyzed by TEM.
XRD Analysis. The phase structures of the Pt(0) nanoparticles
prepared in ILs BMI.PF6, BMI.BF4, and BMI.CF3SO3 were
characterized by XRD. For the XRD analysis, the nanoparticles
were isolated as a fine powder and placed in the sample holder.
The XRD experiments were carried out on a SIEMENS D500
diffractometer equipped with a curved graphite crystal using
Cu KR radiation (λ ) 1.5406 Å). The diffraction data were
collected at room temperature in a Bragg-Brentano θ-2θ
geometry. The equipment was operated at 40 kV and 20 mA
with a scan range between 20° and 90°. The diffractograms were
obtained with a constant step, ∆2θ ) 0.05. The indexation of
Bragg reflections was obtained by a pseudo-Voigt profile fitting
using the FULLPROF code.55
TEM Analysis. The morphology and the electron diffraction
(ED) of the obtained particles were carried out on a JEOL JEM-
2010 equipped with an energy-dispersive X-ray spectroscopy
(EDS) system and a JEOL JEM-1200 EXII electron microscope
operating at accelerating voltages of 200 and 120 kV, respec-
tively. The samples for TEM were prepared by dispersion of
the Pt(0) nanoparticles in ILs at room temperature and then
collected on a carbon-coated copper grid. The histograms of
the nanoparticles size distribution, assuming spherical shape,
were obtained from the measurement of about 300 particles and
were reproduced in different regions of the Cu grid, found in
an arbitrarily chosen area of enlarged micrographs. The HRTEM
images were analyzed in Gatan Digital Micrograph Software
from which the Fourier transform of images was obtained.
controlled decomposition of the organometallic Pt(0) precursor
43-46
Pt2(dba)3
in 1-n-butyl-3-methylimidazolium hexafluoro-
phosphate (BMI.PF6) IL yields stable and catalytically active
Pt(0) nanoparticles with a mean diameter of 2.3 nm.17 Moreover,
we were able to observe by TEM contrast density fluctuations
around the metal nanoparticles in the IL, and different thick-
nesses of mass density between the IL and crystalline structures.
This mass densities difference is an indication of the effective
interaction between the IL and the metal nanoparticles. We wish
to disclose herein full details on the kinetics of formation of
Pt(0) nanoparticles and a study of their structure and morphology
by means of X-ray diffraction (XRD), transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy (XPS),
and small-angle X-ray scattering (SAXS) obtained from the
controlled decomposition of Pt2(dba)3 in hydrophobic and
hydrophilic ILs.
Experimental Section
General. All reactions involving platinum compounds were
carried out under an argon atmosphere in oven-dried Schlenk
tubes. Pt2(dba)3 was prepared according to literature proce-
dures.47 The halide-free BMI.PF6,48 BMI.BF4, and BMI.CF3-
49
SO3 ILs were prepared according to a known procedure and
dried over molecular sieves (4 Å), and their purity was checked
1
by H NMR spectra using the intensity of the 13C satellites of
the imidazolium N-methyl group as an internal standard.50
Solvents, alkenes, and arenes were dried with the appropriate
drying agents and distilled under argon prior to use. All other
chemicals were purchased from commercial sources and used
without further purification. NMR spectra were recorded on a
Varian Inova 300 spectrometer. Infrared spectra were performed
on a Bomem B-102 spectrometer. Mass spectra were obtained
using a GC/MS Shimatzu QP-5050 (EI, 70 eV). Gas chroma-
tography analysis was performed with a Hewlett-Packard-5890
gas chromatograph with an FID and a 30 m capillary column
with a dimethylpolysiloxane stationary phase. SAXS analyses
were performed at the D11A SAXS beam line of the Brazilian
Synchrotron Light Laboratory (LNLS).
XRD analyses were performed on a SIEMENS D500 dif-
fractometer using Bragg-Brentano geometry with a curved
graphite crystal as the monochromator. TEM was obtained using
a JEOL JEM2010 microscope operating at 200 kV and a JEOL
JEM1200ExII operating at 100 kV. A 20 µm objective aperture
and slightly under focused (∆f ≈ -300 nm) objective lens were
used to obtain the bright field TEM images.
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 maintained at 75 °C by a hot-stirring plate
connected to a digital controller (ETS-D4 IKA). A deliberated
stirring of 800 rpm was used (no ionic catalytic solution
projection 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.
XPS Analysis. The photoemission studies were performed
using the VG ESCALAB MkII, equipped with a 150 mm
hemispherical analyzer and Al KR Anode X-ray source. The
overall resolution was 200 meV. The detection angle of the
photoelectrons was 45° with respect to the sample surface.
Samples for analysis were prepared by placing the Pt(0)
nanoparticles on a carbon-conducting tape.
SAXS Data Analysis. The samples of Pt(0) nanoparticles
were analyzed using SAXS at room temperature, at the beam
line of the Brazilian Synchrotron Light Laboratory (LNLS). The
samples (nanoparticles plus IL) were combined immediately for
measurement and injected with a needle syringe in the sample
chamber. The sample chamber was sealed with Mylar film on
Kinetic Data Treatment. The kinetic data for cyclohexene
hydrogenation were fitted using the analytic eq 6 (see later)
with Microcal Origin 5.0. The actual rate constants obtained
from the fitting for the autocatalytic growth (A + B f 2B,
k
2obs) were corrected considering the reaction stoichiometry,

Pt(0) Nanoparticles in Imidazolium Ionic Liquids
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13013
both sides for incident and scattered X-ray. These measurements
were taken out in a transmission geometry with λ ) 1.608 Å.
The time exposure for each sample was around 1200 s, and the
distance between the sample and SAXS detection plane was
963.8 mm. The 2D SAXS spectra were obtained using an image
plate as a detector.
SAXS Data Analysis. The typical SAXS spectra are shown
in Figure 1 for a pure IL and Pt(0) plus IL.
The X-ray scattering is experimentally determined as a
function of the scattering vector, q, whose modulus is given
by56
q ) (4π sinus θ)/λ
(1)
where λ is the X-ray wavelength (in the present case λ ) 1.608
Å) and θ is half the scattering angle (2θ). First of all, the
measured raw SAXS data were corrected for parasitic scattering.
The background SAXS scattering from the pattern (IL) and
primary incident beam was measured from a sample without
nanoparticles and then calculated by subtracting the scattering
intensities of the pattern from those of the nanoparticles in
suspension. After correction of the scattering data by subtraction
of the background noise, the analyses were performed using
the methods presented by the references.57-61
The most common method for determining periodicity is to
use Bragg’s law in the calculation of a domain spacing from
the location of the peak maximum, qm in an I(q)q2xq plot. The
position qm is related by Bragg’s law to the long period
correlation, L, if that correlation exists, by
Figure 1. Typical SAXS spectra pattern for the pure BMI.BF4 and
Pt(0) dispersed in BMI.BF4.
From eq 3, the long period, L, can be estimated as the position
of the first maximum in the correlation function.66,68
In the interface distribution function, g(r), eq 4, the long
period, L is defined as the first minimum. In both functions
(γ(r) and g(r)), the long period, L, is defined as the most
probable distance between the centers of gravity between two
adjacent IL supramolecular aggregates or between two adjacent
nanoparticles dispersed in the IL.60,63 Note that functions γ(r)
and g(r) are normalized by the invariant scattering Q. Sophis-
ticated methods involve the calculation of the invariant Q, which
is independent of the size and shape of the structural hetero-
geneities. Q represents the electron density difference between
the two phases and has been calculated from the area under a
Lorentz-corrected scattering curve (I(q)q2 × q) defined by
qm ) 2π/L
(2)
The procedure for the long period determination is based on a
one-dimensional semicrystalline model comprising disordered
and crystalline phase. This model was adopted in this calculation
because we have a two-phase system represented by IL (a quasi-
disordered phase) and Pt(0) nanoparticles in the crystalline
phase.62 The morphology and shape of the nanoparticles and
ILs were verified using this two-phase model, that is, nano-
crystals, represented by the Pt(0) nanoparticles, dispersed in the
IL. From this model, it was possible to analyze the SAXS
scattering curves via a combination of the correlation function,
γ(r), and interface distribution function, g(r).63 The details about
the calculus of the nanoparticles with the ILs obtained by means
of SAXS techniques have been recently reported for Ir(0)
nanoparticles in imidazolium ILs.64
Q ) ∞ I(q)q2 dq
(5)
∫
0
The application of the correlation and interface distribution
function is accepted only in SAXS range data (0.1 nm-1 < q
< 2.5 nm-1); it must be necessary to extrapolate to both high
and low q values before Fourier transformation. The extrapola-
tion to high q values was performed with the aid of the Porod
law.
Results and Discussion
Characterization of the Nanoparticles. The XRD pattern
(Figure 2) of the isolated material confirmed the crystalline Pt-
(0) and the mean diameter could be estimated from the XRD
diffraction pattern by means of the Debye-Scherrer equation
calculated from full width at half-maximum (fwhm) of the (111),
(200), (220), (311), and (222) planes obtained with Rietveld’s
refinements.
The most representative reflections of Pt(0) were indexed as
face-centered cubic (fcc) with unit cell parameter a ) 3.9231
Å. The simulations of Bragg reflections and Rietveld’s refine-
ment were performed with a pseudo-Voigt function using the
FULLPROF code.
It is worth to pointing out that the use of full width at half-
maximum (fwhm) of a peak to estimate the size of crystalline
grain by means of the Scherrer equation has serious limitations
since it does not take into account the existence of a distribution
of sizes and the presence of defects in the crystalline lattice.
Therefore, the calculation of average diameter of grain from
fwhm of the peak can overestimate the real value since the larger
grains give a strong contribution to the intensity, while the
smaller grains just enlarge the base of the peak. Moreover, the
The correlation function, γ(r), is a measure of the electron
density in a sample defined as the Fourier transform of the
Lorentz-corrected SAXS profile defined by65-67
2
(
∞ I(q)q2 cos(qr)eσ q2 dq)
∫
0
γ(r) )
(3)
Q
The interface distribution function g(r) is the Fourier transform
of the interference function G(q).22 This function is the second
derivative of the correlation function g(r) ) γ′′(r) defined by
eq 4, which represents the probability distribution of finding
two interfaces (between a nanocrystal and the adjacent IL region
at a distance r).
2
(
∞ G(q)cos(qr)eσ q2 dq)
∂2(γ(r))
∂r2
∫
0
g(r) )
)
Q
G(q) ) K - (I(q)q2)q2eσ2q2
(4)

13014 J. Phys. Chem. B, Vol. 110, No. 26, 2006
Scheeren et al.
Figure 2. X-ray diffraction pattern (top) and Rietveld’s refinement (bottom) of the Pt(0) nanoparticles prepared in BMI.PF6 (left) and in BMI.BF4
(right).
Figure 3. X-ray photoelectron spectra of Pt nanoparticles prepared with (a) BMI.BF4 and (b) BMI.PF6 showing the Pt 4f region with the fitting
results (using Shirley type background). The insets of the figures show the F 1s signal observed for each case.
presence of defects in a significant amount causes an additional
enlargement of the diffraction line. Considering this enlarge-
ment, the obtained size can be smaller than the real size of the
grains. These problems can be minimized by the use of
Rietveld’s refinement method. Indeed, these discrepancies are
confirmed by the values found for the average diameter of the
nanoparticles without the structural refinement (3.8 and 5.4 nm)
which significantly differs from those found by means of
Rietveld’s refinement of 2.6 and 3.1 nm for the samples Pt-
(0).BMI.PF6 and Pt(0).BMI.BF4, respectively. These latter
values are much closer to those determined by TEM and SAXS
(see later).
The XPS analysis of the isolated nanoparticles showed the
presence of platinum, fluorine, and carbon (from the support)
for the samples prepared in BMI.BF4 and BMI.PF6. Moreover,
in the case of the sample obtained in BMI.PF6, there is also a
very small contribution of phosphorus. It is clear that the F and
P signals indicate that the isolated nanoparticles contain residues
of the ILs. No other impurities were detected within the
sensitivity of the technique. Figure 3 shows the XPS signal of
the Pt 4f region for both samples.
The Pt 4f spectrum indicates the presence of three chemical
states of Pt at the nanoparticle surface with distinct binding
energies, and in both samples the contribution due to Pt(0) (Pt7/2
at 71 eV, thick line) is present. In Figure 3a (BMI.BF4), besides
the predominant Pt(0) contribution, one notices a component
with the Pt7/2 peak at 72.1 eV (dotted curve), which could be
assigned to Pt2+ and another broad component (Pt7/2 at 75.7
eV, dashed curve), which could correspond to a mixture of Pt3+
and Pt4+ contributions. The F 1s peak observed in this case has
a low intensity at about 687 eV.
For the Pt(0) nanoparticles prepared with BMI.PF6 (Figure
3b), the Pt(0) contribution is less important and another Pt 4f
doublet with the Pt7/2 peak at 74.1 eV (dotted curve) is observed,
which could be assigned to Pt4+. Again, we notice the presence
of a third broad doublet (Pt7/2 at 75.7 eV, dashed curve)

Pt(0) Nanoparticles in Imidazolium Ionic Liquids
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13015
Figure 4. Plots of the correlation function for (a) pure ionic liquids and (b) Pt(0) dispersed in the ionic liquids.
TABLE 1: Long Period, L, Lm, and LM Values Obtained by
corresponding to other chemical bonds at the surface of the
nanoparticles. The F 1s signal (at 690 eV) in this sample is
quite intense, and we may associate it with the appearance of
the Pt4+ component of the Pt 4f at higher binding energy, as
compared with that of the BMI.BF4 case, due to the abundance
of fluorine atoms, which have a higher electronegativity
character.
The chemical states that were observed in the Pt 4f signal
(Pt(0), Pt2+, and Pt4+) are related to different bonds between
the metallic particles and oxygen and fluorine atoms, forming
Pt-F, Pt-O, and Pt-O-F type bonds, whereas the presence
of oxygen is probably due to the brief air exposure during the
sample preparation process. The measured binding energy values
are in accordance with the expected values,69,70 and the broad
peak at higher binding energies could be assigned to mixed
bonds. These results are a strong indication of the effective
interaction of the IL with the metal surface, which may be
responsible for the stabilization of the nanoparticles. More
detailed information about the interaction of the nanoparticles
with the ILs was obtained from SAXS studies.
Figure 4 shows the correlation function obtained from the
SAXS spectra for pure ILs (a) and nanoparticles dispersed in
ILs (b). Through the use of the two-phase model,65-68 it was
possible to obtain the diameter of the platinum nanoparticles
that are larger than the thickness of the IL layer. As reported
earlier, the interface distribution function can be applied to the
experimental data for the calculation of the extended molecular
lengths of pure ILs.64 The calculated values for extended
molecular lengths of pure ILs of 2.8 nm for BMI.BF4 and 3.3
nm for BMI.PF6 are in agreement to those recently reported
(2.8 nm for BMI.BF4 and 4.0 nm for BMI.PF6).64 Interestingly,
these values are very close to those estimated by semiempirical
calculations (AM1) for the molecular length of [(BMI)3(PF6)4]-
(3.4 nm) and [(BMI)2(BF4)3]- (2.4 nm) supramolecular ag-
gregates.39
Correlation Function, γ(r) and Interface Distribution
Functions, g(r)
correlation
function,
γ(r)
interface
distribution functions,
g(r)
sample
L (Å)
Lm (Å)
L (Å)
LM (Å)
Pt(0).BMI.BF4
BMI.BF4
Pt(0).BMI.PF6
BMI.PF6
162
127
166
157
74
52
81
73
160
122
167
151
81
57
87
65
van der Waals volumes, which are 68 and 48 Å3 for the PF6
and BF4 anions, respectively.72
The integrated area under the scattering profile, which
represents the electron density difference between the crystal
(nanoparticle) and IL region, also increased. This increase of
the invariant Q when the Pt(0) nanoparticles are dispersed in
the ILs can be interpreted as an enhancement of the structural
ordering of the nanoparticles plus ILs. On the other hand, the
distance, Lm, can be determined as twice the position of the
first minimum in the correlation function, which is interpreted
as the most probable distance between the centers of gravity of
a nanocrystal (platinum nanoparticle) and its adjacent IL region
(see Figure 5a). In the interface distribution function, g(r), eq
4, the first maximum can be estimated as semi-order ionic-
liquid-phase thickness represented by LM (see Figure 5b). The
model shown in Figure 5 considers that the platinum nanopar-
ticles (ordered phase) are surrounded by anionic supramolecular
aggregates of the type [(BMI)x-n(X)x)]n- (semi-ordered phase).
The subtraction of the calculated LM (ionic liquid molecular
lengths) of Lm, estimated as the ordered phase length (nano-
particle plus the ionic liquid) which has been obtained using
the interface distribution and correlation function, gave mean
diameters of 3.4 and 3.2 nm for the platinum nanoparticles
dispersed in BMI.BF4 and BMI.PF6, respectively.73
A considerable increase in L, with the addition of Pt(0)
nanoparticles, was observed for the two ILs studied. This
increase was significantly higher for the BF4 anion, which may
have suffered a more ordered structural arrangement, as shown
in Figure 4b, where a change of the position of the maximum
associated with the long period shifted toward higher r values.
A good agreement was found also for the results derived from
correlation and interface distribution functions (see Table 1).
As already reported, this difference may be due to the fact
that the PF6 anion is much larger than the BF4 anion, thus
restricting the cation mobility,71 hampering the formation of the
ordered arrangement in relation to the platinum nanoparticle,
despite the greater coordinating ability of the PF6 anion than
the BF4 anion. This can also be correlated with the calculated
It is important to note thatsas was observed for Ir(0)
nanoparticles in the same ionic liquids64scalculations using the
Guinier approximation law at a small-angle region gave
inconsistent results. On opposition, the calculations using Porod
law gave consistent results since it assumed the presence of
two phases: a crystalline phase (nanoparticles) and a semi-
ordered phase (IL).74-76 This clearly indicates that the assump-
tion that transition-metal nanoparticles are simply diluted in the
ILs, that is, the ILs behave as a classical organic solvent, cannot
be applied in these cases.
The TEM micrographs of the nanoparticles in situ in the ILs
BMI.PF6 and BMIBF4, respectively, are presented in Figure 6.
The particles of Pt(0) were also analyzed by energy dispersion
spectrometry (see S1 in the Supporting Information). The values

13016 J. Phys. Chem. B, Vol. 110, No. 26, 2006
Scheeren et al.
Figure 5. Schematic illustration of the proposed two-phase model: (a) corresponds to the correlation function and (b) to the interface distribution
function. (a) Lm, the first minimum, is interpreted as the most probable distance between the center of gravity of a Pt(0) nanoparticle and its
adjacent region. (b) LM, the first maximum, can be estimated as disordered ionic-liquid-phase thickness. If X ) BF4, x ) 2, and if X ) PF6,
x ) 3.
Figure 6. TEM micrographs (top) and histograms (bottom) showing the particle size distribution of Pt(0) prepared in BMI.BF4 (left) and in
BMI.PF6 (right).
of energy are in agreement with the values of energy for the
pattern of Pt(0).
Moreover, the crystalline structure of the materials was
confirmed by selected area diffraction which shows ring patterns

Pt(0) Nanoparticles in Imidazolium Ionic Liquids
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13017
Figure 7. TEM micrograph (negative image, under focus) of the Pt-
(0) in BMI.PF6 ionic liquid showing the contrast density fluctuation
around the metal nanoparticles.
that can be fitted to simulation based on Pt(0) parameters (see
S2 in the Supporting Information) as well as by Gatan Digital
Micrograph Software from FFT.
Figure 8. Experimental HRTEM images of single twinned platinum
nanoparticles in BMI.BF4, observing the planar defects and interplanar
distance indicated by arrows.
Analysis of the micrographs indicate that these Pt(0) nano-
particles display an irregular shape, but evaluating their
characteristic diameter, results in a mono modal distribution with
an average diameter of 2.3 nm for those prepared in BMI.PF6
and 3.4 nm for those in BMI.BF4 estimated from ensembles of
300 particles found in an arbitrarily chosen area of the enlarged
micrographs. The histograms of Figure 6 show that particle size
distributions can be reasonably well fitted by a Gaussian curve.
The detailed analysis of the nanoparticles of Pt(0) dispersed
in the IL BMI.PF6 shows a rather strong fluctuation of the
contrast density (see Figure 7). These fluctuations of contrast
density are characteristic of amorphous substances.
TABLE 2: Mean Diameter of the Pt Nanoparticles
Determined by TEM, SAXS, and XRD
SAXS
(nm)
sample
TEM (nm)a
XRD (nm)b
Pt(0).BMI.BF4 3.4 ( 0.3 (3.4 ( 0.8) 3.4 ( 0.4 3.1 ( 0.4 (5.4 ( 0.7)
Pt(0).BMI.PF6 2.3 ( 0.3 (2.4 ( 0.7) 3.2 ( 0.4 2.6 ( 0.4 (3.8 ( 0.6)
a Isolated and redispersed nanoparticles and in parentheses mean
diameter obtained in situ (see Figure 10). b Mean diameter obtained
without Rietveld’s refinement method in parentheses.
The TEM images were compared with those of a pure carbon
film and with the images from samples containing only IL. The
sample regions containing particles embedded in the IL should
not present a high contrast density fluctuation if the surrounding
IL were to maintain its liquid features as in the pure liquid
observation field. The perimeter and the core of the particle
images also show strong contrast density fluctuations under
larger under focus conditions, and upon increasing under focus,
it becomes difficult to observe, offering strong evidence for the
interaction features of the BMI.PF6 IL with the Pt(0) nanopar-
ticles. The defect in the crystalline lattice was confirmed by
HRTEM measurements of the nanoparticles prepared in BMI.BF4.
The particles that have one twin are shown in the micrograph
presented in Figure 8. Note that twin defects are typical for
small particles, and the presence of such defects in a significant
amount causes an additional enlargement of the diffraction line
as observed by Rietveld’s refinements. Moreover, these defects
are more pronounced in the nanoparticles prepared in BMI.BF4
than those formed in BMI.PF6 suggesting that the particle defects
depend on the binding abilities of the anions. In this HRTEM
image (Figure 8), it was possible, by means of Gatan software,
to obtain the Fourier transform from which a lattice spacing of
2.30 and 2.04 Å were calculated corresponding to an experi-
mental error < 5%. These lattice spacings corresponding to the
interplanar distances (1 1 1) and (2 0 0) of the Pt(0) are depicted
by the arrows in Figure 8 and could be defined as the 0 -
2 2 orientation with the respect to the electron beam.
An excellent agreement was found between TEM, SAXS,
and XRD methods for determining the mean relative diameters
of the nanoparticles in the two ILs. Comparisons of the values
obtained with the different techniques are summarized in Table
2.
SCHEME 1
It is also clear that the nanoparticles prepared in BMI.PF6
that have the stronger binding anion36 are smaller than those
prepared in BMI.BF4.
In situ Investigations of Pt(0) Nanoparticle Formation. To
follow the formation of these Pt(0) nanoparticles in situ, we
made use of an indirect method developed by Finke and co-
workers.52 The Pt(0) nanoparticles catalyzed hydrogenation (H2
uptake) of cyclohexene is used as a reporter reaction via the
pseudo-elementary step concept, Scheme 1 (where A is the
precatalyst Pt2(dba)3 and B is the catalytically active Pt(0)
nanoclusters).
If cyclohexene hydrogenation (step c, Scheme 1) is a fast
reaction, on the time scale of steps a and b, it can serve as a
reporter reaction for the Pt(0) formation, that is, the kinetics of
the overall reaction are those represented only by steps a and b
in Scheme 1. Moreover, the sum of all three steps leads to a
“kinetically equivalent” elementary step which is related the
overall H2 consumption stoichiometry (cyclohexene) with the
formation of Pt(0) nanoparticles, the so-called pseudo-elemen-
tary step d (Scheme 1).

13018 J. Phys. Chem. B, Vol. 110, No. 26, 2006
Scheeren et al.
Figure 9. Cyclohexene hydrogenation curve with the catalyst precursor Pt2(dba)3 dispersed in (a) BMI.PF6 and (b) BMI.BF4 under 6 atm constant
hydrogen pressure at 75°C. The Pt2(dba)3/cyclohexene molar ratio ) 250.
TABLE 3: Rate Constants, k1 and k2, Obtained on the
Hydrogenation of Cyclohexenea by the Catalyst Precursor
Pt2(dba)3 Dispersed in BMI.PF6 and BMI.BF4
nm in BMI.BF4 and 2.3 nm in BMI.PF6, corroborating with
the idea that agglomeration is not dominant in the formation of
these nanoclusters and the small-sized Pt(0) nanoparticles are
the true catalyst responsible for the catalytic hydrogenation
reaction.
Noteworthy, these mean diameters are very close to those
obtained by XRD of the isolated and redispersed Pt(0) nano-
particles (Table 2) in the two ILs.
b
ionic liquid
k1 (h-1
)
k2 (M-1‚h-1
)
BMI.PF6
BMI.BF4
0.86
1.34
2.50
70.50
a 75 °C and under 6 atm of hydrogen (constant pressure). b Rate
constants obtained considering the substrate/catalyst ratio ) 250.
Moreover, attempts to fit the curve with a mechanism
including a third step, the bimolecular aggregation (B + B f
C, k3),77,78 or with the recently discovered double autocatalytic
mechanism,79,80 which includes a fourth step for the formation
of bulk-metal (B + C f 1.5C, k4), did not converge the fit,
which again strongly suggests that agglomeration is not
significant in our case. Noteworthy, no metallic platinum was
observed in these experiments.
It is also noteworthy that the cyclohexene hydrogenation in
BMI.BF4, in which larger nanoparticles are formed, is much
faster than that performed in BMI.PF6 where smaller particles
are formed. This is at first glance contradictory since it is
expected that smaller nanoparticles should in principle display
higher catalytic activity than the larger ones. However, it is most
probable that in this case the nanoparticles surface is more
exposed since the BF4 anion is much less coordinating than the
PF6 anion (see XPS analysis).
This mechanism is equivalent to the two-step mechanism for
transition-metal nanocluster self-assembly, from metal salts
under reductive conditions, proposed by Finke52,54 with one main
conceptual difference. Since Pt2(dba)3 is a Pt(0) nanocluster
precursor in the zero oxidation state, the first step, a in Scheme
1, must be considered the metal-compound decomposition and
not a metal nucleation step as occurs, for example, using a Pt
precursor as in an oxidation state different than zero.
The kinetics for the hydrogenation of cyclohexene catalyzed
by Pt2(dba)3 dispersed in BMI.PF6 and BMI.BF4 are shown in
Figure 9. The kinetic curves above were treated using the
pseudo-elementary step and fitted by the following integrated
rate equation for metal-compound decomposition (A f B, k1)
and autocatalytic nanocluster surface growth (A + B f 2B,
k2). For a more detailed description of the use of a pseudo-
elementary step for the treatment of hydrogenation kinetic data
and derivatization of the kinetic equations, see elsewhere.51,52,54
To verify this hypothesis, the formation of Pt(0) nanoparticles
in BMI.CF3SO3 IL, which possesses a more strongly binding
anion, was also investigated (Figure 11). Indeed, the reaction
rate in BMI.CF3SO3 is much slower than the reaction performed
with the other two ILs. Note that the mean diameter of
nanoparticles prepared in BMI.CF3SO3 is 3.0 nm (see Support-
ing Information), that is, very close to that obtained in BMI.BF4.
Moreover, the nanoparticles prepared in BMI.BF4 were
isolated and redispersed in BMI.PF6, and those isolated from
BMI.PF6 were redispersed in BMI.BF4 (Table 4). These
nanoparticle dispersions were used in the hydrogenation of
1-hexene, and the reactions performed in the IL containing the
more coordinating anion (PF6) are much slower than those
performed in BMI.BF4. Note that the miscibility of 1-hexene is
almost identical in both ionic liquids and that the reactions were
performed at 6 atm of hydrogen pressure, that is, in conditions
where the reaction is not under mass transfer control.82 This
result is another indication that the nanoparticle is surrounded
by the anionic species of the ionic liquid.
[cyclohexene]t )
(k1/k2) + [cyclohexene]0
(6)
k1
exp(k +k [cyclohexene] )t
1
2
0
1 +
k2[cyclohexene]0
As expected, the kinetic curve of Figure 9 shows no induction
period, indicating that the first step, the Pt2(dba)3 decomposition,
is fast and the catalyst nanocluster B is readily available at the
onset of the hydrogenation reaction. The obtained rate constant
values from the fit of Figure 9 are summarized in Table 3.
The kinetic curves shown in Figure 9 can also be fitted by a
simple exponential equation, which is possible because the
surface growth step (k2) is quite slow when compared with the
decomposition step (k1)sone must take into consideration the
species concentration when comparing first and second-order
rate constantssand therefore, k2 has a very small influence on
the global rate for the nanoparticle formation.
These observations are very important when combined with
the data from the TEM analysis of the nanoparticles in situ (see
Figures 6 and 10). Transmission electron microscopy revealed
an average nanoparticle size, after reaction completion, of 3.4
Conclusions. We have demonstrated that the autocatalytic
model can be applied for the formation of Pt(0) nanoparticles
from the decomposition of Pt2(dba)3 in three different ILs
(BMI.PF6, BMI.BF4, and BMI.CF3SO3). XRD analysis of the

Pt(0) Nanoparticles in Imidazolium Ionic Liquids
J. Phys. Chem. B, Vol. 110, No. 26, 2006 13019
Figure 10. In situ TEM micrographs (top) and histograms (bottom) showing the particle size distribution of Pt(0) in BMI.BF4 (left) and in BMI.PF6
(right) after the hydrogenation of cyclohexene.
TABLE 4: 1-Hexene Hydrogenation by Pt(0) Nanoparticles
Isolated from BMI.BF4 (3.4 nm) and Redispersed in
BMI.PF6 and Those Isolated from BMI.PF6 (2.3 nm) and
Redispersed in BMI.BF4 at 75 °C under 6 atm of Hydrogen
(constant pressure) and 1-hexene/Pt ) 250
Pt(0) medium
preparation
Pt(0) size
(nm)
reaction
medium
a
entry
TOF (h-1
)
1
2
BMI.BF4
BMI.PF6
3.4 ( 0.3
2.3 ( 0.3
BMI.PF6
BMI.BF4
404 (1300)
1000 (2380)
a Turnover frequency ) mol(1-hexene)/mol(Pt2(dba)3)‚h and in
parentheses the corrected turnover frequency considering only the
exposed atoms on the nanoparticle surface (3.4 nm, 32%, and 2.3 nm,
42%) using the magic number approach.81
BMI.BF4 can be ascribed to the presence of highly uncoordi-
nated atoms compared with those reactions performed in ILs
containing more coordinating anions (PF6 and CF3SO3). More-
over, the IL anion also plays an important role in the shape of
nanoparticles formation, that is, more imperfect nanoparticles
are formed in processes performed in ILs containing less
coordinating anion (BF4) than those synthesized in ILs contain-
ing the PF6 and CF3SO3 anions.
Figure 11. Cyclohexene hydrogenation curve with the catalyst
precursor Pt2(dba)3 dispersed in BMI.CF3SO3 under 6 atm constant
hydrogen pressure at 75 °C. The Pt2(dba)3/cyclohexene molar ratio )
250 (k1) 0.53 h-1 and k2 ) 11.5 M-1‚h-1).
isolated nanoparticles shows that smaller Pt(0) nanoparticles of
around 2.3 nm are obtained in ILs containing more coordinating
anion (PF6) whereas larger particles with 3.4 nm in mean
diameter were formed in the IL BMI.BF4. XPS analysis clearly
demonstrated the formation of an ionic liquid protective layer
surrounding the nanoparticle surface. This protective layer is
probably composed of semi-organized anionic species present
as supramolecular aggregates of the type [(BMI)x(X)x+1)]- (x
) 2 for X ) BF4 and x ) 3 for X ) PF6) with an extended
molecular length of 2.8 nm for BMI.BF4 and 3.3 nm for
BMI.PF6 as calculated from SAXS data. Moreover, the high
catalytic activity observed for the larger nanoparticles in
Acknowledgment. Thanks are due to the following agen-
cies: CNPq, CAPES, CT-PETRO, FAPERGS, and TWAS for
partial financial support. We also acknowledge the staff of LNLS
(Project D11A-SAXS, 2003) for technical assistance. We also
thank Prof. R. G. Finke and E. E. Finney (Colorado State
University) for help in the curve fitting and for the use of the
MacKinetic program.
Supporting Information Available: Energy dispersion
spectrometry and electron diffraction micrograph of Pt(0)
nanoparticles prepared in BMI.PF6 and X-ray diffraction and

13020 J. Phys. Chem. B, Vol. 110, No. 26, 2006
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