A simple and reproducible method for the synthesis of silica-supported rhodium nanoparticles and their investigation in the hydrogenation of aromatic compounds

Article abstract of DOI:10.1039/b605893k

Colloidal suspensions of rhodium nanoparticles have been easily prepared in aqueous solution by chemical reduction of the precursor RhCl ₃·3H₂O in the presence of the surfactant N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium chloride (HEA16Cl) and further used to immobilize rhodium nanoparticles on silica by simple impregnation. The obtained silica-supported rhodium nanoparticles have been investigated by adapted characterization methods such as transmission electron microscopy and X-ray photoelectron spectroscopy. A particle size increase from 2.4 to 5 nm after the silica immobilization step and total elimination of the surfactant has been observed. This "heterogeneous" catalyst displayed good activities for the hydrogenation of mono-, di- alkylsubstituted and/or functionalized aromatic derivatives in water under atmospheric hydrogen pressure and at room temperature. In all cases, the catalyst could be recovered several times after a simple decantation or filtration and reused without any significant loss in catalytic activity. This supported catalyst has also been tested under higher hydrogen pressure giving rise to TOFs reaching 6430 h ^-1 at 30 bar and in terms of catalytic lifetime 30 000 TTO in 8.5 h for pure anisole hydrogenation at 40 bar. the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b605893k

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A simple and reproducible method for the synthesis of silica-supported
rhodium nanoparticles and their investigation in the hydrogenation of
aromatic compounds
a
a
a
b
´
Vincent Mevellec, Audrey Nowicki, Alain Roucoux,* Christophe Dujardin,
Pascal Granger,b Edmond Payenb and Karine Philippotc
Received (in Montpellier, France) 25th April 2006, Accepted 2nd June 2006
First published as an Advance Article on the web 22nd June 2006
DOI: 10.1039/b605893k
Colloidal suspensions of rhodium nanoparticles have been easily prepared in aqueous solution by
chemical reduction of the precursor RhCl3 ꢀ 3H2O in the presence of the surfactant N,N-dimethyl-
N-cetyl-N-(2-hydroxyethyl)ammonium chloride (HEA16Cl) and further used to immobilize
rhodium nanoparticles on silica by simple impregnation. The obtained silica-supported rhodium
nanoparticles have been investigated by adapted characterization methods such as transmission
electron microscopy and X-ray photoelectron spectroscopy. A particle size increase from 2.4 to 5
nm after the silica immobilization step and total elimination of the surfactant has been observed.
This ‘‘heterogeneous’’ catalyst displayed good activities for the hydrogenation of mono-, di-
alkylsubstituted and/or functionalized aromatic derivatives in water under atmospheric hydrogen
pressure and at room temperature. In all cases, the catalyst could be recovered several times after
a simple decantation or filtration and reused without any significant loss in catalytic activity. This
supported catalyst has also been tested under higher hydrogen pressure giving rise to TOFs
reaching 6430 hꢁ1 at 30 bar and in terms of catalytic lifetime 30 000 TTO in 8.5 h for pure anisole
hydrogenation at 40 bar.
tend to be more active than classical heterogeneous systems
Introduction
under mild conditions.
Among active areas of academic and industrial research, total
and partial hydrogenation of benzene derivatives still attracts
increasing interest, more particularly, with the industrial de-
mand for cyclohexane,1 the most important precursor to
adipic acid used to produce nylon-6,6, and cyclohexene, an
important intermediate in organic synthesis produced by the
Asahi Chemical Industry process.2 Moreover, hydrogenation
of aromatic compounds becomes more and more of an
industrial challenge as refineries attempt to reduce emissions
of volatile organic matter (VOM) during combustion of diesel
oil.3 The catalytic transformation is generally carried out in
drastic conditions with heterogeneous catalysts of group VIII
metals.4 In the past few years, the concept of arene homo-
geneous hydrogenation catalysis has been reviewed since some
catalysts originally assigned as homogeneous have proved to
be precursors to colloidal or nanoparticulate catalysts.5 Con-
sequently, relevant hydrogenation catalytic systems have been
investigated using noble metal nanoparticles stabilized in
water or organic media.6 These nanoheterogeneous systems
For economical and environmental reasons, the develop-
ment of efficient catalysts in water, limiting the use of volatile
organic solvents, seems to be of fundamental interest for the
future.7 Therefore, the synthesis of colloidal nanoparticles
stabilized in aqueous solution may lead to significant progress
in green chemistry. These aqueous systems are potentially
excellent starting materials for the preparation of new sup-
ported catalysts, which are largely more active and selective
than commercially available analogous catalysts. Indeed, sev-
eral papers relate the effective catalytic activities of such metal
nanoparticles after their immobilisation in rigid matrices
(SiO2, Al2O3,. . .).8 In most cases, the particles have been
synthesized by reduction of a metallic precursor in the pre-
sence of an appropriate support. Only some research relates
the anchoring of pre-stabilized metal nanoparticles on an
inorganic support. The immobilization of polymer-stabilized
colloids by a modified coordination capture described by Liu
and co-workers9 led to supported metal nanoparticles with
singular catalytic activity for olefinic and nitro compound
hydrogenation but no aromatic ring hydrogenation has been
observed. The deposition of trioctylamine (TOA)-stabilized
ruthenium nanoparticles over g-Al2O3 has been described by
Marconi and co-workers8i for the reduction of methylbenzoate
to methylhexanoate at 80 1C under 20 atm hydrogen pressure
and the authors have reported higher activities with sup-
ported-nanoparticles prepared in the presence of TOA as a
stabilizer in comparison to the direct reduction of the pre-
cursor in the presence of g-Al2O3. Based on a similar
a CNRS UMR 6226 ‘Sciences Chimiques de Rennes’—Equipe
`
`
´
Synthese Organique et Systemes Organises-Ecole Nationale
Supe´rieure de Chimie de Rennes, Avenue du Gal Leclerc, 35700
Rennes, France. E-mail: Alain.Roucoux@ensc-rennes.fr; Fax: þ33
(0)223238199
b
´
Unite de Catalyse et de Chimie du Solide, UMR 8181, Universite des
Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq, France
´
c Laboratoire de Chimie de Coordination du CNRS, UPR 8241, 205
route de Narbonne, 31077 Toulouse Cedex 04, France
ꢂc
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approach, we report, in this paper, an easy way to synthesize
supported rhodium nanoparticles by simple addition of silica
under vigorous stirring to a pre-stabilized rhodium aqueous
colloidal suspension. The rhodium nanoparticles embedded in
rigid matrices have been characterized by adapted methods
such as TEM after microtomy preparation and XPS experi-
ments. The catalytic activity of these supported systems has
been evaluated in the classical hydrogenation of various
aromatic substrates under mild conditions. Moreover, these
supported catalysts allow an easy product separation and an
easy catalyst recycling as well as the catalytic hydrogenation of
pure substrates in the absence of co-solvents and associated
environmental problems.
Fig. 1 TEM realized on the central zone of Rhcoll/SiO2 at 100 kV and
metal particle size distribution.
visible in certain areas. The mean diameter of the Rh nano-
particles, calculated from about 300 particles found in an
arbitrary chosen area, corresponds to an average diameter of
5 nm indicating that a weak clustering has occurred during the
immobilization step since the mean diameter of the rhodium
particles in the initial aqueous colloidal suspension was
2.4 nm. The microtomy experiments show that surfactant-
stabilized Rh nanoparticles have homogeneously diffused in-
side the silica pores during the immobilization step. Moreover,
the pore sizes of the silica matrix certainly contribute inside to
limit this aggregation phenomenon and finally allow the
control of the particle size.
Results and discussion
Synthesis of silica supported rhodium(0) nanoparticles
The immobilization of metal nanoparticles on supports to
obtain nanocatalysts has already been reported. Most of the
time, the supported nanocatalyst preparation methods require
severe conditions involving repeated oxidation and reduction
steps at high temperature. The approach described here com-
bines in one pot stabilization of metal nanoparticles and their
deposition onto an inorganic support in the same aqueous
media.
The silica-supported rhodium nanoparticles have also been
analyzed by X-ray photoelectron spectroscopy to examine the
oxidation state and the chemical environment of the deposited
Rh nanoparticles. Fig. 2 shows the photopeak related to the
Rh 3d core level recorded on Rh nanoparticles embedded
in silica.
Indeed, we have developed an easy way to immobilize
rhodium nanoparticles on silica from our previously reported
aqueous colloidal rhodium(0) suspension9 prepared by chemi-
cal reduction of an aqueous solution of RhCl3 ꢀ 3H2O in the
presence of highly water soluble ammonium chloride salts as
stabilizing agents.10 The optimized results have been obtained
with the N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammo-
nium chloride salts (HEA16Cl) which efficiently prevent par-
ticle aggregation. Then, the nanoparticle adsorption on a
support (about 0.1%wt Rh) has been performed by the
addition of an adequate amount of silica to this latter colloidal
suspension under vigorous stirring. After 2 h, the coloration of
the aqueous phase turned from black to colorless evidencing a
total adsorption of the particles at the silica surface. After
filtration, the dark-brown powder was washed several times
with clean water to eliminate the surfactant and dried over-
night at 60 1C leading to a solid corresponding to silica-
supported rhodium nanoparticles. It should be noticed that
all the reactions have been carried out at ambient temperature,
under air, without neither use of gas treatment nor a calcina-
tion step.
Two contributions corresponding to different rhodium spe-
cies can be isolated after mathematical decomposition accord-
ing to the procedure described in the Experimental section.
The low binding energy signal (BE of 3d3/2 at 311.8 eV and Rh
3d5/2 at 307.9 eV) is consistent with rhodium metal whereas the
higher one (BE of 3d3/2 at 313.8 eV and Rh 3d5/2 at 308.8 eV)
agrees with the presence of oxidic rhodium species. Subse-
quent comparisons with earlier investigations are consistent
with Rh31 species suggesting the formation of Rh2O3 on the
surface.11 Consequently, a partial oxidation during the sample
preparation and/or the introduction into the spectrometer
occurs. Same observations concerning the presence of the
rhodium oxidized form could be made between the supported
rhodium nanoparticles (Rhcoll/SiO2) and the aqueous colloi-
dal rhodium suspension (Rh-HEA16Cl) with a similar atomic
ratio between the two different forms of rhodium. On the other
hand, no specific core-level peaks associated with N 1s, C 1s
and Cl 2p relative to the surfactant HEA, used to prepare the
aqueous colloidal rhodium suspension, were noticed (Fig. 2)
and the Si 2p and O 1S corresponding to the silica support
were found, respectively, at 103.8 eV and 533.1 eV. Absence of
the Cl signal (BE at 200 eV) indicates that the surface is not
contaminated by ionic species. Therefore, we suggest that the
particles were totally adsorbed on the support during the
preparation and that the surfactant was totally removed after
the washing step. This result justifies the investigation of the
effect of chlorine as already reported12 by a catalytic compar-
ison with the aqueous suspension stabilized by the ammonium
Characterization of the supported nanoparticles
This silica-supported rhodium nanoparticle system has been
characterized by several techniques (TEM, XPS) in order to
evaluate the size and the nature of deposited rhodium nano-
particles.
Fig. 1 shows a transmission electron micrograph and a
particle size histogram obtained by observation of the central
zone of the silica spheres containing metal nanoparticles after
microtomy preparation.
We can observe a homogeneous dispersion of the rhodium
particles in the silica matrix although a few aggregates are
ꢂc
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Fig. 2 (a) X-Ray photoelectron spectra of isolated silica-capped rhodium nanoparticles. (b) X-Ray photoelectron spectra of rhodium 3d core
level in Rhcoll/SiO2.
triphasic system (H2O/hydrocarbon/silica). The catalytic reac-
tions were carried out at room temperature and monitored by
the hydrogen volume consumption and gas chromatography
analysis. The catalytic performances of the Rhcoll/SiO2 nano-
catalyst in the hydrogenation of mono- or di-alkylsubstituted
and/or functionalized benzene derivatives are reported in
Table 1.
Scheme 1 Scheme proposed for the anchoring of rhodium nano-
particles on silica surface.
In all cases, a total conversion of the substrate has been
observed with quite interesting turn over frequencies (TOF),
ranging from 93 hꢁ1 to 129 hꢁ1. In the series of benzene,
toluene, ethylbenzene, propylbenzene and cumene (Entries
1–5), the increased reaction time and decreased catalytic
TOF observed are typical of the increasing steric hindrance
influence of the alkyl group as usually observed with other
catalysts.15 Moreover, cumene (Entry 5) which is usually
difficult to reduce under mild conditions has been hydroge-
nated with a TOF of 93 hꢁ1. In addition, the hydrogenation of
functionalized aromatic compounds such as anisole, phenol,
ethylbenzoate, aniline or styrene (entries 6–10) has been
performed with complete conversion. Nevertheless, arenes
substituted by electron-withdrawing groups such as an ester
group react more slowly (entry 8) whereas the reaction is
accelerated for electron rich substrates such as anisole which is
chloride salts (vide infra). Moreover, in the literature, experi-
mental and theoretical attention has been devoted to under-
standing the interactions between metal and supports.13 Based
on recent work,14 including XAFS experiments coupled with
the density functional theory, we suggest that rhodium nano-
particles might be anchored to silica via oxygen bonding
(Scheme 1).
Catalytic activity in the hydrogenation of aromatic derivatives
The silica-supported rhodium nanocatalyst (Rhcoll/SiO2) pre-
viously described has been tested in the hydrogenation of
various aromatic compounds under mild conditions in a
a
Table 1 Hydrogenation of aromatic compounds with Rhcoll/SiO2
Entry
Substrate
Product (%)b
t/h
3.8
TOF/hꢁ1c
1
2
3
4
5
6
7
8
Benzene
Toluene
Ethylbenzene
Propylbenzene
Cumene
Anisole
Phenol
Ethylbenzoate
Aniline
Styrene
o-Xylene
m-Xylene
p-Xylene
o-Methylanisole
m-Methylanisole
p-Methylanisole
Cyclohexane (100)
Methylcyclohexane (100)
Ethylcyclohexane (100)
Propylcyclohexane (100)
Iso-propylcyclohexane (100)
Methoxycyclohexane (100)
Cyclohexanol (100)
Ethylcyclohexanoate (100)
113
107
100
97
4
4.3
4.4
4.6
3.2
3.2
5.3
93
129
129
80
31
127
63
71
79
67
73
9
Cyclohexylamine (100)
Ethylcyclohexane (100)
14
10
11
12
13
14
15
16
a
4.5
6.8
6.1
5.4
6.4
5.9
5.1
1,2-Dimethylcyclohexane (cis/trans: 95/5)
1,3-Dimethylcyclohexane (cis/trans: 90/10)
1,4-Dimethylcyclohexane (cis/trans: 75/25)
1-Methoxy-2-methylcyclohexane (cis/trans: 98/2)
1-Methoxy-3-methylcyclohexane (cis/trans: 95/5)
1-Methoxy-4-methylcyclohexane (cis/trans: 90/10)
77
Conditions: Rhcoll/SiO2(1 g, 0.07 wt% Rh), [S]/[M] = 100, 10 ml H2O, 1 atm H2, 20 1C, stirred at 1500 minꢁ1
.
Determined by GC
b
c
analysis. Turnover frequency defined as mol of H2 per mol of rhodium per h.
ꢂc
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hydrogenated in less than 4 h (entry 6). Moreover, the hydro-
genation of aniline with the supported catalyst has been
achieved after 14 h (entry 9) and Rhcoll/SiO2 could be reused
after simple filtration with no significant loss of activity (TOF
= 30 hꢁ1 for the second run). This activity is relevant since a
destabilization of the catalytic system and particle agglomera-
tion were observed after the first run for aniline hydrogenation
with our classical aqueous colloidal rhodium suspension.10a
Thus, the adsorption of metal nanoparticles on silica is an
alternative to solve this problem. Finally, the hydrogenation of
di-substituted arenes has also been performed with complete
conversion (entries 11–16). In all cases, the selectivity of the
reaction was unchanged. Indeed, as usually observed with pure
heterogeneous catalysts and with our classical aqueous rho-
dium colloidal suspension,10a the hydrogenation of xylenes
and methylanisoles yields the kinetically favoured cis-product
with a ratio of up to 98% in 1-methoxy-2-methylcyclohexane
(entry 14) with silica supported rhodium nanoparticles. The
cis-product is also the major product with p-xylene but the
ratio of the trans-isomer increases in the same range with
colloidal and supported systems (30 and 25%, respectively). In
this case, this ratio could be explained by a successive dis-
sociation and re-association phenomenon of a partially hydro-
genated intermediate on the catalyst surface.
Table 2 Hydrogenation of anisole with Rhcoll/SiO2 under hydrogen
pressurea
Substrate/
Rh
PH2/
bar
Product
(%)b
TOF/
hꢁ1c
Entry
17
18
19
20
21
22
a
100
100
100
300
1000
10 000
10
20
30
10
10
40
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Methoxycyclohexane (100)
1714
4286
6429
1929
951
5069
Conditions: Rhcoll/SiO2(1 g, 0.07 wt%Rh), water (10 ml),1 atm H2,
.
20 1C, stirred at 1500 minꢁ1
over frequency defined as mol of H2 per mol of rhodium per h.
b
c
Determined by GC analysis. Turn-
Table 3 Comparison of traditional catalysts versus the Rhcoll/SiO2
for the hydrogenation of anisolea
1st run
t/h
TOF/
hꢁ1c
2nd run
t/h
TOF/
hꢁ1c
Catalyst
Rh/C 5 wt%
8
38
—
—
(Degussa type DG 10)
RhIII-NaBH4/SiO2 0,1 wt%
Rhcoll/SiO2 0,07 wt%
Rh-HEA16Clb
6
3.2
3.6
50
129
83
7
3.2
3.9
42
129
77
a
Reaction conditions: Substrate/Rh = 100 (molar ratio), anisole (3.8
ꢃ 10ꢁ3 mol), water (10 mL), 1 atm H2, 20 1C, stirred at 1500
The deposition of the surfactant-stabilized rhodium nano-
particles on silica appears as a rapid and easy way for
preparation of active heterogeneous catalysts under mild
conditions. The so-obtained supported nanocatalyst has
shown an interesting activity in the hydrogenation of various
aromatic compounds at 20 1C and under atmospheric hydro-
gen pressure. However, in order to explore the limits of this
catalyst, other anisole hydrogenation experiments have been
carried out at higher hydrogen pressure (Table 2).
b
minꢁ1
c
.
Turnover frequency defined as mol of H2 per mol of rhodium per h.
Rhodium (3.8 ꢃ 10ꢁ5 mol), HEA16Cl (7.6 ꢃ 10ꢁ5 mol).
adsorption of the particles on the silica surface since in the case
of weak interaction between particles and support, a loss of
activity is usually observed due to metal leaching. In our case,
a slight loss of activity is noticed (e.g. TOF = 129 hꢁ1 for the
first run of anisole hydrogenation and 120 hꢁ1 for the fifth
run) that can be attributed to the loss of a small quantity of the
catalyst during the filtration step.
Silica-capped rhodium nanoparticles reveal an efficient ac-
tivity at higher hydrogen pressure for anisole hydrogenation
since a TOF of 6430 hꢁ1 at 20 1C and 30 bar H2 has been
obtained. This result is certainly one of the most important
Turnovers described in the literature for immobilised metal
nanoparticles (entry 19). Moreover, 30 000 total turnover
(TTO) in 8.5 h were also obtained in terms of catalytic lifetime
under 40 bar H2 (entry 22).
The efficiently obtained activities can be attributed to the
very homogeneous dispersion of rhodium nanoparticles inside
silica pores due to the initial surfactant-stabilization of the
particles, thus avoiding aggregation and allowing a better
control of the particle size. Moreover, the substrate diffusion
rate to rhodium active sites might be increased thanks to the
For comparison, three other catalytic systems have been
tested in anisole hydrogenation at 20 1C and under 1 bar of H2
(Table 3). These catalysts are: the heterogeneous catalyst Rh/C
from Degussa (5 wt%), our classical aqueous rhodium nano-
particles Rh-HEA16Cl suspension and a similar catalyst con-
taining supported metal nanoparticles, RhIII-NaBH4/SiO2,
adapted from the literature.16 In this latter case, the rhodium
salt was adsorbed on the support before chemical reduction.
This comparative study evidences that the best activities are
obtained with the catalyst resulting from immobilization on
silica on the pre-stabilized Rh-HEA16Cl nanoparticles.
Finally, recycling experiments have been investigated on
catalytic systems containing typical substrates such as ben-
zene, anisole and styrene at 20 1C and under 1 atm H2 (Fig. 3).
After each run, the catalyst was recovered by simple filtration
and dried at 60 1C before another run with new addition of
substrate. Five runs could be performed at least with no
significant loss of activity. These results point out the efficient
Fig. 3 Catalytic lifetime of silica-capped rhodium nanoparticles for
benzene, anisole and styrene hydrogenation at 20 1C and under
1 atm H2.
ꢂc
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large covering of the silica matrix in contrast with classically
supported nanoparticles which are usually concentrated on a
localised area of the support.
difference in binding energy levels between both signals as
constraints.
Synthesis and characterization of the aqueous rhodium(0)
suspension
Conclusions
To an aqueous solution of surfactant HEA-16-Cl (95 mL,
7.6 ꢃ 10ꢁ3 mol 1ꢁ1) was added sodium borohydride (36 mg,
9.5 ꢃ 10ꢁ4 mol). Then this solution is quickly added under
vigorous stirring to an aqueous solution (5 mL) of the
precursor RhCl3 ꢀ 3H2O (100 mg, 3.8 ꢃ 10ꢁ4 mol) to obtain
an aqueous Rh(0) colloidal suspension (100 mL). The reduc-
tion occurs instantaneously and is characterized by a color
change from red to black. The obtained suspensions are stable
for several months as confirmed by TEM (the sizes of the
particles remain unmodified over this time frame). The trans-
mission electronic microscopic studies were conducted using a
In summary, we have shown that an aqueous solution contain-
ing pre-stabilized Rh nanoparticles can be used easily for
particle deposition onto silica through simple impregnation.
The so-obtained silica supported-rhodium nanoparticles dis-
played an unexpected activity under atmospheric hydrogen
pressure and at room temperature for the hydrogenation of
various aromatic compounds and could be reused at least for
five runs without significant loss of activity. Moreover, this
catalytic system appears as a good way to solve the problem of
destabilisation which occurs with rhodium nanoparticles in
aqueous suspension towards N-substrates. Finally, since the
preparation of the catalyst has been carried out at ambient
temperature without necessitating neither a calcination step
nor a high hydrogen pressure, the method described in this
paper is an easy way to prepare a heterogeneous nanocatalyst.
FEI Technaı G2 Sphera at 200 kV (cathode LaB6). Samples
¨
were prepared by a dropwise addition of the stabilized nano-
particles in water onto a copper sample mesh covered with
carbon. The colloidal dispersion was partially removed after 1
min using cellulose before transferring to the microscope. The
picture is obtained at X 80.000 with a video GATAN
USC1000 (CCD detector 2048X2048) and associated software
DIGITAL Micrograph. Measurement of about 300 particles
was made with the program SCION Image (NIH) and was
analyzed with the KaleidaGraph program providing histo-
grams of nanoparticle size distribution.
Experimental
Starting materials
Rhodium chloride hydrate was obtained from Strem Chemi-
cals. Rh/C (5 wt%; Degussa-type G10), Sodium borohydride,
and all aromatic substrates were purchased from Aldrich or
Fluka and were used without further purification. Water was
distilled twice before use by conventional methods. The sur-
factant HEA-16-Cl was prepared as previously described in
the literature and was fully characterized.10 Commercial Silica
Synthesis of the silica supported-rhodium nanoparticles
Silica (18.5 g) was added under vigorous stirring to 40 ml of
deionized water. After 2 h, 50 ml of Rh(0)-HEA-16-Cl (1.9 ꢃ
10ꢁ4 mol of Rh) was added under vigorous stirring to this
suspension. The system was kept under stirring for 2 h, after-
wards the black power obtained after filtration was dried
overnight at 60 1C. Anal. found: Si, 41,72; Rh, 0.07.
Si 60 (Merck) was used as received (surface area SBET
364 m2 gꢁ1).
=
Analytical procedures
TEM-analysis. After embedding the sample in a resin (Spurr
R1032) and treatment at 70 1C overnight for polymerization,
the solid was cut into thin lamellae (50 nm) with a microtom.
Analysis of the sample was then realized after deposition of a
lamella onto a carbon covered copper grid using a Hitachi
HU-12 A microscope operating at 120 kV with a resolution of
General procedure for hydrogenation under atmospheric
hydrogen pressure
Reactions were carried out under standard conditions (20 1C,
1 atm of H2). A 25 ml round bottom flask, charged with
appropriated Rh(0) catalyst (10 mL for aqueous suspension
and 1 g in 10 ml of water for Rh/SiO2) and a magnetic stirrer,
was connected with a gas burette (500 mL) with a flask to
balance the pressure. The flask was closed by a septum, and
the system was filled with hydrogen. The appropriate aromatic
substrate ([Substrate]/[Rh] = 100) was injected through the
septum, and the mixture was stirred at 1500 minꢁ1. The
reaction was monitored by the volume of gas consumed and
by gas chromatography. Turnover frequency (TOF) was
determined for 100% conversion.
´
4.5 A, at the service ‘‘Centre de Microscopie Electronique
Applique a la Biologie de la Faculte de Medicine’’ in
Toulouse.
´
´
Gas chromatography. All analysis were performed using a
Carlo Erba GC 6000 with a FID detector equipped with an
Alltech AT1 column (30 m, 0.25 mm i.d.).
X-Ray photoelectron spectroscopy analysis. XPS measure-
ments were performed using a Vacuum Generators Escalab
220XL spectrometer. A monochromatized aluminium source
was used for excitation (Al Ka = 1486.6 eV). The binding
energies were referenced to the photopeak C 1s at 285 eV. The
decomposition of the Rh 3d photopeak was achieved accord-
ing to a ratio Gaussian/Lorentzian equal to 0.3 and account-
ing for the relative intensity I(Rh 3d5/2)/I(Rh 3d3/2) and the
Acknowledgements
Financial support from Region Bretagne is gratefully ac-
´
knowledged. We also thank Leon Gengembre for conducting
XPS measurements, and I. Fourquaux from the service
´
´
‘‘Centre de Microscopie Electronique Applique a la Biologie
ꢂc
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de la Faculte
´
de Me
´
decine’’ in Toulouse for microtomy
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Moreno, W. Oberhauser, R. Psaro, L. Sordelli et and F. Vizza, J.
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preparation.
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