Surface characterisation of phosphine and phosphite stabilised Rh nanoparticles: a model study

Article abstract of DOI:10.1039/c5ra21835g

Small and well defined Rh nanoparticles (<2 nm) stabilised by the model ligands triphenylphosphine 1 and triphenylphosphite 2 were synthesised using various P/Rh ratios. In all cases, the crystalline NPs exhibited a spherical shape and a fcc structure. During the synthesis of these materials, hydrogenation and oxidation of the stabilisers take place and the degree of hydrogenation of the stabilising ligand increased when low ligand to Rh ratio is used during their synthesis. The Rh1 systems mainly contain adsorbed phosphine oxide species at their surface whereas the Rh2 systems include both phosphite and phosphite oxide species. Analysis of the surface of these nanoparticles by infrared spectroscopy revealed the presence of Rh-H at the surface of the NPs and hydride titration experiments revealed a higher hydride coverage for the phosphine stabilized systems. By CO adsorption/infrared experiments, bridging, terminal and geminal Rh(CO)₂ sites were detected. All these systems were active catalysts in the hydrogenation of styrene and the observation of Rh(CO)₂ sites could be correlated with the activity of these species for the hydrogenation of the aromatic ring.

Full text of DOI:10.1039/c5ra21835g

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Surface characterisation of phosphine and
phosphite stabilised Rh nanoparticles: a model
Cite this: RSC Adv., 2015, 5, 97036
study†
a
b
a
a
*
*
Jessica Llop Castelbou, Pascal Blondeau, Carmen Claver and Cyril Godard
Small and well defined Rh nanoparticles (<2 nm) stabilised by the model ligands triphenylphosphine 1 and
triphenylphosphite 2 were synthesised using various P/Rh ratios. In all cases, the crystalline NPs exhibited
a spherical shape and a fcc structure. During the synthesis of these materials, hydrogenation and
oxidation of the stabilisers take place and the degree of hydrogenation of the stabilising ligand increased
when low ligand to Rh ratio is used during their synthesis. The Rh1 systems mainly contain adsorbed
phosphine oxide species at their surface whereas the Rh2 systems include both phosphite and phosphite
oxide species. Analysis of the surface of these nanoparticles by infrared spectroscopy revealed the
presence of Rh-H at the surface of the NPs and hydride titration experiments revealed a higher hydride
coverage for the phosphine stabilized systems. By CO adsorption/infrared experiments, bridging,
terminal and geminal Rh(CO)2 sites were detected. All these systems were active catalysts in the
hydrogenation of styrene and the observation of Rh(CO)2 sites could be correlated with the activity of
these species for the hydrogenation of the aromatic ring.
Received 19th October 2015
Accepted 4th November 2015
DOI: 10.1039/c5ra21835g
www.rsc.org/advances
of the potential of metal nanoparticles in catalysis, there is
a need for a better understanding how the ligands stabilise the
Introduction
NPs and affect their surface properties. Recently, the group of
Chaudret and co-workers demonstrated that combined IR and
NMR spectroscopic studies can allow the very precise location of
carbene ligands at the surface of Ru-NPs.12 They also looked at
the dynamics of adsorbed CO molecules by FTIR spectroscopy
and observed that the presence of the bulky diphosphine as
stabilisers reduces the mobility of these ligands.13 These results
indicated that the use of the appropriate ligands as stabilisers
could lead to the modulation of the surface properties of M-NPs,
and thus open the way to the rational design of selective
nanocatalysts. Moreover, Dyson and co-workers showed that the
addition of phosphines to Rh-PVP NPs of ca. 3 nm of diameter
improves the selectivity of these nanocatalysts in hydrogenation
reactions.14 They postulated that the selective coordination of
these ligands could modify the steric hindrance at specic sites
of the NPs and hence generate selectivity.
In recent years, our research group has reported the appli-
cation of transition metal nanoparticles in selective catalytic
processes using chiral diphosphites as stabilisers.15 In most
cases, the catalytic performances of the resulting catalysts were
clearly inuenced by the ligands, although no clear structure–
reactivity relationship could be achieved. The hydrogenation of
substituted arenes was also investigated using soluble Rh-NPs
as catalysts, and results showed relevant differences in activity
and selectivity depending on the properties of the P-ligand used
for the stabilisation of the NPs.4 More recently, a study on the
behaviour of Ru- and Rh-nanoparticles stabilised by mono- and
Phosphorus based ligands such as phosphines and phosphites
have been extensively and successfully used in homogeneous
catalysis due to their rich coordination chemistry and their
inuence on the stability and selectivity of transition metal
catalysts.1 The ability to nely tune the properties of the catalyst
by selecting the adequate steric and electronic features of the
ligands is a key tool in homogeneous catalysis for the rational
design of new catalysts.2
In recent years, phosphorus based ligands also revealed to
efficiently stabilise transition metal nanoparticles (NPs) that are
active catalysts in several processes such as arene hydrogena-
tion and CC bond formation.3–8 In heterogeneous catalysis,
however, the tuning of the catalyst is usually attempted through
the use of additives such as thiols9 but remains mainly depen-
dant on the size of the particles.10 Furthermore, in contrast with
homogeneous systems where spectroscopic methods can
provide information on where and how the ligands are coordi-
nated to the metal centre, the characterisation of these nano-
catalysts at the atomic level is much more challenging.11 In view
a
´
´
`
Departament de Quımica Fısica I Inorganica, Universitat Rovira I Virgili, C/ Marcel li
Domingo s/n, Campus Sescelades, 43007, Tarragona, Spain. E-mail: cyril.godard@
urv.cat; Fax: +34 977 559563
b
´
`
Departament de Quımica Analitica i Organica, Universitat Rovira I Virgili, C/ Marcel li
Domingo s/n, Campus Sescelades, 43007, Tarragona, Spain
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra21835g
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bidentate phosphines showed clear differences in the selective
hydrogenation of aromatic ketones.16
TGA experiments were carried out in the furnace of a Mettler
Toledo TGA/SDTA851 instrument.
Herein, we describe the synthesis and characterisation of
GS-MS spectroscopy was carried out on a HP 6890A spectrom-
a series of Rh-NPs stabilised by model monodentate phos- eter, with an achiral HP-5 column (0.25 mm ꢁ 30 m ꢁ 0.25 mm).
phorus donor ligands that are active catalysts in hydrogenation
reactions.8 Variation of the amount of ligand during the
General procedure for the synthesis of the Rh-NPs
synthesis was performed and CO adsorption-FTIR spectroscopy
In a typical procedure, the [Rh(h3-C3H5)3] (64 mg, 0.28 mmol)
were used to monitor the effects of the nature of the ligands,
was placed into a Fischer–Porter reactor at ꢂ110 ꢀC (acetone/N2
their steric bulk and their loading at the surface of the NPs.
bath) in 64 ml of dry and deoxygenated THF by freeze–pump–
These features were correlated with the results obtained in the
thaw cycles in the presence of the appropriate ligand. The
hydrogenation of styrene.
Fischer–Porter reactor was then pressurised under 6 bar of H2
and stirred for 30 minutes at room temperature. The solution
was then heated to 40 ꢀC and stirred at this temperature during
Experimental part
24 h. The initial colourless solution became black aer 1 h. A
Reagents and general procedures
small amount (5 drops approx.) of the solution was deposited
All syntheses were performed using standard Schlenk tech-
niques under N2 or Ar atmosphere. Chemicals were purchased
from Aldrich Chemical Co, Fluka and Strem. All solvents were
distilled over drying reagents and were deoxygenated before
use. The precursor [Rh(h3-(C3H5)3)], was prepared following
previously described methods.27 The synthesis of the nano-
particles were performed using 1L Fisher Porter and pressurized
on a high pressure line.
under an argon atmosphere on a carbon-covered copper grid for
transmission electron microscopy analysis (TEM). The rest of
the solution was concentrated under reduced pressure. Precip-
itation and washing with pentane (3 ꢁ 15 ml) was then carried
out, obtaining a black precipitate.
General procedure for the hydrogenation reactions
Autoclave Par 477 equipped with PID control temperature and
reservoir for kinetic measurements was used as reactor for the
hydrogenation reactions. In a typical experiment, the autoclave
was charged in the glove-box with Rh nanoparticles (3.5 mg of
Rh-NPs; the catalyst concentration was calculated based on the
total number of metallic atoms in the NPs) and the substrate
(1.24 mmol, approx. substrate to metal ratio ¼ 55) in 10 ml of
heptane. Molecular hydrogen was then introduced until the
desired pressure was reached. The reaction was stirred during
Characterization techniques
TEM experiments were performed at the “Unitat de Microscopia
`
dels Serveis Cienticotecnics de la Universitat Rovira I Virgili”
(TEM-SCAN) in Tarragona with a Zeiss 10 CA electron micro-
˚
scope operating at 100 kV with resolution of 3 A. The particles
size distributions were determined by a manual analysis of
enlarged images. At least 300 particles on a given grid were
measured in order to obtain a statistical size distribution and
a mean diameter.
XRD measurements were made using a Siemens D5000
diffractometer (Bragg–Brentano parafocusing geometry and
vertical q–q goniometer) tted with a curved graphite diffracted-
beam monochromator, incident and diffracted-beam Soller
slits, a 0.06ꢀ receiving slit and scintillation counter as a detector.
The angular 2q diffraction range was between 26 and 95ꢀ. The
ꢀ
the corresponding time at 80 C. The autoclave was then dep-
ressurised. The solution was ltered over silica and analysed by
gas chromatography. The conversion and the selectivities of the
product were determined using a Fisons instrument (GC 9000
series) equipped with a HP-5MS column.
Results and discussion
data were collected with an angular step of 0.05ꢀ at 16 s per step To evaluate the two model ligands PPh3 1 and P(OPh)3 2 as
and sample rotation. A low background Si(510) wafer was used stabilisers for Rh-NPs,8 the organometallic precursor Rh(h3-
as sample holder. Cuka radiation was obtained from a copper X- C3H5)3 was reduced in THF under 6 bar of hydrogen at 40 ꢀC and
ray tube operated at 40 kV and 30 mA.
in the presence of sub-stoichiometric amounts of these ligands
XPS experiments were performed in a PHI 5500 Multitechnique (Fig. 1).17 The amount of P-based stabilisers 1 and 2 was varied
System (from Physical Electronics) with a monochromatic X-ray from 0.1 to 0.6 equivalent per Rh to study the effect on the
source (Aluminium Kalfa line of 1486.6 eV energy and 350 W), formed systems.
placed perpendicular to the analyser axis and calibrated using the
Initially, the triphenylphosphine ligand 1 was used to syn-
3d5/2 line of Ag with a full width at half maximum (FWHM) of 0.8 thesise the series of NPs Rh1. The amount of ligand used during
eV. The analysed area was a circle of 0.8 mm diameter, and the the synthesis was varied using 0.1, 0.2, 0.4 and 0.6 equivalent
selected resolution for the spectra was 187.5 eV of pass energy and per Rh. However, in the presence of 0.1 equivalent of 1, no
0.8 eV per step for the general spectra and 23.5 eV of pass energy nanoparticles were formed. When the amount of ligand was
and 0.1 eV per step for the spectra of the different elements in the increased, the NPs Rh1-0.2, Rh1-0.4 and Rh1-0.6 were formed
depth prole spectra. A low energy electron gun (<10 eV) was used and isolated.
in order to discharge the surface when necessary. All measure-
Interestingly, the TEM micrographs of Rh1-0.2–Rh1-0.6
ments were performed in a ultra high vacuum (UHV) chamber revealed the formation of very similar nanoparticles with no
pressure between 5 ꢁ 10ꢂ9 and 2 ꢁ 10ꢂ8 torr.
signicant differences in size, shape and distribution since
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Rh, respectively (Fig. 1). It is noteworthy that in contrast with
the result obtained with PPh3 1, in the presence of 0.1 eq. of
ligand 2 per Rh, the stabilisation of small Rh-NPs was this time
efficiently achieved. This result can be explained by the stronger
p-acceptor character of the phosphite ligand, which increases
its coordinating properties for electron-rich metal centres. The
formation of the nanoparticles was conrmed by TEM micros-
copy and the corresponding micrographs and size distributions
are displayed in Fig. 3.
The triphenylphosphite 2 stabilised systems Rh2-0.1–Rh2-
0.6 exhibited similar size and shape with the detection of
spherical nanoparticles with diameters of ca. 1.6 ꢃ 0.3 nm.
Moreover, no difference in crystallinity nor packing (fcc) was
observed by XRD between the systems. Analysis by XPS again
showed that these Rh-NPs are mainly constituted by Rh0. The
systems were also analysed by TGA, revealing that when the
Fig. 1 Synthetic method used for the preparation of the Rh-NPs.
these three systems appeared as spherical NPs with a diameter
of ca. 1.5 ꢃ 0.2 nm (Fig. 2). The nanoparticles were charac-
terised by XRD analysis, that revealed crystalline systems with
fcc packing. XPS measurements demonstrated that these NPs
are mainly constituted by Rh in zero valent state. TGA analysis
of these species revealed that Rh1-0.2 contains 44% in weight of
phosphine while for the systems Rh1-0.4 and Rh1-0.6, the
values are similar (27% and 30% respectively). In view of these
results, no clear correlation could be established between the
amount of phosphine 1 used during the synthesis of the
nanoparticles and the weight losses observed by TGA.
When the triphenylphosphite ligand 2 was used as stabiliser,
the nanoparticles Rh2-0.1, Rh2-0.2, Rh2-0.4 and Rh2-0.6 were
obtained using 0.1, 0.2, 0.4 and 0.6 equivalent of ligand 2 per
Fig. 2 TEM micrographs and size distributions of the Rh1-0.2, Rh1-0.4 Fig. 3 TEM micrographs and size distributions of the Rh2-0.1, Rh2-
and Rh1-0.6 NPs.
0.2, Rh2-0.4 and Rh2-0.6 NPs.
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amount of phosphite 2 is increased from 0.1 to 0.6 equivalent
Therefore, when these NPs are placed under CO pressure,
during the synthesis of these NPs, an increase in ligand content several P-based species adsorbed at their surface are expulsed
was observed from 34% for Rh2-0.1, 42% for Rh2-0.2 and Rh2- from the NP surface and could be consequently detected by
0.4, and 53% for Rh2-0.6 NPs.
NMR in solution. The system Rh1-0.4 thus mainly contains
diphenylcyclohexylphosphine oxide adsorbed at its surface,
together with other phosphine oxides. In the case of Rh2-0.4,
both phosphite and oxidised phosphite species are adsorbed at
NMR studies
The nanoparticles Rh1-0.4 and Rh2-0.4 were analysed by solu- their surface.
tion NMR spectroscopy in d8-THF. For both systems, no signals
were detected when the nanoparticles were analysed by 31P
IR studies
NMR at room temperature, as previously described for solution
NMR analysis of ligand-stabilised M-NPs and explained by the Next, the nanoparticles stabilised with triphenylphosphine 1
inuence of parameters such as the Knight shi, the fast T2 were analysed by infrared spectroscopy using a KBr pellet (see
relaxation resulting from the slow tumbling of the particles in spectra in Fig. S22, ESI†). First, the C–H stretching vibrations
solution and to the surface anisotropy, as described in several zone (ca. 3000 cmꢂ1) was analysed and compared to that of the
reports.4,18
ligand 1. For the nanoparticles Rh1-0.2 (spectra b) only one set
However, when a 31P{1H} NMR spectrum of the solution of signals were detect at low frequencies (2900 cmꢂ1 < n < 3000
containing the system Rh1-0.4, was recorded at low temperature cmꢂ1) attributed to the alkyl n(C–H) signals. However, for the
(ꢂ80 ꢀC), a singlet phosphorus signal was detected at 43.3 ppm. system Rh1-0.4, a small signal attributed to aromatic n(C–H)
This resonance was attributed to diphenylcyclohexylphosphine was observed at higher frequencies (>3000 cmꢂ1). Similar
oxide and conrmed by GC-MS analysis. This observation is in pattern was observed for system Rh1-0.6 (spectra d), signals of
agreement with a recent report that demonstrated the adsorp- both alkylic and aromatic C–H bonds were observed. It was
tion of phosphine oxide at the surface of Ru-NPs by solid state therefore concluded hydrogenation of the phenyl rings of the
NMR spectroscopy.19 This result also indicated that O]PCyPh2 ligand had taken place during the synthesis of the NPs, in
is formed from PPh3 1 during the NP synthesis via hydrogena- agreement with the NMR results obtained with the NPs Rh1-0.4
tion of one of the aryl rings and oxidation of the phosphorus in solution. Furthermore, the intensity of the aromatic C–H
centre, or vice versa, presumably due to traces of water in the bond stretching signals increased when more ligand is used
solvent. The non detection of the phosphorus signal at RT also during synthesis, indicating a higher degree of hydrogenation
indicated that this phosphine oxide species is involved in at lower ligand to Rh ratio. These observations are in agreement
a dynamic process between the surface of the NPs and the THF with results reported for Ru-NPs stabilized by monodentate
solution. In order to gain information on the species adsorbed ligands.3 At ca. 2000 cmꢂ1, a very weak band was observed for
at the surface of Rh1, the sample was placed in a high pressure system Rh1-0.2. This signal was more pronounced in the spec-
sapphire tube and charged with 30 bar of CO pressure at room trum of Rh1-0.4 and Rh1-0.6. Such a frequency typically corre-
temperature. In the corresponding 31P{1H} spectrum, the signal sponds to carbon monoxide ligands coordinated in terminal
previously detected at low temperature was again observed at mode to a metal centre or a metal surface20 or a metal hydride
43.3 ppm but with higher intensity and together with three stretching. For instance, in a study on Rh/Al2O3 catalyst under
other singlet signals at 37.7, 27.4 and 21.6 ppm. Interestingly, H2, Worley and co-workers attributed a similar band at 2013
no signal for PPh3 1 was detected, which indicated that no cmꢂ1 to rhodium hydride species.21 To conrm the identity of
untransformed ligand remained aer the NP synthesis.
these species, the authors repeated the reaction under D2,
For the system Rh2-0.4 stabilised by triphenylphosphite 2, observing the disappearance of this band and the detection of
the low temperature (ꢂ80 ꢀC) spectrum showed a signal at ꢂ16 a new signal at 1441 cmꢂ1
.
ppm, assigned to triphenylphosphite oxide and indicated that
Therefore, a fresh sample of Rh1-0.4 was synthesised under
this species was adsorbed at the NP surface and is involved in D2 as instead of H2 and an IR spectrum of Rh1-0.4-D2 was
a dynamic process between the NP surface and the solution. recorded (see Fig. S23, spectrum c vs. a, ESI†). In this spectrum,
Aer exposure of the sample to 30 bar of CO pressure, new the intense band previously observed at 1973 cmꢂ1 was not
singlet signals at 1.15 and 7.02 ppm attributed to oxidised detected, clearly indicating that this stretching was arising from
derivatives of the stabilising ligand 2 were detected. Moreover, a Rh-hydride moiety. However, in our case, the displacement of
two very broad resonances centred at ca. 120 and 135 ppm were this band to lower frequencies could not be observed due to
also observed. In view of their chemical shis, these latter overlapping with other signals. It was therefore concluded from
signals were attributed to phosphite ligands and the broadness these observations that the signal observed at 1973 cmꢂ1 arises
of these signals could indicate a dynamic process involving the from the presence of Rh-H species at the surface of the NPs.
P centres of these ligands and the surface of the nanoparticles.
When the 4 sets of nanoparticles Rh2 stabilised by the tri-
These signals were attributed to partially hydrogenated phos- phenylphosphite 2 were analysed by IR spectroscopy, similar
phite ligands. It was therefore concluded that when the phos- results than with Rh1 were obtained.
phite 2 is used as stabiliser, the resulting nanoparticles are
Comparing the spectra of Rh2-0.1–Rh2-0.6, no relevant
covered by a mixture of partially hydrogenated phosphite and differences were observed, except for the C–H stretching
phosphite oxide ligands.
frequencies at ca. 3000 cmꢂ1 (Fig. 4). Similarly to the results
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a mixture of both oxidised and unoxidised phosphites are
adsorbed at the surface of Rh2. Moreover, a higher hydride
coverage is present at the surface of the phosphine stabilised
Rh1-NPs than at that of the phosphite stabilised Rh2 nano-
particles. It is noteworthy that these values are lower than those
reported for related P-ligand stabilised ruthenium nano-
particles of similar size (ca. 1.1 mol M-H/Ru surface atom).4
To gain further information on the surface of the rhodium
nanoparticles, freshly synthesised NPs were reacted in the solid
state with 1 atm of CO at room temperature and infrared (IR)
spectra of the sample in KBr pellets were recorded.
Initially, a series of samples of Rh1-0.4 were placed under CO
pressure during 2, 4, 6 and 12 h and the resulting samples
analysed by IR spectroscopy. The spectra of Rh1-0.4 exposed to 1
atm of CO aer 2, 4 and 6 h were very similar: two large
absorptions at 1870 cmꢂ1 and 2032 cmꢂ1 were observed and
attributed to bridging and terminal carbonyl ligands, respec-
tively, in agreement with previously reported measurements
(Fig. S25, ESI†).22 As no shi to higher frequency was observed
when the time of the CO exposure increased, it was deduced
that the total CO coverage had been reached during this time.23
However, aer 12 h of CO exposure, a new band at 2090 cmꢂ1
was detected. In previously reported studies on Rh surfaces,
such a high frequency band was attributed to one component of
the signal for geminal Rh–(CO)2, the other component being
Fig. 4 Selected region of the IR spectra of (a) P(OPh)3 2, (b) Rh2-0.1,
(c) Rh2-0.2, (d) Rh2-0.4, (e) Rh2-0.6.
described for PPh3 1 as stabilizers, complete hydrogenation of
the ligand 2 was indicated for Rh2-0.1 since only aliphatic C–H
stretching bands were detected. When higher amount of ligand
was used during the synthesis, the detection of both aromatic
and aliphatic C–H stretching bands suggested that the reduc-
tion of the ligand was only partial.
detected at ca. 2020 cmꢂ1 20,21,24 Here, this latter band could not
.
In agreement with the NMR results, these observations
demonstrated that total or partial hydrogenation of the ligand
takes place during the synthesis of both sets of nanoparticles
Rh1 and Rh2.
Moreover, the spectra of Rh2 exhibited a small and broad
band at 1999 cmꢂ1. When the NPs Rh2-0.4 were synthesised
under D2 and analysed by IR spectroscopy, the corresponding
spectrum did not exhibit such a band (see spectra in Fig. S24,
ESI†). This result thus indicated that rhodium hydrides are also
present at the surface of the nanoparticles Rh2. It is noteworthy
that the Rh-H stretching band had a lower intensity in the case
of Rh2-0.4 than for Rh1-0.4.
To quantify the amount of Rh hydride present at the surface
of Rh1 and Rh2, titration experiments were performed on these
nanoparticles following a reported methodology: freshly syn-
thesised samples of Rh1 and Rh2 were reacted with norbornene
and the quantity of norbornane produced was determined by
GC-MS.4 The results showed that in the case of the phosphine
stabilised systems Rh1, ca. 0.4, 0.8 and 0.1 mol of hydride per
Rh surface atom are present in the systems Rh1-0.2, Rh1-0.4 and
Rh1-0.6 respectively while for the phosphite stabilised nano-
particles Rh2, values between 0.15 and 0.3 mol of hydride per
Rh surface atom were obtained.
To summarise, the Rh2-0.4 NPs stabilised by the triphenyl-
phosphite ligand 2 exhibit similar properties than its phos-
phine counterpart Rh1-0.4 such as size, shape, and structure
packing. Furthermore, NMR and IR studies revealed that
hydrogenation and oxidation of the ligands take place during
the synthesis of both sets of NPs and that in both cases, Rh-H
species are present at the surface of the NPs. The Rh1-NPs
revealed to contain only oxidised ligands at their surface while
be detected due to the overlapping with the signal centred at
2032 cmꢂ1 and corresponding to terminal CO ligands. In order
to investigate the effect of CO pressure, the sample Rh1-0.4 was
exposed to 40 bar of CO during 12 h, depressurised and ana-
lysed by IR. The high frequency band reported for geminal Rh–
(CO)2 was slightly more intense and a shoulder at ca. 2000 cmꢂ1
was revealed under this conditions, conrming the assignment
of these signals (see Fig. S26, ESI†).
These results therefore indicated that CO adsorption on
various sites of the NPs Rh1-0.4 had taken place and evolved
with time. During the rst 6 h, coordination of CO in bridging
(faces) and in terminal mode (apexes and edges) at the surface
of the nanoparticles Rh1-0.4 was observed. At longer reaction
times, a geminal Rh(CO)2 stretching was detected, indicating
that adsorption of CO to form these groups is slow compared to
initial coordination of CO. It was also shown that the formation
of these species is favoured when the sample is exposed to
higher CO pressure although the small differences between the
spectra aer exposure to 1 and 40 bar of CO could indicate that
the formation of these species is not very sensitive to CO pres-
sure or that it is a reversible process and that in situ experiments
are needed to appreciate the corresponding changes.
The detected geminal Rh(CO)2 can be formed at the apexes
and edges of the nanoparticles. It is usually assumed that the
stabilising P-based ligands are situated on the apexes of
metallic nanoparticles, as demonstrated for the well charac-
terised Au55Cl6(PPh3)12.25 The Rh atoms in these positions
should thus not be able to coordinate to two molecules of CO.
However, the displacement of the stabilising agents by CO in
the solid state cannot be discarded.
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The nanoparticles Rh1-0.2 and Rh1-0.6 were placed under
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It should be noted that the band previously observed for Rh1-
CO pressure under the same previously described conditions 0.4 at ca. 2000 cmꢂ1 and attributed to Rh-H at the surface of the
for Rh1-0.4 and the results obtained for these systems were NPs could not be detected in these samples. This was attributed
compared with those for Rh1-0.4 NPs (Fig. 5).
to the removal of hydrides under CO exposure, as previously
In all these spectra, a broad band at ca. 1800–1900 cmꢂ1 was reported.13
attributed to bridging carbonyl and a more intense band at 2038
The same experiments were performed for the systems sta-
cmꢂ1 was related to terminal carbonyls. In the spectrum of Rh1- bilised with triphenylphosphite 2 ligand. For the Rh2-0.4
0.2, only these two bands were observed. However, at higher system, the effect of the CO exposure time was monitored at 2,
ligand/Rh ratio, the bands corresponding to Rh(CO)2 were 4, 6 and 12 h (Fig. S27, ESI†). Aer 2 h of CO exposure, two main
observed to increase in intensity. For Rh1-0.6, the bands for signals at 2035 and 2002 cmꢂ1 were detected and attributed to
these latter sites were clearly detected at 2090 and 2007 cmꢂ1
.
terminal carbonyl ligands, together with a very weak band at
Furthermore, at higher ligand/Rh ratio, the band for the 1830 cmꢂ1 corresponding to bridging COs. Under these condi-
bridging CO ligands was observed to shi to lower frequency tions, the signals for geminal Rh(CO)2 were not detected. At
(from 2038 cmꢂ1 for Rh1-0.2 to 2028 cmꢂ1 for Rh1-0.6) and to longer reaction time, the band for bridging carbonyl was
decrease in intensity. These observations could indicate that at observed to increase in intensity while those for terminal COs
higher ligand concentration during the NPs synthesis, the sta- remained unchanged. This result indicated that CO could rst
bilising ligands coordinate on the faces of the NPs, resulting in coordinate to the terminal positions, namely edges and apexes,
an increase in electron density of the system and a lower CO to later get displaced to bridging positions located at the faces,
coverage on these sites, both of which contribute to the which is in contrast with previous IR observations on Ru-NPs.26
lowering of the frequencies for these signals. The IR spectra of Rh2-0.1–Rh2-0.6 aer CO adsorption are
Comparing the spectra obtained for the systems Rh1-0.2, displayed in Fig. 6.
Rh1-0.4 and Rh1-0.6, a clear trend can be deduced: at higher
Interestingly, the CO bridging bands were found to shi to
ligand/Rh ratio, the coordination of the stabiliser on the faces of lower frequency when the ligand coverage increased, as ex-
these NPs and the formation of geminal Rh(CO)2 sites are fav- pected for an increase in electronic density of the surface. With
oured. The steric hindrance induced by the presence of the this system, a small band at ca. 2090 cmꢂ1 attributed to
ligands on the faces of the nanoparticles might hamper their a terminal Rh–(CO)2 stretching was only observed at low P-
coordination on the NP edges and thus enhance the formation coverage, indicating that the corresponding position is occu-
of geminal Rh(CO)2 sites when placed under CO.
pied by the ligand at higher P-content.
Next, the surface analysis of nanoparticles Rh1-0.4 by CO
This observation contrasts with the results obtained for the
adsorption-IR spectroscopy as a function of time and with system Rh1 and clearly indicates that the stabilizers 1 and 2 do
various equivalents of ligand 1 show that both the CO exposure not coordinate at the NP surface in the same manner.
(time and pressure) and the amount of ligand used during
synthesis inuence the surface environment of these NPs since
the formation of Rh(CO)2 sites was increased at longer expo-
sure, at higher pressure and at higher ligand/Rh ratio.
Catalytic experiments
The nanoparticles previously described were used as catalysts in
the hydrogenation of styrene at 80 C during 16 h. The results
ꢀ
are summarised in Table 1.
Fig. 5 Carbonyl region of the IR spectra of (a) Rh1-0.2, (b) Rh1-0.4 and Fig. 6 Carbonyl region of the IR spectra of systems (a) Rh2-0.1, (b)
(c) Rh1-0.6 after 12 h under CO atmosphere.
Rh2-0.2, (c) Rh2-0.4 and (d) Rh2-0.6 after CO adsorption.
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Table 1 Catalytic hydrogenation of styrene using the Rh-NPsa
The amount of stabilizer was varied during the synthesis
between 0.1 to 0.6 equivalent of ligand per Rh. No variation in
size nor shape were observed when this parameter was varied
but in both cases, a higher degree of hydrogenation of the sta-
bilising ligand was observed at low ligand to Rh ratio during the
synthesis. Furthermore, solution NMR experiments showed
that the Rh1 systems mainly contained adsorbed phosphine
oxide species at the surface whereas in the case of the phosphite
stabilized NPs Rh2, both phosphite and phosphite oxide species
could be identied.
Entry
NPs
% conv.b
% 4b
% 5b
1
2
3
4
5
6
7
Rh1-0.2
Rh1-0.4
Rh1-0.6
Rh2-0.1
Rh2-0.2
Rh2-0.4
Rh2-0.6
100
100
100
100
100
100
100
—
—
—
26
87
97
100
100
100
100
73
13
3
Hydride titration experiments were performed on these
systems, revealing a higher hydride coverage in the case of the
phosphine stabilized systems. Analysis of the surface of these
nanoparticles by infrared spectroscopy revealed the presence of
0
a Rh-H stretching band at 1950–2000 cmꢂ1
.
a
General conditions: 1.24 mmol of substrate, 2 mol% of Rh-NPs, 10 ml
of heptane, T ¼ 80 ꢀC, P ¼ 40 bar H2, t ¼ 16 h. b Determined by GC.
CO adsorption/infrared experiments were performed and
several conclusions can be drawn:
- Three types of carbonyl signals were observed: bridging
(1800–1900 cmꢂ1), terminal (1950–2070 cmꢂ1) and geminal
Rh(CO)2 (ca. 2100 and 2020 cmꢂ1), associated with the faces,
and the apexes and edges of the NPs, respectively.
In all experiments, full conversion of the substrate was
observed. However, the selectivity for ethylbenzene 4 or for the
fully hydrogenated product ethylcyclohexane 5 revealed to be
strongly affected by the ligand coverage on these catalysts.
When the reaction was carried out using the nanoparticles
stabilized with triphenylphosphine 1, only the complete
hydrogenation product ethylcyclohexane 5 was observed for all
the systems Rh1-0.2, Rh1-0.4 and Rh1-0.6 (entries 1, 2 and 3).
With the nanoparticles stabilized by triphenylphosphite 2,
the selectivity to ethylbenzene 4 was observed to steadily
increase when the ligand coverage on the catalyst was
increased. When system Rh2-0.1 was used 73% of product
ethylcyclohexane 5 was observed (entry 4), while only 13% and
3% of this product was detected using Rh2-0.2 and Rh2-0.4
nanoparticles, respectively (entries 5 and 6). However, 100%
selectivity towards the hydrogenation of the vinyl group was
reached when Rh2-0.6 was used, and only ethylbenzene 4 was
detected (entry 7). It was therefore concluded that at high ligand
coverage, the sites responsible for the hydrogenation of arenes
are occupied by the stabilizer and thus, only the vinyl unit of the
substrate can be hydrogenated.
- For the PPh3 system Rh1, at higher ligand/Rh ratio, the
coordination of the stabiliser on the faces is observed and the
formation of geminal Rh(CO)2 sites was favoured. The forma-
tion of these Rh(CO)2 species was also favoured when the
sample was exposed to higher CO pressure (40 bar). For the
systems stabilised by the triphenylphosphite 2, the opposite
trend was observed and increasing the amount of stabilising
agent caused the disappearance of the signal for geminal
carbonyls Rh(CO)2. It was concluded that at high ligand/Rh
ratio, the corresponding sites are occupied by the ligand.
All these systems were active catalysts in the hydrogenation
of styrene. The systems stabilised by triphenylphosphine 1
showed no selectivity as only the complete hydrogenation
product ethylcyclohexane 5 was detected using these systems.
However, for the triphenylphosphite 2 stabilised systems, the
selectivity towards the ethylbenzene 4 product increased at
higher ligand/Rh ratio, obtaining 100% selectivity using Rh2-0.6
nanoparticles. It was therefore concluded that the coordination
of the ligand at the site detected as Rh(CO)2 inhibits the
hydrogenation of the aromatic ring.
CO adsorption-infrared spectroscopy experiments showed
that the sites detected as Rh(CO)2 were occupied by the ligands
at high phosphite coverage while these sites were available for
CO adsorption at low ligand coverage. A correlation could
therefore be deduced between the availability of these sites of
the NPs and their ability to hydrogenate the aromatic ring of the
substrate.
Acknowledgements
´
We are grateful to the Spanish Ministerio de Economıa y
Competitividad for nancial support (CTQ2010-14938,
CTQ2013-43438-R, CTQ2013-46404-R and 2014 SGR 670,
Ramon y Cajal to C. G.)
Conclusions
Notes and references
All the nanoparticles synthesized in this work using triphenyl-
phosphine 1 and triphenylphosphite 2 as stabilisers exhibited
a small diameter (<2 nm) with narrow size distributions,
a spherical shape and a good dispersion. These crystalline
materials exhibit a fcc structure and are mainly constituted by
Rh in the zero valent state.
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