Selective confinement of ruthenium nanoparticles in silica nanotubes

Article abstract of DOI:10.1002/ejic.201300909

One hundred percent confinement of ruthenium nanoparticles inside silica nanotubes was reached for the first time in bulk by a simple synthesis strategy involving the use of carbon nanotubes as template. Carbon nanotubes (CNTs) are used as a template for the preparation of Ru nanoparticles confined in silica nanotubes (SiO₂NTs). The simple three-step procedure provides a useful tool for the preparation of confined metal or metal oxide nanoparticles. Copyright

Full text of DOI:10.1002/ejic.201300909

SHORT COMMUNICATION
DOI:10.1002/ejic.201300909
Selective Confinement of Ruthenium Nanoparticles in
Silica Nanotubes
T. Trang Nguyen,[a] Ovidiu Ersen,[b] and Philippe Serp*[a]
COVER PICTURE
Keywords: Confinement / Nanoparticles / Nanotubes / Ruthenium / Silica
One hundred percent confinement of ruthenium nanopar-
ticles inside silica nanotubes was reached for the first time in
bulk by a simple synthesis strategy involving the use of car-
bon nanotubes as template.
A few studies have been reported with the use of metal-
supported SiO2NT catalysts. Pt/CNTs were covered with
silica layers by hydrolysis of 3-aminopropyltriethoxysilane
and/or tetraethoxysilane (TEOS). The silica-coated Pt/CNT
showed high catalytic activity for electrochemical reactions
in aqueous H2SO4 electrolyte, despite the uniform coverage
of the Pt metal with silica layers.[10] Pt/CNT[11] and PtAu/
CNT[12] catalysts were covered by a porous SiO2 layer to
improve their thermal and electrochemical stability.
SiO2NTs were synthesized with CNTs as the template and
used as a support to prepare a ruthenium-based catalyst by
a slurry impregnation method. The Ru NPs were mainly
located inside the SiO2NTs. Compared to Ru/SiO2, the Ru/
SiO2NT catalyst exhibits higher activity for Fischer–
Tropsch synthesis.[13] Similar results were reported for Co/
SiO2NT catalysts.[14]
Introduction
The possibility to perform chemistry in nanoreactors, in
which the different phases are subject to confinement ef-
fects, has recently attracted great interest from the scientific
community.[1] The modified behavior of the confined phase,
including transport, adsorption, phase transitions, dif-
fusion, and structure, might directly affect the course of
chemical reactions, including hydrogen storage.[2] In the
case of supported catalysts, metal nanoparticles (NPs) can
be entrapped and stabilized within well-defined pores to
prevent aggregation. The orientation and configuration of
metal NPs can also be modified as a result of spatial restric-
tion, which could directly influence their catalytic activity
through interaction with reactants, intermediates, and/or
products. In addition to zeolites,[3] aluminosilicates,[3a]
mesoporous
silicates,[4]
and
carbon
nanotubes
In the present communication, we report a simple
method for the ultraselective confinement of Ru NPs inside
SiO2NTs. To the best of our knowledge, no example has
been reported for the 100% confinement of NPs in nano-
tubes. The synthetic strategy is outlined in Scheme 1. It con-
sists of: (1) the preparation of a Ru/CNT by wet impregna-
tion from a ruthenium precursor, (2) the deposition of a
silica coating on the Ru/CNT by hydrolysis of TEOS to
produce SiO2/Ru/CNT, and (3) a calcination step in air to
burn the CNT template, followed by reduction of the Ru
NPs to produce Ru@SiO2NT.
(CNTs),[1a,1b,5] silica nanotubes (SiO2NTs) represent a new
type of material with a well-defined pore structure.[6] Dif-
ferent strategies have been proposed to fill CNTs with metal
NPs, including two-step biphasic impregnation,[7] impreg-
nation and selective washing,[8] and molecular recogni-
tion.[9] However, none of these approaches allow 100%
selectivity to be reached in the confinement, particularly for
nanotubes with small internal diameters (Ͻ15 nm), which is
very important to accurately determine confinement effects.
Currently, the definition of efficient techniques for filling
small nanotubes at the bulk scale is still a synthesis chal-
lenge. As far as SiO2NTs are concerned, no selective ap-
proach for NP confinement has been reported up to now.
[a] Laboratoire de Chimie de Coordination UPR CNRS 8241,
composante ENSIACET, Université de Toulouse UPS-INP-
LCC
4 allée Emile Monso, BP 44362, 31030 Toulouse Cedex 4,
France
E-mail: philippe.serp@ensiacet.fr
http://www.lcc-toulouse.fr/lcc/spip.php?article265
[b] IPCMS – Groupe Surfaces et Interfaces, CNRS – ULP UMR
7504,
23 Rue du Loess, BP 43, 67034 Strasbourg, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300909
Scheme 1. Synthetic route for the production of ruthenium NPs
confined in SiO2NTs.
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plate (calcination at 650 °C, followed by reduction at
300 °C), showed a filling yield of 100% (Figure 2b). Metal
Results and Discussion
Given that a hydrophilic CNT surface is desirable for loadings, NP size, and filling yields are reported in Table 1.
good adherence of the SiO2 coating, we used nitric acid
oxidized multiwalled CNTs (SBET = 271 m2 g–1; exter-
nal diameter: 12.3 nm). HNO3 oxidation involved the initial
rapid formation of carbonyl groups, which were consecu-
tively transformed into phenol and carboxylic groups.[15]
HNO3 oxidation also increased the BET surface area and
pore volume as a result of CNT tip opening. The use of
CNTs as a template for the growth of SiO2NTs has already
been reported. SiO2NTs have been produced (1) by grafting
Figure 2. TEM micrographs of (a) Ru1/SiO2NT and (b) Ru1@
SiO2NT.
2-aminoethyl 3-aminopropyltrimethoxysilane on oxidized
CNTs,[16] (2) by treating acyl chloride functionalized CNTs
with 3-aminopropyltriethoxysilane,[17] (3) by hydrolysis of
TEOS by using polyethylene glycol functionalized CNTs,[18]
and (4) by a facile sol–gel process involving acid-oxidized
CNTs and hydrolysis of TEOS.[19]
In a first step, we validated that SiO2NTs were effectively
produced by TEOS hydrolysis on the oxidized CNT tem-
plate without Ru NPs on its surface. The as-produced
SiO2NTs were characterized by TEM (Figure 1), nitrogen
adsorption, and thermogravimetric analysis (TGA; Fig-
ure S1, Supporting Information). The internal diameter and
wall thickness of the SiO2NTs are in the 9–15 nm and 10–
12 nm ranges, respectively. The specific surface area of the
SiO2NTs is 129 m2 g–1; the porous volume is 0.59 cm3 g–1
and the majority of the pores range between 2 and 9 nm.
Table 1. Metal loading, NP size, and filling yield of the prepared
materials.
Sample
Ru loading NP size NP size
(% w/w)
Filling yield
(%)
(nm)[a]
(nm)[b]
Ru1/SiO2NT
Ru1/CNT
Ru1@SiO2NT 1.17
Ru2/CNT 1.90
Ru2@SiO2NT 0.37
Ru3/CNT 1.30
Ru3@SiO2NT 0.24
2.43
2.83
3.5
3.8
5.5
1.4
4.8
1.1
2.5
–
–
6.7
–
5.6
–
75
40
100
58
100
52
1.9Ϯ0.6 95
[a] From TEM. [b] From XRD.
For Ru1/SiO2NT, the mean particle size ranged between
2 and 6 nm (mean diameter 3.5 nm), and most of the NPs
were spherical, whereas for Ru1@SiO2NT, the mean par-
ticle size ranged between 5 and 8 nm with a mean diameter
of 5.5 nm (compared to 2–8 nm, mean diameter 3.8 nm for
the Ru1/CNT sample), and most of the NPs were faceted.
The presence of faceted nanoparticles on the Ru1@SiO2NT
sample should arise from a strong metal–support interac-
tion on the CNT support. The formation of faceted Ru
nanoparticles on the CNT surface has already been re-
ported.[20] The larger particle size obtained for the
Ru1@SiO2NT sample should also arise from the nature of
the support, SiO2 versus CNTs, and from the high-tempera-
ture calcination treatment. No severe aggregation of the
NPs was noticed, as was reported for Pt NPs.[10,21] XRD
analyses confirmed the presence of metallic ruthenium for
both reduced samples. The lower Ru loading obtained for
Ru1@SiO2NT relative to that obtained for Ru1/CNT
should be correlated to the fact that 40% of the Ru NPs of
Ru1/CNT are confined in the inner cavity of the CNTs. We
believe that these particles do not anchor on the SiO2NT
Figure 1. TEM micrographs of (a) SiO2NT/CNT, (b) SiO2NT, and
(c) internal and external diameters of SiO2NT.
In a second step, we checked that our approach led to surface during the calcination step.
better selectivity for confinement than that already reported
In a third step, given that the nature of the CNT/SiO2NT
for the slurry impregnation method.[13] Ruthenium-sup- interface should play a role in silica grafting, we studied the
ported catalysts were prepared from [Ru3(CO)12] on oxid- influence of the dispersion of the Ru NPs on the structure
ized CNTs or on preformed SiO2NTs. The sample prepared of the final material. Three 3%Ru/CNT samples were pre-
from the slurry impregnation method (reduction tempera- pared on oxidized CNTs by wet impregnation, from three
ture 300 °C), named Ru1/SiO2NT, presented 75% different Ru precursors, [Ru3(CO)12] (Ru1/CNT), RuCl3
of the Ru NPs inside the SiO2NTs (Figure 2a). The (Ru2/CNT), and [Ru(cod)(cot)] (Ru3/CNT; cod = 1,5-cy-
Ru1@SiO2NT sample, prepared by using CNTs as a tem- clooctadiene, cot = 1,3,5-cyclooctatriene). After reduction,
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the mean particle size (Figure S2, Supporting Information) 4–5 nm (1.9 nm from XRD). For the Ru1@SiO2NT and
ranged between 2 and 6 nm for Ru1/CNT (mean diameter Ru2@SiO2NT samples, 100% of the nanoparticles were
3.8 nm, 40% of confined Ru NPs), 1 and 3 nm for Ru2/ confined inside the silica nanotubes. For the Ru3@SiO2NT
CNT (mean diameter 1.4 nm, 53% of confined Ru NPs), sample, a few (Ͻ5%) small-diameter particles were also ob-
and Ͻ1.1 nm for Ru3/CNT (52% of confined Ru NPs). Af- served on the external walls of the SiO2NTs. The presence
ter silica deposition, the thickness of the SiO2 coating was of Ru NPs on the external surface of the SiO2NTs could be
10–12 nm for SiO2NT/Ru1/CNT with a homogeneous coat- due to diffusion/migration of small Ru/RuO2 clusters dur-
ing (Figure S2, Supporting Information). If the Ru disper- ing the calcination step. There also, the lower loading ob-
sion was increased, as for Ru2/CNT, the silica coating of tained for the SiO2NT-confined Ru NPs related to the Ru/
SiO2NT/Ru2/CNT was still homogeneous, but its thickness CNT samples should find its origin in the percentage of Ru
decreased to 6–9 nm (Figure S3, Supporting Information). NPs confined in the Ru/CNT samples.
Finally, in the case of SiO2NT/Ru3/CNT, the measured
SiO2 thickness ranged between 6 and 9 nm, but we occa-
sionally noticed a non-homogeneous coating (Figure S3,
Supporting Information). This phenomenon could be due
to the presence of a higher density of small Ru clusters on
the CNT surface that hinders the SiO2-shell–oxidized-CNT
interaction. Accordingly, a decrease in the C–O–Si bonds in
favor of the Ru–O–C bonds reduces the efficiency of silica
coating. After the calcination step at 650 °C to remove the
CNT template, all of the ruthenium was present as RuO2
(Figure S4, Supporting Information), as verified by XRD
of the three Rux@SiO2NT samples. Notably, such high-
temperature calcination may induce SiO2 segregation to the
surface of RuO2, which results in the formation of SiO2
patches.[22] The XRD patterns of the three Rux@SiO2NT
reduced samples (reduction temperature 350 °C) show
peaks at 2θ ≈ 22.5° (large) that correspond to SiO2NTs and
reflections at 38.7, 42.5, 44.2, 58.6, and 69.8° that corre-
spond to the (100), (002), (101), (102), and (110) planes of
metallic ruthenium, respectively, and these data are consis-
tent with a hexagonal crystal structure and establish the
presence of Ru0 (Figure S4, Supporting Information). Trace
amounts of RuO2 were also detected in the three samples.
This might be due to (1) air autocatalytic oxidation of the
smaller Ru NPs,[23] (2) SiO2-induced modification of the
surface of the RuO2 NPs,[22] or (3) a strong Ru–SiO2 inter-
action.[24] The Rux@SiO2NT samples were observed by
TEM to evaluate the confinement efficiency. These TEM
observations revealed another effect of Ru dispersion. As
the Ru dispersion increased inside the Rux@SiO2NT, TEM
observation under the electron beam was almost impossible,
as most of the Ru2@SiO2NT samples, and particularly the
Ru3@SiO2NT samples, were damaged under irradiation.
Figure 3. Cryo-TEM micrographs (–100 °C) of (a) Ru1@SiO2NT,
(b) Ru2@SiO2NT, and (c) Ru3@SiO2NT and typical slices through
the reconstructed volumes obtained by cryo-TEM tomography on
Indeed, the presence of numerous small NPs in the SiO2NT
induced the immediate collapse of the SiO2 structure under
irradiation to produce Ru NPs encapsulated inside SiO2
nanowires (Figure S5, Supporting Information). The for-
mation of such core–shell structures, which can function as
microcapsular-like reactors, can also be of interest for catal-
ysis applications.[25] Therefore, to evaluate the confinement
some
representative
agglomerates
of
(d) Ru1@SiO2NT,
(e) Ru2@SiO2NT, and (f) Ru3@SiO2NT.
Finally, to evaluate the accessibility of the confined Ru
efficiency, we performed 2D and 3D cryo-TEM observa- nanoparticles, we investigated their catalytic activity in the
tions (Figure 3). The dispersion of the three samples hydrogenation of cinnamaldehyde (20 bar H2, 100 °C). All
followed the order Ru3@SiO2NT Ͼ Ru2@SiO2NT ≈ the confined samples showed high activity, turnover fre-
Ru1@SiO2NT. The measured Ru particle size ranged from quencies ranging between 175 and 200 h–1, and selectivity
5 to 8 nm for the Ru1@SiO2NT (6.7 nm from XRD) and values of approximately 45% for hydrocinnamyl alcohol,
Ru2@SiO2NT (5.6 nm from XRD) samples and from 40% for hydrocinnamaldehyde, and 15% for cinnamyl
1.5 to 2.5 nm for Ru3@SiO2NT with few a particles at alcohol.
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Ex Situ Method: SiO2NT (50 mg) were dispersed in a solution of
[Ru3(CO)12] in acetone (5 mL, to achieve a theoretical value of
Conclusions
In summary, we have demonstrated a useful and effective 3 wt.-% Ru) by ultrasonication for 30 min. After ultrasonic treat-
ment, the mixture was stirred at room temperature until the acetone
had evaporated. The [Ru3(CO)12]@NTs samples were dried over-
night and then reduced at 300 °C for 2 h in a flow of H2/Ar.
method for the ultraselective confinement of ruthenium
nanoparticles in silica nanotubes. The key to a successful
coating consists in using Ru/CNTs as a template. We believe
that this approach can be extended to many metal nanopar-
ticles and will provide a useful tool for the research com-
munity whose interests involve confined metal or metal ox-
ide nanoparticles.
Characterization: Thermogravimetric analysis (TGA) in air was
conducted with a Setaram apparatus by using a temperature pro-
gram of 25–1000 °C with a heating rate of 10 °Cmin–1. X-ray dif-
fraction (XRD) was performed with a Panalytical MPD Pro pow-
der diffractometer at room temperature by using Cu-Kα radiation
(λ = 0.15418 nm). TEM images and electron tomography data were
acquired with a JEOL 2100F transmission electron microscope
with a field emission gun operating at 200 kV equipped with a
probe corrector and a GATAN Tridiem energy filter. Before obser-
vation, the powders were dispersed in ethanol by sonication, and
several droplets were deposited onto a cooper grid covered by a
carbon holey membrane. For the tomographic experiment, the tilt
series was acquired by tilting the specimen over a range of Ϯ60°;
an image was recorded every 2° in the Saxton mode. The acqui-
sitions were carried out at low temperature (about 100 K) to reduce
irradiation damage in the organic part of the layer during the total
duration of the acquisition process (1 h). The images of the tilt
series were initially aligned by using a cross-correlation algorithm.
Refinement of this initial alignment was obtained by considering
the centers of several Ru nanoparticles as fiducial markers. The
volume reconstructions were computed by using iterative algo-
rithms based on algebraic reconstruction techniques implemented
in the TOMOJ software, with a number of 20 iterations. Visualiza-
tion and quantitative analysis of the final volumes were done by
using ImageJ software.
Experimental Section
CNT Oxidation: Multiwalled carbon nanotubes (90% purity) were
supplied from the same batch by Arkema, France, under the trade-
mark Graphistrength®. The as-received CNTs were purified with
a mixture of concentrated H2SO4/water (1:1) at 140 °C for 3 h to
remove any residual metal catalyst. Then, the purified multiwalled
CNTs (2 g) were suspended in concentrated nitric acid (65%,
80 mL) and heated at reflux at 140 °C for 3 h. After cooling, the
CNTs were filtered, washed with distilled water until neutralization
of the filtrate, and dried in air at 110 °C for 2 d.
Preparation of Ru/CNT: Three different ruthenium precursors were
used and dissolved in appropriate solvents with a concentration
calculated so as to give a theoretical value of 3 wt.-% Ru:
[Ru3(CO)12] (Ru1) in acetone, RuCl3 (Ru2) in 2-propanol, and
[Ru(cod)(cot)] (Ru3) in heptane. To prepare the catalysts from
[Ru3(CO)12] and RuCl3, oxidized CNTs (200 mg) were dispersed in
a solution of the precursor (20 mL) by ultrasonication for 20 min.
After ultrasonic treatment, the mixture was stirred at room tem-
perature until the solvent had evaporated and then dried overnight
at 110 °C. For [Ru(cod)(cot)], the catalysts were prepared in a glove
box by impregnation: The oxidized CNTs (100 mg) were immersed
into a heptane solution (50 mL) of [Ru(cod)(cot)] and heated at
reflux at 70 °C for 24 h under an atmosphere of argon. After cool-
ing, the solid was filtered and dried overnight at 110 °C. For all of
the prepared catalysts, the dried solids were reduced at 300 °C for
2 h in a furnace under flowing H2/Ar (10% H2 v/v) to transform
the ruthenium precursor into the metal.
Supporting Information (see footnote on the first page of this arti-
cle): TGA curves, TEM micrographs, and XRD diagrams.
Acknowledgments
T. T. N. thanks the Université des Sciences et Technologie d’Hanoï
(USTH) Consortium and the Ministry of Education and Training
of Vietnam for her PhD grant. The authors acknowledge financial
support from the French Centre National de la Recherche Sci-
entifique (CNRS) (FR3507) and the Commissariat à l’Énergie Ato-
mique et aux Énergies Alternatives – Microscopie Electronique et
Sonde Atomique (CEA METSA) network.
Preparation of SiO2NT: The oxidized CNTs (50 mg) and a mixture
of ethanol (250 mL) and ammonium hydroxide (8 mL) were
charged into a 500 mL flask and mixed by ultrasonication for
30 min and stirring for 15 min. After dispersion, TEOS (6 mL) was
injected quickly into the flask, and the solution was vigorously
stirred at room temperature for 24 h. Upon completion of the reac-
tion, the composite was washed with ethanol to remove secondary
silica particles and dried overnight at 110 °C. Finally, SiO2NTs
were prepared by calcination of the silica-coated CNTs at 650 °C
for 2 h in air.
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Published Online: September 23, 2013
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