A sustainable aqueous route to highly stable suspensions of monodispersed nano ruthenia

Article abstract of DOI:10.1039/c1gc15769h

Highly stable suspensions of monodispersed ruthenia nanoparticles have been prepared via a sustainable aqueous oxidative pathway. The nanoparticles (2 nm) have been thoroughly characterized by TEM, XRD, XPS, MS-TGA and thermodiffraction. The addition of hydrogen peroxide in the RuCl₃ solution provokes a fast oxidation of Ru(iii) ions into Ru(iv). This increases the rate of the hydrolysis/condensation reactions and further promotes the nucleation over the growth of the particles. The very high stability conditions of the colloidal suspension have been studied. This aqueous one-step process, which uses no organic solvent or toxic pollutant additive, is quick and produces calibrated ruthenia nanoparticles in high yields. It presents a green alternative to the preparation and use of ruthenia. As examples, two applications are presented. In the first, RuO₂ coatings have been tested for their electrical capacitance. In the second, RuO₂/TiO ₂ catalysts, prepared from the controlled deposition of ruthenia nanoparticles on TiO₂ particles, have been proven to be highly effective for the production of methane from CO₂. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1gc15769h

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PAPER
A sustainable aqueous route to highly stable suspensions of monodispersed
nano ruthenia
a,b,c
a,b,c
a,b,c,d
d
Capucine Sassoye,*
Sophie Cassaignon,
Guillaume Muller,
Christian Pizarro,
Damien P. Debecker,
Alejandro Karelovic,
a,b,c
a,b,c
d
a,b,c
Patricio Ruiz and Cl e´ ment Sanchez
Received 29th June 2011, Accepted 9th August 2011
DOI: 10.1039/c1gc15769h
Highly stable suspensions of monodispersed ruthenia nanoparticles have been prepared via a
sustainable aqueous oxidative pathway. The nanoparticles (2 nm) have been thoroughly
characterized by TEM, XRD, XPS, MS-TGA and thermodiffraction. The addition of hydrogen
peroxide in the RuCl solution provokes a fast oxidation of Ru(III) ions into Ru(IV). This increases
3
the rate of the hydrolysis/condensation reactions and further promotes the nucleation over the
growth of the particles. The very high stability conditions of the colloidal suspension have been
studied. This aqueous one-step process, which uses no organic solvent or toxic pollutant additive,
is quick and produces calibrated ruthenia nanoparticles in high yields. It presents a green
alternative to the preparation and use of ruthenia. As examples, two applications are presented. In
the first, RuO
catalysts, prepared from the controlled deposition of ruthenia nanoparticles on TiO
have been proven to be highly effective for the production of methane from CO
2
coatings have been tested for their electrical capacitance. In the second, RuO
2
/TiO
2
2
particles,
2
.
Introduction
All properties exhibited by RuO
strongly dependent on the degree of crystallinity, texture, surface
area and degree of hydration of the ruthenium oxide phase.
As an example in electrochemistry, RuO
films exhibiting capacitance up to 1000 F g at 10 mV s
2
based nanomaterials are
Nowadays the need of advanced nanostructured materials is
critical. Potential applications of nanomaterials are numerous
in wide ranges of practical fields. Among them, electrochemical
energy storage/conversion device and catalysis have attracted
much attention. In particular, ruthenium oxide based nanoma-
terials are versatile and attractive candidates for various tech-
2
mesoporous thin
-1
-1
1
–4
5
were recently prepared from ruthenium peroxo-based sols and
19
amphiphilic block copolymers. The fine tuning of the precursor
solution and thermal treatment allowed the combined control
of the film thickness, mesoporosity and nanocrystallinity. These
characteristics explain the excellent capacitance of the materials.
Nanostructured catalysts must present high activity and selectiv-
ity and good stability over the cycles. This requires a controlled
and homogeneous size of the active particles. Materials with
high surface area and good particle dispersion are generally
required. In addition, the interactions between the particles and
the support must be sufficient to prevent sintering. Fine tuning
of the preparation process is therefore crucial.
6,7
8
nological applications such as supercapacitors, electrodes,
9,10
or catalysts. RuO
2
electrodes demonstrate interesting perfor-
1
1
mances for chlorine electrogeneration, water electrosplitting
into hydrogen or oxygen,
a catalyst, ruthenium and ruthenium oxide have shown high
performance in various reactions such as alkane combustion,
12,13
9,14
or CO oxidation in sensors. As
15
16
17,18
CO
2
methanation and HCl oxidation.
Moreover, thanks to
its high chemical stability in both acidic and alkaline media,
RuO can be used under a large range of conditions.
2
Several pathways have been used to process RuO nano-
2
materials with controlled morphologies and increased surface
areas. From the first syntheses (using the pyrolysis of ruthe-
a
UPMC Univ Paris 06, Chimie de la Mati e` re Condens e´ e de Paris, UMR
7
574, F-75005, Paris, France
20
nium(III) precursors, or the addition of strong base or water to
b
CNRS, Chimie de la Mati e` re Condens e´ e de Paris, UMR 7574, F-75005,
Paris, France
21
ruthenium precursors) to the hydrothermal synthesis, highly
agglomerated nanoparticles with broad size distribution have
c
Coll e` ge de France, Chimie de la Mati e` re Condens e´ e de Paris, UMR
7
574, 11 place Marcelin Berthelot, 75231, Paris cedex 05, France
been obtained. In order to better control RuO surface reactivity,
2
d
Institute of Condensed Matter and Nanoscience - MOlecules, Solids and
reactiviTy (IMCN/MOST), Universit e´ catholique de Louvain, Croix du
Sud 2/17, 1348, Louvain-La-Neuve, Belgium.
other synthetic pathways have been proposed, leading to porous
14
RuO
RuO
2
aerogels, mesoporous RuO
2
@SiO where connected
2
22,23
2
crystallites are trapped within SiO
nanorods, nanotubes and mesoporous powders. Recently,
2
aerogels,
RuO
2
E-mail: capucine.sassoye@upmc.fr; Fax: +33 1 44 27 15 04; Tel: +33 1
4
24
25
26
4 27 14 34
3
230 | Green Chem., 2011, 13, 3230–3237
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1
.6 nm dispersed RuO
2
particles have been prepared through
Crystalline planes are visible on the high resolution TEM
(HR–TEM) images (Fig. 2). For the non heated suspensions, the
a non-green synthetic route (use of toluene, pyridine etc.)
that requires several steps. RuO
catalytic activity in the oxidation of benzyl alcohol have been
prepared with the help of bifunctionnal ligands that act as
metallic Ru nanoparticle stabilizers and sol–gel promoters.
27
˚
˚
2
@SiO
2
nanocomposites with
measured planes (mostly distances of 1.86 A and 1.97 A) do not
correspond to any identified crystalline phase; the distances do
not match with those from crystalline ruthenia nor the precursor
28,29
RuCl , meaning that the particles only present a chlorinated pre-
3
These processes are complicated, need a strictly controlled
atmosphere, present several steps with unknown yields, are
energy/time consuming and use toxic or pollutant reactants.
Our study deals with the green preparation of highly stable
and well defined monodispersed aqueous colloidal suspensions
of small ruthenia nanoparticles. The high stability of this
suspension (several months without any other additive) allows
the further implementation of the nanoparticles in various
processes. To demonstrate this, two examples of applications
ruthenia structure. After 2 h of heating, HR–TEM images show
the 200 or 111 planes which correspond to anhydrous crystalline
˚
˚
ruthenia (2.25 A and 2.22 A). It is noteworthy that this high
organization is only seen on HR–TEM images after a certain
time of observation under the electron beam. It is reasonable to
put forward that a “pre-ruthenia” structure is initially formed,
which easily evolves towards well-crystallized RuO .
2
Under a less intense electron beam (Fig. 1), planes are not
detected, even after a few minutes of electron exposure, showing
low organization in the initial rutile structure, as is well known
for hydrous ruthenia. However, the crystallinity is detected
through large electron diffraction rings. The radii of the rings
are perfectly consistent with the 101, 200, 111 and 210 planes
are presented. RuO
suspensions were tested for their electrical capacitance. Nanos-
tructured RuO /TiO catalysts prepared from the controlled
deposition of the ruthenia particles on TiO were used in the
2
thin films prepared from the nano ruthenia
2
2
2
production of methane from CO
2
.
of ruthenia. This crystallinity is optimum for the sample heated
◦
2
h at 95 C and does not improve with a longer heating time,
meaning that the hydrous rutile structure is obtained from 2 h
of heating.
Results and discussion
Accordingly, the X-ray diffraction (XRD) patterns of the
particles heated for 2 h show two slight broad shoulders that
The particles are easily prepared from an aqueous solution of
◦
◦
RuCl
3
(heated from 30 min to one week at 95 C) to which an
are consistent with the 110 and 101 planes around 2q = 30 and
◦
excess of hydrogen peroxide solution is added. No addition of
alkaline solution is required. The solution turns immediately
from dark brown to dark black. After being heated at 95 C and
the 211 plane around 54 (Fig. 3a). After 6 h of heating the
peaks become more distinguishable, but still remain broad. This
broadness is related to the small size of the crystalline domain
and to the intrinsic disorder due to hydration. Indeed, ruthenia
can accommodate structural water (responsible for protonic
conduction and high capacity). This structural water disrupts the
3D rutile structure, destroying part of the connection between
◦
cooled at room temperature, the suspension remains stable. The
particles are so stabilized as colloids that even after 4 h of high
centrifugation, only partial recovery of the powder is possible
indeed the black supernatant remains black. The yield, estimated
from the weight of the recovered powder after centrifugation and
the RuO
crystallinity.
6
octahedra, inducing disorder, and thus lowering the
◦
31
calcination at 400 C, reaches 75% for 2 h of heating.
Transmission electron microscopy (TEM) grids have been
prepared right after the addition of hydrogen peroxide, and then
after 30 min, 2 h, 6 h and one week of heating. As highlighted
on TEM pictures (Fig. 1–2), the ruthenia particles are well
separated and monodispersed in size. The average diameter of
particles reaches 2.0 ± 0.2 nm. This size is somewhat comparable
to the one obtained with hydrothermal methods (from 1.6 to 2.6
Thermogravimetric analysis coupled to mass spectrometry
(MS–TGA) was performed under air with the particles obtained
from the 2 h heated suspension (Fig. 4). At first continuous and
slow weight loss is observed and attributed to the departure
of water molecules. Accordingly, thermodiffraction (Fig. 3b)
shows an increase in intensity and a narrowing of the rutile
diffraction peaks, which is consistent with an increase of the
crystallinity; the departure of the water molecules allows a better
organization of the rutile structure. When the particles obtained
21,30
nm).
No drastic evolution in the size of the particles with the
heating time has been observed.
◦
Energy dispersive X-ray analyses (EDX) on the non washed
grids show a dramatic decrease in the amount of chlorine
with the heating of the sol (Cl : Ru ratio from 1.0 with no
heating to less than 0.1 with 2 h of heating measured by EDX
analysis). XPS spectra on well washed powders coming from 2 h
heated suspensions show only 0.5 at.% of chlorine. The yield of
recovered powder increases from only 20% at 0 or 30 min to more
than 75% after 2 h and 92% after 6 h of heating. The suspension
remains stable after up to 5 h of heating. On the contrary,
the particles in suspension strongly agglomerate with almost
no reversibility past 6 h of heating. The supernatant becomes
perfectly clear and transparent. The apparent increase in the
centrifugation yield with the duration of synthesis is related to
the strong evolution of the aggregation state of the particles, and
thus their ability to be easily centrifuged.
from suspensions heated for 2 h at 95 C are further heated in
◦
◦
air at 300 C or 450 C, ruthenia 110 planes are clearly visible
in the TEM image (Fig. 5). In parallel, the particles grow and
◦
sinter (at 300 C, mainly 5 nm diameter particles mixed with a
few 50 ¥ 15 nm rods; at 450 C, elongated 20 ¥ 50 nm particles).
◦
As mentioned in the previous paragraph, the dark colloidal
suspensions (heated from 30 min to 5 h) are stable for up
to several months after being cooled at room temperature.
Concentration of the sol, up to 10 times is possible under
◦
vacuum at 40 C without any precipitation. TEM images of
the concentrated or re-diluted suspension clearly show that
the suspension remains colloidal, without any aggregation or
growth of the particles (Fig. 7). The measured pH after 2 h
of heating is 2 (and then does not evolve for months at room
temperature). When the pH of the suspension is increased by
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◦
Fig. 1 TEM images, size histograms and selected area electron diffraction (SAED) of as-synthesized RuO
a), 30 min (b); 2 h (c); 6 h (d).
2
·xH
2
O particles heated at 95 C: 0 min
(
NaOH addition, irreversible RuO
sedimentation start around pH 5 (see Fig. 6). The addition
2
agglomeration and then
The isoelectic point (IEP) has been estimated directly from a
2 h heated suspension. The intersection of the different titration
curves at the same point (pH 5) shows that the IEP is close
to 5. This value is in accordance with the ones established on
of AgNO
3
also leads to agglomeration, conjointly with AgCl
-
precipitation. Cl anions in the suspension seem to play a role
in the stability of the colloids. As silver chloride precipitates, it
is clear that at least some of the Cl anions are not trapped in
32
hydrous and anhydrous ruthenia. During calcinations, it has
-
◦
been shown that this value can increase to 6 (at 500 C).
the rutile structure. In view of further applications, this excess
of chlorine will advantageously be removed.
Hydrogen peroxide has the ability to form complexes with
most of the transition metal ions. It has been particularly
33
3
232 | Green Chem., 2011, 13, 3230–3237
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Fig. 2 High resolution TEM pictures of the (a) non heated and (b) 2 h
heated solution showing the organization that had occurred under the
intense electron beam: ruthenium oxo-chloride precursor of ruthenia
from the non heated solution (a) and rutile structure for the particles
from the 2 h heated solution (b).
34,35
well documented and studied for the titanium case.
More
than the ability to use the titanium(IV) peroxo complexes for
analytical application, peroxo ligands are widely used in order
to stabilize precursors. In the case of Ru, there is little literature
concerning the use of hydrogen peroxide. Few complexes have
36,37
been reported and always involve other organic ligands.
In
the literature, whenever hydrogen peroxide use is mentioned for
3
8
the synthesis of ruthenia, only gel or highly agglomerated
Fig. 4 Thermogravimetric and differential curves of the particles (from
39
◦
particles are obtained. In aqueous medium, most reported
syntheses of ruthenia involve the addition of a large amount of
basic solution, that leads to the precipitation of a black powder.
The oxidation of the precursor Ru(III) into Ru(IV) is never
questioned and it is assumed to take place during crystallization.
In our case, the large excess of hydrogen peroxide allows an
immediate oxidation of the ruthenium precursor. It seems to
allow fast hydrolysis/condensation rates. Since Ru(IV) is not
stable in aqueous solution, this fast nucleation leads to highly
dispersed hydrous ruthenia nanoparticles without any growth of
the particles. Performing the synthesis in the absence of hydrogen
peroxide (with all other parameters being the same) only leads
to a black solution. A longer heating of the solution (3 days) is
then needed to recover ruthenia which appears to be irreversibly
and highly agglomerated. Hydrogen peroxide is thus critical to
obtain highly dispersed nanoparticles.
2 h, 95 C suspensions) associated with the main results of the mass
◦
-1
spectroscopy showing the departure of water (5 C min , under air).
electrons and protons. On one hand, the hydrous region (mostly
corresponding to the surface) allows facile proton permeation
through the material for efficient charge storage. On the other
hand, the bulk crystalline anhydrous RuO network accounts
2
for the electronic conduction. High capacitances are the result
of the best compromise between both electronic and protonic
conduction channels. The charge storage mechanism on RuO2
based electrodes has been explained by the insertion of electrons
and protons at the surface of the material via the global
31
equation:
(IV)
+
-
(III)
Ru
O
a
(OH)
b
+ dH + de = Ru
Oa-d (OH)b+d
In order to evaluate the electrical capacity of the produced
◦
materials, one drop of ruthenia suspension (2 h, 95 C) was
Electrochemical properties of the particles
allowed to dry on a fluorinated tin oxide (FTO) substrate and
◦
Ruthenia is well known for its pseudo-capacitive behavior. This
behavior is related to the ability of ruthenia to conduct both
was further heated for 1 h at 150 C in air. The measured exposed
2
surface was 0.07 cm . Cyclic voltammetry measurements at
◦
Fig. 3 XRD patterns at room temperature (a) of the as-synthesized nanoparticles heated at 95 C from 0 min to 8 days, and thermodiffraction
◦
◦
patterns; (b) of the 2 h particles from 50 to 475 C. The main XRD pattern peaks are indexed as the ruthenia structure (ICDD n 40-1290). * stands
for the sample holder.
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◦
◦
Fig. 5 TEM pictures of the particles heated for 12 h in air at (a) 300 C and (b) 450 C.
of the current density–potential curves (J–E) reveals the perfect
electrochemical reversibility of the redox reactions. This is
characteristic of ruthenia; the measured capacity stands at 240
-
1
-2
F g . The surface capacity reached 33 mF cm , which is
25
higher than the latest results on nanotubes or pyridine-capped
27
ruthenia nanoparticles. Thin films can also be processed
from an ethanol/water colloidal suspension of RuO particles.
Field emission gun scanning electron microscope (FEG–SEM)
pictures show that the substrate is entirely covered by RuO
2
2
,
with some porosity. These films also present constant pseudo-
capacitive behavior over 20 cycles. These results clearly show
that the pseudo-capacitance, one of the well known properties
of ruthenia, is preserved in the present system. However, the
measured values are lower than the best published ones, but one
has to keep in mind that the shaping of the films has not been
optimized here. Further works will have to consider textural
aspects, possible association with other matrices or particles
and optimized thermal treatments. The high stability of the
ruthenia suspension will most probably promote those further
developments.
Fig. 6 (left) Volumetric titration of the 2 h heated solution by NaOH
.02 mol L- with different NaNO
1
concentrations: (a) 0.1, (b) 0.05 and
0
3
-1
(
c) 0.01 mol L ; (right) stable colloid suspensions after the addition of
NaOH until pH 4 and 5, and sedimentation of the particles at pH 6
and 9.
Catalytic behavior
In connection with current environmental concerns a decisive
Fig. 7 TEM picture from the 10 times concentrated RuO
rediluted RuO sol (b).
2
sol (a), and
challenge is to master the catalytic hydrogenation of CO
2
2
40
in methane. The development of highly active methanation
41–44
different scan rates exhibit a pseudo-capacitance signature
Fig. 8) which is stable over 20 cycles. The rectangular shape
catalysts is thus an actual field of research.
In this con-
(
text, supported Ru-based materials were already identified as
◦
Fig. 8 (a) Cyclic voltammetry of a one drop colloidal suspension deposited on a FTO substrate, heated for 1 h at 150 C in air; (b) cyclic voltammetry
◦
◦
of thin film heated at 200 C showing a good stability over 20 cycles; (c) FEG-SEM picture of the 200 C heated film.
3
234 | Green Chem., 2011, 13, 3230–3237
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1
promising catalysts and the formation of small nanoparticles
was pointed out as a criterion to obtain active and selective
Table 1 Catalytic methanation of CO
of CO (10 vol.%), H (40 vol.%) diluted in He at 200 C
2
. Feed: 20 mL min- composed
◦
2
2
16
catalysts. Catalysts were thus prepared from the as-synthesized
RuO nanoparticle suspensions via deposition on a commercial
TiO powder (Degussa P25; 80% anatase, 20% rutile) classically
Metal loading Reduction Activity
(wt%)
-2
-1 -1
2
Sample
conditions (10 molCH4 molRu s )
2
◦
45
Nano-Ru/TiO2 2.2
200 C, 2 h 1.17
used as catalyst support. The deposition is followed by
◦
◦
350 C, 1 h 0.70
calcination in air at 450 C, to yield a RuO
2
/TiO
2
catalyst.
◦
WI-Ru/TiO
2
2.0
2.0
2.0
200 C, 2 h 0.42
◦
Ruthenia nanoparticles were quantitatively transferred onto
the TiO particles. The supernatant was transparent and the
final Ru loading in the dried powder was equal to the nominal
WI-Rh/TiO
WI-Rh/TiO
2
200 C, 2 h 1.09
◦
2
350 C, 1 h 0.83
2
one (2.2 wt%), as verified by ICP-AES. The 2 nm-sized RuO
nanoparticles are distinguished on the surface of the TiO
2
2
2
A reference Rh/TiO
catalyst ) was prepared by WI and tested under the same
conditions. Its activity was similar to the activity reached by our
2
catalyst (a well-recognized methanation
42,46
particles (Fig. 9a). Even if the contrast between TiO and RuO
2
is low, it is clear that sintering appears negligible even after
nano-Ru/TiO catalyst. In both cases selectivity is 100%. Bear-
2
◦
calcination at 450 C (Fig. 9b).
ing in mind that Ru is much cheaper than Rh and considering
that the method presented here is not more complicated than
the WI method, we believe this new preparation route presents
indisputable competitive advantages.
Conclusion
A highly stable suspension of monodispersed ruthenia nanopar-
ticles has been prepared via a sustainable aqueous oxidative
pathway. The addition of hydrogen peroxide to the ruthenium
precursor is directly responsible for the fast oxidation of the
Ru(III) species. Hydrogen peroxide seems to increase the speed of
the hydrolysis/condensation reactions and promotes nucleation
over the growth of the particles. The as-synthesized particles, of
which diameters reach 2 nm, present low crystallinity, mainly
explained by the small particle size and the intrinsic disorder
due to structural water. This combination of partial crystallinity
and hydration is desired for electrochemical properties: thin
Fig. 9 TEM pictures of TiO
tions: heated 16 h in air at (a) 150 C and (b) 450 C. The 450 C sample
were treated under hydrogen for (c) 2 h at 200 C and (d) 1 h at 350 C
and tested for catalysis. The inset in (a) represents TiO
adsoption.
2
/RuO
2
powders under different condi-
◦
◦
◦
◦
◦
films prepared from ethanolic aqueous RuO suspensions have
2
2
before RuO
2
shown pseudo-capacitive behavior. Dehydration and further
calcination of the particles lead to anhydrous highly crystalline
ruthenia powder with increased particle size. In order to obtain
effective catalysts, a composite of ruthenia and titania has been
prepared. As expected, the interaction of these particles on a
titanium oxide surface is strong enough to prevent sintering
during calcination. The prepared titania-supported ruthenium
catalysts have proven to be highly effective for the production
Before reaction, the catalyst is reduced in situ under H
After a 2 h treatment at 200 C, ruthenium nanoparticles are
clearly visible on TEM images (Fig. 9c–d), and on the surface
2
.
◦
◦
of the TiO
2
particles (Fig. 9c). At 200 C, this nano-Ru/TiO
2
-
2
-1 -1
catalyst exhibits a specific activity of 1.2 ¥ 10 molCH4 molRu
Table 1). This corresponds to a CO conversion of 38%.
At this relatively low operation temperature, the selectivity
moles of methane formed/moles of CO reacted) is 100%.
No other compounds containing carbon was formed. The
superior activity of the nano-Ru/TiO catalyst over a reference
sample (WI-Ru/TiO , Table 1) prepared by conventional wet
s
(
2
of methane from CO
examples of applications presented here, the high stability
several months) of this suspension without any other additive
2
with 100% selectivity. Beyond the two
(
2
(
opens the door for multiple further uses.
2
2
Experimental section
impregnation (WI) is clearly demonstrated. The activity of
the new nanostructed catalyst is 3-fold as compared to the
All chemicals were available commercially and used without
further purification.
-
2
-1 -1
reference (0.42 ¥ 10 molCH4 molRu s ). It can be noted that
catalytic performance in the same order were obtained with
16
other Ru/TiO catalysts prepared by a sputtering method. The
2
Nanoparticle preparation
experimental conditions (activation, reaction temperature and
contact time) were however not strictly comparable.
When the reduction is carried out at 350 C for 1 h, the
A dark brown solution was made from 0.1308 g (0.63 mmol)
◦
of RuCl
stirring, a diluted solution of hydrogen peroxide (3 mL of 30%
aqueous H in 20 mL of H O) was added dropwise. The
3
dissolved in 40 mL of water. After one minute of
ruthenium particles sinter, form a heterogeneous layer on the
surface of the TiO
activity of 0.7 ¥ 10 molCH4 molRu
2
particles (Fig. 9d) and exhibit a lower specific
2
O
2
2
-
2
-1 -1
s
(Fig. 9c). This confirms
pH of the suspension was measured after 5 min of stirring
and had dropped from 2.4 to 2.1. The solution was then put
that small nanoparticles are the actual active species.
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◦
in an oven at 95 C, for a duration of 30 min to one week
Transmission Electronic Microscopy (TEM) images were ac-
quired with a FEI Tecnai 120 Twin microscope operating at 120
kV and equipped with a Gatan Orius CCD numeric camera
without stirring. After cooling, a black colloidal suspension
is obtained after up to 5 h of heating. At room temperature,
this suspension is stable for several months without further
stirring or additives. After 6 h of heating, black powder was
recovered from the bottom of a transparent solution. To verify
the influence of temperature, immediately after the addition of
hydrogen peroxide, a suspension was centrifuged after 10 min of
stirring, without heating.
to assess the dispersion, size and crystallinity of the RuO
2
nanoparticles. Samples were prepared by evaporating a drop
of aqueous diluted suspensions of the Ru particles on a carbon
coated copper grid.
High Resolution Transmission Electronic Microscopy (HR–
REM) images were taken on a JEOL 2010 operating at 200 keV,
equipped with a Gatan camera.
Thin film preparation
FEG-SEM images were taken from a field emission gun scan-
ning electron microscope (FEG–SEM Hitachi S4200) performed
on a broken piece of coated FTO substrate.
X-Ray Diffraction (XRD) measurements (10–80 ) were per-
formed using Cu Ka radiation in a Bruker D8 Advance
diffractometer equipped with a Lynx eye detector.
Temperature XRD measurements experiments were recorded
on a Panalytical X’PertPro diffractometer equipped with an
Anton Paar HTK1200N furnace.
◦
One drop (7.2 mL) of the ruthenia particle suspension (2 h, 95 C)
was allowed to dry on a FTO substrate and was further heated
◦
◦
for 1 h at 150 C in air, in order to allow quantification of the
2
RuO
2
. The measured exposed surface was 0.07 cm .
The black suspension was concentrated to obtain a
.08 mol L- concentration. An equal volume of anhydrous
1
0
ethanol was then added, with 0.061 g of Pluronic F127
HO(C O)106H) . The solution was cast
by dip-coating onto FTO or silicon wafers at a controlled
(
2
H
4
O)106(C
3
H
6
O)70(
C
2
H
4
Thermogravimetric analyses (TGA) were performed on a
Netzsch STA 409 PC thermobalance coupled with a QMS403C
analyzer for mass spectrometry. Solids were heated in an alumina
◦
-1
temperature (80 C) and controlled withdrawal speed (2 mm s ).
The as-synthesized brown films were immediately heated under
◦
-1
◦
crucible with a heating rate of 5 C min up to 800 C under air.
◦
◦
air at different temperatures (from 120 C to 300 C) for 30 min.
TEM grids from the concentrated water colloid solution and
from the ethanol/water solution showed that the particles were
still well separated.
Electrochemical properties
The RuO
2
thin films, or the deposited RuO drops were mounted
2
in a three-electrode cell. In the case of the films, part of the
2
Catalyst preparation
working electrode was masked so that an area of 28.3 mm was
in contact with the electrolyte solution. A Pt wire was used
as the counter electrode while Ag|AgCl|KClsat was used as
For the nano-Ru/TiO
prepared (2 h, 95 C) and cooled RuO
2
catalyst, an entire batch of previously
colloidal suspension
◦
2
◦
◦
the reference electrode (E = 0.197 V vs. SHE, -25 C). The
was added to TiO
2
P25 (4 g) from Degussa. The mixture was
-1
electrolyte was H
2
SO
4
(1 mol L in water). Solutions were
◦
stirred magnetically and heated for 16 h at 50 C. Water was
then evaporated at 50 C under vacuum (50 mbar). The black
powder was then calcined for 16 h at 450 C under air and
washed scrupulously 3 times in water.
de-aerated by argon bubbling for 5 min prior to experiments.
The cyclic voltammetry experiments were conducted with a
potentiostat galvanostat, Princeton Applied Research, 263A,
◦
◦
-1
-1
with a scan rate from 10 mV s to 50 mV s . All the CV curves
represent J–E; J being the current density (I divided by the
geometric surface of the electrode). Capacitance values were
calculated from the voltammetric profiles and normalized to
The WI-Rh/TiO
pared by the wet impregnation of TiO
solution of RhCl or RuCl for 18 h. Water was then evaporated
2
and the WI-Ru/TiO
2
catalysts were pre-
2
P25 with an aqueous
3
3
under reduced pressure. The catalysts were then dried and
RuO
the films.
2
·xH
2
O content for the drops, to the exposed surface for
◦
calcined in air at 450 C for 16 h.
RuO
2
catalytic test
Iso electric point (IEP). The IEP was determined by poten-
tiometric titrations of stable ruthenia suspensions, as obtained
after 2 h of heating at 95 C, meaning that chloride ions are
Catalytic tests were carried out using a quartz reactor (U-
shaped) with a 0.4 cm internal diameter. A section in the center
of the tube is expanded to a diameter of 1 cm, in which catalyst
◦
present. The ionic strength was adjusted by addition of NaNO
The sols were titrated with NaOH (0.02 mol L ) using Metrohm
tritrando 808 potentiograph.
3
.
-1
(200 mg) ( 200–315 mm particle size) was placed and supported
by a quartz frit. A thermocouple was in contact with the central
part of the catalyst bed and was used to measure and control
the temperature. The reaction was carried out at atmospheric
Inductively coupled plasma-atomic emission spectroscopy
(ICP-AES). Weight percentages of Ru and Ti were measured
by ICP-AES on an Iris Advantage apparatus from Jarrell Ash
-
1
pressure, after activation in a 30 mL min
H flow during 2 h at
2
◦
◦
◦
-1
◦
2
00 C or 1 h at 350 C (10 C min ). Afterwards, the reactor
was cooled to 50 C and the reaction mixture was admitted
Corporation. The materials were dried at 105 C prior to
◦
measurements.
(
20 mL min- composed of CO
1
2
(10 vol%), H (40 vol%) diluted
2
in He). The reactor was heated stepwise to 50, 100, 150 and
Acknowledgements
◦
200 C, holding 1 h at each temperature. Gas effluent concentra-
tions were determined using a gas chromatograph. All transfer
lines were maintained at 120 C to avoid water condensation.
D. P. Debecker thanks the FNRS for his post-doctoral researcher
position. A. Karelovic thanks the Comision Nacional de
◦
3
236 | Green Chem., 2011, 13, 3230–3237
This journal is © The Royal Society of Chemistry 2011

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Investigacion Cientifica y Tecnologica (Chile) for his PhD
student position. The two laboratories are partners of the
23 K. E. Swider, C. I. Merzbacher, P. L. Hagans and D. R. Rolison,
J. Non-Cryst. Solids, 1998, 225, 348–352.
4 D. Susanti, D.-S. Tsai, Y.-S. Huang, A. Korotcov and W.-H. Chung,
J. Phys. Chem. C, 2007, 111, 9530–9537.
2
“
Inanomat” IUAP network, sustained by the Service public
f e´ d e´ ral de programmation politique scientifique (Belgium).
Authors from UCL also acknowledge the Cost Action D41.
25 C.-C. Hu, K.-H. Chang, M.-C. Lin and Y.-T. Wu, Nano Lett., 2006,
6, 2690–2695.
2
2
6 S. H. Oh and L. F. Nazar, J. Mater. Chem., 2010, 20, 3834–3839.
7 Y. Lin, N. Zhao, W. Nie and X. Ji, J. Phys. Chem. C, 2008, 112,
1
6219–16224.
References
2
8 M. Tristany, K. Philippot, Y. Guari, V. Colliere, P. Lecante and B.
1
2
M. Armand and J. M. Tarascon, Nature, 2008, 451, 652–657.
Chaudret, J. Mater. Chem., 2010, 20, 9523–9530.
P. G. Bruce, B. Scrosati and J.-M. Tarascon, Angew. Chem., Int. Ed.,
29 V. Matsura, Y. Guari, C. Reye, R. J. P. Corriu, M. Tristany, S. Jansat,
K. Philippot, A. Maisonnat and B. Chaudret, Adv. Funct. Mater.,
2009, 19, 3781–3787.
2
008, 47, 2930–2946.
3
A. Seguatni, M. Fakhfakh, M. J. Vauley and N. Jouini, J. Solid State
Chem., 2004, 177, 3402–3410.
30 K.-H. Chang, C.-C. Hu and C.-Y. Chou, Chem. Mater., 2007, 19,
4
5
6
C. R. Sides and C. R. Martin, Adv. Mater., 2005, 17, 125–128.
A. T. Bell, Science, 2003, 299, 1688–1691.
2112–2119.
31 D. A. McKeown, P. L. Hagans, L. P. L. Carette, A. E. Russell, K. E.
Swider and D. R. Rolison, J. Phys. Chem. B, 1999, 103, 4825–4832.
32 S. Ardizzone, A. Daghetti, L. Franceschi and S. Trasatti, Colloids
Surf., 1989, 35, 85–96.
33 D. Bayot and M. Devillers, Coord. Chem. Rev., 2006, 250, 2610–2626.
34 G. Schwarzenbach, J. Muehlebach and K. Mueller, Inorg. Chem.,
1970, 9, 2381–2390.
35 J. Lu, L. P. Bauermann, P. Gerstel, U. Heinrichs, P. Kopold, J. Bill
and F. Aldinger, Mater. Chem. Phys., 2009, 115, 142–146.
36 M. M. T. Khan, M. A. Moiz and A. Hussain, J. Coord. Chem., 1991,
23, 245–256.
G. Li, Y. Fan, T. Zhang, T. Ge and H. Hou, J. Coord. Chem., 2008,
6
1, 540–549.
7
8
T. R. Jow and J. P. Zheng, J. Electrochem. Soc., 1998, 145, 49–52.
O. Delmer, P. Balaya, L. Kienle and J. Maier, Adv. Mater., 2008, 20,
5
01–505.
Z.-P. Liu, P. Hu and A. Alavi, J. Chem. Phys., 2001, 114, 5956–
957.
9
5
1
0 J. P. Popic, M. L. Avramov-Ivic and N. B. Vukovic, J. Electroanal.
Chem., 1997, 421, 105–110.
1
1
1
1 S. Trasatti, Electrochim. Acta, 1991, 36, 225–241.
2 E. R. Koetz and S. Stucki, J. Appl. Electrochem., 1987, 17, 1190–1197.
3 A. Mills, P. A. Duckmanton and J. Reglinski, Chem. Commun., 2010,
37 M. M. T. Khan, A. Hussain, K. Venkatasubramanian, G.
Ramachandraiah and V. Oomen, J. Mol. Catal., 1988, 44, 117–127.
38 J. Pagnaer, D. Nelis, D. Mondelaers, G. Vanhoyland, J. D’Haen,
M. K. Van Bael, H. Van den Rul, J. Mullens and L. C. Van Poucke,
J. Eur. Ceram. Soc., 2004, 24, 919–923.
4
6, 2397–2398.
1
4 K. Reuter and M. Scheffler, Phys. Rev. B: Condens. Matter Mater.
Phys., 2006, 73, 045433–045417.
5 J. Okal and M. Zawadzki, Appl. Catal., B, 2009, 89, 22–32.
6 T. Abe, M. Tanizawa, K. Watanabe and A. Taguchi, Energy Environ.
Sci., 2009, 2, 315–321.
7 N. Lopez, J. Gomez-Segura, R. P. Marin and J. Perez-Ramirez,
J. Catal., 2008, 255, 29–39.
8 C. Mondelli, A. P. Amrute, F. Krumeich, T. Schmidt and J. Perez-
Ramirez, ChemCatChem, 3, 657–660.
1
1
39 K.-H. Chang and C.-C. Hu, J. Electrochem. Soc., 2004, 151, A958–
A964.
40 S. K. Hoekman, A. Broch, C. Robbins and R. Purcell, Int. J. Green-
1
1
1
2
2
2
house Gas Control, 2010, 4, 44–50.
41 S. Sharma, Z. Hu, P. Zhang, E. W. McFarland and H. Metiu, J. Catal.,
2011, 278, 297–309.
42 S. Eckle, H.-G. Anfang and R. J. Behm, Appl. Catal., A, 2011, 391,
9 C. Sassoye, C. Laberty, H. Le Khanh, S. Cassaignon, C. Boissiere, M.
Antonietti and C. Sanchez, Adv. Funct. Mater., 2009, 19, 1922–1929.
0 G. Lodi, C. De Asmundis and P. F. Rossi, Mater. Chem., 1977, 2,
325–333.
43 V. Jimenez, P. Sanchez, P. Panagiotopoulou, J. L. Valverde and A.
Romero, Appl. Catal., A, 2010, 390, 35–44.
44 M. Jacquemin, A. Beuls and P. Ruiz, Catal. Today, 2010, 157, 462–
466.
45 D. P. Debecker, C. Faure, M.-E. Meyre, A. Derre and E. M.
Gaigneaux, Small, 2008, 4, 1806–1812.
46 A. Beuls, C. Swalus, M. Jacquemin, G. Heyen, A. Karelovic and P.
Ruiz, Appl. Catal., B, 2011, DOI: 10.1016/j.apcatb.2011.02.033.
1
03–108.
1 K.-H. Chang and C.-C. Hu, Electrochem. Solid-State Lett., 2004, 7,
A466–A469.
2 J. V. Ryan, A. D. Berry, M. L. Anderson, J. W. Long, R. M. Stroud,
V. M. Cepak, V. M. Browning, D. R. Rolison and C. I. Merzbacher,
Nature, 2000, 406, 169–172.
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