Chemoselective reduction of heteroarenes with a reduced graphene oxide supported rhodium nanoparticle catalyst

Article abstract of DOI:10.1039/c8cy01046c

Rhodium nanoparticles immobilized on reduced graphene oxide were obtained from the microwave-induced thermal decomposition of Rh₆(CO)₁₆ in the ionic liquid [bmim][BF₄] (bmim = 1-butyl-3-methylimidazolium cation) in the absence of additional stabilizing agents. The resulting rhodium nanoparticles are <4 nm in diameter and are evenly dispersed on the reduced graphene oxide. The composite was evaluated as a catalyst for the chemoselective hydrogenation of quinoline and other heteroarenes containing N- and O-heteroatoms. A high selectivity for the heterocyclic ring was achieved, typically >99%, without interfering with other reducible groups, and with high conversions. Related catalysts prepared using conventional thermal heating were prepared for comparison purposes and were found to be considerably less active.

Full text of DOI:10.1039/c8cy01046c

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Chemoselective reduction of heteroarenes with a
reduced graphene oxide supported rhodium
nanoparticle catalyst†
Cite this: DOI: 10.1039/c8cy01046c
Alena Karakulina, Aswin Gopakumar,
Zhaofu Fei
and Paul J. Dyson
*
Rhodium nanoparticles immobilized on reduced graphene oxide were obtained from the microwave-
induced thermal decomposition of Rh IJCO)16 in the ionic liquid [bmim]ijBF ] (bmim = 1-butyl-3-
6
4
methylimidazolium cation) in the absence of additional stabilizing agents. The resulting rhodium nano-
particles are <4 nm in diameter and are evenly dispersed on the reduced graphene oxide. The composite
was evaluated as a catalyst for the chemoselective hydrogenation of quinoline and other heteroarenes
containing N- and O-heteroatoms. A high selectivity for the heterocyclic ring was achieved, typically
Received 22nd May 2018,
Accepted 20th August 2018
>
99%, without interfering with other reducible groups, and with high conversions. Related catalysts pre-
DOI: 10.1039/c8cy01046c
pared using conventional thermal heating were prepared for comparison purposes and were found to be
considerably less active.
rsc.li/catalysis
1
4,15
16
chemical sensors,
energy-related devices,
electrodes for fuel cells and other
Introduction
1
7–19
and as catalysts for hydrogenation
2
0
11,21
The direct and selective hydrogenation of nitrogen containing
aromatic compounds, particularly quinolines, remains a chal-
lenging reaction although numerous homogeneous and
reactions and transformations of bio-based substrates.
The general route used to obtain metal NP–graphene com-
posite materials involves the simultaneous reduction of metal
salts adhered to or in proximity to the graphene oxide sur-
face. Beller and co-workers reported the preparation of cobalt
1
,2
heterogeneous catalysts have been reported. The develop-
ment of novel heterogeneous systems able to operate under
mild conditions is of significant interest despite their high
vulnerability to poisoning by adsorbed hydrogenated quino-
line intermediates. A range of heterogeneous catalysts, i.e.
nanoparticles on a nitrogen-doped graphene and Al
2 3
O sup-
port and used the nanocomposite as a catalyst for the selec-
2
2
tive hydrogenation of heteroarenes.
Subsequently, they
3
metal nanoparticles (NPs) supported on dendrimers, metal
reported an iron–nitrogen-doped graphene/core–shell catalyst
4
,5
6
7
oxides,
silica hollow nanospheres, titania, and meso-
and applied it in the oxidative dehydrogenation of N-hetero-
8
23
porous carbon nitride have been evaluated in the hydrogena-
tion of quinoline compounds.
cycles. The nanocomposite was obtained at high tempera-
tures (up to 800 °C) and required ligands such as 1,10-
phenanthroline. Rh NPs derived from RhIJNO3)3 and
supported on rGO have been used to catalyze the hydro-
Carbon materials with large specific surface areas are par-
ticularly attractive supports for nanoparticle catalysts, and, in
the last decade, graphene has received special attention due
to its two-dimensional structure and outstanding electronic
2
4
formylation of 1-hexene.
Microwave induced thermal decomposition of metal pre-
9
and mechanical properties. Graphene oxide possesses reac-
cursors in ionic liquid (IL) media represents a promising vari-
1
0
25,26
tive oxygen-containing functional groups on the surface,
ation of this approach,
as ILs efficiently absorb micro-
2
7,28
which enhance the polarity of the material, thus promoting
its use in liquid-phase reactions. The oxygen-containing
groups can act as nucleation sites that anchor metal ions to
waves due to their high dielectric constants.
The
microwave-assisted IL method combines the advantages of
traditional microwave synthesis (fast and energy efficient)
and IL solvents (microwave absorbers and structure-directing
1
1–13
facilitate the dispersion of metal NPs.
Graphene-
2
9
supported metal NP composite materials have been used as
agents). Metal carbonyl complexes are known to decompose
under microwave heating in ILs yielding metal NPs with rela-
3
0,31
tively small and uniform sizes.
Starting from Rh IJCO) ,
6 16
the only side product is CO, no additional ligands are re-
quired and no salt by-products are formed. This approach has
been used to prepare metal and metal oxide particles and
Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de
Lausanne (EPFL), CH-1015 Lausanne, Switzerland. E-mail: paul.dyson@epfl.ch
†
Electronic supplementary information (ESI) available: Additional characteriza-
3
2,33
tion details. See DOI: 10.1039/c8cy01046c
biomass-based nanocomposites.
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Herein, we describe the application of microwave induced
thermal decomposition of a discrete hexarhodium cluster,
Rh IJCO)16, in an ionic liquid to afford Rh NPs immobilized
rapid microwave heating allows improved morphological con-
3
3,40
trol of NPs compared to conventional heating.
Thus, the
6
Rh IJCO)16–rGO suspension in [bmim]ijBF ] was irradiated with
6
4
on reduced graphene oxide (rGO). The resulting composite
material is an excellent catalyst for the selective reduction of
quinolines and other heteroarenes.
microwaves (20 W) for 6 minutes to afford Rh NPs of ca. <4
nm in diameter that adhere to the rGO surfaces (Fig. 1). The
Rh NPs are dispersed over the rGO sheets although in some
regions the density of the Rh NPs is high, but nonetheless
they remain dispersed.
Results and discussion
The Rh content in the Rh NP–rGO material was deter-
mined by atomic adsorption spectroscopy (AAS) after washing
the suspension with water in order to remove the IL. The Rh
content was found to be 18.31% (similar to that of the mate-
rial obtained by conventional thermal decomposition,
3
4
The rGO substrate was prepared using a literature method
see the ESI†) and characterized by transmission electron
(
microscopy (TEM) (Fig. S1, ESI†). The rGO sheets comprise
large stacked sheets and possess typical corrugation/wrinkles,
which are characteristic of this material.
22.14%) and in good agreement with the value of 19.6%
35,36
The surface area
obtained from XPS analysis (Fig. 2). XPS was also used to ex-
plore the surface oxidation state of the Rh NPs in the Rh NP–
rGO composite prepared via microwave treatment (Fig. 2).
The survey spectrum indicates the presence of Rh, C and O
in the composite material (Fig. 2). The Rh 3d XPS spectrum
contains two peaks corresponding to the Rh 3d5/2 and 3d3/2
states from spin–orbital splitting. Each peak was fitted using
the doublets corresponding to zero-valent and oxidized rho-
dium. Rh 3d5/2 can be deconvoluted into two distinguishable
peaks with different binding energies, i.e. 307.20 eV and
of the rGO was determined by nitrogen adsorption, with the
nitrogen adsorption–desorption isotherm corresponding to
3
7
type IV and possessing a hysteresis loop and a specific sur-
2
−1
face area of 470 m
g
(Fig. S2, ESI†). X-ray photoelectron
spectroscopy (XPS) was used to determine the degree of reduc-
tion of the rGO sheets, revealing the presence of the expected
38
type and abundance of surface groups (Fig. S3, ESI†).
The rGO sheets and the hexanuclear carbonyl cluster,
IJCO)16, were dispersed in the IL [bmim]ijBF ] (bmim =
-butyl-3-methylimidazolium cation) without any stabilizing
Rh
6
4
1
308.18 eV. The value of 307.20 eV of the curve-fitted Rh 3d5/2
additives. Conventional thermal decomposition and micro-
wave dielectric heating were applied to generate the Rh NP–
rGO composites from the suspension as shown in Scheme 1.
TEM analysis shows that the Rh NP–rGO composite pre-
pared using conventional thermal decomposition contains
Rh NPs that are not uniformly dispersed on the rGO as well
as free Rh NPs present in the suspension (Fig. S4, ESI†). The
Rh NPs on the surface of the rGO sheets have a mean diame-
ter of 20 nm (for the NPs on the surface of the reduced
graphene oxide). The prolonged thermolysis of NP precursors
in the presence of an IL and rGO has previously been shown
component unambiguously corresponds to metallic
38,41
Rh(0).
The other curve fitted to the deconvoluted Rh 3d5/2
peak is centered around 308.18 eV. Since the peaks in the re-
gion 307.6–309.6 eV are assigned to RhIJI), and the peaks be-
24,42
tween 308.8–311.3 eV are attributed to RhIJIII),
the binding
energy at 308.18 eV may correspond to rhodium in the RhIJI)
and RhIJIII) oxidation states.
A similar assignment applies to the Rh 3d3/2 peak. From
the curve-fitting procedure, the fraction of rhodium oxide in
the sample was estimated to be around 40%. The formation
of an oxide layer on the surface of the Rh NPs is indirectly
supported by the increase in the oxygen content after deposi-
tion of Rh NPs on the rGO surface. The oxygen content of the
rGO sample, evaluated by XPS, was found to be 11.1% and in-
creased to 26.1% in the Rh NP–rGO composite, presumably
3
3,39
to result in the formation of large metal particles,
and
4
3,44
due to a surface oxide coating on the Rh NPs.
The pow-
der X-ray diffraction (XRD) patterns of the Rh NP–rGO
Fig. 1 TEM images of the Rh NP–rGO material comprising Rh NPs
supported on rGO obtained by microwave heating (a) with the
enlarged region showing a high-density region (inset of a) (b) and the
corresponding Rh NP size distribution (c).
Scheme 1 Illustration of the route used to prepare the Rh NP–rGO
composites applying conventional thermal and microwave dielectric
heating methods.
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5). Note that the related composite prepared by thermal
heating is less active, i.e. under the same conditions it af-
fords the 1,2,3,4-tetrahydroquinoline product in 74% yield, al-
beit with equally high selectivity. The yield and selectivity
achieved with the Rh NP–rGO catalyst are comparable with
those of Rh NPs in Lewis acidic ionic liquids that are sensi-
5
0
tive to moisture and N-graphene-modified cobalt nano-
22
particles (Co O –Co/NGr@α-Al O )
which operate under
3
4
2 3
harsher conditions.
The substrate scope of the Rh NP–rGO catalyst, obtained
by the microwave-assisted method, was explored under mild
reaction conditions (Table 2). Quinoline derivatives were
converted to their respective 1,2,3,4-tetrahydroquinolines in
high yields of up to 87% (Table 2, entries 1–7). Only the re-
duction of the heteroarene ring was observed. The functional
group tolerance of the catalyst was studied using some
challenging substrates with various functional groups at-
tached to different parts of the heteroarene structure. The
hydrogenation of hydroxyl, aldehyde and amine substituted
quinolines using the Rh NP–rGO composite gave the corre-
sponding products in high yields and reduction of the func-
tional group was not observed. Notably, in the presence of an
aldehyde group, which is highly reactive towards reduc-
Fig. 2 XPS survey spectrum of the Rh NP–rGO composite obtained by
microwave irradiation (top) and Rh 3d spectrum (bottom).
composite prepared via microwave treatment contain the
characteristic reflections corresponding to the (111), (200),
51,52
(
220), (311) and (222) planes of crystals of metallic Rh (Fig.
tion,
only the heteroarene ring was hydrogenated (Table 2,
4
5–47
S5, ESI†).
The minor signals in the F 1s region at lower
entry 5), demonstrating the excellent chemoselectivity of the
catalyst. Indeed, the presence of electron donating or with-
drawing substituents on the aromatic ring does not affect the
selectivity of the reaction.
energy possibly correspond to the interactions of the anion
4
8,49
in the IL with the Rh NP surface as described elsewhere.
The hydrogenation of pyridine derivatives, which is im-
portant for the chemical and pharmaceutical industries,
usually requires harsh reaction conditions. The Rh NP–rGO
Evaluation of the catalytic activity of the Rh NP–rGO composite
The Rh NP–rGO composite, prepared using microwave dielec-
53
tric heating, was re-dispersed in [bmim]ijBF ] and evaluated
4
catalyst is able to hydrogenate pyridine and methyl-
as a catalyst in the hydrogenation of quinoline at a loading of
substituted pyridines at 80 °C under 10 bar of H
the corresponding piperidine derivatives in excellent yields
Table 2, entries 8–10). Indole can also be hydrogenated into
,2,3,4-tetrahydroindole in 72% yield under mild conditions
2
affording
1
mol% (based on Rh). The selective reduction of the hetero-
aromatic ring of quinoline to afford 1,2,3,4-
tetrahydroquinoline takes place in quantitative yield under 30
(
1
bar of H at 80 °C (Table 1, entry 1). By extending the reac-
2
(
Table 2, entry 11).
tion time, 1,2,3,4-tetrahydroquinoline could be obtained
quantitatively at a lower pressure of 10 bar H (Table 1, entry
2
The Rh NP–rGO catalyst was also assessed in the hydroge-
nation of oxygen containing heterocycles. The reduction of
furan to tetrahydrofuran in quantitative yield was achieved at
room temperature (Table 3, entry 1). NMR spectroscopy and
ESI-MS of the reaction mixture before and after the catalysis
indicate that the IL is stable (Fig. S6–S10, ESI†). The Rh NP–
rGO catalyst was also evaluated in water and glycerol. The
yield remains higher in these solvents for N-heterocyclic sub-
strates (Table S1, ESI†).
Table 1 Optimization of the Rh NP–rGO catalyzed hydrogenation of
quinoline to 1,2,3,4-tetrahydroquinoline in [bmim]ijBF
4
] (note that no
a
other products were observed)
b
Entry
Pressure (bar)
Time (h)
Yield (%)
The utility of the catalyst was evaluated in the hydrogena-
tion of complex molecules, i.e. furanochromone and
furanocoumarin derivatives. Visnagin, 8-methoxypsoralen and
1
2
3
4
5
6
7
8
30
20
10
1
10
10
10
10
12
12
12
12
24
6
>99
98
93
6
>99
75
2
khellin were efficiently converted under 10 bar of H at 80 °C
with the respective products obtained in high yields (Table 3,
entries 2–4). Remarkably, only the unsaturated CC bonds
in the two heterocyclic rings in visnagin are reduced
affording tetrahydrovisnagin in 94% yield, and the catalyst is
completely selective towards the aromatic ring in the hydro-
genation of other complex molecules.
4
1
67
38
a
Reaction conditions: Rh NP–rGO (1 mol% in [bmim]ijBF
4
]),
b
substrate (1.0 mmol), 80 °C. GC yield.
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Table 2 Evaluation of the substrate scope of the Rh NP–rGO catalyst in
Table 3 Hydrogenation of oxygen containing heterocycles using the Rh
a
a
[
bmim]ijBF
4
]
4
NP–rGO catalyst in [bmim]ijBF ]
b
b
Entry
1
Substrate
Product
Yield (%)
Entry
1
Substrate
Product
Yield (%)
c
86
71
75
99
2
3
4
94
92
60
2
3
Visnagin
c
4
5
59
8-Methoxy psoralen
c
87
Khellin
Reaction conditions: substrate (1 mmol), Rh NP–rGO (1 mol%),
c
6
7
80
a
b
c
[
4 2
bmim]ijBF ] (2.0 mL), H (10 bar), 80 °C, 36 h. Isolated yield. 25 °C.
76
c
8
9
99
sphere in a glovebox or using Schlenk techniques. Microwave
synthesis was carried out using a laboratory microwave sys-
tem CEM Discover. Transmission electron microscopy (TEM)
of the composites on a Ted-Pella standard carbon-coated cop-
per grid (methanol was used to disperse the composites) was
performed on an FEI Talos F200S electron microscope operat-
ing at 300 eV. The obtained micrographs were processed
using Fiji software. Surface area analysis was performed with
c
78
c
1
1
0
1
82
d
72
a
Micromeritics 3Flex instrument. The samples were
−
1
outgassed at 3 mTorr and 120 °C (2 °C min ) for 4 h prior to
analysis. The Rh content in the graphene-supported rhodium
catalysts was determined by atomic absorption spectroscopy
a
Reaction conditions: Rh NP–rGO (1 mol% in [bmim]ijBF
4
], 2.0 mL),
b
c
d
substrate (1.0 mmol), H
2
(10 bar) 80 °C, 15 h. GC yield. 8 h. 70 h.
(
AAS) using a Shimadzu AA-6650 spectrometer with an air–
acetylene flame from the diluted solutions in aqua regia (1/3
v/v HNO /HCl). The sample (10 mg) was digested in hot aqua
3
The recyclability of the Rh NP–rGO catalyst was studied as
catalysts typically used in the hydrogenation of quinolines
are often difficult to recycle due to poisoning by reaction in-
termediates. The reaction time and conditions for the
recycling experiments were optimized for the reduction of
quinoline over several runs (Fig. 3), with only a slight loss of
catalytic activity observed over the five cycles. The loss of ac-
tivity can be easily compensated, by increasing the reaction
time or adding a small amount of fresh catalyst. The selectiv-
ity towards the 1,2,3,4-tetrahydroquinoline product remained
at 100% throughout the five cycles.
regia (20 mL, HCl (37%) : HNO
dryness and the residue was re-dissolved in concentrated HCl
3
(65%) 3 : 1), slowly heated to
Experimental
Materials and methods
All chemicals were obtained commercially and used without
further purification except for [bmim]ijBF ], which was dried
4
4
Fig. 3 Recycling of the Rh NP–rGO catalyst in [bmim]ijBF ] in the
hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline. Reaction
and deoxygenated under vacuum prior to use. All manipula-
conditions: Rh NP–rGO (30 mg, 0.05 mmol Rh), quinoline (5.0 mmol),
tions with Rh IJCO) were carried out under an inert atmo-
4 2
[bmim]ijBF ] (10.0 mL), H (10 bar), 80 °C, 60 h.
6
16
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(
20 mL, 37%). The solution was filtered, aqua regia was
h under a nitrogen atmosphere until a homogeneously dis-
persed mixture was formed. Then, the mixture was divided
into portions of 1.5 mL (due to the configuration of the
microwave system) and placed into microwave vials under a
nitrogen atmosphere. The mixtures were subjected to micro-
wave irradiation (“fixed power” regime, 20 W, 6 min, maxi-
mal temperature 300 °C, “control” mode, no ventilation) un-
der a nitrogen atmosphere, and the irradiation and heating
regime were kept constant. After cooling to room tempera-
ture, an aliquot of the mixture was collected for TEM charac-
terization. The slurry was then degassed under vacuum and
added to a total volume of 50 mL and the solution was ana-
lyzed by AAS. X-ray photoelectron spectroscopy (XPS) mea-
surements were carried out using a PHI VersaProbe II
scanning XPS microprobe (Physical Instruments AG, Ger-
many). Analysis was performed using a monochromatic Al
Kα X-ray source of 24.8 W power with a beam size of 100 μm.
The spherical capacitor analyser was set at 45° take-off angle
with respect to the sample surface. The pass energy was
4
6.95 eV yielding a full width at half maximum of 0.91 eV for
the Ag 3d5/2 peak. Curve fitting was performed using the PHI
Multipak software. Powder X-ray diffraction (XRD) was
performed using a PANalytical Empyrean XRD with a Bragg–
Brentano geometry fitted with a PANalytical PIXcel-1D detec-
tor using Ni filtered Cu-Kα radiation (λ = 1.54056 Å). Data
were collected at 45 kV and 40 mA in continuous mode with
4
washed with distilled water (30 mL) to remove [bmim]ijBF ].
The black suspension was centrifuged (2 × 20 min, 10 000
rpm), the liquid water–ionic liquid phase was decanted and
this procedure was repeated three times. Finally, the precipi-
tate was dispersed in water (30 mL), filtered and dried under
vacuum, yielding black-grey flakes corresponding to the Rh
NP–rGO composite. The Rh loading in the material was deter-
mined by AAS.
−
1
a scanning rate of 0.01° min in the 2θ range between 4 and
4°. The catalytic reactions were performed under H in high
9
2
pressure Parr autoclaves. Quantitative analysis was performed
using a GC/MS Agilent 7000C equipped with a 30 m HP-5 0.5
mm column with a 0.25 μm coating and a flame ionization
detector.
rGO was prepared according to reference protocols by trans-
forming graphite into graphite oxide using the Hummers/
General procedure for the hydrogenation reactions
An autoclave with a glass inlay was used to conduct the hy-
drogenation reactions. In a typical reaction, the Rh NP–rGO
catalyst (0.01 mmol Rh, 6 mg of the catalyst powder) was
placed into the vessel and the substrate (1.0 mmol) and
54
34
Offeman process followed by reduction with hydrazine.
[
bmim]ijBF
4
] (2.0 mL) were added. The autoclave was sealed
, pressurized to 10 bar and then heated
Synthesis of the Rh NP–rGO composite via thermal
decomposition
and purged with H
2
to 80 °C. After the appropriate time, the heating was stopped,
the autoclave cooled to room temperature and the residual
gas pressure released. Ethyl acetate (20 mL) was added and
the mixture was stirred for 48 h. The ethyl acetate layer was
separated and analyzed by GC (using decane as an internal
standard). Note that prior to catalytic tests, mass transfer lim-
itations in the system were evaluated using the stirring rate
criterion. The quinoline conversion levels achieved at differ-
ent stirring rates indicated that the conversion was not af-
fected by the stirring rate if the stirring rate was set between
1200 and 1500 rpm. Thus, the stirring rate was maintained at
a constant speed of 1500 rpm.
rGO (9.6 mg, 0.2 wt% based on [bmim]ijBF
degassed [bmim]ijBF ] at room temperature for 20 h with ul-
trasound treatment during the first 2 h. Rh IJCO) (83 mg)
4
]) was dispersed in
4
6
16
was added to the mixture (1 wt% metal, relative to 4.8 g
bmim]ijBF ]) and the suspension was stirred for 18 h under a
nitrogen atmosphere. The solution was heated to 230 °C for
8 h under stirring. After cooling to room temperature, an al-
[
4
1
iquot of the mixture was collected for TEM characterization.
The slurry was then degassed (to remove any residual carbon
monoxide) under vacuum and washed with distilled water
(
10 mL) to remove the ionic liquid from the Rh NP–rGO com-
posite. The black suspension was centrifuged (2 × 20 min,
0 000 rpm) and the water–ionic liquid phase was decanted.
1
Recycling experiments for the hydrogenation of quinoline
The resulting precipitate was washed with water and sepa-
rated by centrifugation and decantation three times. Finally,
the residue was dispersed in water (10 mL), filtered and dried
under vacuum, yielding black-grey flakes corresponding to
the Rh NP–rGO composite. The Rh loading in the material
was determined by AAS.
The Rh NP–rGO catalyst (0.05 mmol Rh, 30 mg of the catalyst
powder) was placed in an autoclave and quinoline (5.0 mmol)
and [bmim]ijBF ] (10.0 mL) were added. The autoclave was
4
sealed and purged with H , pressurized to 10 bar and then
2
heated to 80 °C. After 60 h, the heating was stopped, the au-
toclave cooled to room temperature and the residual gas
pressure released. Ethyl acetate (100 mL) was added and the
mixture was stirred for 48 h. The extraction mixture was sepa-
rated and the organic layer was analyzed by GC (using decane
Synthesis of the Rh NP–rGO composite via microwave
dielectric heating
rGO (57.6 mg, 0.2 wt% based on the ionic liquid [bmim]ijBF4])
as an internal standard). The [bmim]ijBF ] layer containing
4
was dispersed in deoxygenated [bmim]ijBF ] at room tempera-
ture for 20 h with additional ultrasound treatment during the
the Rh NP–rGO catalyst was collected and any residual vola-
tiles were removed under vacuum. The catalyst was placed in
the autoclave and a new portion of quinoline (5.0 mmol) was
added to repeat the catalytic procedure.
4
first 2 h. Rh IJCO) (249 mg) was added to the mixture (1
6
16
wt% metal, relative to 14.4 g [bmim]ijBF ]) and stirred for 18
4
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K. Junge and M. Beller, J. Am. Chem. Soc., 2015, 137,
Conflicts of interest
There are no conflicts to declare.
1
1718–11724.
Acknowledgements
2
2
2
3 X. Cui, Y. Li, S. Bachmann, M. Scalone, A.-E. Surkus, K.
Junge, C. Topf and M. Beller, J. Am. Chem. Soc., 2015, 137,
1
4 M. Tan, G. Yang, T. Wang, T. Vitidsant, J. Li, Q. Wei, P. Ai,
We thank the Swiss National Science Foundation and EPFL
for financial support. We are also grateful to CIME (Interdisci-
plinary Centre for Electron Microscopy, EPFL) and Dr.
Thomas LaGrange for the assistance with the characterization
of the NPs. We thank Dr. Pierre Mettraux for the assistance
with XPS measurements and Dr. Arnaud Magrez for the assis-
tance with XRD analysis.
0652–10658.
M. Wu, J. Zheng and N. Tsubaki, Catal. Sci. Technol.,
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5 K. Schütte, J. Barthel, M. Endres, M. Siebels, B. M. Smarsly,
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