Small and Narrowly Distributed Copper Nanoparticles Supported on Carbon Prepared by Surface Organometallic Chemistry for Selective Hydrogenation and CO2 Electroconversion Processes

Article abstract of DOI:10.1002/cctc.201901414

Copper nanoparticles (Cu NPs) are intensively investigated in recent years due to their promising catalytic properties, e. g. selective alkyne hydrogenation and CO₂ electrocatalytic reduction. While dispersing small supported Cu nanoparticles is relatively straightforward on most oxides, obtaining the corresponding small and well dispersed nanoparticles on carbon supports is more challenging because of weaker metal-support interactions resulting typically in larger particles and broader distribution. Here, we show that Surface Organometallic Chemistry can be applied on carbon support and allows the generation of small and narrowly dispersed Cu NPs (4.0+/?1.4 nm) supported on carbon. The thus-obtained Cu nanoparticles are catalytically active in the selective semihydrogenation of an alkyne and the hydrogenation of ethyl cinnamate into the corresponding saturated ester. Moreover, these Cu NPs dispersed on a conductive support catalyze the electroconversion of CO₂ towards C₁ (CO, HCOO^?, CH₄) and C₂ (C₂H₄) reduction products, with high Cu-specific activity towards methane.

Full text of DOI:10.1002/cctc.201901414

Accepted Article
Title: Small and Narrowly Distributed Copper Nanoparticles Supported
on Carbon Prepared by Surface Organometallic Chemistry for
Selective Hydrogenation and CO2 Electroconversion Processes
Authors: Christos Mavrokefalos, Nicolas KAEFFER, Hsueh-Ju Liu,
Frank Krumeich, and Christophe Copéret
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To be cited as: ChemCatChem 10.1002/cctc.201901414
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10.1002/cctc.201901414
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Small and Narrowly Distributed Copper Nanoparticles Supported
on Carbon Prepared by Surface Organometallic Chemistry for
Selective Hydrogenation and CO2 Electroconversion Processes
Christos K. Mavrokefalos‡[a], Nicolas Kaeffer‡*[a], Hsueh-Ju Liu[a], Frank Krumeich[a], Christophe
Copéret*[a]
Abstract: Copper nanoparticles (Cu NPs) are intensively investigated
in recent years due to their promising catalytic properties, e.g.
selective alkyne hydrogenation and CO2 electrocatalytic reduction.
While dispersing small supported Cu nanoparticles is relatively
straightforward on most oxides, obtaining the corresponding small
and well dispersed nanoparticles on carbon supports is more
challenging because of weaker metal-support interactions resulting
typically in larger particles and broader distribution. Here, we show
that Surface Organometallic Chemistry can be applied on carbon
support and allows the generation of small and narrowly dispersed Cu
NPs (4.0 +/- 1.4 nm) supported on carbon. The thus-obtained Cu
large earth-abundancy of Cu makes high dispersion of Cu NPs on
support very appealing.
Our group has shown that surface organometallic chemistry
(SOMC) is a powerful and general approach to generate small,
narrowly-dispersed nanoparticles onto high surface area oxide
supports.[8] This strategy relies on the controlled grafting of a
molecular precursor, usually an organometallic compound, on an
oxide support with a pre-defined OH density, followed by a
reductive treatment under H2 to generate supported particles. The
thus-obtained nanoparticles have typically small size (1-10 nm
range) that depends mainly on the metal and less on the support
itself. For instance, this approach has been successfully applied
to generate 2-5 nm particles of coinage metals as Cu[8a-d] but also
Ag[8e] or Au[8f] on silica, alumina, zirconia, titania as well as tailored
supports. In the specific case of Cu, we have shown that highly
dispersed nanoparticles supported on silica (Cu/SiO2) efficiently
catalyze the semihydrogenation of alkynes in the presence of
organic ligands,[8c, d, 9] with performance reaching that of noble
metal catalysts such as Lindlar Pd. We also discovered that
Cu/SiO2 can selectively catalyze the hydrogenation of the olefinic
bond of a,b-conjugated esters, whereas commercial
Cu/ZnO/Al2O3 or large Cu nanoparticles are inactive for that
transformation, underlining the single behaviour of small and
dispersed nanoparticles.[10]
In the past two decades, carbon materials have drawn attention
as supports for heterogeneous catalysts,[11] because these
materials are typically inert toward strongly acidic (or alkaline)
conditions, in contrast to oxides that show poor stability under
such conditions. Owing to their electronic conductivity, these
supports are also particularly suited for electrocatalytic
applications, as CO2 electroconversion. We thus reasoned that
the advantages offered by the carbon supports, the specific
catalytic properties of copper nanoparticles and the superior
metal optimization of small particles could be brought together
using SOMC as a method of choice to generate small, well
dispersed Cu nanoparticles on carbon. In this work, we
demonstrate that SOMC can be transposed to carbon supports
and used to produce small (4.0 ± 1.4 nm) and dispersed Cu NPs
supported on a common carbon support, carbon black. The
resulting material (Cu/CB) is active in selective hydrogenations of
unsaturated C-C bonds and, owing to the electronically
conductive nature of the support, efficient in the electrocatalytic
conversion of CO2 to C1 and C2 reduction products.
nanoparticles
are
catalytically
active
in
the
selective
semihydrogenation of an alkyne and the hydrogenation of ethyl
cinnamate into the corresponding saturated ester. Moreover, these
Cu NPs dispersed on
a
conductive support catalyze the
electroconversion of CO2 towards C1 (CO, HCOO–, CH4) and C2
(C2H4) reduction products, with high Cu-specific activity towards
methane.
Introduction
Nanoparticles (NPs) prepared from earth-abundant and low-cost
transition metals display catalytic properties towards a broad
range of chemical processes employed both in industry and
academia.[1] Size, shape, dispersion as well as the support play
an important role in their final catalytic properties. Decreasing
particle size – to increase dispersion (surface to volume ratio) and
optimally use the metal – has led to a significant research effort,
in particular since smaller particles are often associated with
increased reactivity. Supported metal nanoparticles are
specifically interesting in this regard because strong metal-
support interactions tend to favour high dispersion.[1b, 2] Among
various metal nanoparticles, Cu nanoparticles[3] have received
renewed
interest
for
their
applicability
in
organic
transformations,[3-4] such as hydrogenations[5] or couplings,[6] and
for their unique activity in the electroconversion of CO2 to valuable
chemicals and fuels.[7] These catalytic properties together with the
[a]
Dr. C. K. Mavrokefalos, Dr. N. Kaeffer, Dr. H.-J. Liu, Dr. F.
Krumeich, Prof. C. Copéret
Department of Chemistry and Applied Biosciences
ETH Zürich
Vladimir-Prelog Weg 1-5 CH-8093 Zürich (Switzerland)
E-mail: nkaeffer@ethz.ch, c.coperet@inorg.chem.ethz.ch
‡These authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201901414
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a)
b)
c)
Results and Discussion
Preparation of carbon-supported Cu nanoparticles
Treatment of carbon-based materials under harsh oxidizing
conditions allows the generation of surface groups such as, inter
alia, hydroxyl or carboxyl groups (denoted here as -OH),[12] that
could be exploited for SOMC. Here, carbon black is submitted to
refluxing nitric acid followed by heat treatment at 200°C under
high vacuum to generate the CB functionalized support (figure 1)
displaying a preserved surface area (73 m2·g-1; figure S1) and a
concentration of reactive surface -OH groups around 0.41
mmolOH×g–1 (ca 3.4 OH·nm-2). This concentration of -OH groups is
within the same order of magnitude as that of silica used in the
Figure 2. Representative HAADF-STEM images of a) [Cu(Mes)]Ox/CB, b)
CuOx/CB and c) particle size distribution at CuOx/CB.
This material was then further exposed to a slow oxidation in air,
affording CuOx/CB (figure 1). Elemental analysis of CuOx/CB gives
a Cu loading of 4.23 wt%, preserved from [Cu(Mes)]Ox/CB, and
TEM images and EDX analysis of CuOx/CB (figures 2b and S2b,
respectively) evidence the formation of small copper
nanoparticles, which are well-dispersed over the CB support. The
particle size is ca. 4.0 ± 1.4 nm (figure 2c), higher than that found
for Cu NPs prepared on SiO2-700 support using the same approach
(1.2 ± 0.6 nm, 4.6 Cu wt%).[9a] Despite the lower dispersion of Cu
on CB compared to SiO2-700 of higher surface area (73 vs 206
m2·g-1, respectively), the thus-obtained particles range among the
smallest ones reported for Cu NPs on carbon supports,[3, 14] while
maintaining a quite narrow size distribution and a marked Cu
weight loading. These results further highlight the potentiality
offered by the SOMC route to carbon-supported nanoparticles. To
increase the Cu NPs loading on carbon black, we evaluated the
possibility to use a second cycle of grafting of [Cun(Mes)n] and H2
treatment, taking Cu/CB as a starting material (figure S3). The
resulting material contains an increased Cu loading, reaching
5.75 wt%, while preserving a high Cu dispersion and a rather
small size for Cu NPs of 6.0 ± 1.8 nm (figure S4). The good
retention of size for small supported (Cu) particles upon
increasing metal loading was also observed with silica as a
support[8b] and appears as an intrinsic property of the SOMC
approach.[8i]
9a]
SOMC preparation of supported Cu nanoparticles.[8a-c,
Contacting CB for 24h with a toluene solution of [Cun(Mes)n] (n =
4,5), a classical precursor for the SOMC routes to supported Cu
nanoparticles,[8c, 9] affords [Cu(Mes)]/CB after washing and drying
(figure 1).
O[Cum(Mes)m-1
]
Cu
OH
n
OH
[(Mes)m-1Cum]O
[(Mes)m-1Cum]O
HO
O[Cum(Mes)m-1
]
[Cun(Mes)n] (n = 4,5)
toluene
OH
HO
O[Cum(Mes)m-1
]
-MesH
CB
[Cu(Mes)]/CB
OH
OH
COuHOx
CuOx
OH
Cu
OH
Cu
OH
H2, 300°C
Air
HO
HO
OH
OH
-MesH
Cu/CB
CuOx/CB
Figure 1. Synthetic route for the preparation of Cu/CB and CuOx/CB.
Titration of the remaining precursor (excess) in the combined
supernatant and washings infers the grafting of ca 0.66 mmole of
[Cu(Mes)] unit per gram of CB, releasing mesitylene upon
protonolysis, quantified to ca 0.30 mmolMesH·g–1 in a separate
NMR-scale grafting experiment in C6D6. These figures imply that
ca 73% of the surface -OH groups reacted and that grafted Cu
mesityl is present as multinuclear species ([Cum(Mes)m-1]-O-CB
with m = 2-5; ca 5.4 Cu·nm–2), in line with what is found for silica.[8c,
Alkyne hydrogenation
We then investigated the use of Cu/CB in alkyne hydrogenation
since the corresponding silica-supported Cu nanoparticles via
SOMC showed good catalytic performances towards the
d, 9a]
semihydrogenation of alkynes.[8c,
1-Phenyl-1-propyne was
13]
used as a prototypical substrate (S, table 1a) with Cu/CB (0.8 Cu
mol%). The substrate S is fully converted within 24 h under mild
conditions (at 60°C under 10 or 20 bar H2, respectively entries #1
and #2, table 1b), but yields the overhydrogenated alkane product
S4H (table 1a), as observed with silica-supported copper
nanoparticles Cu/SiO2 (entry #9, table 1b). Decreasing the
reaction temperature to 40°C limits the conversion of S to 98%
and affords high selectivity (92%; entry #3, table 1b) to the desired
semihydrogenated cis-olefin (Z)-S2H (table 1a). These results are
in agreement with S inhibiting the overhydrogenation reaction, a
process already evidenced for Cu/SiO2 catalyst.[9a]
The substoichiometric amount of surface -OH groups reacted
likely results from the steric bulkiness of grafted multinuclear Cu
species. After slow oxidation in air, the resulting [Cu(Mes)]Ox/CB
material was characterized by elemental analysis, transmission
electron microscopy (TEM) and energy-dispersive X-ray
spectroscopy (EDX). Elemental analysis reveals a Cu loading of
3.96 wt%, confirming the immobilization of the copper precursor
on CB surface, as also evidenced from EDX data (figure S2a).
This Cu loading is also in good agreement with that calculated
from titration of the unreacted [Cun(Mes)n] (ca 3.7 Cu wt%). TEM
picture of [Cu(Mes)]Ox/CB does not show the substantial
formation of copper/copper oxide nanoparticles (figure 2a)
pointing to the high dispersion of Cu (small Cu clusters or discrete
Cu sites) over the CB support upon grafting. [Cu(Mes)]/CB was
then treated at 300 °C under a flow of hydrogen gas to give Cu/CB.
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a)
semihydrogenation of related phenylalkyl- and diarylalkynes,
since similar reactivity and selectivity patterns have been
established with the silica-supported Cu NPs.[8c, 9a] Overall, the
hydrogenation activity obtained here with Cu/CB further highlights
the reach of the SOMC approach to generate catalytically active
Cu NPs on carbon support.
H2
With/Without
ligand L
others
(oligomers,…)
+
+
+
OH
Cu
OH
Cu
OH
OH
(Z)-S2H
(E)-S2H
S
S4H
HO
Cu/CB
Ethyl cinnamate hydrogenation
We also investigated the catalytic hydrogenation of a,b-
conjugated esters to further compare the catatytic property of
Cu/CB with respect to Cu/SiO2.[10] At 10 bar H2 and 60°C, the
hydrogenation of the a,b-conjugated ester ethyl cinnamate (S2,
Table 2a) into the corresponding ethyl dihydrocinnamate S22H
occurs, though with limited conversion within 24 h (30%; entry #1,
table 2b). While increasing the H2 pressure to 20 bars does not
significantly improve the conversion (35%; entry #2, table 2b),
increasing the temperature by only 20 °C, from 60 to 80 °C,
greatly enhances the conversion to 85% at 24 h (entry #3, table
2b). Full conversion and high selectivity could be successfully
achieved in doubling the loading in Cu/CB catalyst to 2.6 Cu mol%
(entries #3-4, table 2b) or using the twice-loaded CuCu/CB
catalyst (1.8 Cu mol% loading; entry #6, table 2b). The latter
observation further shows the higher catalytic activity of CuCu/CB,
which features a higher Cu loading. Note that the hydrogenation
tested with only carbon CB (entry #7, table 2b) does not show any
conversion, confirming that this reaction is catalysed by copper.
We note that the activity observed here with Cu/CB reproduces
well that observed with Cu/SiO2 under identical conditions
(compare entries #5 and #8, table 2b).
b)
#
Catalyst
L*
P(H2)
T
Conv. Sel. S2H
Sel. S4H Sel. others
(bar) (°C) (%)
(%) (Z/E) (%)
(%)
<1
<1
<1
<1
<1
1
2
3
4
5
6
7
8
9
Cu/CB
Cu/CB
Cu/CB
Cu/CB
Cu/CB
–
–
–
20
10
20
20
20
20
20
20
20
20
60
60
40
60
60
60
60
60
60
60
>99 <1
>99 2 (74:16) 98
98 94 (98:2)
>99 86 (98:2) 15
>99
6
PCy3
IMes
–
>99 95 (99:1)
>99 <1
5
†
CuCu/CB
>99
–
<1
–
CB
–
<1
<1
–
–
CB
IMes
–
–
–
‡
Cu/SiO2
>99 <1
>99
<1
<1
‡
10 Cu/SiO2
IMes
>99 73 (93:7) 27
Table 1. a) Reaction scheme and b) conversions and overall selectivities for
hydrogenation of S after 24 h of catalytic run (6.7 μmol Cu, 0.8 Cu mol%, †except
for CuCu/CB 1.1 Cu mol%). *L/S = 1:50. ‡9.3 mg material (6.7 μmol Cu, 0.8 Cu
mol%).
Similar to what was found for S alkyne hydrogenation, the Cu NPs
at Cu/CB are well retained in morphology and size (5.1 ± 1.7 nm)
upon catalytic conversion of S2 (Figure S8).
a)
On the other hand, introducing 2%mol of tricyclohexylphosphine
(PCy3) in the catalytic mixture allows reaching a selectivity of 84%
towards (Z)-S2H at full conversion of S (entry #4, table 1b). The
selectivity towards (Z)-S2H was further increased to 91%
selectivity upon using the more efficient N-heterocyclic ligand
IMes in place of PCy3, (entry #5, table 1b).[8d] The increased
selectivity obtained with IMes originates from the competitive
binding of this ligand on Cu NPs, which enables the adsorption of
S and its conversion to S2H, but adsorbs preferentially than the
generated S2H, hindering overhydrogenation.[9a] Of note, under
the same conditions Cu/SiO2 only provides 73% selectivity
towards S2H at full conversion (entry #10, table 1b), likely because
overhydrogenation is faster compared to Cu/CB, as suggested by
H2 uptake curves (figure S5). Besides, control experiments on
bare CB show no conversion of S (entries #7 and #8, table 1b)
confirming that Cu NPs are the catalytic entities in Cu/CB.
Interestingly, the morphology and size distribution of Cu NPs on
Cu/CB are well retained after catalytic tests. After full conversion
of S (24 h run), the Cu/CB catalyst exhibits a Cu NPs particle size
of 5.5 ± 1.7 nm (Figure S6), very close to the initial value of 4.0 ±
1.4 nm. A similar size (4.8 ± 1.5 nm) is found when IMes is
introduced as selectivity-driving ligand in the catalytic batch
(Figure S7).
O
O
H2
others
O
O
+
(oligomers,…)
OH
Cu
OH
Cu
OH
S22H
S2
HO
OH
Cu/CB
b)
#
Catalyst
n(S2)
m(cat) P(H2)
T
Conv. Carbon balance
(µmol) (mg)
(bar)
(°C)
(%)
(%)
1
2
3
4
5
6
7
8
Cu/CB
Cu/CB
Cu/CB
Cu/CB
Cu/CB
500
500
500
250
500
10.0*
10.0*
10.0*
10.0†
20.0‡
10.0§
10.0
10
60
60
80
80
80
80
80
80
30
98
20
35
96
20
85
98
20
>99
>99
>99
<1
98
20
98
CuCu/CB 500
20
>99
98
CB
500
500
20
Cu/SiO2
18.6‡
20
>99
95
Table 2. a) Reaction scheme and b) conversion of S2 after 24 h of catalytic run
(*6.7 μmol Cu, 1.3 Cu mol%; †6.7 μmol Cu, 2.6 Cu mol%; ‡13.4 μmol Cu, 2.6
Cu mol%; §9.1 μmol Cu, 1.8 Cu mol%).
While the scope of this reaction was not investigated, we
anticipate that such system will be active for the
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CO2 electro-conversion
which is attributed to catalytic H+ reduction. By contrast, under
CO2-saturated conditions, we observe an initial catalytic reduction
In order to investigate the electrochemical properties of the
carbon-supported Cu NPs, we deposited the air-oxidized
CuOx/CB material onto a gas diffusion layer (GDL) electrode using
Nafion® as an ion-exchanging binding material, giving
CuOx/CB/GDL electrodes. TEM picture and local EDX
spectroscopy of the electrode top layer together confirm that Cu
NPs are preserved after electrode templating (figure S9). The
signatures of Nafion® are evident on the C 1s X-ray photoelectron
spectrum (XPS) of the CuOx/CB/GDL electrode (peak at 292.5 eV
for CF2 groups;[15] see figure S10 and table S1), confirming the
casting of the ionomer. To provide an estimate of the amount of
electroactive Cu at CuOx/CB/GDL, we recorded cyclic
voltammetry at this electrode in 1 M KOH aqueous electrolyte,
after a reductive equilibration of the electrode at –1.3 V vs NHE
(–0.47 V vs RHE) to convert CuOx layers into Cu. Upon scanning
potential upward, a broad, ill-defined oxidation event is observed
at around 0.04 V vs NHE (0.87 V vs RHE) as shown in figure S11a.
Such event, which is not present on the blank CB/GDL electrode,
is attributed to the oxidation of top Cu layers of the nanoparticles
at the CV timescale. We assign this event to the oxidation of
electroactive Cu(0) species at the NPs surface to Cu(I) and further
Cu(II) species, namely soluble [Cu(OH)2]– and [Cu(OH)4]2– ions
under the pH conditions used (pH = 14),[16] leading to the partial
dissolution of the NPs. By integration of this wave, we obtain an
upper estimate of the density of electroactive Cu species around
that rises from potential shifted upward (around –1.86 V vs Fc+/0
)
compared to CO2-free conditions; this current is thus attributed to
the specific electrocatalytic reduction of CO2. However, we note
that the products actually evolved from CO2 electroreduction can
differ significantly between organic and aqueous electrolytes and
further electrocatalytic tests were performed under fully aqueous
conditions.
Figure 3. LSVs recorded at CB/GDL (black) and CuOx/CB/GDL (red) electrodes
in a) CO2-saturated 0.5 M KHCO3 aqueous electrolyte and b) 0.1 M nBu4PF6
MeCN (5%v water) under Ar (dashes) or CO2 (lines) saturation.
G
CV(Cu) = 1.6 10–8 mol·cmgeom–2. A similar density was found from
CVs recorded in 0.5 M KHCO3 electrolyte (figure S11b). The total
The actual CO2ER activity of CuOx/CB/GDL was then assessed
by potentiostatic electrolyses in CO2-saturated 0.5 M KHCO3
aqueous media, coupled with online analysis of gas-phase
products (figure 4a). At low overpotentials (–0.4 to –0.6 V vs RHE),
H2 is the principal volatile product, as a result of the side-reduction
of protons. From –0.6 V vs RHE down, evolution of CO is
observed with increasing faradaic efficiency (F.E.) up to 5% at –
1.1 V vs RHE (figure 4b). From –1.1 V vs RHE, C2H4 and CH4 are
also evolved as CO2-reduction products with increasing F.E. as
potentials are scanned downward (figures 4a,b).
From –1.3 V vs RHE, the current density among CO2-reduction
products is mostly directed towards CH4 (figure 4c), which
reaches 13 % F.E. at –1.6 V vs RHE (figure 4b). The increase in
F.E. for C2H4 and CH4 correlates with a decrease in CO F.E. That
observation is consistent with the current proposal that CO is a
primary intermediate in the formation of C2H4 and CH4 over Cu
electrocatalysts.[18] Beside volatile products, we also observed the
production of HCOO– in an total F.E. of 14% as measured at the
end of 4-hour electrolysis run at –1.0 V vs RHE (figure S12). Note
that the Cu-free control electrode CB/GDL yields H2 as the major
volatile, with only traces of CO (< 1%; figures 4d and S13),
unequivocally confirming that Cu NPs in CuOx/CB/GDL are
responsible for the generation of the CO2 reduction products. At
high overpotentials, the magnitude of the overall current density
at CB/GDL approaches that of the H2 partial current density at
CuOx/CB/GDL (figure 4d). This observation suggests that H+ side-
reduction at CuOx/CB/GDL mostly arises from the carbon black
support and the gas diffusing layer and further underlines that Cu
is crucial in that material to drive the conversion of CO2. Partial
current densities obtained after subtraction of the background
density in Cu per geometric electrode area Gtot(Cu) is estimated
–2
around 2.6 10–7 mol·cmgeom (see supplementary information),
giving a dispersion in electrocatalytic Cu species of ca 0.06. For
comparison, the density of surface Cu atoms estimated from the
particle size obtained by TEM and assuming a spherical shape of
the particles is found at ca GTEM(Cu) = 6.8 10–8 mol·cmgeom–2 (see
supplementary information). This coverage falls consistently
within the same order as that obtained from CV, although with a
higher value. Assuming a spherical shape for the particle size
distribution obtained by TEM tends to give overestimate in Cu
coverages,[8b] which can explain this higher value compared to CV
estimate. In addition, part of the CuOx/CB domains may not be in
electronic contact with the GDL back electrode, thus not
addressed by the electroactive Cu coverage GCV(Cu).
Having established that Cu NPs at CuOx/CB/GDL can be
electrochemically addressed, we studied their activity in CO2
electroreduction, given the well-known ability of Cu nanoparticles
to electrocatalyze this process.[7a, b, h, 17] In CO2-saturated KHCO3
aqueous electrolyte, the linear scan voltammogram at
CuOx/CB/GDL shows a catalytic reduction current that onsets
around – 0.84 V vs NHE (– 0.43 V vs RHE), shifted more than 150
mV upward compared to the CB/GDL control electrode (figure 3a).
Such shift suggests that Cu NPs present on CuOx/CB/GDL are
electrocatalytically active. To further probe the origin of catalytic
reductive processes, LSVs were recorded in 0.1 M nBu4PF6
MeCN electrolyte using water (5%v) as a proton source, in the
absence or presence of CO2 (figure 3b). In the CO2-free
electrolyte, a reduction current onsets from ca –2.24 V vs Fc+/0
,
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activity of CB/GDL shows that CuOx/CB/GDL has Cu-specific
mass activity (using the total Cu loading Gtot(Cu)) going from H2 to
predominantly CH4 as potential is lowered (figures 4c and S14).
loading at CuOx/CB/GDL thus corroborates its moderate overall
F.E. for C-products and its high intrinsic selectivity for CH4 within
C-products, also favoured by high Cu dispersion. We note that
subsurface Cu atoms remaining partially oxidized under
electroreductive conditions and possibly impacting activity, as
debated in the literature,[21] may be present at our CuOx/CB/GDL
electrode, although we did not address this point in further details.
Long-term electrolyses over 24 hours show a gradual decay of
the activity towards CO2 reduction (figure S15). To understand the
origin of this decay, we analysed by TEM the CuOx/CB material at
several durations of electrolysis. After a 1-hour electrolysis at –
1.0 V vs RHE, we observed Cu nanoparticulate species of ca 1-3
nm (figure S16a,b) together with the formation of larger cube-
shape nanoparticles of ca 20 nm (figure S16c-e), possibly grown
from this small Cu NPs (figure S16f). Interestingly, longer
electrolyses lead to the gradual disappearance of the larger cubic
nanoparticles towards more of the 1-3 nm Cu nanoparticles (after
4-hour electrolysis; figures S17-19) and further nanometric
clusters (after 24-hour electrolysis; figure S20). Consistent with
this behaviour, Hwang and co-workers have recently reported a
similar observation that small Cu-based nanoparticles first
agglomerate into larger cube-shaped nanoparticles, which further
undergo fragmentation back into smaller Cu NPs.[7k] Electrolyses
at lower potential (–1.4 V vs RHE) also led to the re-dispersion of
Cu into smaller Cu NPs (1-3 nm) after 4 and 24 hours (figures
S21,22). We note that the presence of potentially low-coordinated
Cu sites at these nanoparticles also correlates with the high Cu
22]
mass activity observed for H2 and CH4.[19,
Overall,
reconstruction of Cu NPs under electrocatalytic conditions into
more dispersed Cu species having higher intrinsic activity for H2
evolution explains the observed decrease in selectivity towards
CO2 reduction products. While Cu loading is known to directly
affect the reconstruction of Cu NPs under electrocatalytic
conditions,[7h] we propose that the moderate Cu loading at
CuOx/CB favours re-dispersion of the initial Cu NPs into smaller
ones. Also, strong interactions with surface groups present on the
CB support, such as alcohol- or ketone-type moieties evidenced
on the C 1s and O 1s XPS spectra[23] of CuOx/CB/GDL electrode
(figure S10, table S1) may further stabilize these small Cu NPs.
Figure 4. a) Total current density (black squares, left scale) and faradaic
efficiencies for gas-phase products (dots, right scale) at CuOx/CB/GDL. b) Zoom
on faradaic efficiencies for CO2-reduction gas-phase products at CuOx/CB/GDL.
c) Background-subtracted Cu-mass partial current densities at CuOx/CB/GDL.
d) Total (black) and H2 partial (wine) current densities at CB/GDL (triangles) and
CuOx/CB/GDL (squares). Conditions: CO2-saturated 0.5 M KHCO3 aqueous
electrolyte (pH = 7). Colors: all gas-phase products (gray), all gas-phase CO2-
reduction products (navy), H2 (wine), CO (blue), CH4 (green) and C2H4 (purple).
This high specific activity for H2 and CH4 is a consistent feature
within precedent reports on small Cu NPs (< 10nm)[7a, b, 17c] and is
proposed to arise from low-coordinated Cu surface sites.[19] We
observe here that the CH4 specific current density continuously
increases in magnitude with increasing overpotential at
Conclusions
–1
CuOx/CB/GDL (–540 A·gCu at –1.6 V vs RHE), whereas it
We successfully translated the SOMC strategy to prepare
narrowly-dispersed and small (ca 4 nm) Cu nanoparticles on
carbon black. We demonstrated that these carbon-supported Cu
NPs are active for the semihydrogenation of a prototypical alkyne
(1-phenyl-1-propyne). The use of a ligand (e.g. IMes) permits to
reach high selectivity for the desired cis-olefin at full conversion of
the starting alkyne. We also showed that the Cu/CB material can
efficiently catalyze the selective hydrogenation of a,b-conjugated
esters like ethyl cinnamate into ethyl dihydrocinnamate under mild
conditions. This activity in selective hydrogenations parallels that
of the counterpart silica-supported Cu NPs and demonstrates the
successful translation of the SOMC route to generate catalytically
active Cu NPs on carbonaceous material. Besides, taking
advantage of the electronic conductivity brought by the carbon
plateaus or decays for unsupported Cu NPs.[7a, b, 17c] Among the
few reports on CO2ER catalyzed by Cu NPs supported on high
surface area carbon,[20] Baturina and co-workers observed that
relatively small Cu NPs (8-13 nm) on Vulcan carbon black (40
wt% Cu)[20a] have high activity towards C-products at low
potentials but produce a mixture of mostly C2H4 and CH4 (ca 47
and 30 % overall F.E., respectively, at –1.6 V vs RHE). At the
same potential, CuOx/CB/GDL displays lower overall F.E. towards
C-products but predominantly evolves CH4 within C-products (F.E.
(CH4) = 13% vs F.E.(C2H4) = 4%). This difference can be
explained by a much higher Cu loading for Cu NPs on Vulcan
carbon[20a] (40 vs 4.2 wt% Cu for CuOx/CB/GDL), given that
increasing Cu loading is known to promote activity towards C-
products and C-C couplings, thus C2+ products.[7a, h, 21] The low Cu
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10.1002/cctc.201901414
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FULL PAPER
μm2. The spectrometer was pre-calibrated by performing measurement on
a clean silver surface, whereby the Ag 3d5/2 peak was aligned to a binding
energy of 368.25 eV with a full width at half-maximum of 0.78 eV at a pass
energy of 30 eV. Spectra were recorded with a dwell time of 50 ms, using
a pass energy of 30 eV and in 0.05 eV binding energy steps. Spectra were
acquired with a take-off angle of 90°. C 1s, respectively O 1s, spectra were
calibrated by adjusting the binding energy of the C sp2 contribution to 284.8
eV, respectively of the C=O contribution to 531.1 eV. Nitrogen adsorption
isotherms were recorded on a Bel-Mini apparatus (BEL Japan, Inc.) at 77
K. Approximately 100 mg of the samples were loaded into cells. Prior to
the adsorption measurement, the samples were evacuated under vacuum
(ca. 10−3 mbar) at R.T. for 1 h then at 200 °C for 2 h. Data were fitted
according to a BET isotherm to obtain the total surface area.
support, Cu NPs obtained by SOMC and templated within an
CuOx/CB/GDL electrode assembly can be electrochemically
addressed. The electrode is competent for the electroconversion
of CO2 to carbonaceous products as CO, HCOOH, C2H4 and CH4
under fully aqueous conditions. The partial reconstruction of the
Cu NPs observed upon electrocatalytic turnover namely yields
discrete Cu species, possibly responsible for the high Cu-specific
activity towards H2 and CH4.
Demonstrating the SOMC route to Cu NPs on carbon reconciles
the innate properties of this support (electronic conductivity,
hydrolytic stability) with the high dispersion of supported metallic
nanoparticles at its surface. We anticipate that the approach can
be straightforwardly expanded to produce supported
nanoparticles of different metals and on alternative carbon
supports. Expanding the library of supports available for SOMC to
carbon materials and more broadly to electronic conductors will
open new opportunities in the application scope of supported,
well-dispersed NPs towards various catalytic conversions,
including electrocatalysis. We are currently investigating this
direction.
Preparation of CB. Carbon black (4.4 g) was first refluxed in 100 ml HNO3
fuming solution for 24 hours, to generate reactive surface groups.[11a] The
material was filtered on a filter paper, washed with large portions of distilled
water, then ethanol and dried under high vacuum (10–5 mbar) at 200 °C for
24 hours to yield the activated carbon support CB. Titration of the activated
support CB with [Mg(Bn)2(THF)2] gave a density of reactive protic groups
of ca 0.41 mmolgroup·g–1. The surface area measured using nitrogen
adsorption-desorption isotherms is 73 m2·g-1 (figure S1).
Preparation of Cu/CB and CuOx/CB. A mixture of [Cu5(Mes)5]·toluene
(0.54 g, 0.54 mmol, 2.68 mmolCu) and activated carbon black CB (1.44 g,
ca 0.59 mmolgroup) in 30 mL toluene was stirred for 24 hours at room
temperature. The mixture was then filtered on a glass frit, washed twice
with 10 mL toluene, then once with 10 mL pentane and dried under
vacuum to afford [Cu(Mes)]/CB. [Cu(Mes)]/CB was then submitted to
hydrogenation under a flow of H2 using the following temperature program:
from R.T. to 300 °C at 60 °C·h–1, then 12 h at 300 °C. The reactor was
then evacuated under high-vacuum at 300 °C for 4 h, to afford the Cu/CB
material, which was stored in an Ar-filled glovebox. Upon slow exposure
to air, [Cu(Mes)]/CB yields [Cu(Mes)]Ox/CB and Cu/CB yields CuOx/CB.
Elemental analysis gives Cu loadings of 3.96 wt%, respectively 4.23 wt%,
for [Cu(Mes)]Ox/CB, respectively CuOx/CB.
Experimental Section
General procedures. Unless otherwise noted, all experiments were
conducted with dry, oxygen-free solvents using standard Schlenk
techniques or in N2 or Ar-filled gloveboxes and reagents were obtained
from commercial suppliers and used as received. Toluene and pentane
were purified by passage through double solvent purification alumina
columns (MBraun). CO2 (quality 48) was purchased from Pangas. Carbon
black (acetylene, 50% compressed, 99.9+%, surface area 75 m2·g–1, bulk
density 80-120 g·L–1) was purchased from Alfa-Aesar. Gas diffusion layer
(GDL 39 BC) was graciously provided by SGL CARBON GmbH Company.
Anion-exchanging membrane (Fumasep® FAA-3-PK-130) was purchased
from Fumatech. Tricyclohexylphosphine was purchased from Aldrich and
recrystallized from pentane. 1-Phenyl-1-propyne and ethyl cinnamate
were purchased from Sigma-Aldrich, degassed by three freeze-pump-
thaw cycles and then high-vacuum transferred in Rotaflo-type Schlenk
vessels, stored in a glove-box over 3 Å molecular sieves and passed over
Preparation of CuCu/CB and CuCuOx/CB. The grafting/hydrogenation
procedure was repeated a second time on the Cu/CB material. Briefly, a
mixture of [Cu5(Mes)5]·toluene (80 mg, 80 µmol, 400 µmolCu) and Cu/CB
(0.2 g) in 10 mL toluene was stirred for 24 hours at room temperature. The
mixture was then filtered on a glass frit, washed twice with 5 mL toluene,
then once with 5 mL pentane and dried under vacuum. The obtained
material was submitted to hydrogenation under a flow of H2 using the
following temperature program: from R.T. to 300 °C at 60 ºC·h–1, then 12
h at 300 °C. The reactor was then evacuated under high-vacuum at 300 °C
for 4 h, to afford the CuCu/CB material, which was stored in a Ar-filled
glovebox. Slow oxidation by exposure to air yields the CuCuOx/CB material,
for which a Cu loading of 5.75 wt% was found by elemental analysis.
activated
alumina
just
before
use.
IMes
(1,3-bis(2,4,6-
procedure
trimethylphenyl)imidazol-2-ylidene) was synthetized by
a
adapted from the literature.[24] [Cu5(Mes)5]·toluene was prepared
according to procedures from the literature.[25] Cu/SiO2 (4.6 Cu wt%) was
prepared as reported previously.[9a]
Characterization methods. Elemental analysis of the supported material
was performed by the Mikroanalytisches Labor Pascher, Remagen,
Germany. Liquid-state NMR spectra were recorded on a Bruker DPX-300
instrument operating at the denoted spectrometer frequency given in MHz
for the specified nucleus. The 1H and 13C chemical shifts are calibrated
with the residual solvent peak. Electron microscopy (HAADF-STEM and
EDX) was performed at the ScopeM facility on a HD2700CS (Hitachi)
instrument or on a Talos F200X (Thermo Fischer Scientific) instrument
both operated at an acceleration voltage of 200 kV. Samples for electron
microscopy were dry-casted on Cu mesh grids (Lacey C only; 300 mesh
Cu; TED PELLA INC.) unless otherwise noted for Au grids (UC-A on
Lacey; 300 mesh Au; TED PELLA INC.) X-ray photoelectron spectroscopy
measurements were performed at ca. 2∙10−9 mbar in a VG ESCALAB
220iXL spectrometer (Thermo Fisher Scientific) using focused
monochromatized Al Kα radiation (1486.6 eV) with a beam size of ca. 500
Preparation of CuOx/CB/GDL electrodes. 5 mg of CuOx/CB were
dispersed in a solution 100 mL ethanol containing 50 μL of Nafion 5 wt%.
The solution was sonicated for around 30 minutes. The final solution was
then filtered on a gas diffusion layer (GDL) support of surface 12.6 cm2
(disc). The obtained CuOx/CB/GDL material was then sliced into pieces to
prepare electrodes, the working area of which was delimited to 1 cm2
geometric using epoxy glue. Samples for TEM imaging and XPS analysis
were prepared by scratching with a plastic spatula the side of the electrode
where CuOx/CB is deposited.
Alkyne hydrogenation. Hydrogenation studies were performed on an 8-
parallel reactor autoclave (Endeavour, Biotage) operated inside a glove
box. The H2 feed was passed through activated Cu/Al2O3 sorbent (BASF)
and activated 4-Å molecular sieves. Stock solutions of substrates in
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10.1002/cctc.201901414
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FULL PAPER
toluene (1 M), ligands in toluene (10 mM) and tridecane (100 mM) in
toluene were prepared using volumetric flasks. GC analysis was
performed on Shimadzu-QP 2010 Ultra equipped with parallel FID and MS
detectors using an HP-5 column, N2 as a carrier gas (30 mL·min–1) and the
following temperature program: ramp of 15 ºC·min–1 from 70 ºC to 260 ºC
and then holding at 260 ºC for 15 min. In a typical experiment, the glass
liner reactors were charged with 10.0 mg of material unless otherwise
noted, 800 μL of 1M substrate (800 μmol) solution and 800 μL of a 100
mM tridecane (internal standard) solution or 800 μL of a mixture of
substrate (1M; 800 μmol) and tridecane (100 mM) and, for corresponding
experiments, 1600 μL of a 10 mM ligand L solution (n(L) = 16 μmol; L/S
ratio of 1:50). The glass liners were then filled with toluene until a total
volume of 4400 μL was reached. 400 μL of the thusly prepared initial
mixtures were then aliquoted and submitted to GC analysis. The reaction
media were then set to the desired pressures and temperatures and stirred
at 500 rpm for 24 h. At the end of the catalytic run, 400 µL of the final
reaction mixtures were drawn and analyzed by GC. Areas of GC peaks
were normalized against the area of the tridecane peak. Comparison of
normalized areas of initial mixture with that of final ones allowed to
determine conversions and selectivities, using the same response factors
for all substrates. All carbon balances were found >99%. Hydrogen
consumption curves are reported in moles of H2 consumed per volume of
batch solution. Post-activity samples for TEM imaging were collected at
the end of catalytic runs. The supernatant solution was removed, the spent
material dried under vacuum and stored in glovebox before slow exposure
to air.
was bubbled first in the counter electrode compartment for 30 min then in
the working electrode compartment for 30 min. Potentiostatic electrolyses
were conducted for 1 h and under a constant flow of CO2 (20 mL·min–1
)
flushing the electrolyte at the working electrode compartment. The outlet
stream directly fed the injection loops of an in-line gas chromatograph-
mass spectrometer (GC-MS) for analysis of gas-phase products. In a
typical experiment, the first electrolysis was recorded at –1.6 V vs RHE,
and following ones at potentials incremented by 0.1 V up to –0.4 V vs RHE.
Identification and quantification of the gas-phase products was realized by
in-line GC-MS (Agilent Technologies 7890 gas chromatograph coupled to
an Agilent Technologies 5977B MSD mass spectrometer) setup. The
outlet stream of the electrochemical cell flushed injection loops of two
parallel lines and samplings are realized every 30 min. The first line was
eluted with Ar carrier gas on a Porapak Q column followed by a GS-
carbonplot column to a thermal conductivity detector (TCD) followed by a
Ni-catalyst methanizer used to methanize the CO fraction and bypassed
otherwise and finally to a flame ionization detector (FID). The other line fed
a split-splitless port that injected in the split mode with He carrier gas on a
GS-Carbonplot capillary column plugged to a mass selective detector
(MSD) operated in the scan mode in the m/z range 0-100. Elution was
realized using the following temperature program: 40 °C for 2 min, then
ramp to 150°C at 55°C·min–1 and 150°C for 3 min. The in-line GC-MS
system was calibrated with gas standards containing known molar ratios
(0.1, 1 or 10% each) of H2, CO, CO2, CH4, C2H4 and C2H6 in CO2 (balance
gas). Long-term electrolyses were performed with CuOx/CB/GDL at –1.0
V and –1.4 V vs RHE, using a CO2 flow of 20 mL·min–1. Investigation of
gas-phase products were performed as described above. At the end of an
electrolysis, the working electrode was immediately disconnected and
dried under a flow of Ar. The side of the electrode where CuOx/CB was
deposited was scratched using a plastic spatula and the resulting black
powder deposited on TEM grids for imaging. Samples of electrolyte were
aliquoted from the cathodic and anodic compartments for characterization
of liquid-phase products. Liquid-phase products were analysed and
quantified using nuclear magnetic resonance (NMR) spectroscopy.
Samples were prepared by mixing 380 µL of analyte with 40 µL D2O and
8 µL of a 0.5 M DMSO (standard) aqueous solution in a NMR tube,
according to a procedure modified from the literature.[27] Quantification was
performed on a 400 MHz Bruker spectrometer using a water suppression
(CW) NMR sequence.
Ethyl cinnamate hydrogenation. The procedure for the hydrogenation of
ethyl cinnamate is identical to that for alkyne hydrogenation except for the
following alterations: a stock solution containing the substrate (1 M) and
tridecane (100 mM) in toluene as an internal standard was prepared. The
glass liner reactors were charged with the desired amount of material
(typically 10.0 mg), of substrate solution (typically 500 μL; 500 μmol) and
filled with toluene to reach a total volume of 4400 μL. 400 μL of the thusly
prepared initial mixtures were then aliquoted and submitted to GC analysis.
The reaction media were then set to desired pressures and temperatures
and stirred at 500 rpm for 24 h. At the end of the catalytic run, 400 µL of
the final reaction mixtures were aliquoted and analyzed by GC. GC
analysis was performed with the following temperature program: 8 min at
40 ºC, then ramp of 20 ºC·min–1 to 200 ºC followed by a ramp of 40 ºC·min–
1 to 260 ºC and then holding at 260 ºC for 4.5 min.
Acknowledgements
Electrochemical investigation. Electrochemical experiments were
carried out with an Autolab PGSTAT 128N potentiostat/galvanostat.
Experiments were conducted at room temperature (24 ± 1 °C), using a Pt
wire counter electrode and a AgCl/Ag (in KCl 3M), respectively a
AgNO3/Ag (10 mM in 0.1 M nBu4PF6 MeCN), reference electrode for
experiments conducted in aqueous, respectively organic, electrolyte.
Potentials measured in aqueous electrolyte were referenced against the
normal hydrogen electrode (NHE) and reversible hydrogen electrode
(RHE) using conversion from the literature.[26] Potentials measured in
organic electrolyte were referenced versus an internal Fc+/0 reference
added at the end of the experiment. Cyclic voltammetries (CV) and linear
sweep voltammetries (LSV) were performed in a single compartment cell
using 1 M KOH or 0.5 M KHCO3 aqueous electrolytes or 0.1 M nBu4PF6
MeCN with H2O 5%v electrolyte, under Ar or CO2-saturated conditions.
The authors thank the SCCER Heat and Energy Storage and the
Competence Center of Energy and Mobility CCEM for the
financial support. The development of in-line analysis of CO2
electroreduction products was funded by a grant of the ETHZ
Scientific Equipment Program. N. K. acknowledges support from
the ETHZ Postdoctoral Fellowship Program, from the Marie Curie
Actions for People COFUND Program and from an ETHZ Career
Seed Grant (SEED-09 17-2). The authors thank Dr J. Mendes-
Burak for the assistance with gas adsorption and hydrogenation
experiments, Dr J.-D. Compain for the assistance with the GC/MS
measurements, Dr D. Lebedev for additional TEM images and Dr
J. Herranz (PSI, Villigen) for the acquisition of XPS spectra. The
authors are grateful to the SGL CARBON GmbH Company for the
gracious gift of GDL support.
Unless otherwise noted, CVs and LSVs were recorded at 100 mV·s–1
Potential-dependence of currents and evolved products was evaluated
under potentiostatic conditions in two-compartment cell. The
.
a
compartment containing the working and reference electrodes was
separated from that containing the counter electrode by a porous glass frit
or by a Fumasep FAA-3-PK-130 anion-exchanging membrane. A mass
flow meter (Bronkhorst EL-Select), was used to control the flow of CO2
Keywords: Supported copper nanoparticles • surface
organometallic chemistry • carbon support • alkyne
flushed through the electrolyte. Prior to electrolysis, CO2 (20 mL·min–1
)
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10.1002/cctc.201901414
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10.1002/cctc.201901414
ChemCatChem
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Expanding support for SOMC route to Cu NPs
Carbon-supported copper
nanoparticles prepared via
surface organometallic chemistry
are active catalysts in selective
hydrogenation and CO2
Christos K. Mavrokefalos, Nicolas
Kaeffer*, Hsueh-Ju Liu, Frank
Krumeich, Christophe Copéret*
Previous works
This work
OH
Cu
Cu
Cu
Cu
OH
Page No. – Page No.
electroconversion.
Small and Narrowly Distributed
Copper Nanoparticles
Supported on Carbon Prepared
by Surface Organometallic
Chemistry for Selective
Cu/CB
Cu/SiO2
Alkyne semireduction
Ethyl cinnamate reduction
CO2 reduction
Alkyne semireduction
Ethyl cinnamate reduction
CO2 electroreduction
Hydrogenation and CO2
Electroconversion Processes
This article is protected by copyright. All rights reserved.