Palladium-gold catalyst for the electrochemical reduction of CO2 to C1-C5 hydrocarbons

Article abstract of DOI:10.1039/c6cc03717h

Copper is a unique electrocatalyst for CO₂ reduction, since it is one of the few catalysts able to produce methane, ethylene and ethane from CO₂ with decent faradaic efficiencies. Here we report on the design and synthesis of a new non-copper-containing catalyst able to reduce CO₂ to C₁ to C₅ hydrocarbons. This catalyst was designed by combining a metal that binds CO strongly, Pd, with a metal that binds CO weakly, Au, in an attempt to tune the binding energy of CO. We show that a mixture of C₁-C₅ hydrocarbons and soluble products are produced from an onset potential of -0.8 V_RHE. We propose that the higher hydrocarbons are formed via a polymerization of -CH₂ groups adsorbed on the catalyst surface.

Full text of DOI:10.1039/c6cc03717h

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Palladium-gold catalyst for the electrochemical reduction of CO2
to C -C hydrocarbons
Received 00th January 20xx,
Accepted 00th January 20xx
1
5
[
a]
[a]
[a]
[b]
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DOI: 10.1039/x0xx00000x
R. Kortlever,* I. Peters, C. Balemans, R. Kas, Y. Kwon, G. Mul, and M.T.M. Koper*
www.rsc.org/
Copper is a unique electrocatalyst for CO
2
reduction, since it is one containing copper. Although CO binds strongly on Pd, Pd has been
6
of the few catalysts able to produce methane, ethylene and shown to be a catalyst for the production of CO from CO
ethane from CO with decent faradaic efficiencies. Here we report it is not expected that Pd readily produces hydrocarbons.
on the design and synthesis of a new non-copper-containing
Pd-Au bimetallic catalysts have been studied as both
2
, however
2
7
8
catalyst able to reduce CO
2
to C
1
to C
5
hydrocarbons. This catalyst heterogeneous catalysts and electrocatalysts, and have recently
9
was designed by combining a metal that binds CO strongly, Pd,
with a metal that binds CO weakly, Au, in an attempt to tune the
also been employed as CO reduction catalysts. Plana et al. showed
2
2
that the faradaic efficiency toward CO reduction on Pd shell Au
binding energy of CO. We show that a mixture of C
1
-C
5
9a
core nanoparticles increases with decreasing Pd shell thicknesses.
More recently, they showed that increasing the thickness of the Pd
shell leads to the production of methane and ethane, whereas thin
hydrocarbons and soluble products are produced from an onset
potential of -0.8 VRHE. We propose that the higher hydrocarbons
are formed via a polymerization of –CH
catalyst surface.
2
groups adsorbed on the
9b
Pd shells only produce formic acid. A study by Hahn et al. showed
that formic acid was the dominant product of CO
2
reduction on a
thin PdAu alloy film synthesized by an electron-beam co-deposition
The electrocatalytic reduction of CO
2
to valuable products,
9
c
method.
especially hydrocarbons, is a topic that has attracted the interest of
1
In this study the combination of Pd and Au was made by
electrodeposition of Pd on an Au substrate. Remarkably, we will
many electrochemists and inorganic chemists recently. A major
breakthrough in the electrochemical reduction of CO
2
was the
to
1 5
show that this combination of Pd and Au is able to produce C to C
discovery in 1985 by Hori et al. that copper is able to reduce CO
2
hydrocarbons. To the best of our knowledge, this is the first non-
copper-containing electrocatalyst that produces such a variety of
hydrocarbons such as methane and ethylene with good faradaic
2
efficiencies. Since then ample research has been conducted to
3
multi-carbon products form CO
Pd overlayers on an Au electrode (Pd-Au) were prepared by
electrodeposition of Pd onto Au from a 0.1 M H SO + 1 mM PdCl
2
.
determine the special catalytic ability of copper for this reaction.
In early work, CO was found to be a key intermediate in the
4
formation of hydrocarbons on copper and recent calculations by
2
4
2 -
solution at 0.27 V vs. SCE for 60 seconds, using a procedure for the
Peterson et al. have indicated that a good catalysts for the
10
deposition of Pd on Pt. The structure of the layers of Pd on Au was
visualized with scanning electron microscopy (SEM), showing that
Pd covers the Au substrate (see Fig. S1). Since the Pd layer shows a
rough nanostructured surface, it is difficult to determine the exact
layer thickness. From the coulombic charge of Pd deposition, which
was typically around 4.5 mC, an average thickness of 65-75
monolayers of Pd was estimated. Cu UPD measurements showed
an increase in surface area of 1.18 times compared to a
polycrystalline Pd electrode.
reduction of CO to methane should catalyze the protonation of CO
2
5
to HCO or COH efficiently. If a catalyst binds CO too weakly, CO will
desorb before this protonation can take place, while if a catalyst
binds CO too strongly, significant overpotentials are required for
this reduction step since the formation of HCO or COH is
thermodynamically unfavorable. In this paper we combine a metal
which binds CO too weakly, in this case Au, with a metal which
binds CO too strongly, namely Pd, in an effort to create an active
2
electrocatalyst for the reduction of CO to hydrocarbons not
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6
4
2
00
00
00
0
c)
a)
b)
0
c)
0
-5
-10
Pd-Au
Pd-Au
-
10
15
-
20
-30
40
-
-
-
200
400
600
Pd
Au
Pd-Au
CO
-
2
Blank
-20
-
0
.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
E (V) vs. RHE
-1.5
-1.0
-0.5
0.0
-1.5
-1.0
-0.5
0.0
E (V) vs. RHE
E (V) vs. RHE
Figure 1. Cyclic voltammograms of: a) a Pd electrode (red dashed line), Au electrode (blue dashed line) and a Pd-Au electrode (green solid line) in a 0.5 M H
scan rate of 20 mV/s; b) a Pd-Au electrode recorded in a 0.1 M KH PO / 0.1 M K HPO
first voltammogram of a Pd-Au electrode in a CO saturated 0.1 M KH PO / 0.1 M K HPO
2
SO
4
electrolyte at a
2
4
2
4
electrolyte (pH 6.7) at a scan rate of 50 mV/s purged with either Ar (green) or CO
2
(blue); c)
2
2
4
2
4
electrolyte (pH 6.7).
The electrochemical characterization of the Pd-Au electrode the peak for formic acid, indicating that the concentration of
in 0.5 M H SO (see Fig. 1a) shows both Pd and Au features. methanol is also substantially lower than the concentration of
Hydrogen adsorption and desorption is seen as well as an (surface) formic acid. Ethanol showed small peaks in the H-NMR spectrum
oxide reduction peak around 0.7 VRHE related to the presence of Pd and its identity was confirmed by the coupling of the -CH and -CH
on the surface, in combination with an (surface) Au-oxide reduction peaks of ethanol in 2D homonuclear COSY (see Fig. S30).
2
4
1
3
2
8
b
peak around 1.2 VRHE
The presence of both Pd and Au near the surface is confirmed observed.
by XPS spectra of the Pd-Au electrodes (see Fig. S4-6). The Pd 3d The reduction of CO on Pd-Au studied with OLEMS shows the
.
Furthermore, a peak indicating the presence of acetic acid was
2
spectra showed two separate peaks at 340.7 eV and 335.4 eV, production of hydrocarbons (Fig. 2). Both methane (m/z = 15) and
corresponding to the 3d3/2 and 3d5/2 of metallic Pd. These values ethylene (m/z = 26) were observed starting from -0.8 VRHE and,
show a slight shift to higher binding energies compared to literature interestingly, higher mass fractions pointing to the production of C3
data that report the 3d3/2 peak at 340.4 eV and the 3d5/2 peak at hydrocarbons (m/z = 39, m/z = 42 and m/z = 43), propane and/or
1
1
3
4
35.1 eV. Small peaks were observed in the Au 4f spectral region. propylene, and the production of C hydrocarbons (m/z = 56 and
Both the peaks for Au 4f5/2 and 4f7/2 showed no shift compared to a m/z = 58), butane and/or butene, were detected. Hydrocarbons are
blank measurement with the Au substrate. The atomic percentage produced with an onset potential of -0.8 VRHE, and the intensity of
of Pd detected by XPS is significantly higher than Au indicating that the detected hydrocarbons decreases from C
1
4
to C . Furthermore,
the surface is highly enriched with Pd.
all hydrocarbon mass fragments show a decrease at potentials close
Cyclic voltammograms (CVs) of a Pd-Au electrode in a pH 6.7
-
1.5
-1.0
-0.5
2
phosphate buffer purged with either argon or CO , shown in Fig. 1b,
-
9
show a high degree of similarity with the CVs measured with a Pd
electrode (Fig. S33). A reduction peak around -0.2 VRHE was seen in
the first scan (see Fig. 1c), which was not present in the later scans.
16x10
1
Hydrogen
m/z = 2
2
8
4
1
0
At Pd-Pt a similar peak was observed around -0.4 VRHE
.
The
-9
Methane
m/z = 15
reduction of CO2 starts around -0.6 VRHE and a plateau is observed
around -1.1 ─ -1.2 VRHE as is also seen with Pd.
30x10
2
0
0
1
Formic acid was observed as the dominant soluble product
from CO2 reduction on Pd-Au with online HPLC (see Fig. S26). The
production of formic acid on Pd-Au shows many similarities to the
-9
0x10
1
Ethylene
m/z = 26
8
6
4
-9
1
0
production of formic acid on Pd-Pt. In both cases
overpotential peak in the production of formic acid is seen, which is
correlated with the reduction peak around -0.2
a low
1
0x10
9
8
7
6
5
m/z = 39
V
RHE in the
voltammetry, followed by a continuous production of formic acid at
higher overpotentials (≤ -1.0 VRHE). In the case of Pd-Pt the low
overpotential production of formic acid was the result of (in-)direct
bicarbonate reduction to formic acid, while the high overpotential
-9
2.8x10
m/z = 42
m/z = 56
2
2
1
.4
.0
.6
production of formic acid was due to the reduction of CO
2
to formic
1
0,12
-
12
acid.
Considering the similarities between the production of
7
70x10
7
00
30
560
90
420
formic acid on both catalysts, the same explanation is assumed to
be valid for Pd-Au.
6
4
Formic acid was however not the only dissolved product
1
detected after 15 hours of electrolysis at -1.2 VRHE
.
H-NMR shows
-1.5
-1.0
-0.5
the production of methanol, ethanol and acetic acid (see Table S1
and Fig. S29). The peak for methanol was substantially lower than
E (V) vs. RHE
Figure 2. Formation of hydrogen (m/z = 2), methane (m/z = 15) ethylene (m/z =
6), C hydrocarbons (m/z = 39 and 42) and C hydrocarbons (m/z = 56) from CO
reduction on a Pd-Au electrode followed with OLEMS in a pH 6.7 phosphate
2
3
4
2
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
2 4 2 4
buffer electrolyte (0.1 M KH PO / 0.1 M K HPO ).
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to -1.5 VRHE suggesting that the optimal potential for the production
of hydrocarbons lies between -1.1 and -1.4 VRHE
Furthermore, C
5
hydrocarbons were detected below -1.2 VRHE.
.
Pentane was produced dominantly, followed by 2-methyl-butane
The onset potential of -0.8 VRHE for the reduction of CO to
2
hydrocarbons on Pd-Au is 0.4 V less negative than the onset
potential for the production of hydrocarbons, methane and a small
amount of ethylene, on a Pd electrode (see Fig. S18). Also, this
onset potential is comparable with the onset potential of CO
2
reduction to hydrocarbons on copper in the same electrolyte, which
3
b
is -0.6 VRHE for ethylene and -0.8 VRHE for methane. It is however
more negative than a recently reported NiGa catalyst able to
produce methane, ethylene and ethane at an onset potential of -
1
3
0
2
.48 VRHE. Furthermore, the reduction of CO on copper catalysts
is generally reported to produce only methane, ethylene and
ethane as hydrocarbon products, though the production of higher
1
4
hydrocarbons on polycrystalline copper has been observed, with a
Figure 3. Faradaic efficiencies for the reduction of CO
2
on Pd-Au in a pH 6.7 phosphate
recent study showing the production of C -C hydrocarbons on a
1
4
15
buffer electrolyte (0.1 M KH
products.
2 4 2 4
PO / 0.1 M K HPO ) toward different C -C hydrocarbon
1
5
2
Cl-induced biphasic Cu O-Cu catalyst.
To confirm that CO is a key intermediate in the production of
4
and pentene, respectively. The highest faradaic efficiency toward C
5
hydrocarbons on Pd-Au, as is the case with copper, the reduction
hydrocarbons, 0.07%, was achieved at -1.4 VRHE
.
of CO on Pd-Au was studied. All mass fragments that were
3
The production of C and higher hydrocarbons on PdAu
previously observed with OLEMS for CO
2
reduction are also
electrodes may go through different mechanistic pathways than the
production of hydrocarbons on Cu electrodes, since Cu generally
detected when CO reduction is performed (see Fig. S20). The onset
potential for the reduction of CO to hydrocarbons lies between -0.6
1
d,3
only produces C
1
and C
2
hydrocarbons.
We propose that the
V ─ -0.7 VRHE while the onset potential for CO
reduction to
2
production of these higher hydrocarbons on PdAu proceeds via a
coupling of -CH2 groups adsorbed on the electrode, as is the case
for the production of ethylene on Cu at higher applied
hydrocarbons lies around -0.8 VRHE, indicating that the reduction of
CO
2
to CO would be the potential determining step.
To determine the faradaic efficiencies toward the production
16
overpotentials, or via CO insertion in a Fischer-Tropsch-like step.
This also explains the production of branched hydrocarbons such as
isobutane and 2-methyl-butane. We postulate that the formation of
of the hydrocarbons and CO on Pd-Au and to further elucidate the
identity of the hydrocarbon products, chronoamperometry
experiments with a custom-made flow cell with a high exposed
cathode surface area and a low electrolyte volume were performed.
At all potentials studied the production of CO was observed
2
these -CH groups on the PdAu surface is possible due to a decrease
in CO binding energy, making protonation of CO to CH2
thermodynamically more favorable in comparison to bulk Pd. Since
such a coupling mechanism is essentially a polymerization of
(
see Fig. 3a). The production of CO reached a peak at -0.6 VRHE, with
a maximum faradaic efficiency of 30.9%. Below -0.8 VRHE the
production of hydrocarbons was observed, in agreement with the
onset potential detected with OLEMS. The dominant hydrocarbon
that was produced was methane, reaching a maximum faradaic
efficiency of 2% at -1.4 VRHE (see Fig. 3b). Both ethylene and ethane
absorbed -CH
2
groups, it should give a hydrocarbon product
distribution that obeys the Flory-Schulz distribution. The chain
growth probability is commonly given as a function of the weight
14
fractions of the products:
2
are produced as C hydrocarbons, with a higher faradaic efficiency
2
n
w
n
= (ln
a
) · n
a
(1)
toward ethane. The superior faradaic efficiency toward ethane is
believed to be due to high amounts of hydrogen produced at the
in which w
number n, and
Schulz distribution is plotted for the total amounts of C
n
is the weight fraction of the product with carbon
is the chain growth probability. In Fig. 4 a Flory-
-C
PdAu electrode. A maximum faradaic efficiency of 0.7 % toward C
hydrocarbons is achieved at -1.4 VRHE
Below -0.9 RHE the production of propane, propylene,
2
a
.
1
5
V
hydrocarbons produced at -1.5 VRHE, and Fig. S36-38 show Flory-
Schulz distributions for -1.4, -1.3 and -1.2 VRHE. A clear linear
correlation can be seen at all these potentials, providing additional
support for the hypothesis that these hydrocarbons are produced
butane, 1-butene and isobutene is detected, as shown in Fig. 3c.
The production of propane shows higher faradaic efficiencies than
the production of propylene, as was the case with the
hydrocarbons, with total faradaic efficiency toward
C
C
2
3
a
2
by the coupling of -CH groups.
hydrocarbons of approximately 0.3% at -1.4 VRHE. 1-butene was the
dominant C species, indicating that the hydrogenation of 1-butene
is less favorable than the hydrogenation of ethylene and propylene.
Since the initial structure of the PdAu electrode was highly
enriched in Pd we speculate that the catalytic site producing the
multi-carbon products is a Pd rich PdAu alloy site. The formation of
such a surface alloy can be due to the migration of small amounts of
Au toward the surface, as Au has a lower surface energy than Pd.
This would also explain the small shift in binding energies in the Pd
4
The amount of C
efficiency at -1.3 VRHE
4
hydrocarbons constituted to 0.16% faradaic
.
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3d3/2 and 3d5/2 peaks after 30 minutes of electrolysis at -1.2 VRHE Au electrode, the activity toward hydrocarbon production
(see Fig. S7-9).
increases. This is an indication that the active catalyst for the
To support this hypothesis, Pd monolayers (MLs) were production of hydrocarbons is a Pd rich PdAu surface alloy.
deposited on an Au electrode via galvanic displacement of a Cu UPD
layer and were employed for the reduction of CO . A difference was
Notes and references
2
observed in hydrocarbon distribution for different Pd layer 1.
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89, ed. C.G. Vayenas, R.E. White, M.E. Gamboa-Aldeco,
1
0
Springer, New York, 2008; (b) M. Rakowski DuBois, D.L.
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Sánchez-Sánchez, V. Montiel, D. A. Tryk, A. Aldaz, A.
Fujishima, Pure Appl. Chem., 2001, 73, 1917-1927; (d) R.
Kortlever, J. Shen, K. J. P. Schouten, F. Calle-Vallejo, M. T.
M. Koper, J. Phys. Chem. Lett., 2015, 6, 4073-4082.
Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett., 1985, 11,
-
-
-
-
-
1
2
3
4
5
1
2
Carbon number (n)
3
4
5
2.
1
695-1698.
(a) M. Gattrell, N. Gupta and A. Co, J. Electroanal. Chem.
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der Ham, Z. Qin, M. T. M. Koper, Chem. Sci., 2011, 2,
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3
.
,
Figure 4. Flory-Schulz product distribution of the hydrocarbon products produced
2
at -1.5 V vs. RHE on a Pd-Au electrode in a pH pH 6.7 phosphate buffer electrolyte
(
2 4 2 4 n
0.1 M KH PO / 0.1 M K HPO ). w : weight fraction, n : carbon number.
1
thicknesses (see Fig. S21-24). For 1 ML of Pd on Au the production
of C to C hydrocarbons from CO is observed, while with 2, 3 and 4
1
1
3
2
Jaramillo, Energy Environ. Sci., 2012, 5, 7050.
MLs of Pd C
4
species are observed. In general, the OLEMS results 4.
(a) Y. Hori, A. Murata, R. Takahashi and S. Suzuki, J. Am.
Chem. Soc., 1987, 109, 5022-5023; (b) Y. Hori, A. Murata,
T. Tsukamoto, H. Wakebe, O. Koga and H. Yamazaki,
Electrochim. Acta, 1994, 39, 2495-2500.
show higher intensities of higher hydrocarbon mass fragments with
increasing Pd layer thicknesses, indicating that a high Pd/Au ratio
on the surface is beneficial for the reduction of CO
2
to
5
6
.
.
A. A. Peterson, J. K. Nørskov, J. Phys. Chem. Lett., 2012, 3,
hydrocarbons. Faradaic efficiency measurements with 4 MLs of Pd
on Au showed an increase in faradaic efficiency towards
hydrocarbons with respect to PdAu which is related to the decrease
2
51-258.
D. Gao, H. Zhou, J. Wang, S. Miao, F. Yang, G. Wang, J.
Wang, X. Bao, J. Am. Chem. Soc., 2015, 137, 4288-4291.
F. Gao and D. W. Goodman, Chem. Soc. Rev., 2012, 41,
in hydrogen production due to the decrease of Pd on the surface 7.
8
009-8020.
(a) Y. Suo, I. M. Hsing, Electrochim. Acta, 2011, 56, 2174-
183; (b) J. S. Jirkovský, I. Panas, S. Romani, E. Ahlberg, D.
(
see Fig. S32). This is an indication that the faradaic efficiencies
8
9
.
.
toward hydrocarbons can be fine-tuned by optimizing the catalytic
surface. The experiments with Pd monolayers on Au also indicate
2
J. Schiffrin, J. Phys. Chem. Lett., 2012, 3, 315-321.
2
that the production of hydrocarbons from CO is caused by the
(a) D. Plana, J. Florez-Montano, V. Celorrio, E. Pastor and
D. J. Fermin, Chem. Commun., 2013, 49, 10962-10964; (b)
J. J. L. Humphrey, D. Plana, V. Celorrio, S. Sadasivan, R. P.
Tooze, P. Rodriguez, D. J. Fermin, ChemCatChem, 2016, 8,
intrinsic catalytic properties of the PdAu catalyst and is not due to a
morphology effect caused by the electrodeposition of Pd. To further
emphasize this, it was found that electrodeposited Pd on an Ag
electrode did not show production of higher hydrocarbons from
9
52-960; (c) C. Hahn, D. N. Abram, H. A. Hansen, T.
Hatsukade, A. Jackson, N. C. Johnson, T. R. Hellstern, K. P.
Kuhl, E. R. Cave, J. T. Feaster, T. F. Jaramillo, J. Mater.
Chem. A, 2015, 3, 20185-20194.
R. Kortlever, C. Balemans, Y. Kwon, M. T. M. Koper, Catal.
Today, 2015, 244, 58-62.
CO
2
(see Fig. S25), only methane and small amounts of ethylene
were detected.
In conclusion, a PdAu electrocatalyst is able to reduce CO to a
2
10.
mixture of C
1
to C
5
hydrocarbons, consisting of methane, ethylene,
ethane, propylene, propane, 1-butene, isobutane, butane, 2- 11.
methyl-butane, pentane and pentene, and soluble products such as
formic acid, methanol, ethanol and acetic acid. The observation of
(a) J. F. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomben,
Handbook of X-ray Photoelectron Spectroscopy, Physical
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Anderegg, P. A. Thiel, D. W. Lynch, Phys. Rev. B, 1996, 54,
higher hydrocarbons (C
that future electrocatalysts for the reduction of CO
hydrocarbons can be designed by fine-tuning the CO binding
strength on the catalyst surface. We propose that the higher 13.
hydrocarbons are produced by polymerization of -CH
intermediates adsorbed on the surface. The production of
hydrocarbons from CO starts at an onset potential of -0.8 VRHE
3
-C
5
) is unprecedented, and is an indication
6
301-6306.
2
to (higher)
1
2.
R. Kortlever, I. Peters, S. Koper, M. T. M. Koper, ACS
Catal., 2015, 5, 3916-3923.
D. A. Torelli, S. A. Francis, J. C. Crompton, A. Javier, J. R.
Thompson, B. S. Brunschwig, M. P. Soriaga, N. S. Lewis,
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a
2
1
1
1
4.
5.
6.
H. Shibata, J. A. Moulijn, G. Mul, Catal. Lett., 2008, 123,
2
,
1
86-192.
S. Lee, D. Kim and J. Lee, Angew. Chem. Int. Ed., 2015,
27, 1-6.
while the reduction of CO to hydrocarbons starts at a slightly lower
onset potential of -0.6 VRHE, showing that CO is a key intermediate
in the production of hydrocarbons on PdAu and that the reduction
1
(a) K. J. Schouten, Z. Qin, E. P. Gallent, M. T. Koper, J. Am.
Chem. Soc., 2012, 134, 9864-9867; (b) X. Nie, M. R. Esopi,
M. J. Janik, A. Asthagiri, Angew. Chem. Int. Ed., 2013, 52,
of CO
2
to the CO intermediate is the potential determining step.
Furthermore, we show that by increasing the amount of Pd on an
2
459-2462.
4
| J. Name., 2012, 00, 1-3
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