Electrohydrogenation of Carbon Dioxide using a Ternary Pd/Cu2O–Cu Catalyst

Article abstract of DOI:10.1002/cssc.201901636

A simple one-pot method has been developed to synthesize a palladium/cuprous oxide-copper (Pd/Cu₂O–Cu) material with a well-defined structure, by modification of Cu₂O–Cu with Pd through a galvanic replacement reaction. Compared with the well-known copper/cuprous oxide (Cu/Cu₂O) catalysts, the Pd/Cu₂O–Cu material can catalyze the electroreduction of CO₂ into C₁ products with much higher faradaic efficiencies at lower overpotentials in a CO₂-saturated 0.5 m NaHCO₃ solution. In particular, the highest faradaic efficiencies of 92 % for formate and 30 % for methane were achieved at ?0.25 and ?0.65 V (vs. the reversible hydrogen electrode), respectively. The improvement is suggested to be the result of a synergistic effect between PdH and the catalytically active copper sites during electrochemical CO₂ reduction.

Full text of DOI:10.1002/cssc.201901636

Accepted Article
Title: Electrohydrogenation of Carbon Dioxide Using a Ternary Pd/
Cu2O-Cu Catalyst
Authors: Jie Zhang, Jin Li, Si-Xuan Guo, Feng Li, Fengwang Li,
Xiaolong Zhang, Jiantai Ma, Douglas MacFarlane, and Alan
Bond
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Electrohydrogenation of Carbon Dioxide Using a Ternary
Pd/Cu2O-Cu Catalyst
Jing Li,‡[a,b] Si-Xuan Guo,‡[a,c] Feng Li,[d] Fengwang Li,[a,c] Xiaolong Zhang,[a] Jiantai Ma,*[d] Douglas R.
MacFarlane,[a,c] Alan M. Bond [a,c] and Jie Zhang*[a,c]
‡ These authors contributed equally to this work
Abstract:
A
simple one-pot method has been developed to
ease of the competing hydrogen evolution reaction (HER), the
efficiency of CO2 reduction is low.[3] In the past two decades, it
has been demonstrated that metal catalysts, such as Ag, Au, Cu,
Pb, and Sn, can be designed in a way so that they can lower the
overpotential for CO2 reduction while suppressing the HER.[4]
Among these catalysts, only Cu can catalyze CO2 reduction to
hydrocarbons with reasonably high faradaic efficiencies (FEs),
other than products such as CO, HCOO–, and H2. For example,
Baturina et. al investigated four carbon-supported Cu catalysts
toward CO2 electroreduction to hydrocarbons. The ratio of
C2H4/CH4 faradaic efficiencies was found to be dependent on the
particle size, with higher ratios observed for smaller Cu
nanoparticles.[5] Manthiram et. al demonstrated that n-Cu/C
electrocatalyst exhibits an average faradaic efficiency for
methane of 80% during extended electrolysis.[6] Unfortunately,
these Cu catalysts usually require a large overpotential (>0.7 V)
for CO2 reduction which limits its energy efficiency.[7] Thus, there
remains a need to design efficient copper catalysts, and to
understand the mechanistic details on the catalytic surface.
Intensive research efforts, both experimental (e.g. through
nanostructuring, oxide-derivation etc.) and theoretical (e.g.
identification of key intermediates and reaction pathways etc.),
have been devoted to reducing the overpotential and improving
the product selectivity of CO2 reduction catalyzed by Cu-based
catalysts.[8] It has been shown that oxide-derived Cu films or
cubes significantly lowered the overpotential and enhanced the
product selectivity compared to polycrystalline Cu metal
catalysts.[9] For example, Lee et al. reported that the bi-phasic
Cu2O–Cu catalyst showed remarkable catalytic ability towards the
generation of C2–C4 species in KCl electrolyte;[9a] Huan et al.
reported that the addition of nitrogen-based organic compounds,
such as tetramethyl cyclam, in solution significantly promoted CO2
reduction on Cu/Cu2O-modified FTO, and produced formic acid
with a FE of almost 90% at -2.0 V versus Fc+/Fc (Fc=Ferrocene)
in a CO2-saturated DMF/H2O (99:1, v/v) solution;[9b] Kas et al.
reported that CO2 reduction catalyzed by copper nanoparticles,
derived from Cu2O with optimized morphologies and surface
coverages, selectively produced ~8 to 12 times more ethylene
than methane.[9c] Some other studies have suggested that the
introduction of noble metals to Cu-based catalysts, e.g. AuCu
nanoparticles or CuPd nanoalloys, is beneficial to breaking the
scaling relation and stabilizing the intermediates, and thus lowers
the overpotential and improves the product selectivity.[10] For
example, the ordered CuPd catalyst prepared by Kenis et al. can
produce CO with a faradaic efficiency over 80% at a potential of
about -0.55 V vs. the reversible hydrogen electrode (RHE).[10b]
Other forms of PdCu materials, such as PdCu alloy, Pd-Cu
bimetallic aerogel, Cu-modified Pd nanoparticles, and Cu
synthesize a palladium/cuprous oxide-copper (Pd/Cu2O-Cu) material
with well-defined structure, by modification of Cu2O-Cu with Pd
through a galvanic replacement reaction. Compared to the well-known
copper/cuprous oxide (Cu/Cu2O) catalysts, the newly synthesized
Pd/Cu2O-Cu material can catalyze the electroreduction of CO2 into C1
products with much higher faradaic efficiencies at lower overpotentials
in a CO2-saturated 0.5 M NaHCO3 solution. In particular, the highest
faradaic efficiencies of 92% for formate, and 30% for methane were
achieved at -0.25 V and -0.65 V vs. the reversible hydrogen electrode,
respectively. The presence of Pd enhances the current density and
reduces the overpotential for electrocatalytic CO2 reduction. The
improvement is suggested to be the result of a synergistic effect
between PdH and the catalytically active copper sites during
electrochemical CO2 reduction.
Introduction
Carbon dioxide (CO2) is a major greenhouse gas that causes
serious environmental issues. However, it also provides a cheap
and abundant C1 building block for the synthesis of value added
chemicals and fuels.[1] Among all the methods reported so far,
electrochemical reduction of CO2 offers an effective way to
convert CO2 back to fuels and other valuable chemicals, provided
renewable electrical energy is used.[2] However, due to the high
stability and low solubility of CO2 in aqueous media, and the large
overpotential required for its electrochemical reduction versus the
[a]
[b]
Dr. J. Li, Dr. S.-X. Guo, Dr. F. Li, Dr. X. Zhang, Prof. D. R.
MacFarlane, Prof. A. M. Bond, Prof. J. Zhang.
School of Chemistry, Monash University, Clayton, Victoria 3800,
Australia
E-mail: jie.zhang@monash.edu; majiantai@lzu.edu.cn
Dr. J. Li.
State Key Laboratory of High-efficiency Coal Utilization and Green
Chemical Engineering, College of Chemistry and Chemical
Engineering, Ningxia University, Yinchuan，Ningxia, 750021, P. R.
China.
[c]
[d]
Dr. S.-X. Guo, Dr. F. Li, Prof. D. R. MacFarlane, Prof. A. M. Bond,
Prof. J. Zhang.
ARC Centre of Excellence for Electromaterials Science, Monash
University, Clayton, Victoria 3800, Australia
Dr F. Li, Prof. J. Ma.
State Key Laboratory of Applied Organic Chemistry (SKLAOC), The
Key Laboratory of Catalytic Engineering of Gansu Province, College
of Chemistry and Chemical Engineering, Lanzhou University,
Lanzhou, Gansu, 730000, P. R. China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201901636
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overlayers on tetrahexahedral Pd nanocrystals with high-index
facets have also shown promising CO2 reduction activity and
selectivity.[11]
Encouraged by the above literature results, in this study, we
report a facile one-pot synthesis of a new ternary Pd/Cu2O-Cu
catalyst which has excellent electro-catalytic activity for CO2
reduction and achieves selective formation of C1 species in an
applied potential range of -0.15 V to -0.75 V vs. RHE in a CO2-
saturated 0.5 M NaHCO3 solution. In particular, electrolysis at -
0.25 V produces formate with ~ 92 % faradaic efficiency, while at
-0.65 V, 30% FE for methane is obtained. The origin of the
synergistic effects of Pd modification and the Cu2O-Cu structure,
which result in much improved activity, stability, and product
selectivity, was discussed.
Fig. 1 (a, b) TEM and (c) HR-TEM images of the Pd40/Cu2O-Cu nanoparticles;
(d) HAADF-STEM image of a pile of Pd40/Cu2O-Cu nanoparticles, and HAADF-
STEM-EDX elemental mapping images of O-K, Pd-K and Cu-K. (e) Cross-
sectional compositional line profiles of O, Cu and Pd in Pd40/Cu2O-Cu
nanoparticles recorded along the line shown in the HAADF-STEM image (inset).
(f) EDX spectrum of the Pd40/Cu2O-Cu nanoparticles.
Scheme I. Synthesis procedures of Pd/Cu2O-Cu Nanoparticles.
The XRD patterns of the Pd40/Cu2O-Cu, PdCu and Cu2O-Cu
catalysts are shown in Fig. 2a. In the case of Pd40/Cu2O-Cu, the
five main diffraction peaks located at 29.6º, 36.5º, 42.4º, 61.3º and
73.9º are ascribed respectively to the (110), (111), (200), (220)
and (311) reflections of a face centered cubic (fcc) Cu2O crystal
(JCPDS Card No. 65-3288). The three major diffraction peaks
located at 40.4º, 46.7º, 68.4º are respectively indexed to the (111),
(200) and (220) reflections of a fcc Pd crystal (JCPDS Card No.
01-1201), which are consistent with those of the PdCu nanoparticle
catalyst. The Cu diffraction peaks in Pd40/Cu2O-Cu is not obvious
due to its low content. Quantitative analysis of compositions of the
Cu2O-Cu catalyst was carried out by analyzing the XRD data
using the Relative intensity ratio (RIR) method (the calculation
details are described in the ESI†),[12] and the results showed that
the Cu2O-Cu catalyst consists of 10.35 wt % Cu and 89.65 wt%
Cu2O. The XPS Pd 3d spectrum of the Pd40/Cu2O-Cu catalyst (Fig.
2b) can be deconvoluted into two pairs of peaks, which are
attributed to metallic Pd0 and Pd2+ species.[13] In the Cu 2p region
of the XPS spectrum (Fig. 2c), the peaks at 932.6 and 952.5 eV
Results and Discussion
Characterizations of the nanomaterials Pd/Cu2O-Cu, PdCu
and Cu2O-Cu.
The morphology of the Pd/Cu2O-Cu catalyst (Fig. 1a and b) was
examined by TEM under different magnifications. The low
magnification TEM image (Fig. 1a) shows a uniform structure,
while the high-resolution TEM image (Fig. 1c) reveals the lattice
fringe spacings of 2.45 Å, 2.20 Å and 2.08 Å, attributed to the
(111) facet of Cu2O, Pd and Cu, respectively. A HAADF-STEM
image together with the EDX line scans and spectrum (Fig. 1 d, e
& f) were obtained to determine the elemental distribution of the
Pd/Cu2O-Cu catalyst. Both Pd and Cu elements distribute
uniformly over the entire sample (Fig. 1d), however, the Cu
content is higher than that of Pd when the EDX line was scanned
in the sample area (Fig. 1e). The exact loading of Pd on Pd/Cu2O-
Cu is 40% (denoted as Pd40/Cu2O-Cu), as determined by ICP-
AES. Elements C, Ni, Pd, Cu and O are also detected in the EDX
spectrum (Fig. 1f). The C and Ni signals arise from the sample
holder. PdCu nanoparticles show similar uniform morphology as
the Pd40/Cu2O-Cu catalyst, but with larger particle size, while
Cu2O-Cu catalyst shows various shapes such as cubes and
spheres with different sizes, as well as irregular shapes (Fig. S3†).
are consistent with the binding energies of Cu+ 2p3/2 and Cu+ 2p1/2
,
respectively. Compared to the Cu 2p XPS spectrum of Cu2O-Cu,
a negative shift of ~0.2 eV in binding energy was observed from
the Pd40/Cu2O-Cu catalyst (Fig. S4†), which confirms that charge
transfer occurred between Cu and Pd and the presence of an
intimate contact between them, consistent with the TEM
observation (Fig. 1c). Since the binding energy values of Cu and
Cu2O are similar, it is very difficult to distinguish them by the XPS
spectra based solely on the Cu 2p data. Thus, the X-ray-
generated Auger Cu LMM spectrum has been obtained for a more
reliable assignment for Cu and Cu2O. As shown in Fig. 2d, the Cu
LMM spectrum for the Pd40/Cu2O-Cu catalyst shows two peaks at
916.3 and 918.0 eV, which are attributed to Cu2O and Cu,
respectively, confirming their presence.[12c] The PdCu catalyst is
also characterized by EDX and XPS measurements (Fig. S1†).
The highly sensitive XPS results show the presence of residual
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Cu0 in the PdCu catalyst (~ 1 %, atomic concentration, see detailed
discussion in the ESI†).
The distribution of gaseous and liquid products obtained during
the electrochemical reduction of CO2 using the Pd40/Cu2O-Cu
catalyst under bulk electrolysis conditions in a CO2–saturated 0.5
M NaHCO3 aqueous solution (pH 7.2) in the potential range from
-0.15 V to -0.75 V vs. RHE is presented in Fig. 4a. The amount of
charge passed as a function of electrolysis time is provided in Fig.
S5†. As the applied potential changes from ⁃0.15 V to -0.25 V,
the FE for HCOO– increases from about 80% to 92%, while that
for H2 increases slightly. As the applied potential becomes more
negative, the FE for HCOO– decreases and is less than 10% when
the potential is more negative than -0.65 V, while that for H2
increases significantly and reaches 82% at a potential of -0.75 V.
A small amount of CH3OH is detected in the potential range of
⁃0.35 to -0.65 V, with a maximum FE of about 6% obtained at
⁃0.55 V. Both CH4 and CO are detected only in the potential range
of -0.55 V to -0.75 V, with a maximum FE of ~30% for CH4
obtained at -0.65 V, and ~4% for CO at -0.75 V vs. RHE. No C2+
products were obtained with the Pd40/Cu2O-Cu catalyst. Although
the presence of Cu2O was reported to favor the formation of C2H4
during CO2 reduction, the formation of C2+ products only occurs
at highly negative potentials at high pH,[14] which can lower the C-
C coupling energy barrier and suppress the primary competing
reactions including H2 and CH4 formations. Furthermore, in this
work, PdH is formed during the catalytic reaction, and the
adsorption/desorption of CO on PdH becomes thermoneutral,
thus CO adsorption is weakened and the C-C coupling reaction is
unfavored.[15] Representative NMR and GC spectra obtained are
presented in Figs. S6 and S7†. At PdCu nanoparticles modified
electrodes, a maximum FE of ~73% for HCOO– is obtained at
⁃0.15 V vs. RHE as shown in Fig. 4b, which decreases as the
applied potential becomes more negative and reaches around 2%
at -0.75 V. In contrast, for the Cu2O-Cu catalyst, HCOO– could
only be produced when the potential is more negative than -0.55
V with very low FEs, and a maximum FE of ~6% is obtained at
⁃0.75 V vs. RHE. The partial current densities for HCOO–
production with different catalysts are compared in Fig. 4c. The
partial current density of HCOO– reaches ~4.2 mA cm-2 with
Pd40/Cu2O-Cu and ~1.5 mA cm-2 with PdCu at -0.25 V vs. RHE,
respectively. From Fig. 4d, it can be seen that the partial current
density for H2 production remarkably increases when the applied
potential becomes more negative (since the H2 partial current
densities below -0.35 V are low, a broken Y-axis presentation was
used for convenient comparison in the whole potential range),
indicating that the rate of H2 production accelerates at more
negative applied potentials. An earlier study by Li and Kanan,[16]
using a thick Cu2O film on Cu as the electrode, achieved ~38%
FE for HCOO– at -0.55 V and ~42% FE for CH4 at -0.97 V vs. RHE,
respectively. Recently, Melchionna et al. demonstrated that CO2
could be converted to HCOOH at near zero overpotential in an
aqueous solution using an electrode modified with
Pd@TiO2/single walled carbon nanoHorns.[17] Data reported with
other catalysts are summarized in Table S1†, which show that the
Pd40/Cu2O-Cu catalyst has excellent electrocatalytic performance
for CO2 reduction in terms of product selectivity at low
overpotentials, particularly with respect to HCOO– and CH4.
Pd/Cu2O-Cu catalysts with different Pd loadings on Cu2O-Cu
(20 wt. % and 60 wt. %, denoted as Pd20/Cu2O-Cu and
Fig. 2 (a) XRD patterns obtained for the Pd40/Cu2O-Cu, PdCu and Cu2O-Cu
nanoparticles; XPS spectra of (b) Pd 3d and (c) Cu 2p; and (d) Cu LMM
spectrum of Pd40/Cu2O-Cu..
Linear sweep voltammetric (LSV) studies of the Pd40/Cu2O-Cu,
PdCu and Cu2O-Cu catalysts were conducted in a 0.5 M NaHCO3
aqueous solution saturated with CO2 at a scan rate of 5 mV s–1
without iR compensation (Fig. 3a). All data were recorded after 50
cycles of potential scan in the range from 0 to -0.8 V vs. RHE at a
scan rate of 50 mV s–1, to reduce CuO and the majority of Cu2O
to the thermodynamically more stable Cu0 and a small amount of
metastable Cu+. Since the reduction of these copper species
contribute significantly to the current response in the first few
cycles of potentials, linear sweep voltammograms obtained from
a freshly prepared Pd/Cu2O-Cu modified electrode were not
shown to avoid confusion. The results in Fig. 3a show that the
Pd40/Cu2O-Cu catalyst has the most positive onset potential (-0.1
V) and is 0.2 V and 0.5 V more positive than those of -0.3 V and
-0.6 V for the PdCu and Cu2O-Cu catalysts, respectively. The
current density increases rapidly when the potential is more
negative than -0.2 V and -0.3 V, and reaches ~ 25.5 mA cm–2 and
~19.4 mA cm–2 at ⁃0.8 V, under CO2 and N2 atmospheres,
respectively (pH values 7.2 and 8.5, respectively) (Fig. 3b).
Fig. 3 (a) Linear sweep voltammograms of Pd40/Cu2O-Cu (red line), PdCu (blue
line) and Cu2O-Cu (green line) modified GCEs in a CO2–saturated 0.5 M
NaHCO3 aqueous solution, (b) Pd40/Cu2O-Cu modified GCE in a CO2–saturated
(red line) and N2–saturated (brown line) 0.5 M NaHCO3 aqueous solutions. Scan
rate 5 mV s–1
.
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Pd60/Cu2O-Cu, respectively) were tested to investigate its
influence on the product distribution of CO2 reduction. The
distribution of gaseous and liquid products obtained under the
same electrolysis conditions are presented in Fig. S8†. When the
loading of Pd is 20 wt. % and 60 wt. %, the FE for HCOO– is ~80 %
and ~65 % respectively at -0.25 V, which is inferior to Pd40/Cu2O-
Cu. Furthermore, a maximum FE of ~7 % for CH4 was obtained
at ⁃0.65 V on the Pd20/Cu2O-Cu catalyst, while no CH4 was
detected on Pd60/Cu2O-Cu. Based on the above observations, it
is concluded that the synergistic effect is optimal when the loading
of Pd is 40 wt. %.
of PdH (2.29 Å), where H occupies metal vacancy as a dopant to
form PdH and hence changes the unit cell volume.[19] However,
there is no obvious PdH diffraction peaks in Fig. 5a, because the
amount of stabilized PdH is relatively small. In a control
experiment, the Pd40/Cu2O-Cu was firstly fully reduced to Pd/Cu
in a CO2-saturated NaHCO3 solution (confirmed by the XRD data
shown in Fig. S9†) before being used in electrolysis. Much lower
faradaic efficiencies for HCOO– were obtained over the whole
potential range studied (Fig. 5c), suggesting the important role of
Cu2O in CO2 reduction to HCOO–. It has been reported that Pd-
based catalysts and the enzyme formate dehydrogenase can
catalyze the reduction of CO2 to HCOO– at close to its equilibrium
potential.[20] Our results show that the Pd/Cu2O-Cu catalyst can
drive this CO2 reduction reaction under moderate overpotentials.
A HCOO– partial current density as high as 4.2 mA cm-2 was
obtained at -0.25 V vs. RHE on the Pd40/Cu2O-Cu modified
electrode, compared to that of 1.5 mA cm-2 on the PdCu catalyst
under the same conditions. On most catalysts, the initial step for
CO2 reduction involves the transfer of an electron to CO2 to form
CO2˙- which requires a large overpotential.[21] It has been reported
that supported Pd can effectively catalyze the reduction of
bicarbonate to formate in the presence of H2.[22] Kanan et al. have
shown that Pd nanoparticles dispersed on carbon support
catalyzed CO2 reduction to formate with a partial current density
as high as 10 mA cm-2 at -0.2 V vs. RHE with a mass loading of
Pd as low as 50 μg cm-2.[20b] Such high activity of Pd/C was
attributed to the electrohydrogenation of CO2 to formate by the
electrochemically generated PdH on the catalyst surface. In this
work, the formation of PdH on the catalysts surface is confirmed
by the XPS spectra taken after electrolysis at -0.25 V (Fig. S10†).
In addition to the peaks at the binding energies of 335.4 and 337.5
eV corresponding to Pd0 3d5/2 and Pd2+ 3d5/2, respectively, the
peak at 335.9 eV is attributed to PdH. The electrokinetic
measurements support a rate law given by Eq. 2 where
electrohydrogenation of CO2 is the rate-limiting step. [20c]
Fig. 4 CO2 reduction activities over the Pd40/Cu2O-Cu, PdCu and Cu2O-Cu
catalysts in a CO2-saturated 0.5 M NaHCO3 solution. (a) FEs of gaseous and
liquid products obtained using Pd40/Cu2O-Cu as the catalyst; (b) FEs of HCOO–
obtained on different catalysts; (c) HCOO– and (d) H2 partial current densities
obtained on different catalysts.
It has been reported that the product selectivity in CO2
reduction was influenced by the oxide phase of the Cu-based
catalysts, as well as the structure of metallic copper.[16, 18] In order
to explore this issue, XRD measurements were undertaken after
controlled potential electrolysis with 10 C of charge at -0.25 V and
-0.75 V vs. RHE using the Pd40/Cu2O-Cu catalyst. As shown in
Fig. 5a, peaks located at 30º and 42.4º ascribed to the (110) and
(200) facets of Cu2O are clearly observed, although their
intensities are weak. The (110) diffraction peaks of Cu2O shift to
slightly higher 2θ compared to those of the original Pd40/Cu2O-Cu
catalyst, suggesting the formation of compressed lattices of Cu2O
during the electrolysis process. It seems that only a small amount
of charge was used to reduce Cu+, resulting in no significant
diffraction peaks of Cu in the XRD. In the XPS spectra (Fig. 5b),
some Cu2+ appears whereas no XRD pattern associated with
CuO was observed. This fact suggests that the Cu2+ peak is most
likely from the CuO surface layers generated after electrolysis due
to the oxidation of Cu in air. The Cu+ peaks observed in the XPS
spectra imply that the non-reduced Cu2O layer exists after
electrolysis. The morphology of the Pd40/Cu2O-Cu particles
become more regular (Fig. 6) after electrolysis. A slight increase
in the lattice spacing of Pd (2.20 Å) is attributed to the formation
j=k [CO2] exp(-aEF/RT)
(1)
where k is the rate constant and a is the transfer coefficient and
E is the applied potential, R is the gas constant and T is the
absolute temperature. A Tafel slope of 118 mV dec-1 is expected
if α = 0.5. In this study, a Tafel plot was constructed in the potential
range from -0.17 to -0.25 V vs. RHE, and the Tafel slope value of
135 mV dec-1 was obtained (Fig. 5d). This value is close to the
value of 118 mV dec-1. Therefore, in the presence of Pd, CO2 is
proposed to be converted to HCOO– by PdH which was generated
in situ electrochemically. A schematic representation of the
mechanism for CO2 reduction on Pd40/Cu2O-Cu is shown in
Scheme II. For the generation of CH4, the initial formation of *CO
and the following hydrogenation to *H3CO (methoxy) are
proposed, followed by the oxophilicity of the Pd40/Cu2O-Cu
surface which can break the C-O bond, and the final formation of
CH4 by the hydrogenation process.[21b, 23]
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distribution at their interface and hence efficiently stabilizing Cu+
at negative applied potentials as suggested by the XPS data (Fig.
S4†). At an applied potential of -0.25 V, the amounts of formate
generated were measured after consuming 10 C, 20 C, 30 C and
40 C of charges (Fig. 7b). The FEs of HCOO– remain essentially
unchanged from 10 C to 40 C (92% at 10 C vs 89% at 40 C). XRD
results (Fig. S11†) suggest that Cu2O is present even though its
content significantly decreased in comparison with that found in
the pristine Pd40/Cu2O-Cu catalyst. This result agrees with those
obtained by Sargent and coworkers which show that Cu+ is
present at an applied potential as negative as -1.2 V vs. RHE for
over 1 h and plays an important role in the catalytic CO2
reduction.[24] Controlled potential electrolysis was also undertaken
in a N2 atmosphere at -0.15, -0.45 and -0.75 V vs. RHE (Fig.
S12†). H2 was the main product with a faradaic efficiency of at
least 90% under all conditions.
Fig. 5 (a) XRD powder patterns of Pd40/Cu2O-Cu before and after electrolysis
in CO2 saturated 0.5 M NaHCO3; (b) Cu 2p XPS spectra of Pd40/Cu2O-Cu after
electrolysis at ⁃0.75 V for 10 C; (c) Faradaic efficiency for HCOO– (red bars) and
H2 (black bars) production at Pd/Cu as a function of applied potential. (d) Partial
current density of HCOO– vs. applied potential on Pd40/Cu2O-Cu. The Tafel
slope is 135 mV dec–1
.
Fig. 7 (a) Chronoamperometric i-t data obtained for a Pd40/Cu2O-Cu modified
electrode for reduction of CO2 in a CO2-saturated 0.5 M NaHCO3 solution under
stirring at applied potentials over the range of -0.15 V to -0.75 V vs. RHE; (b)
jtotal–time relationship (black trace) and FE of HCOO– (blue triangles) obtained
for the CO2 reduction in a CO2-saturated 0.5 M NaHCO3 solution under an
applied potential of -0.25 V vs. RHE.
Conclusions
Fig. 6 TEM (a) and HR-TEM (b-c) images of Pd40/Cu2O-Cu after potentiostatic
electrolysis at -0.75 V for a total charge of 10 C
A synergistic effect between the Cu2O-Cu nanostructure and the
introduced Pd atoms in electrocatalytic reduction of CO2 has been
recognized for the first time. We found that the modification of
Cu2O-Cu nanoparticles with Pd facilitates the formation of C1
products at low overpotentials. The Pd/Cu2O-Cu catalyst has
been characterized in considerable detail and achieves
remarkable catalytic activity and excellent stability for the
reduction of CO2 to produce C1 species. In particular, the highest
faradaic efficiencies of 92% for HCOO–, and 30% for CH4 were
obtained at -0.25 V and -0.65 V vs. RHE respectively. A simple
one-pot synthesis provides a straightforward strategy towards the
generation of this new electrocatalyst.
Scheme II. Electrohydrogenation mechanism for CO2 reduction to form formate
on Pd/Cu2O-Cu.
The potentiostatic i-t response associated with bulk electrolysis
was used to evaluate the stability of the electrocatalyst. A series
of i-t curves (Fig. 7a) obtained at applied potentials over the range
of -0.15 V to -0.75 V vs. RHE show that the Pd40/Cu2O-Cu catalyst
is stable on the corresponding time scale. This high stability is
probably due to the fact that the work function of Pd (~5.12 eV) is
higher than that of Cu2O (~4.3 eV), facilitating the electron
transfer from Cu2O to Pd to equilibrate the electron Fermi
Experimental Section
Materials
NaHCO3 (ACS grade), acetone (absolute GR, 99.7%), dimethylsulfoxide
(DMSO), and Nafion (5 wt. %) were purchased from Merck; anhydrous
copper chloride (CuCl2), disodium tetrachloropalladate (Na2PdCl4, 98%),
and sodium borohydride (NaBH4) were from Sigma–Aldrich. All chemicals
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10.1002/cssc.201901636
ChemSusChem
FULL PAPER
were used without further purification. Water from a Milli Q water
purification system (18.2 MΩ cm) was used for the preparation of all
aqueous solutions.
Instruments) working electrodes were employed. The Ag/AgCl reference
was calibrated against the RHE reference scale following a procedure
reported in the literature,[25] and the formula given in Eq. 2 was derived
(see the ESI† for details). All potentials reported herein are with respect to
the RHE scale unless otherwise noted.
Synthesis of the Pd/Cu2O-Cu catalyst
E (vs. RHE) = E (vs. Ag/AgCl) + 0.205 V + 0.059 V ×pH
(2)
Prior to catalyst synthesis, all solvents used were purged with N2 for 30
min to remove oxygen. The synthesis process is schematically illustrated
in Scheme I. In detail, 15.73 mg of anhydrous copper chloride (CuCl2) was
dissolved in 25 mL of deionized water under magnetic stirring. Next, 30 mL
of freshly prepared aqueous NaBH4 (10 mg) solution was added dropwise
until the mixture turned dark purple under vigorous stirring to form Cu2O-
Cu nanoparticles. The mixture was stirred overnight to make sure all
residual NaBH4 is decomposed. Next, 1.5 mL of Na2PdCl4 (17.65 mg)
aqueous solution was added to the mixture, and the Pd/Cu2O-Cu catalyst
was obtained by galvanic replacement which involved the reduction of
To prepare the catalyst modified GC working electrode, 5 mg of the
catalyst was dispersed in a mixture of 2.5 mL deionized water, 1.25 mL
0.5 % Nafion solution and 1.25 mL ethanol (v:v:v = 2:1:1) by sonication for
30 min. 5 μL of this dispersion was then drop cast onto a polished GCE
and dried under an infrared lamp.
Bulk electrolysis was operated in an airtight H-shape electrolysis cell with
two compartments separated by a glass frit at room temperature and
pressure. A platinum (Pt) gauze counter electrode, a modified glassy
carbon plate (1 cm×1 cm, loaded with 50 μL catalyst dispersion) working
electrode were utilized along with the same reference electrode used for
voltammetric measurements. Before electrolysis, the solution was bubbled
with CO2 gas for at least 30 min to obtain a CO2-saturated electrolyte
solution, and then quickly sealed tightly. The solutions in both
compartments were stirred during electrolysis.
2-
PdCl4 to Pd metal and the oxidation of Cu. The entire synthesis was
performed over one day with continuous stirring at room temperature
under a N2 atmosphere to prevent the oxidation of Cu by O2 in air. Finally,
the black product was filtered, washed and stored in acetone. For
comparison, Pd nanoparticles were also synthesized using galvanic
replacement reaction by adding excess amount of Pd2+ precursor to
oxidize Cu to Cu2+, and the mixture was stirred overnight. The as-obtained
Pd nanoparticles are denoted as PdCu since it contains small amount of Cu
based on XPS analysis (vide infra).
Acknowledgements
Characterization of the catalysts and products of electrochemical
CO2 reduction.
This work was supported by the Australian Research Council
through the ARC Centre of Excellence for Electromaterials
Science and the Fundamental Research Funds for the Central
Universities in China (lzujbky-2017-kb11). The authors gratefully
acknowledge technical assistance from Mr. Philip Holt for the GC
measurements and Dr. Peter Nichols for the NMR measurements.
J.L. is specifically thankful to the China Scholarship Council for a
scholarship. The authors acknowledge the use of facilities in the
Monash Centre for Electron Microscopy.
Transmission electron microscopic (TEM) and High-angle annular dark-
field scanning transmission electron microscopic (HAADF-STEM) images
were obtained with a FEI Tecnai G2 T20 TWIN TEM Instrument. X-ray
powder diffraction (XRD) data were collected with a Bruker D2 PHASER
powder diffractometer (Cu Kα radiation, λ = 0.15406 nm). The relative
intensity ratio (RIR) method (the calculation details are presented in the
Supporting Information) was used to estimate the mass fraction of Cu and
Cu2O in the catalyst when analyzing the XRD data. The compositions of
the nanomaterials were determined by Inductively Coupled Plasma Atomic
Emission Spectroscopy (ICP-AES) (PerkinElmer Optima 4300 DV). The X-
ray photoelectron spectroscopic (XPS) spectra obtained with
PerkinElmer PHI-5702 instrument were calibrated by the position of the C
1s peak.
a
Keywords: CO2 electroreduction • ternary nanocatalyst •
palladium • copper • formate • methane
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Entry for the Table of Contents
FULL PAPER
Ternary Pd/Cu2O-Cu efficiently
catalyzes electroreduction of CO2 to
formate and methane in bicarbonate
media via an electrohydrogenation
pathway.
Jing Li,‡ Si-Xuan Guo,‡ Feng Li,
Fengwang Li, Xiaolong Zhang, Jiantai
Ma,* Douglas R. MacFarlane, Alan M.
Bond and Jie Zhang*
Page No. – Page No.
Electrohydrogenation of Carbon
Dioxide Using a Ternary Pd/Cu2O-Cu
Catalyst
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