Selective electro- or photo-reduction of carbon dioxide to formic acid using a Cu-Zn alloy catalyst

Article abstract of DOI:10.1039/c7ta00353f

A copper-and-zinc (Cu-Zn) alloy material was synthesized using a vacuum sealing method, in which evaporated zinc was reacted with copper film or nanoparticles to form a homogeneous Cu-Zn alloy. This alloy was evaluated as an electrocatalyst and/or cocatalyst for photocatalysis to selectively reduce carbon dioxide to formic acid. Based on the optimised alloy composition, the Cu₅Zn₈ catalyst exhibited efficient electrochemical CO₂ reduction. Furthermore, we constructed a photoelectrochemical (PEC) three-electrode system, in which the Cu₅Zn₈ film functioned as the cathode for CO₂ reduction in the dark and strontium titanate (SrTiO₃) served as the photoanode for water oxidation. The PEC system also selectively reduced CO₂ to formic acid with a faradaic efficiency of 79.11% under UV-light and the absence of an applied bias potential. SrTiO₃ particles decorated with nanoparticles of the Cu-Zn alloy also photocatalytically reduced CO₂ to formic acid under UV-light. Isotope trace analysis demonstrated that water served as the electron donor to produce oxygen and organic molecules under UV light, similar to photosynthesis in plants. The Cu-Zn alloy material developed in the present study is composed of ubiquitous and safe materials, and can catalyse CO₂ conversion by means of various kinds of renewable energies.

Full text of DOI:10.1039/c7ta00353f

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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DOI: 10.1039/C7TA00353F
Journal Name
ARTICLE
Selective electro- or photo-reduction of carbon dioxide to formic
acid using Cu-Zn alloy catalyst
†
§*
§
‡
†
†
Ge Yin , Hideki Abe , Rajesh Kodiyath , Shigenori Ueda , Nagarajan Srinivasan , Akira Yamaguchi ,
Masahiro Miyauchi†*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A copper-and-zinc (Cu-Zn) alloy material was synthesized using a vacuum sealing method, in which evaporated zinc was
reacted with copper film or nanoparticles to form a homogeneous Cu-Zn alloy. This alloy was evaluated as an
electrocatalyst and/or cocatalyst for photocatalysis to selectively reduce carbon dioxide to formic acid. Based on the
www.rsc.org/
optimised alloy composition, the Cu
constructed a photoelectrochemical (PEC) three-electrode system, in which the Cu
CO reduction in the dark and strontium titanate (SrTiO ) served as the photoanode for water oxidation. The PEC system
also selectively reduced CO to formic acid with a Faradaic efficiency of 79.11% under UV-light and the absence of an
applied bias potential. SrTiO particles decorated with nanoparticles of the Cu-Zn alloy also photocatalytically reduced CO
5
Zn
8
catalyst exhibited efficient electrochemical CO
2
reduction. Furthermore, we
5
Zn
8
film functioned as the cathode for
2
3
2
3
2
to formic acid under UV-light. Isotope trace analysis demonstrated that water served as the electron donor to produce
oxygen and organic molecules under UV-light, similar to photosynthesis in plants. The Cu-Zn alloy material developed in
the present study is composed of ubiquitous and safe materials, and can catalyse CO
2
conversion by means of various
kinds of renewable energies.
production of various organic agents through thermal
9
1
. Introduction
The increasing emission of carbon dioxide (CO ) into the
2
treatments or catalytic reactions, and can be used in
agricultural and animal husbandry as a silage additive.10 At the
industrial scale, HCOOH is mainly produced from methyl
formate, which is synthesised from CH OH and CO in the
3
presence of a strong salt base. However, this method requires a
large excess of water and the disposal of accumulated by-
products. As the market for formic acid is estimated to exceed
600 million by 2019, it is highly desirable to develop clean
and renewable HCOOH production processes based on CO
conversion with solar-, wind- and/or hydropower.
atmosphere from the consumption of fossil fuels has become a
global problem. The non-renewable nature of hydrocarbon-
based fossil fuels necessitates the use of CO
sources like solar energy, wind power, and hydropower. The
development of CO -recycling systems based on such
renewable energy sources is of growing importance to establish
the desired carbon-neutral society. In particular, the rational
design of high-performance catalysts is in critical demand to
2
-free energy
1
1
2
1
2
$
2
In previous studies, Ru complex catalysts were used for
2
efficiently reduce CO to valuable organic products for different
renewable energy sources under moderate conditions through
electrocatalysis, photoelectrocatalysis and/or photocatalysis.
3
selective CO
2
reduction to HCOOH. Although high efficiency
(
Faradic efficiency over 70%) was achieved, the Ru-complex was
precluded from broad use due to its high cost and complicated
synthesis. To achieve low-cost and easy preparation, numerous
The main products of CO
2
reduction in aqueous media are
types of metals and alloys were proposed as CO
2
-reduction
, Cu-Au , etc.
formic acid (HCOOH), carbon monoxide (CO), formaldehyde
13-23
24,25
26-34
35-37
38,39
40
catalysts
such as Zn
, Cu
, Ag
, Au
1
-7
(HCHO), and methane (CH
4
).
Among the possible CO
2
These materials can catalyse CO
2
reduction owing to their d-
and low surface CO adsorption
strengths . However, only a few metals can achieve selective
CO to HCOOH conversion with low efficiency and high
overpotential. It is known that different CO
products are generated via different pathways depending on
the balance of the adsorption strengths of reduction
anion radicals and CO radicals) on the
Thus we anticipated that the selective
to HCOOH can be realized by tuning the
chemisorption strengths of the reduction intermediates on the
reduction products, HCOOH is highly desirable as it is widely
used in the energy and chemical industries as a fuel and as a raw
material for H
10
electron availability
2
41
8
2
generation. HCOOH is also important for the
2
42
2
reduction
†
Department of Materials Science and Engineering, School of Materials and
Chemical Technology, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku,
Tokyo 152-8552, Japan.
•
—
•
§
Advanced Electronic Materials Center, National Institute of Materials Science, 1-1 intermediates (CO
2
Namiki, Tsukuba 305-004, Japan
7,29,43-46
catalyst surface.
conversion of CO
‡
1
*
Synchrotron X-ray Station at SPring-8, National Institute for Materials Science, 1-
-1 Kouto, Sayo, Hyogo, 679-5148, Japan
E-mail: mmiyauchi@ceram.titech.ac.jp
2
；ABE.Hideki@nims.go.jp
Electronic Supplementary Information (ESI) available.
See DOI: 10.1039/x0xx00000x
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catalyst surface through rational alloying of elemental metals
with different adsorption abilities.
Pure Cu nanoparticles were first deposited onto the surface
3 3
of the SrTiO powder to obtain Cu nanoparticle-loaded SrTiO
View Article Online
DOI: 10.1039/C7TA00353F
Herein, we report that an alloy of Cu and Zn (Cu-Zn), which is powder. An aliquot (2 g) of SrTiO
historically called “poor man’s gold” due to its abundance, non- mL of distilled water that contained 5.28 mg of CuCl
toxicity, and brilliant finish, can selectively convert CO to solution was stirred overnight in an Ar atmosphere and cooled
HCOOH in aqueous media at room temperature probably in an ice bath. An adequate amount of NaBH solution (> 0.61
and CO radicals mL of 1 M solution) was added to this solution to precipitate Cu
to HCOOH nanoparticles. The obtained Cu nanoparticle-loaded SrTiO
powder was collected by centrifugation and dried under
3
powder was dispersed in 40
2
∙2H O. This
2
2
4
•
—
•
because the chemisorption strengths of CO
2
were regulated to an ideal condition for the CO
conversion.
2
3
In previous studies, a Cu-Zn alloy electrode was reported to vacuum at 80 ˚C for 2 hours. The Cu nanoparticle-loaded SrTiO
3
4
7,48
be used in electrocatalytic CO
2
reduction
and a Cu-modified powder was then vacuum-sealed in quartz ampoules with pure
zinc oxide was reported to function as a CO
catalyst at high temperatures.
2
hydrogenation Zn grains and heated at elevated temperatures for 2 days to
However in this study, we yield the desired Cu-Zn nanoparticle-loaded SrTiO powder.
have successfully materialized Cu-Zn alloy catalysts as film
electrodes and SrTiO
-supported nanoparticles for the 2.4 Electrochemical CO
electrochemical/photoelectrochemical reduction and
The prepared Cu-Zn film electrodes, pure Pt plates, and
photocatalytic conversion of CO . The CO reduction activity of Ag/AgCl electrodes were used as working, counter, and
the Cu-Zn catalysts was firstly optimised by tuning the alloy reference electrodes, respectively. Air-tight electrochemical
composition of the film electrodes to Cu:Zn= 5:8 (Cu Zn ). The cells were half filled with a 0.1 M aqueous solution of KHCO3.
Cu Zn film catalyst further exhibited enhanced The Cu-Zn film electrodes were cleaned by 10 cycles of repeated
photoelectrochemical performance for the conversion of CO to cyclic voltammetry in the electrolyte solution: starting bias (E )=
49-51
3
2
-reduction performance tests
3
2
2
5
8
5
8
2
0
HCOOH under UV light when used as the cathode in -0.9 V, 1st operating bias (E ) = -1.8 V, and 2nd operating bias
1
photoelectrochemical cells (PEC) equipped with mesoporous (E )= -0.1 V, with slope 90 mV/sec. Either CO or Ar gas was
2
2
SrTiO
3
photoanodes. It was finally demonstrated that Cu-Zn purged into this electrolyte solution prior to the performance
can also serve as an efficient test.
nanoparticle-loaded SrTiO
3
photocatalyst that selectively promotes the desired CO
2
-to-
HCOOH conversion under UV irradiation.
2.5 Photoelectrochemical CO
An H-type glass cell was used for the photoelectrochemical
CO reduction over the Cu-Zn film electrodes. The Cu-Zn film
electrodes were used as the cathode, while the prepared
mesoporous SrTiO electrodes were used as the photoanode
2
-reduction performance tests
2
2
. Experimental and Evaluation
2
.1 Preparation of Cu-Zn film electrodes
Titanium (Ti) metal sheets (99.5%, Nilaco Co.) were used as
3
(see Figure S1 in the Supporting Information). The cathodic- and
anodic sides of the glass cell were separated by a Nafion
the substrate to fabricate electrodes. Pure Cu films with a
thickness of 50 nm were sputtered onto the surface of the Ti
sheets. These Cu-coated Ti sheets served as pure Cu electrodes
in the control group. The prepared Cu-coated Ti sheets were
placed in vacuum-sealed quartz ampoules with pure zinc (Zn)
metal grains. The vacuum-sealed ampoules were then heated
in an electric oven to temperatures higher than the melting
point of Zn (> 419 °C at ambient pressure) such that the Cu-
coated Ti sheets were annealed in Zn vapour for 1 h to yield
the Cu-Zn film electrodes. The Zn concentration of the Cu-Zn
film electrode was controlled by tuning the Zn vapour
membrane. The electrolyte on the cathodic side was a CO
purged 0.1 M KHCO solution and that on the anodic side was a
.01 M NaOH solution containing 0.1 M KCl. During the
performance tests, the anodic side was irradiated with UV light
Hg-Xe lamp (240-300nm), LA-410UV, Hayashi Ltd., see the
spectrum of the light source in the Supporting Information
Figure S2)). The gaseous products in the headspace such as CH
CO, and H were quantified with a gas chromatograph (BID,
2
-
3
0
(
(
4
,
2
SHIMADZU). The liquid products in the electrolyte solution such
as HCOOH were quantified with an ion chromatograph
(SHIMADZU).
pressure, which was dependent on the heating temperature.
2
.6 Photocatalytic CO
A 100 mg aliquot of the prepared Cu-Zn nanoparticle-loaded
powder SrTiO powder was dispersed in a CO -purged 0.1 M KHCO
2
-reduction performance tests
2
.2 Preparation of SrTiO
SrTiO paste was prepared from commercial SrTiO
ALDRICH, nanopowder, < 100 nm particle size, > 99.5% trace solution that was contained in an air-tight glass reactor
metal basis). Mesoporous SrTiO films 8 µm thick were prepared equipped with a quartz window. The same light source and
on FTO glass substrates by screen-printing the SrTiO paste over chromatograph as mentioned above were used for the
the surface, which were further annealed at 450 ˚C for 1 h to performance test. A rectangular quartz cell (10 x 10 x 50 mm
yield mesoporous SrTiO
electrodes. Exposed surface of the FTO was used for isotope tracing experiments. For the tracing of 13C,
substrate was covered with heat-resistant insulating tapes for the Cu-Zn nanoparticle-loaded SrTiO powders were dispersed
3
photoanodes
3
3
3
2
3
(
3
3
3
)
3
3
an electrochemical measurement.
in 1 mL of 0.1 M KH13CO solution into which 13CO gas was
purged prior to irradiation. For the tracing of 18O, H218O was
used as the solvent for the electrolytes. The gaseous products
3
2
2
3
.3 Preparation of Cu-Zn nanoparticle-loaded SrTiO powder
2
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were quantified with a gas chromatograph-mass spectrometer
GC-MS, SHIMADZU).
View Article Online
DOI: 10.1039/C7TA00353F
(
2
3.2 Electrocatalytic CO reduction
Linear sweep voltammetry (LSV) curves of the prepared Cu-
Zn electrode under CO and Ar purging are shown in Fig. 2. Pure
2
.7 in situ Fourier transform infrared (FT-IR) spectroscopy analysis.
2
The in situ FT-IR spectra were measured using FTIR6100 (JASCO),
the catalytic reaction was conducted in a gas-flow reactor with a
quartz window (S.T. Japan), in which Cu-Zn nanoparticle-loaded
SrTiO powders were put in a porous ceramic cup. For pre-
3
treatment, pure Ar gas was introduced to the reactor at the flow
rate of 100ml/min at 573K for 1 hour. Heater was only used for pre-
treatment. During the catalytic reaction, Ar gas or CO gas were
2
introduced into the reactor through bubbling of water to flow
humid gas. UV light (Hg-Xe lamp (240-300nm), LA-410UV, Hayashi
Cu and Zn electrodes were also evaluated as control groups. The
onset potential of the Cu-Zn electrode was found to be −0.65 V
vs. Ag/AgCl at the lowest, which was markedly lower than those
of pure Cu (-1.0 V vs. Ag/AgCl) and Zn (−1.2 V vs. Ag/AgCl). The
atomic ratio of Cu-Zn in the alloys was controlled by varying the
annealing temperature under vacuum, as described in the
experimental section. Specifically, with increasing heating
temperature, the Zn concentration in the Cu-Zn electrode also
increased. Figure 2d shows the influence of the Zn
concentration in the Cu-Zn alloy electrode on the onset
Ltd, was used as light source. The Cu nanoparticles loaded SrTiO
3
potential for CO
Zn concentration of ~60 atom%. The Zn concentrations in Figure
d were determined using X-ray fluorescence (XRF) elemental
2
reduction. The onset potential was lowest at a
prepared according to the session 2.3 was also analyzed as a control
group. The schematic illustration of the in situ FT-IR analysis was
shown in our supporting information (Figure S3).
2
analysis (raw data shown in Table S1 in the Supporting
Information), and the optimised Zn concentration of ~60% was
consistent with the crystal structure (Cu
XRD (Figure 1c).
5 8
Zn ) determined by
3
. Results
3.1. Characterisation
Fig. 1 HAXPES spectra of a) Cu 2p and b) Zn 2p core-levels in the Cu-Zn alloy
electrode. The HAXPES spectra of pure copper and zinc are also shown. c) The XRD
pattern of the Cu-Zn alloy electrode prepared at 500 ˚C. Ti metal sheet was used
5 8
as the substrate. ▲, Cu Zn (cubic); ■, pure Ti
Fig. 2 Linear sweep voltammetry (LSV) curves of a) Cu
Cu, and c) pure Zn. The onset potential for CO reduction at the Cu-Zn electrode
as a function of Zn atomic ratio is also shown (d).
5 8
Zn alloy electrode, b) pure
2
The prepared Cu-Zn film electrodes were characterised using
hard X-ray photoelectron spectroscopy (HAXPES; h = 5.95 keV)
at the undulator beamline BL15XU of SPring-8 as shown in Fig.
a and 1b. The Cu 2p and Zn 2p peak positions for the Cu-Zn film

5 8
The reaction products generated by the Cu Zn electrode,
pure Cu electrode, and pure Zn electrode under different bias
potentials are also shown in Fig. 3. Notably, the scales of the
5
2
1
electrode (Cu:Zn=5:8, see Fig. 2 and the corresponding
paragraph) were different from those for pure Cu and Zn metals,
indicating that the Cu-Zn film was neither pure Cu nor Zn, but
an alloy of Cu and Zn. Figure 1c shows the X-ray diffraction (XRD)
profile of the same Cu-Zn film electrode. The reflection peaks at
vertical axis for the Cu
about 10 times larger than that for the pure Cu electrode. The
amount of HCOOH generated at the Cu Zn alloy electrode was
markedly higher than that produced at the pure Cu electrode (<
.03 mol), which generated CH and CO as the main reaction
products. The Cu Zn alloy electrode also generated more
HCOOH than the pure Zn electrode (< 1 mol). Specifically,
under −1.0 V vs. Ag/AgCl, the selectivity of the Cu Zn alloy for
5 8
Zn electrode and pure Zn electrode are
5
8
0

4
3
5.97, 37.89, 43.23, and 62.96
°
correspond to the (222), (321),
Zn
5
8
(330), and (600) reflections of the atomically ordered Cu
5
8

alloy (cubic), which is consistent with the expected chemical
composition of the Cu-Zn film, Cu:Zn=5:8. As discussed in the
5
8
HCOOH generation was obviously better than pure Cu or pure
Zn, as shown in Fig. 3d. It is noted that the Faradaic efficiency of
next section, this Cu
enhanced electrochemical CO
5
Zn
8
film electrode exhibited significantly
reduction.
2
5
the Cu Zn
8
alloy for HCOOH generation was 71.11% at a bias
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application of −1.0 V vs. Ag/AgCl. The CO
2
to HCOOH conversion
View Article Online
DOI: 10.1039/C7TA00353F
efficiency of the Cu
5 8
Zn alloy catalyst was higher than that of
47,48
the previously reported Cu-Zn alloy electrocatalysts.
Fig. 4 Amount of HCOOH produced by a PEC cell equipped with a Cu
as the cathode and CO -purged 0.1 M KHCO as the electrolyte. PEC cells equipped
with a pure Cu cathode and a pure Zn cathode in CO -purged 0.1 M KHCO
electrolyte) were used as controls.
5 8
Zn electrode
2
3
2
3
(
We also conducted the photoelectrochemical performance
test under argon (Ar) gas at the cathodic side as a control
experiment (see Figure S4 in the Supporting Information). The
C1 molecule production was quite limited, while H
was significant with Ar bubbling. These results indicate that C1
molecules are produced by CO reduction on our Cu Zn
electrode. In addition to the HCOOH shown in Figure 4, other
gaseous products such as CO, CH , and H in the headspace of
2
production
Fig. 3 Products generated by a) Cu
Zn electrode in electrochemical cells under different applied bias potentials for 1
h. Bias potentials were applied from -0.6 to -1.4 V vs. Ag/AgCl. d) The CO
reduction product distribution generated at Cu Zn electrode, pure Cu electrode,
and pure Zn electrode under -1.0 V vs. Ag/AgCl.
5 8
Zn electrode, b) pure Cu electrode, and c) pure
2
5
8
2
5
8
4
2
3
.3 Photoelectrochemical CO
2
reduction
the reactor were quantified and are shown in our supporting
information (Figure S5). Based on these data, we estimated the
We constructed a photoelectrochemical (PEC) cell, in which
Faradaic efficiency for HCOOH production of the Cu
5 8
Zn
the Cu
Zn
5 8
electrode served as the cathode for CO
was used as a photoanode for oxygen production
photoanode was
irradiated with UV light, water was oxidised at the anode by the
photogenerated holes in the valence band of SrTiO , whereas
the excited electrons were transferred to the cathodic Cu Zn
electrode and used for CO reduction. The production of
HCOOH at the cathodic side under rest potential conditions
without any bias application) is shown in Fig. 4. The amount of
HCOOH generated by the Cu Zn electrode was markedly higher
than those by the pure Cu- or Zn electrodes, which were
2
reduction
cathode combined with a SrTiO photoanode. It is noteworthy
3
and SrTiO
3
that the Faradaic efficiency of HCOOH production was as high
as 79.11%, which is markedly higher than those of pure Cu
from water (Figure S1). When the SrTiO
3
(
28.54%) or Zn (47.68%). In the case of pure Zn, hydrogen was
mainly produced through water splitting and only limited CO
reduction activity was observed. The Cu Zn electrode also had
excellent stability, with the turnover number (TON) for CO
reduction reaching 1458.9, which was twice of that of pure Cu
731.3) under the same experimental conditions. The quantum
3
2
5
8
5
8
2
2
(
(
5
8
efficiency of photoelectrochemical CO
electrode was 0.13%.
2
reduction by the Cu-Zn
consistent with the results of electrochemical CO
shown in Fig. 3.
2
reduction
3
.4 Photocatalytic CO
SrTiO powder
Cu-Zn nanoparticle-loaded SrTiO
2
reduction by Cu-Zn nanoparticle-loaded
3
3
powder was prepared
using a similar vacuum annealing method to that used for film
fabrication, as mentioned in the experimental section. The
prepared Cu-Zn nanoparticle-loaded SrTiO
3
powder was
characterized by TEM, XPS, and UV-visible optical absorption,
and the results are shown in Fig. 5. As can be seen in the TEM
image, the Cu-Zn nanoparticles were approximately 3-5 nm in
3
diameter and uniformly distributed on the SrTiO surface. The
deposition of alloy nanoparticles was also confirmed by UV-
visible spectra, which absorbed visible light photons according
to their metallic plasmon absorptions. The spectral shift
observed in the XPS spectra demonstrated that the Cu
4
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nanoparticles were successfully converted to Cu-Zn alloy Even though the two materials were prepared using the same
View Article Online
DOI: 10.1039/C7TA00353F
nanoparticles.
vacuum annealing method, the Zn concentration of the Cu-Zn
nanoparticles was higher than that of the Cu-Zn electrode
(
Table S2). The XRD pattern of Cu-Zn nanoparticles loaded
SrTiO (5%wt) also proved that pure Zn exists (Figure S8
supporting information). In the case of the particulate system,
Zn vapour would be directly loaded on the surface of SrTiO
3
3
without reacting with Cu during annealing. The present
modification method with Cu-Zn nanoparticles is facile and
applicable to various light harvesting materials to enhance their
2
photocatalytic CO reduction activity.
3
Fig. 5 a) the TEM image of the Cu-Zn alloy nanoparticle-loaded SrTiO nanopowder,
b) the XPS Cu 2p and Zn 2p spectra of the Cu-Zn alloy nanoparticle-loaded SrTiO
3
nanopowder, compared with that of pure Cu and pure Zn. Measurement condition:
1
0 cycles, 3 repetitions, one step = 0.15 eV, 40 ms. c) The UV-Visible spectra of
Fig. 6 Photocatalytic CO
2
reduction by Cu-Zn alloy nanoparticle-loaded STO
were generated.
pure SrTiO
3
(STO), Cu nanoparticle-loaded SrTiO
3
, Zn nanoparticle-loaded SrTiO
3
,
powders. HCOOH, CH , CO, and H
4
2
and the Cu-Zn alloy nanoparticle-loaded SrTiO
3
.
Under UV irradiation of the Cu-Zn nanoparticle-loaded SrTiO
powder, CO was mainly converted to HCOOH, whereas CH
CO productions were limited (Fig. 6). A negligible amount of
hydrogen was detected in the reactor headspace, indicating
3
2
4
and
4
. Discussion
The high selectivity of the Cu-Zn alloy electrode toward
that the Cu-Zn co-catalyst was highly selective for CO
Oxygen production through water oxidation would be between CO
2
reduction. HCOOH generation can be explained based on the relationship
reduction pathways and the state of the catalyst
3
proceeded by photogenerated holes in SrTiO , but oxygen surface. Among numerous studies on CO
2
reduction
2
molecules that cannot be ignored already existed in the head pathways,14, 43, 44, 55 most papers suggest that the formation of
-
space of our reactor and/or were adsorbed on the surface of HCOOH is initiated by a CO
2
radical formed near or weakly
53
5,42,56
the photocatalyst powder as H
2
O
2
molecules. Therefore, we adsorbed on the catalyst surface.
As shown in Figure 7,
•
—
tried to prove that the water molecule was an electron donor when the catalyst surface has weak adsorption for CO
2
anion
radicals
2
O was used as the aqueous would be predominantly formed in a state where the C atom is
3
,54
•—
using isotope trace analysis,
as shown in the Supporting radicals, one electron would transfer to CO
2 2
and CO
18
Information (Figure S6). When H
electrolyte solution, 1
8
O
(m/z=36) generation was detected. available for hydrogenation, leading to the generation of
2
—
This result strongly suggests that water molecules served as the HCOO . The Zn surface has very weak adsorption strength for
electron donor in our photocatalyst system, which is consistent CO
with photosynthesis in natural plants. We also analysed the gas- converted into HCOO as the main reaction product. However,
•
—
2
anion radicals; thus, during CO
2
reduction, CO
2
is
—
1
3
phase products in the headspace under CO
2
(m/z = 45) isotope due to this weak adsorption strength, proton reduction (water
bubbling with light irradiation. As a result, gas phase 13CO (m/z splitting) competes with CO
29) was detected, indicating that the extracted electrons from and the selectivity of zinc for CO
2
reduction is quite low.1
reduction in aqueous electrolytes,
2
0,14
=
water molecules would be excited into the conduction band of
In contrast, when the catalyst surface has high adsorption
to produce strength for the CO •
—
anion radical, the radical formed on the
SrTiO
3
, and these excited electrons react with CO
2
2
fuel molecules.
metal surface is stabilized by d-electron back donation and
•
According to the XPS analysis of the Cu-Zn nanoparticles- would be subsequently converted to the CO radical. If the
•
loaded SrTiO
3
powder collected after the CO
2
reduction, as adsorption strength for the CO radical is weak, the radical
shown in the supporting information (Figure S7), there was no would desorb from the metal surface, resulting in the
valence change of Cu and Zn during the reaction, indicating that production of CO. In contrast, if the adsorption strength is
•
our catalyst was stable under photon irradiation in aqueous appropriately strong to stabilize the CO radical on the catalyst
2
media. The TON for the CO reduction of the particulate system surface, the radical is further converted to CH
4 3
and/or CH OH.
was 11.46, which is lower than that of the Cu-Zn thin film Notably, if the adsorption strength is sufficiently strong, the
•
electrode system under the same experimental conditions. metal surface will be occupied by the CO radical, leading to
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Journal Name
deactivation of the catalyst. In the case of Cu and Cu oxides, demonstrating that the catalyst had high selectViievwitAyrtictolewOnalirnde
although the CO
reduction properties were previously HCOOH generation. The Cu-Zn electrodDeOaI:l1s0o.10h3a9d/C7eTxAc0e0l3le5n3Ft
their adsorption and desorption strength stability with the turnover number (TON) for CO reduction
was so strong that the CO radicals can be adsorbed reaching 1458.9, which was twice that of pure Cu (731.3). The
on the Cu surface, causing the generation of CH and/or quantum efficiency of the photoelectrochemical CO reduction
CH OH. by the Cu-Zn electrode was 0.13%. In addition, we also achieved
By combining Cu with Zn at the atomic scale, the surface nanoparticlisation of the Cu-Zn alloy catalyst. When the Cu-Zn
adsorption and desorption would take place at the interface nanoparticles were loaded onto the surface of SrTiO , the
between Cu and Zn in the alloy compound. Specifically, the photocatalytic reduction of CO reduction was promoted under
anion radical and CO• UV-light. We demonstrated that the Cu-Zn alloy catalyst
radical decreased, and thus, HCOOH was mainly generated as a exhibited high efficiency, stability, and selectivity for CO
CO reduction product. By combining Cu and Zn at the atomic reduction in the electrode system as well as the particulate
scale in an alloy, high selectivity for HCOOH formation and photocatalyst. The ability to selectively regulate CO reduction
2
26,28,34,57-61
reported,
with CO
2
2
2
6
2
4
2
3
3
2
•
—
adsorption strength for both the CO
2
2
2
2
suppression of hydrogen production were achieved. This using the Cu-Zn alloy catalyst described here may prompt the
proposal is also supported by computational study on the free development of new types of alloy catalysts with high selectivity
*
binding energies of hydrogen and COOH of intermetallic alloys for CO
2
reduction.
63
including CuZn alloy . In the present study, in situ FT-IR analysis
was operated to verify our proposal, as shown in the supporting
Author Contributions
information (Figure S9), in humid CO
bidentate surface carbonate were recognized in the case of Cu-
Zn nanoparticles loaded SrTiO powders while monodentate
surface carbonate was detected from Cu nanoparticles loaded
SrTiO powders. These results provide a plausible explanation
for the high selectivity of HCOOH generation of Cu-Zn catalyst.
2
under UV irradiation,
GY acquired all experimental data and wrote the manuscript
in discussion with all co-authors. HA and NR established the
synthesis method of the Cu-Zn alloy catalyst. NR, HA, and SU
conducted the XPS measurements. NS and AY provided
important suggestions on the electrochemical analyses. MM
and HA conducted the study and wrote the manuscript.
3
3
Acknowledgements
This work was supported by a Grant-in-Aid from the Advanced
Catalytic Transformation Program for Carbon Utilization (ACT-
C), Japan Science and Technology Agency (JST); Japan Society
for the Promotion of Science (JSPS) KAKENHI Grant Number
2
6410234 and 16J08808. The XPS measurements were
performed under the approval of the NIMS Synchrotron X-ray
Station (Proposal #2015A4603 and 2015B4605). We thank Mr
K. Hori at the Center for Advanced Materials Analysis, Tokyo
Institute of Technology, for help with TEM characterisation.
We appreciate members of the MANA Foundry for assistance
with metal sputtering, and the Material Analysis Station, NIMS
for support with the analysis of XRF and ICP data. We also
acknowledge Mr. Greg Newton for a critical reading of the
manuscript.
2
Fig. 7 The speculated pathways for CO reduction.
Notes and references
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The Cu-Zn alloy catalyst developed here was prepared from
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