Electrocatalytic conversion of carbon dioxide to formic acid over nanosized Cu6Sn5 intermetallic compounds with a SnO2 shell layer

Takao Gunji*a, Hiroya Ochiaia, Yu Isawaa, Yubin Liua, Fumihiro Nomuraa, Masahiro Miyauchib and Futoshi Matsumotoa
aDepartment of Material and Life Chemistry, Kanagawa University, 3-27-1, Rokkakubashi, Kanagawa-ku, Yokohama, Kanagawa 221-8686, Japan. E-mail: tgunji@kanagawa-u.ac.jp
bDepartment 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

Received 1st August 2019 , Accepted 19th October 2019

First published on 21st October 2019


A novel ordered intermetallic compound of carbon-black-supported Cu6Sn5 nanoparticles (Cu6Sn5 NP/CB) in which Cu6Sn5 has a NiAs-type structure was successfully prepared through a wet chemical method using lithium triethylborohydride as a reducing agent. The prepared ordered intermetallic compound was characterized using X-ray diffraction (XRD), transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray absorption fine structure spectroscopy (XAFS). The XRD measurements confirm the formation of the NiAs-type ordered intermetallic Cu6Sn5. XPS and STEM-X-ray energy dispersive spectroscopy measurements allowed us to confirm the Cu6Sn5 structure. The surface of the intermetallic Cu6Sn5 was found to be covered by SnO2, indicating that a core–shell structured intermetallic compound (i.e., Cu6Sn5 core/SnO2 shell) had formed. The Cu6Sn5 NP/CB material exhibited a faradaic efficiency of 65.3% at −0.6 V for HCOO formation via electrochemical CO2 reduction, which is superior to those of the non-intermetallic Cu NP/CB and Sn NP/CB samples. From the XAFS measurements, we determined the Sn–Sn distance in the SnO2 on the surface of the Cu6Sn5 NPs, and the key factor affecting the high selectivity was found to be the 4.9% compressive strain of the SnO2 shell layers on the Cu6Sn5 compared to that of the Sn NP/CB sample.


1. Introduction

The electrochemical reduction of CO2 is a high-value reaction because it can be used to reduce the amount of this greenhouse gas in the atmosphere.1 In addition, the development of an electrocatalyst enabling highly selective CO2 conversion could allow the creation of a co-catalyst for photocatalytic CO2 reduction because both electrocatalytic and photocatalytic reactions are electron transfer processes. However, CO2 is a very stable compound because of the strength of the C[double bond, length as m-dash]O bonds, and, thus, it is extremely hard to convert CO2 into other compounds. Moreover, potassium bicarbonate, which is commonly used as the electrolyte for electrochemical CO2 reduction, decomposes on most catalysts at the potentials required to reduce CO2, forming hydrogen (H2) by the reduction of protons (H+) and reducing the faradaic efficiency (FE) for the electrochemical reduction of CO2. Therefore, it is important to use a catalyst that can prevent the H2 evolution reaction (HER).

Widely studied electrocatalysts for electrochemical CO2 reduction include precious metals, such as Pd, Ag, and Au.2–6 It is known that precious metals can efficiently reduce CO2 to carbon monoxide (CO) or formate (HCOO) with high selectivity at low overpotentials. When precious metals, such as Pd nanoparticles (NPs), are used, formic acid can be formed by the electrochemical reduction of CO2 at −0.1 to −0.45 V, meaning that much lower overpotentials are required for the formation of HCOO;7–10 in contrast, bulk Pd requires a potential of less than −0.8 V.11 These results indicate that it is important to control the particle size, shape, and structure of the catalyst to enhance the catalytic activity and selectivity. Although non-precious metals such as Zn, Sn, and Bi are promising electrocatalysts, transforming CO2 into HCOO with these elements requires high overpotentials.

Tin and tin oxide electrode materials are candidate CO2 reduction electrocatalysts because Sn is a non-noble and low toxicity metal that shows relatively high selectivity toward the formation of HCOO. In particular, SnO2 exhibits superior selectivity and activity toward the production of HCOO compared to pure Sn metal in electrolytic CO2 reduction.22 Recently, several Sn, SnO2, and Sn-based electrocatalysts with remarkable selectivities for CO2 reduction have been reported, e.g., bulk Sn, Sn-based bimetallic compounds, and core–shell structured Sn-based materials, as shown in Table 1. For example, bulk Sn shows some HER activity at the CO2 reduction potential, whereas nanosized Sn and SnO2 allow the highly selective electrocatalytic reduction of CO2. In fact, Meyer et al. reported that the selectivity of the electrochemical reduction of CO2 on Sn strongly depends on the particle size; for example, 5 nm Sn NPs show a three-times-higher FE at −0.9 V (vs. the reversible hydrogen electrode (RHE)) than bulk Sn.16 In addition, the catalytic activity and selectivity for CO2 reduction over Sn and SnO2 can be enhanced by alloying with transition metals such as Cu, Ag, and Bi.13,20,21 For example, Sun et al. prepared bimetallic 7.0 nm Cu NP seeds with a 1.8 nm SnO2 layer through a two-step method. This structure doubled the FE for HCOO production from CO2 compared to that of pure Sn.13 The bulk metals and their oxides are not attractive electrocatalysts for reduction reaction of CO2 because there exhibit a low surface area and poor conductivity. In the case of nanosized electrocatalysts, there have a high surface area compared with bulk materials. Therefore, it can be expected decreasing of overpotentials for reduction reaction of CO2 by using nanosized electrocatalysts. However, unlike precious metals such as Pt and Pd, it is difficult to control the size of non-precious metal nanoparticles (<8 nm), such as Sn, by chemical reduction processes or electrodeposition without the use of surfactants, which would disturb the electrochemical reactions. Therefore, the development of new methods for the preparation of nanosized non-precious metals is crucial; however, the changes to the structure of the SnO2 surface on alloying with transition metals have not been sufficiently studied.

Table 1 Summary of FEs for the formation of formate by the electrochemical reduction of CO2 over various Sn and Sn-based electrocatalysts
Catalyst Morphology FEformate/% E/V vs. RHE Ref.
a E/V vs. Ag/AgCl.b Saturated calomel electrode.
Sn Bulk 87.5 −1.1 12
Sn Bulk 65 −1.0 13
Sn6O4(OH)4 <60 nm NP 79 −1.8a 14
Sn on Sn sheet Bulk 4 −0.8 15
Sn NPs 5 nm NP 82 −1.8b 16
Sn/Sn oxide Film 40 −0.7 17
Sn Bulk 68 −1.0 18
SnS2 Nanosheet 84.5 −1.4a 19
Bi–SnO2 Bi NP on SnO2 Nanosheets 96 −1.1 20
Cu/SnO2_1.8 nm <9 nm NP 85 −0.9 13
Cu55Sn45 Bulk 89.5 −1.1 12
Ag3Sn core SnO2 shell <20 nm NP <80 −0.9 21
Cu6Sn5 NP/CB NP 65.3 −0.6 This work


Recently, we successfully prepared ordered intermetallic compounds that exhibit enhanced electrocatalytic activity for fuel cell electrocatalysts (e.g., the oxygen reduction reaction and formic acid oxidation).23,24 The intermetallic core can be used to modify the surface structure and electronic state because of long-range chemical ordering. Therefore, an ordered intermetallic core can be expected to have surface modifying effects that could enhance the electrocatalytic activity. However, concerning CO2 reduction electrocatalysts, the study of the electrocatalytic activity and selectivity of ordered intermetallic compounds is insufficient compared to those of bimetallic compounds. One of the reasons for this is the difficulty in finding suitable, mild synthetic conditions. We have attempted to create a novel ordered intermetallic alloy of Sn with Cu that can modify the electronic state and crystal structure of the active sites of the SnO2 surface because of the atomic radius mismatch between Sn and Cu.

Herein, we report a novel approach for the preparation of ordered intermetallic Cu6Sn5 NPs on carbon black (Cu6Sn5 NP/CB) in which the Cu6Sn5 has a NiAs-type structure. A tin oxide shell layer that functions as the active site for electrochemical CO2 reduction was expected to form on the surface of the ordered intermetallic Cu6Sn5 NPs. The NPs were prepared by the co-reduction of Cu and Sn precursors because Sn2+ ions are more difficult to reduce than Cu2+ ions; consequently, SnO2 (the surface of Sn is oxidized in the atmosphere) should be present on the NP shell. The prepared Cu6Sn5 intermetallic NPs exhibit not only significantly improved electrocatalytic activity for CO2 reduction but also highly selective conversion of CO2 to formic acid. In this paper, the structural differences in the surface SnO2 compared to Sn and Cu6Sn5, which result in the high activity and selectivity toward the CO2 reduction reaction, are discussed.

2. Experimental

2.1 Materials

The Cu and Sn precursors, Cu(acac)2 and SnCl2, respectively, were purchased from Sigma-Aldrich and Wako Pure Chemicals Co., (Japan), respectively. Carbon black (Vulcan), which was used as a supporting material, was purchased from Premetek Co. In addition, 0.1 M LiEt3BH (Super Hydride) in tetrahydrofuran (THF), which was used as the reducing agent, was purchased from Sigma-Aldrich. Potassium hydrogen carbonate (99.0%), ethanol (99.8%), and THF (dehydrated and stabilizer free) were purchased from Wako. Nafion (5 wt% equivalent weight (EW): 1100) was obtained from Sigma-Aldrich. Water was purified using a Millipore system (resistance: 18.2 MΩ cm at 25 °C).

2.2 Preparation of the carbon-supported ordered intermetallic Cu6Sn5 core–SnO2 NPs shell NPs

The intermetallic Cu6Sn5 NPs supported on CB were prepared by reducing the precursors under an argon atmosphere. Cu(acac)2 (53.7 mg) and SnCl2 (77.2 mg) were used as the precursors in a Cu[thin space (1/6-em)]:[thin space (1/6-em)]Sn molar ratio of 6[thin space (1/6-em)]:[thin space (1/6-em)]10 and dispersed in 50 mL dehydrated THF in a 100 mL flask. Then, the solution was ultrasonically dispersed with 50 mg carbon black with vigorous magnetic stirring for at least 1 h, which resulted in a 40 wt% loading of metal in total weight. As a reducing agent, lithium triethylborohydride (4 mL) was injected into the dispersion. During the reaction, the flask containing the solution was held in a water bath at 50 °C, which was key to the formation of the ordered Cu and Sn intermetallic structure. In addition, bimetallic Cu–Sn was also prepared using the same procedure but without a water bath as a comparison. After being held in the water bath for one night, the dispersion was centrifuged, washed with hexane several times, and dried under vacuum at 25 °C. The obtained powder was sealed in a Cu tube under an argon atmosphere and annealed at 150 °C. The annealed powder was used as an electrocatalyst for the subsequent experiments. Cu NP/CB and Sn NP/CB were synthesized by similar procedures.

2.3 Physicochemical characterization

Powder X-ray diffraction (pXRD) measurements were carried out on a Rigaku MiniFlex 300/600, and diffraction patterns were collected at a scanning rate of 3° min−1 from 20° to 80° in 2θ. Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and elemental mapping profiles were obtained using a 200 kV transmission electron microscope (JEM-ARM200F, JEOL) equipped with two aberration correctors (CEOS GmbH) for the image- and probe-forming lens systems, and X-ray energy-dispersive spectroscopy (EDS) measurements (JED-2300T, JEOL) were employed for compositional analysis. Both aberration correctors were optimized to obtain TEM and STEM resolutions of 1.3 and 1.1 Å, respectively. X-ray photoelectron spectroscopy (XPS) measurements (JEOL, JP-9010 MC) were performed to examine the chemical states (Cu 2p and Sn 3d regions) of the electrocatalysts. A Mg Kα X-ray source with an anodic voltage (10 kV) and current (10 mA) was used for XPS measurements. All the XPS spectra of the samples were obtained with a take-off angle of 45°. X-ray absorption fine structure (XAFS) analysis was conducted using the BL01B1 beamline at the SPring-8 (JASRI, Hyogo, Japan) facility. The Cu and Sn K-edge XAFS spectra were measured using quick XAFS (QXAFS) in transmission mode, and a double-crystal Si(111) monochromator was employed to obtain the XAFS data. The energy levels during XAFS measurement were calibrated using the pre-edge peak in the spectrum of the Cu plate at 8980.3 eV. All the XAFS spectra were analyzed using Athena.25

2.4 Electrochemical measurements

The working electrode was prepared by airbrushing the catalyst ink onto a carbon paper with a gas diffusion layer (GDL, Torey Co.). The ink was prepared by ultrasonically dispersing 10 mg of catalyst for 15 min in a mixture of 2 mL of ethanol with 0.05 mL of Nafion. The dispersion was then painted onto a GDL-coated carbon paper with an airbrush (AIRTEX. Co., Ltd.), heated to 80 °C, and then dried and cut into 2 cm × 2 cm pieces. The ink was painted on one side of the carbon paper, whereas the other side was masked by an adhesive tape (Nitto Denko Corporation, NITOFLON no. 903UL) to prevent electrolysis.

A modified 100 mL gas-tight Teflon cell with two compartments separated by a Nafion 117 membrane (Sigma-Aldrich) was employed for all electrochemical experiments. A 0.1 M KHCO3 solution was used as the electrolyte. Each compartment in the cell contained 90 mL electrolyte that had been saturated with pure CO2 by bubbling for at least 30 min CO2 until the pH reached 6.8.

A BioLogic VMP3 potentiostat was used for all electrochemical measurements including chronoamperometry (CA) measurements at room temperature (25 °C). A Pt plate and KCl-saturated Ag/AgCl were used as counter and reference electrodes, respectively. The electrode potential in Ag/AgCl was converted to the RHE scale using the following equation: E(RHE) = E(Ag/AgCl) + 0.059 pH + 0.196. All potentials reported in this work are reported on the RHE scale. The pH after bubbling CO2 through the 0.1 M KHCO3 solution was 6.8. Electrocatalytic CO2 reduction was carried out for 2 h at various potentials, and the gas phase produced was sampled every 20 min. The liquid phase was analyzed at the end of the reaction.

2.5 Product identification

To identify the gas product, 0.5 mL gas was withdrawn from the gas phase of the cell with a clean gas-tight syringe (VICI, 1 mL) and injected into a gas chromatograph (GC) to measure the hydrogen and CO concentrations. The GC was equipped with a 13X molecular sieve column and a thermal conductivity detector (TCD, SHIMADZU). Argon was used as the carrier gas for H2 and CO analysis. A Sunpak-A column with a flame ionization detector (FID, SHIMADZU) with N2 carrier gas was used for the analysis of CH4, C2H6, and other possible gas phase products. To analyze the formate produced, a high-performance liquid chromatograph (HPLC, SHIMADZU) equipped with an organic acid analysis system (Rezex ROA-organic acid H+ 8%), which allows the sensitive detection of formic acid, was used.

3. Results and discussion

Fig. 1 shows the XPS profiles of the Sn 3d region for the Sn plate and Sn NP/CB and Cu6Sn5 NP/CB samples. The XPS data of the Sn NP/CB and Cu6Sn5 NP/CB samples were calibrated using the binding energy of the C 1s peak at 284.5 eV. For all electrodes, the Sn 3d5/2 and Sn 3d3/2 peaks were observed at 496.0 and 487.6 eV, respectively, which can be assigned to tin(IV) oxide,26 and the metallic Sn component was not observed. However, the electronic state of the Sn surface in the Cu6Sn5 NP/CB samples is distinctly different from that of the Sn NP/CB sample. The tin oxide peak in the 3d5/2 region of the Cu6Sn5 NP/CB sample spectrum (495.5 eV) is lower than the binding energy observed in both the spectrum of Sn NP/CB and that of the Sn plate (487.2 eV). This small negative shift in the binding energy in the Sn region indicates that the electronic state of Sn in SnO2 had been modified by the intermetallic Cu6Sn5 core, unlike the Sn in the Sn NP/CB sample. The molar ratio of Sn and Cu is 80.1[thin space (1/6-em)]:[thin space (1/6-em)]19.9, indicating that a tin-oxide-rich shell layer had been formed by the one-pot synthesis process (Table S1). This mechanism for the formation of the Sn oxide shell on the intermetallic Cu6Sn5 core may be explained considering that, during the co-reduction of the Sn and Cu precursors, it is more difficult to reduce Sn2+ ions than Cu2+ ions: (E0(Sn/Sn2+) = −0.137 V and E0(Cu/Cu2+) = 0.337 V vs. standard hydrogen electrode (SHE)). Thus, the Sn2+ ions had a lower rate of reduction than the Cu2+ ions, and Sn metal is present at the shell region on the intermetallic Cu6Sn5 core, and metallic Sn is oxidized in the air atmosphere. At the same time, the Sn metal core–Sn oxide shell structure could be formed in the case of Sn NPs/CB. In addition, the green and blue lines of the fitting curves in the Sn 3d and Cu 2p (Fig. S1) regions indicate peaks corresponding to metals and their oxides, respectively, as determined from analysis of the XPS profile obtained for the Sn plate. Peaks corresponding to Sn metal in the Sn 3d region are present in the XPS profile after Ar+-etching treatment for 6 s, as show in Table S1, reaching a value of 52.8%, which is close to 45.5% (Cu6Sn5). The XPS profile in the Cu 2p region for Cu6Sn5 before and after Ar+-etching treatment is shown in Fig. S1. In addition, the Cu satellite peaks around 944 and 963 eV are present, indicating the presence of some copper oxides on the surface of the Cu6Sn5 NPs. However, we believe that the small amount of copper oxides will not affect the electrocatalytic activity for CO2 reduction. The copper oxide was not observed after Ar+-etching treatment for 6 s, indicating the formation of intermetallic compounds in the core region.
image file: c9cy01540j-f1.tif
Fig. 1 XPS profiles for (a) Sn plate, (b) Sn NP/CB, and (c) Cu6Sn5/CB in the Sn 3d region (A) before and (B) after Ar+-etching treatment.

To estimate the bulk structure of the Sn and Cu6Sn5 NP/CB samples, pXRD patterns were obtained. Fig. 2 shows the pXRD profiles of Cu NP/CB, Sn NP/CB, and Cu–Sn NP/CB prepared with and without the 50 °C water bath, and the solid bars represent the calculated pXRD peaks for Cu (Fm3m, a = b = c = 0.3615 nm, ICDD 00-004-0836), Sn (I41/amd, a = b = 0.583 nm, c = 0.318 nm, ICDD 00-004-0673), and Cu6Sn5 (C2/c, a = 0.110 nm, b = 0.729 nm, c = 0.983 nm, ICDD 00-045-1488). The obtained diffraction patterns for the Sn and Cu NPs coincide with the reference patterns of Sn and Cu metals, which are body-centered cubic (BCC) and face-centered cubic (FCC) structures, respectively. The ordered intermetallic Cu6Sn5 prepared in the 50 °C water bath can be assigned to an ordered intermetallic phase (NiAs-type structure), whereas the Cu–Sn prepared at room temperature only shows broad peaks of the FCC-type structure. The ordered intermetallic Cu6Sn5 produced diffraction peaks at 30.3°, 43.2°, 43.3°, 53.5°, 56.7°, 60.1°, and 62.8° which can be indexed a NiAs-type structure, and peaks corresponding to pure Cu or Sn could not be detected. These results indicate that the intermetallic Cu6Sn5 NPs had been successfully prepared, and the reduction reaction at 50 °C in the water bath is the key process for the formation of the ordered intermetallic Cu and Sn structures. In addition, peaks corresponding to tin oxide were not observed using XRD. It is believed that the tin oxide is present on the surface of the Sn NP and Cu6Sn5 NP as a thin layer or as an amorphous compound. The CB-supported intermetallic Cu6Sn5 core/tin oxide shell structure is denoted “Cu6Sn5 NP/CB.”


image file: c9cy01540j-f2.tif
Fig. 2 pXRD patterns for (a) Cu NP/CB, (b) Sn NP/CB, (c) disordered Cu–Sn NP/CB without water bath treatment, and (d) ordered intermetallic Cu6Sn5 NP/CB prepared in a 50 °C water bath.

Fig. 3 shows the TEM images of the Sn NP/CB, Cu–Sn NP/CB, and Cu6Sn5 NP/CB samples and their corresponding size distribution histograms, which were obtained by measuring one-hundred NPs. The NPs on the carbon surface can be seen as dark spots, and they are uniformly dispersed on the CB surface. The average particle sizes of Sn NP/CB, Cu–Sn NP, and Cu6Sn5 NP on the carbon surface were determined to be 15.7, 4.1, and 8.1 nm, respectively. A surfactant is often used to control the particle size and morphology. However, the surfactant not only disturbs the catalytic reaction but is difficult to remove from the catalyst surface. In the present case, an electrocatalyst with small NPs can be prepared without the use of any surfactants.


image file: c9cy01540j-f3.tif
Fig. 3 (A–C) Low-magnification TEM images and (D–F) size distribution histograms on (A and D) Sn NP/CB, (B and E) Cu–Sn NP/CB, and (C and F) Cu6Sn5 NP/CB.

Fig. 4 shows typical high-angle annular dark-field imaging (HAADF)-STEM images of the Cu6Sn5 NP/CB sample and the corresponding EDS mapping profiles, which give further surface details. No regions associated solely with Sn or Cu were observed in the Cu6Sn5 NPs. In the core region, the molar ratio of Cu and Sn was determined to be 45[thin space (1/6-em)]:[thin space (1/6-em)]55, consistent with the XPS data. On the other hand, the tin oxide shell layer enclosing the intermetallic Cu6Sn5 core can be clearly observed in the surface region, as expected from the XPS data. In addition, the presence of a SnO2 layer on the Cu6Sn5 NPs was confirmed by the high-magnification TEM image, as shown in Fig. S2. These results suggest that Sn atoms were the active sites for the CO2 reduction reaction.


image file: c9cy01540j-f4.tif
Fig. 4 (A) Low- and (B) high-magnification STEM images and their corresponding compositional mapping images of Cu6Sn5 NP/CB.

Fig. 5 shows the relationship between the potential and the FE for the electrochemical reduction of CO2 over the Sn plate and the Sn NP/CB, Cu–Sn NP/CB, and Cu6Sn5 NP/CB samples. The Sn plate electrode has an FE of 70.0% for H2 formation at −0.4 V vs. RHE, as shown in Fig. 5A. The FE for H2 production decreased with increasing reduction potential, whereas the FE for HCOO formation gradually increased with increasing reduction potential; this is consistent with other reports.13 The FE for HCOO production over the Sn NP/CB sample at −0.2 V vs. RHE, which is similar to the CO2/HCOO reduction/oxidation potential,1 was 23.1%, whereas the FEs for the production of H2 and CO were 56.1% and 12.2%, respectively. In contrast, the Cu6Sn5 NP/CB showed an FE of 36.3% for HCOO production at −0.2 V vs. RHE, which is 1.6-times higher than that of Sn NP/CB, indicating that Cu6Sn5 NP/CB can efficiently reduce CO2 to HCOO with a low overpotential. Moreover, a reduction current at −0.2 V could not be obtained on the Sn plate. Thus, the bulk Sn electrode is not an attractive electrocatalyst for CO2 reduction. It should be noted that HCOO and CO were formed from CO2 evolved from the KHCO3 electrolyte solution because HCOO and CO could not be detected without CO2 (Fig. S3). In the case of the Cu plate and Cu NPs, although small amounts of organic compounds such as CH4 and CH2CH2 were detected for bulk Cu in the potential range of −1.0 to −1.5 V, the FEs for the CO2 reduction reaction on bulk Cu and Cu NP/CB show a much narrower potential range with good selectivity, between −0.2 and −1.1 V, as shown in Fig. S4. In addition, the FE for the formation of H2 at −0.2 V over the bimetallic Cu–Sn NP/CB was 83.0%, which is close to that of the Cu NP/CB sample. Haruyama et al. reported the FEs toward the electrochemical reduction of CO2 on various Cu–Sn bimetallic compounds.12 They suggested that the tin oxide surface rather than the Cu–Sn alloy contributes to the formation of HCOO. In the current case, the poor FE toward CO2 reduction observed on the Cu–Sn NPs is not a result of the formation of Sn or Sn oxide on the surface. This indicates that SnO2 on the surface of the NP contributes to the electrochemical reaction. As the potential was reduced, the FE for H2 production, starting from 50.0%, decreased significantly, reaching a minimum value of 15% at −0.6 V, indicating that CO2 can be reduced selectively on the Cu6Sn5 NP/CB sample. On the other hand, the FE of HCOO increased gradually with increasing reduction potential, reaching a maximum value of 65.3% at −0.6 V, which are 1.6 and 1.4 times higher than those of the Sn plate and Sn NP/CB. Moreover, the Cu6Sn5 NP/CB electrocatalyst showed selectivity toward the formation of HCOO, as shown in Fig. S5. On the other hand, to confirm the dependence on the metal loading weight of the selective reduction of CO2, Cu6Sn5 NP/CB samples with different metal loadings were prepared. On the basis of our tests, 40 and 60 wt% loadings show similar performance, whereas the 20 wt% metal loading catalyst had a low selectivity for the CO2 reduction reaction (Fig. S6). In addition. The loss of the electrocatalytic selectivity towards the formation of HCOO over the Cu6Sn5 NP/CB catalyst after five cycles was as low as 2%, as shown in Fig. S7.


image file: c9cy01540j-f5.tif
Fig. 5 Potential dependence of the FE (left) and corresponding current densities (right) for the electrochemical reduction of CO2 over (A) Sn plate, (B) Sn NP/CB, (C) Cu–Sn NP/CB, and (D) Cu6Sn5 NP/CB.

Earlier studies of the species involved in the electrochemical reduction of CO2 suggest that the intermediate differs depending if CO or HCOO is formed. In the case of CO formation, the carbon in CO2 is adsorbed at the electrode surface (i.e., *COOH formation). On the other hand, a different intermediate is formed when HCOO is formed by CO2 reduction. Jaramillo et al. proposed that *OCHO is the key intermediate for the electrochemical reduction of CO2 to HCOO.18 When the *OCHO adsorption energies on the surface of the electrocatalyst are weak, such as for Au and Ag, the electrocatalytic activity for the formation of HCOO is limited. In contrast, electrocatalysts having high *OCHO adsorption energies would also have limited activities. That is, there exists an optimal value of the *OCHO adsorption energies that correlates with selectivity for the formation of HCOO (the so-called Sabatier principle in the field of chemical catalysis). Sn is the best electrocatalyst for the formation of HCOO of the pure metals because it has the optimal adsorption energy between the oxygen in *OCHO and the Sn atoms. In addition, Yin et al. reported that bridge-type *OCO* intermediates can be detected by in situ Fourier transform IR spectroscopy on Cu–Zn bimetallic co-catalysts used for photocatalysis, which demonstrates the superior photocatalytic activity for the formation of HCOO from the photoreduction of CO2.27 These results indicate that two neighboring Sn active sites are required to adsorb *OCO* chemically as an intermediate on the catalyst surface. Therefore, it is crucial to control the Sn–Sn atomic distance in the SnO2 on the surface of the intermetallic Cu6Sn5 NPs.

To estimate the local structure of the Sn atoms (i.e., the Sn–Sn and Sn–O separations and the electronic state of Sn), the prepared electrocatalysts were further characterized by synchrotron radiation XAFS at the BL01B1 beamline at the SPring-8 facility. Fig. 6A shows the Sn K edge X-ray absorption near-edge structure (XANES) spectra of the Sn plate, Sn NP/CB, and Cu6Sn5 NP/CB samples. The Sn K-edge peak tops for the prepared Sn NP/CB and Cu6Sn5 NP/CB samples were 1.19 and 1.20, respectively, both higher than the white line intensity of the Sn plate (1.12). This result indicates that there are tin oxide components in the Sn NP/CB and Cu6Sn5 NP/CB samples, as also shown by the XPS and EDS data.


image file: c9cy01540j-f6.tif
Fig. 6 (A) XANES normalized Sn K-edge spectra for (a) Sn plate, (b) Sn NP/CB, and (c) Cu6Sn5 NP/CB. (B) The k3-weighted Fourier-transform from the Sn K-edge EXAFS spectra.

The Sn K-edge extended XAFS (EXAFS) spectra were obtained by Fourier transforming the XAFS oscillations (k3χ(k)). Table 2 shows the result of the analysis of the bond type, distance, and R-factor between a Sn atom and its nearest neighbor. The Fourier transformed EXAFS spectrum contain peaks at 2.26 and 2.80 Å, indicating a BCC-type Sn structure and suggesting close 1st and 2nd nearest Sn atoms, as shown in Fig. 6B. The Sn–O bond for SnO2 on the Sn plate could not be detected because synchrotron radiation is bulk sensitive. A peak at 1.57 Å was observed in both the spectrum of the Sn NP/CB sample and that of the Cu6Sn5 NP/CB sample, and this is assigned to the Sn–O bond in SnO2 because this peak is not present in the Sn metal sample, indicating that the prepared Sn NP/CB and Cu6Sn5 NP/CB samples contain both Sn (or intermetallic) metal and SnO2 components. Fig. S8 and Table S2 show the EXAFS spectra of the Cu K-edge of the Cu plate, Cu NP/CB, and Cu6Sn5 NP/CB and the corresponding analyses. In the case of fitting with Cu metal for Cu NP/CB, the R-factor is 0.059, which is significantly higher than the fitting result obtained for copper oxides and Cu metal. This result indicates that Cu NPs contain both copper metal and copper oxide components, whereas the intermetallic Cu6Sn5 NP/CB contain only Cu metal and no copper oxides. Therefore, Cu is not present on the Cu6Sn5 NP/CB surfaces. Importantly, the Sn–Sn separation in the SnO2 in Cu6Sn5 NP/CB was determined to be 3.09 Å, which is shorter than that for Sn NP/CB (3.25 Å), as shown in Table 2. This difference in the Sn–Sn separation in SnO2 on the surface of Cu6Sn5 NP/CB could be due to surface modification by the intermetallic core containing Cu and Sn, which have mismatched atomic radii. As mentioned, the Sn–Sn distance in SnO2 on the surface of Cu6Sn5 NP/CB is 3.09 Å, representing compressions of 4.7% and 4.9% compared to that of bulk SnO2 (3.24 Å) and the prepared Sn NP/CB (3.25 Å), respectively. We assumed that the ordered intermetallic Cu6Sn5 NP core induces the surface-compressive strain of the SnO2 layers, resulting in an ideal surface for the adsorption of CO2 on SnO2. This compressive Sn–Sn strain in the SnO2 on the surface of the NPs is a key factor for the selectivity toward HCOO formation via CO2 reduction.

Table 2 Summarized bond types and distance between the nearest neighbors (R) estimated from the Sn K-edge EXAFS analysis
  Bond R(ref.)/Å R(expt.)/Å R-Factora
a The R-factor represents the quality of fitting.
Sn plate Sn–Sn 2.92 2.97 0.015
Sn–Sn 3.18 3.09
Sn NP/CB Sn–Sn 2.92 3.04 0.013
Sn–O(oxide) 2.09 2.05
Sn–Sn(oxide) 3.24 3.25
Cu6Sn5 NP/CB Sn–Cu 2.66 2.63 0.010
Sn–Cu 2.77 2.73
Sn–Cu 2.82 2.78
Sn–Sn 3.32 3.29
Sn–Sn 3.48 3.45
Sn–O(oxide) 2.09 2.03
Sn–Sn(oxide) 3.24 3.09


4. Conclusions

In this study, we successfully prepared ordered intermetallic Cu6Sn5 NPs coated with SnO2 on CB in one step through a wet chemical synthesis procedure. To synthesize the ordered intermetallic Cu6Sn5 NP/CB, the reduction reaction must be carried out at 50 °C. The prepared Cu6Sn5 NP/CB sample was characterized by XPS, XRD, TEM, and XAFS techniques. The Cu6Sn5 NPs are deposited on the CB surface as disperse NPs. Using XPS and STEM-EDS, the presence of a SnO2 shell layer on the surface of the intermetallic Cu6Sn5 NPs was determined. The FE value of Cu6Sn5 NP/CB was found to be 1.6 and 1.4 times, respectively, higher than those of Sn plate and Sn NP/CB at −0.4 V, supporting the superior CO2 reduction electrocatalysis of the Cu6Sn5 NPs. The Sn–Sn separations of the SnO2 on the surface of the Cu6Sn5 NPs were smaller by 4.7% and 4.9% than those of SnO2 bulk (0.324 nm) and the prepared Sn NP/CB sample (0.325 nm), respectively, as determined by XAFS. We believe that the compressive strain of the SnO2 surface layer is a key factor in improving the electrocatalytic selectivity for the formation of HCOO via the reduction of CO2. The present results can be used to prepare novel electrocatalysts with ordered intermetallic Cu6Sn5 NPs on CB and may reveal the structural characteristics of SnO2-based materials required for the highly selective formation of HCOO.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful to Dr. Toshiaki Ina for help with QXAFS measurements at the BL01B01 beamline of the SPring-8 facility (Proposal No. 2019A1316). This work was financially supported by the Izumi Science and Technology Foundation of Japan.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cy01540j

This journal is © The Royal Society of Chemistry 2019