Highly selective palladium-copper bimetallic electrocatalysts for the electrochemical reduction of CO2 to CO

Author’s Accepted Manuscript Highly Selective Palladium-Copper Bimetallic Electrocatalysts for the Electrochemical Reduction of CO2 to CO Zhen Yin, Dunfeng Gao, Siyu Yao, Bo Zhao, Fan Cai, LiLi Lin, Pei Tang, Peng Zhai, Guoxiong Wang, Ding Ma, Xinhe Bao www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(16)30216-6 http://dx.doi.org/10.1016/j.nanoen.2016.06.035 NANOEN1353

To appear in: Nano Energy Received date: 27 April 2016 Revised date: 24 May 2016 Accepted date: 18 June 2016 Cite this article as: Zhen Yin, Dunfeng Gao, Siyu Yao, Bo Zhao, Fan Cai, LiLi Lin, Pei Tang, Peng Zhai, Guoxiong Wang, Ding Ma and Xinhe Bao, Highly Selective Palladium-Copper Bimetallic Electrocatalysts for the Electrochemical Reduction of CO2 to CO, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.06.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highly Selective Palladium-Copper Bimetallic Electrocatalysts for the Electrochemical Reduction of CO2 to CO Zhen Yin,a║ Dunfeng Gao,b║ Siyu Yao,c Bo Zhao,c Fan Cai,b LiLi Lin,c Pei Tang,c Peng Zhai,c Guoxiong Wang,*b Ding Ma,*c Xinhe Baob a State Key Laboratory of Separation Membranes and Membrane Processes, Department of Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, P. R. China, b State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China, c Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China Author Information Corresponding Author *E-mail: [email protected]; [email protected] ║ Z. Y. and D. G. contributed equally.

Abstract Selective and efficient conversion of carbon dioxide (CO2) to a reusable form of carbon via the electrochemical reduction of CO2 has attracted much attention recently, as it is a promising approach for the storage of renewable energy. Herein, we synthesize palladium-copper bimetallic nanoparticles with different compositions, which serve as a well-defined platform to understand their fundamental catalytic activity in CO2 reduction. Among PdCu/C and Pd/C catalysts tested, Pd85Cu15/C catalyst shows the highest CO Faradaic efficiency of 86%, CO current density of 6.9 mA cm-2 and mass activity for CO production of 24.5 A g-1 at −0.89 V vs. RHE in CO2-saturated 0.1 M KHCO3 solution, which is about 5 times, 8 times and 2.2 times higher than Pd/C catalyst, respectively. It was suggested from EXAFS and CO TPD-MS studies that the highly selective CO production on Pd85Cu15/C catalyst is due to the presence of an optimum ratio of the copper element and low-coordination sites over monometallic Pd active for H2 evolution with low overpotential. We believe that controllable size and composition for the bimetallic nanoparticles are critical to the CO2 reduction activity enhancement and high CO Faradaic

efficiency. The insights gained through this work may shed light in a foundation for designing efficient catalysts for electrochemical reduction of CO2.

Keywords: CO2 conversion; electrocatalysis; bimetallic nanocatalysts; Pd-based nanoparticles

Graphical Abstract The bimetallic palladium-copper nanoparticles with different compositions were loaded on the carbon support to obtain bimetallic PdCu/C catalysts towards CO2 electrochemical reduction. Among PdCu/C and Pd/C catalysts tested, Pd85Cu15/C catalyst shows the highest CO Faradaic efficiency of 86%, CO current density of 6.9 mA cm-2 and mass activity for CO production of 24.5 A g-1 at −0.89 V vs. RHE in CO2-saturated 0.1 M KHCO3 solution, which is about 5 times, 8 times and 2.2 times higher than Pd/C catalyst, respectively.

Introduction Emissions of carbon dioxide (CO2) in the atmosphere become a serious environmental threat due to the ever-increasing worldwide consumption of fossil fuels. In order to meet the fuel and chemical demands in a sustainable way, chemical conversion of CO2 into usable chemicals has attracted intensive research during recent years [1, 2]. The waste CO2 can be utilized as chemical feedstock via chemical reduction process to get CO-rich feeds, hydrocarbons, even alcohols, which is of high interest for

industrial production. Of all chemical ways to convert CO2 into useful products, electrochemical reduction of CO2 is regarded as a potentially “clean” method [3-6]. Moreover, CO as one of the desired products from the CO2 reduction can be further reduced to oxygenates and hydrocarbons electrochemically [7-9]. However, the high overpotential and thus low energetic efficiency become the current bottleneck due to the high thermodynamic barriers of the CO2-to-CO2- electron uptake process [10, 11]. Hence, one of key challenges for practical application of electrochemical CO 2 conversion is the development of highly efficient catalysts that can reduce CO2 at low overpotentials [4, 12, 13]. Moreover, selectivity was the other essential issue for CO2 reduction, as there are a range of possible products and water reduction reaction competing with CO2 reduction, thus leading to unwanted products. During past few decades, bimetallic nanoparticles, composed of two different metal elements, have received tremendous research attention due to their intriguing catalytic behaviors with respect to that seen with monometallic nanoparticles, such as bimetallic Pd-based nanoparticles [14-17]. It’s well known that the size, composition and/or morphology-controlled metallic nanoparticles could

exhibit superior

electrocatalytic performance, either mono- or bi- metallic nanoparticles [9, 18-23]. Recently, Strasser and co-workers studied the activity-selectivity-size relationship for CO2 reduction via the size-controlled Cu nanoparticles in the 2-15 nm mean size range [24]. Their results show that similar catalytic selectivity for hydrocarbon formation were observed over Cu nanoparticles in a size range between 5 and 15 nm compared to Cu bulk surfaces. However, for nanoparticles below 5 nm, the catalytic activity and selectivity for H2 and CO dramatically increase along with decreasing Cu particle size, while hydrocarbon (methane and ethylene) selectivity depressed unexpectedly. Meanwhile, Yang and co-workers have demonstrated that the ordered gold-copper monolayers assembled with bimetallic AuCu nanoparticles show superior catalytic performance for CO2 reduction due to synergistic effects of electronic effect and geometric effect [25]. Thus, fabrication of bimetallic nanoparticles with controllable size and/or composition to investigate the catalytic activity for the electrochemical CO2 reduction would be one potential direction for the development

of electrochemical CO2 reduction catalysts. In our previous works, we prepared Pd-based nanoparticles via emulsion-assisted ternary ethylene glycol (EG) method, which supplied well-defined nanocatalysts for the electrocatalytic reaction [26]. Significantly by tuning the size or composition, Pd-based bimetallic nanoparticles displayed superior electrocatalytic activity in methanol oxidation reaction, even better than the commercial Pt/C catalysts [27-29]. However, according to the previous reports, the bulk polycrystalline Pd electrode in Group VIII has poor activity for the electrochemical reduction of CO2 [30, 31]. The total current efficiency of the electrochemical reduction of CO2 was far below 100% (about 60% at -1.2 V vs. NHE) on the bulk polycrystalline Pd electrode, where the efficiency for CO formation was 28.3% and for H2 evolution was 26.2%. Furthermore, H2 evolution was inevitable and competed with CO2 reduction during the electro-reduction process, where the substantial amount of generated H2 may be absorbed by Pd and the hydrogen atoms might be strongly trapped and stabilized in interstitial lattice sites of Pd nanoparticles [32]. If the Cu element can be introduced as the second component and form alloy, it would be beneficial for the enhancement of current efficiency and selectivity for CO production due to bimetallic alloy effects [19, 30]. Moreover, in our previous work, we report the prominent size-dependent activity/selectivity in the electrocatalytic reduction of CO2 via finely controlling size of Pd nanoparticles, which displayed high CO Faradaic efficiency and current density [9]. Hence, the investigation of CO2 reduction with the bimetallic Pd-Cu nanoparticles would be one attractive topic, although numerous publications have reported electrocatalytic activities based on the copper electrodes [33, 34]. Here, carbon supported palladium-copper bimetallic alloy nanoparticles (< 5 nm) with different compositions were used as a well-defined catalyst for electrochemical reduction of CO2. We find that the bimetallic Pd-Cu alloy nanoparticles supported on the carbon exhibit excellent catalytic activities with high CO selectivity, outperforming conventional Pd-based or Cu-based catalysts for the CO2 reduction due to synergistic effects between Pd and Cu. Ex-situ extended X-ray absorption fine structure (EXAFS) and temperature programmed desorption-mass spectrometry

(TPD-MS) experiments were used to investigate the PdCu/C catalysts with varying precursor composition to get in-depth understanding of the relationships between the composition-structure and structure-activity of Pd-based bimetallic nanoparticles and to seek valid descriptors of their electrochemical activity towards CO2 reduction. Understanding gained through the present work will help to design and prepare more efficient CO2 electrochemical reduction catalysts.

Experimental Preparation of Catalysts. Bimetallic PdCu catalysts were prepared through a modified process based on our previous work. Firstly, the Pd-Cu nanoparticles were prepared under an argon atmosphere. Taking the synthesis of Pd85Cu15 nanoparticles as an example, the oleic acid (OA) and oleylamine (OAm) were mixed with ethylene glycol (EG). Following this,

two

metal

precursor

solutions

of

Pd(CH3COO)2

in

acetone

and

Cu(CH3COO)2∙H2O in water with a molar ratio of 5:1 were added. The mixture was stirred vigorously and first heated to 120 ℃ for 30 min. Subsequently, the temperature was raised to 200 ℃ and kept at reflux for 90 min. The as-prepared colloidal nanoparticles dispersions were separated through extraction with hexane. The nanoparticles were re-dispersed in hexane after washing procedure with hexane and ethanol. Two kinds of Pd-Cu nanoparticles with different compositions can be obtained via adjustment of the molar ratio between palladium acetate and copper acetate. Secondly, the PdCu nanoparticles were subsequently loaded on Vulcan XC-72R carbon black (Cabot). Capping ligands on the particle surface were removed through thermal decomposition at high temperature, i.e. 350 °C for 4 h under H2 atmosphere and then passivation with O2/N2 (5% O2) at room temperature. The carbon-supported Pd nanoparticles catalyst (Pd/C) was prepared in a similar way. Characterization. Transmission electron microscopy (TEM) images were recorded on a FEI F30 microscope, which was equipped with a Gatan CCD camera that operated at 200 kV. The actual metal loadings in the bimetallic PdCu catalysts were analyzed by

inductively coupled plasma-atomic emission spectrometry (ICP-AES, LEEMAN, PLASMA-SPEC-II). X-ray Absorption Fine Structure (XAFS) spectra of Cu K edge (8979 eV) were measured at 1W1B beamline of Beijing Synchrotron Radiation Facility (BSRF) in transmission mode using ion chamber detector to collect the data. Cu foil, CuO and Cu2O were measured as standards. Pd K edge (24350 eV) XAFS spectra were measured at BL14W1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) in transmission mode using ion chamber detector. Pd foil was measured as standard. In both measurements, the tested samples were sealed in a 2 mm thick chamber with Kapton film windows under Ar protection in glove boxes after activation by H2. Meanwhile, the Pd and Cu foils were measured simultaneously with the PdCu/C in reference mode for X-ray energy calibration and data alignment at each absorption edge. All XAFS spectrum were processed using Ifeffit package. The EXAFS oscillation were fitted according to back-scattering equation, using feff models generated from crystal structure of Cu (Fm-3m), Cu2O (Fm-3m) and Pd (Fm-3m) respectively. Electrochemical Measurements of the CO2 Reduction. Porous electrode preparation Carbon

black

ink

containing

Vulcan

XC-72R

carbon

black

and

polytetrafluoroethylene (PTFE, Sigma-Aldrich) was painted onto a piece of Toray carbon paper (Toray TGP-H-060, Toray Industries Inc.) to form a microporous layer. The carbon black loading was about 1 mg cm-2 and the PTFE content in the microporous layer was 40 wt%. To fabricate the catalyst layer, the as-prepared catalyst and Nafion ionomer solution (5 wt%, DuPont) were ultrasonically suspended in a water/alcohol mixture and then brushed onto the microporous layer. The loading of Pd/C catalyst was 2.0 ± 0.1 mg cm-2, and the Nafion content in the catalyst layer was 10 wt%. Electrochemical measurements The electrochemical measurements were carried out in an H-cell (separated by Nafion 115) system [9]. The Toray carbon paper with the catalyst layer was cut into a size of 1 cm×2 cm acting as the working electrode. The Pt wire and Ag/AgCl electrode were

used as the counter electrode and reference electrode, respectively. The potentials were controlled by an Autolab potentiostat/galvanostat (PGSTAT 302N). All potentials in this study were measured against the Ag/AgCl reference electrode and converted to the RHE reference scale. Electrocatalytic reduction of CO2 was conducted in CO2-saturated 0.1 M KHCO3 (Sinopharm Chemical Reagent Co. Ltd.) aqueous solution (pH=6.8) at room temperature and under atmospheric pressure. After CO2 was purged into the KHCO3 solution for at least 30 min to remove residual air in the reservoir, controlled potential electrolysis was performed at each potential for 30 min. The oxygen generated at the anode was vented out of the reservoir. The gas products of CO2 electrocatalytic reduction were monitored by an on-line micro gas chromatography (GC) (Agilent 490) equipped with a TCD detector and Molsieve 5A column once every three minutes. The KHCO3 solution after electrolysis was collected and analyzed on a Bruker AVANCE III 400 MHz NMR spectrometer to quantify liquid products. Standard curve was made by using sodium formate (HCOONa∙2H2O, Sinopharm Chemical Reagent Co. Ltd.) and the internal standard (1-Propanesulfonic acid 3-(trimethylsilyl) sodium salt, DSS, Sigma-Aldrich). A 0.5 mL sample of the KHCO3 solution after electrolysis was mixed with the addition of 0.1 mL D2O and 0.1 mL 6 mM DSS solution as an internal standard. The

1

H spectrum was measured with water

suppression by a pre-saturation method. The area ratio of the formate peak to the DSS peak was compared to the standard curve to quantify the concentration of formate. Temperature programmed desorption-mass spectrometry (TPD-MS) analysis In order to avoid the influence of the carbon, the PdCu and Pd nanoparticles were supported on the fumed silica (50 mg), loaded in a quartz reactor heated by an electric furnace controlled by a thin thermocouple located within the catalyst bed. Helium (purity 99.9999 vol.%) was employed as carrier gas. H2 (purity 99.9995 vol.%) was used for pre-reduction of catalyst in TPD-MS analysis, and then CO (10 vol% in helium) was employed for the adsorption in TPD-MS experiments. The outlet gas was analyzed by means of a quadrupolar mass spectrometer (QMS) (LC-D200M PRO, by Tilon Group Technology, USA).

The catalyst was first treated in 20 cm3 min-1 flowing H2 at 350 oC in order to remove absorbed water and pre-reduce the nanoparticles, and then the carrier gas was switched to helium (20 cm3 min-1) till lowering temperature to 30 oC. Thermal desorption experiments were performed after exposing the sample to the CO gases at 30 oC, subsequent flowing helium till the signals relative to H2O (m/z=18), CO (m/z=28), CO2 (m/z=44) and O2 (m/z=32) became stable. TPD experiments were performed with a 10 oC/min temperature ramp from 30 oC up to 500 oC. Results and Discussion. The Pd-Cu nanoparticles with different compositions were prepared by emulsion-assisted EG ternary system with the presence of OAm and OA as the stabilizing agents. As shown in TEM images, the sizes of Pd-Cu nanoparticles were 3.3 ± 0.3 nm for Pd85Cu15 (Fig. 1A) and 2.1 ± 0.2 nm for Pd56Cu44 (Fig. 1B). The as-prepared bimetallic PdCu nanoparticles can be easily separated from the mixture through extraction process and loaded onto a carbon black (Carbot, Vulcan XC-72R). Most of stabilizing ligands can be readily removed by heat-treatment under H2 atmosphere at 350 oC. It can be seen that the bimetallic Pd-Cu nanoparticles were successfully loaded onto a carbon support with uniform dispersion. Even after heat-treatment, the bimetallic PdCu/C catalysts didn’t show any obvious nanoparticles morphological changes or aggregation. Of course, the median size of the Pd85Cu15 nanoparticles slightly increased from 3.3 to 4.2 nm, indicating that some particle growth occurred. Similarly, the median size of the Pd56Cu44 increased from 2.1 nm to 3.5 nm after heat treatment (Fig. S1, Supporting Information). In addition, the similar procedure was applied for the Pd nanoparticles and Pd/C catalyst (Fig. S2). In order to understand the Pd and Cu atom distribution and/or coordination inside the bimetallic nanoparticles, further detailed analysis was conducted by EXAFS studies, which has been widely used as a well-established tool for investigating the structure of bimetallic nanomaterials and studying the local environment of atoms of each component. The Pd K edge X-ray absorption near edge structure (XANES) spectra of both alloy particles exhibits similar near edge features to that of the Pd foil

sample, indicating the predomination of metallic palladium in these two samples. At Cu K edge, the XANES spectra showed that the copper in PdCu alloy were partially oxidized. According to the near edge characters of Cu, the oxidized degree of Pd56Cu44/C sample is higher than the Pd enriched counterparts because of the high Cu content. To gain deeper understanding for the local structure of bimetallic PdCu catalysts, the EXAFS spectra were fitted at both Pd K and Cu K edges. The Fourier transform magnitudes of k3-weighted EXAFS data and theoretical fits for Pd and Cu K edges of bimetallic PdCu/C catalysts were shown in Fig. 2. The fitting of parameters, such as R-bond lengths, N-coordination numbers, and the Debye-Waller terms, △σ2 (a measure of disorder), were listed in Table 1. Table 1. Curve-fitting results of Cu/Pd catalysts. sample

Edge

bond

N

r, Å

△E, eV

△σ2

R-factor

Pd85Cu15/C

Pd

Pd-Cu

0.8±0.1

2.62±0.01

-8.9

0.002

0.02

Pd-Pd

7.2±0.8

2.69±0.01

Cu-Cu

0.7±0.3

2.57±0.03

Cu-Pd

5.8±0.7

2.62±0.01

0.009

Cu-O

1.3±0.6

1.88±0.02

0.004

Pd-Cu

1.9±1.1

2.60±0.01

Pd-Pd

4.6±1.1

2.69±0.02

0.009

Pd-O

1.0±0.7

1.94±0.05

0.004

Cu-Cu

1.5±0.5

2.57±0.04

Cu-Pd

2.5±0.5

2.60±0.02

0.005

Cu-O

1.1±0.4

1.89±0.02

0.004

Cu

Pd56Cu44/C

Pd

Cu

0.009 -9.3

-8.4

1.4

0.002

0.005

0.004

0.003

0.02

0.05

For Pd85Cu15/C sample, consistent with the XANES results, the first coordination shells of Pd were influenced by both Pd-Cu and Pd-Pd scatterings, while Cu atoms have additional scattering component of neighbour O. The ratio of NPd-Pd/NPd-Cu and NCu-Pd/NCu-Cu were larger than the composition of Pd/Cu determined by elemental analysis, suggesting that Pd atoms were enriched locally in the particles. Additionally, part of Cu atoms as the form of copper oxides is distributed on the surface of the particles. The bond length of Pd-Pd was fitted as 2.69 Å, smaller than the theoretical value of Pd-Pd bond of the

bulk materials (2.75 Å). On the other hand, the Cu-Cu bond was expanded to 2.57 Å, slightly larger than the value of metallic copper (2.56 Å). As the percentage of Cu increases, the scattering intensity of Pd-Cu and Cu-Cu shells increased considerably. For the Pd56Cu44/C sample, the NPd-Cu and NCu-Cu increased to 1.9 and 1.5 respectively. Based on the relative intensity of scattering shells and the chemical composition obtained from ICP-AES results, it can be seen that Pd atoms in Pd56Cu44/C sample were also enriched in the core of the particles, similar to the Pd85Cu15/C sample, resulting from the difference of the nucleation temperature of Pd and Cu. The as-prepared electrode with bimetallic PdCu/C or Pd/C catalysts was tested for CO2 reduction in a gas-tight two-compartment electrochemical cell using CO2-saturated 0.1 M KHCO3 aqueous solution as electrolyte. The results of CO2 reduction on different catalysts were shown in Fig. 3. Compared with Pd/C catalyst, the bimetallic PdCu/C catalysts exhibited a distinct difference in selectivity of CO production (Fig. 3A). At applied potentials from -0.6 to -1.2 V vs. RHE, CO was produced as almost the sole product of CO2 reduction. When the electrode was more negatively polarized, the conversion of CO2 to CO was improved, achieving faradaic efficiency (FE) for CO production of ~80% with Pd85Cu15/C, ~55% with Pd56Cu44/C at -0.69 V vs. RHE. The maximum FE for CO production was 86% with Pd85Cu15/C and 73% with Pd56Cu44/C, respectively, at -0.89 V vs. RHE. In contrast, the CO2 reduction to CO catalyzed by Pd/C achieved maximum FE for CO production of 16 % at -0.99 V vs. RHE. It should be noted that the FE for CO production over Pd/C catalyst was relatively low compared to our previous result [9], which was probably caused by different preparation methods in combination with heating-treatment in the present method and the different Pd loading on carbon support. In addition, the FE for CO production on the PdCu/C catalysts decreases at high applied potentials, which probably resulted from the limited mass transport of CO2 in 0.1 M KHCO3 solution. The comparison of partial current density for CO production with different catalysts has been shown in Fig. 3B. It can been seen that the partial current density for CO production would remarkably increase along with the raise of polarization potential,

indicating that reaction rate of CO2 reduction accelerates at high applied potentials. The current density of CO reached about 6.9 mA cm-2 with Pd85Cu15/C, ~5.9 mA cm-2 with Pd56Cu44/C at -0.89 V vs. RHE, 6.7 times and 5.6 times higher compared to Pd/C (0.9 mA cm-2). For the Pd/C catalyst in Fig. 3C, the partial current density for H 2 production increased dramatically along with the raise of polarization potential, approximate 5 mA cm-2 at -0.89 V vs. RHE, indicative of H2 as the main product from the water. Fig. 3D compares the CO mass activity for these nanocatalysts with different compositions at different potentials. Obviously, the mass activity raised clearly when more negative potential was applied. We found that the CO mass activity sharply increased to 24.5 A g-1 for Pd85Cu15/C, 19.7 A g-1 for Pd56Cu44/C, meaning ca. 1.2 times and 70% higher than Pd/C (11.3 A g-1) at -0.89 V vs. RHE, which was slightly higher than that on Pd/C in our previous report [9], on 4 nm Au NPs [20] in the case of a similar metal loading on carbon and was comparable to that on ultrathin Au nanowire [35]. Furthermore, one point should be noticed that the mass activity of Pd NPs was measured based on porous electrode, which can be directly used in practical compact electrolysis cells. Moreover, compared with the bulk Pd electrode, the Pd-based catalysts consisted of the bi- or mono- metallic nanoparticles had higher electrocatalytic activity towards CO2 reduction, which was quite different from those in bulk metals, such as FECO < 6% over Pd wire electrode [36] and FECO < 25% over Pd wire electrode modified with Cu at all different copper coverage [37]. Hence, these PdCu/C nanocatalysts displayed significant potential in practical applications of the CO2 electrocatalytic reduction process. Obviously, the bimetallic PdCu nanoparticles with size smaller than 5 nm, especially Pd85Cu15, could significantly enhance the catalytic activity and selectivity for CO production, while the selectivity of formate etc. restrained dramatically (Table S1). The similar increase in selectivity for CO production with Cu nanocatalyst was also proved using In, Sn and Zn as the second metals [18], which was analogous to the bimetallic PdCu nanoparticles. Usually, the electrochemical reduction mechanism of CO2 to CO in an aqueous solution was suggested to include the following three steps [20, 38]:

CO2 (g) + ∗ + H+ (aq) + e- ↔ COOH∗ -

(1)

+

COOH∗ + e + H (aq) ↔ CO∗ + H2O (l) (2) CO∗ ↔ CO (g) + ∗

(3)

where the asterisk (*) of above means the catalytic site at which a species can adsorb. Firstly, the CO2 was adsorbed on the catalyst surface and then reduced by the incoming electrons, coupled with the resident protons, leading to the formation of COOH*. Second, the COOH* reacted with another proton and electron to produce CO*. Third, CO desorbed from

the

electrode surface, which

was

the

non-electrochemical process. According to the previous works [38], the conversion of CO2 to COOH* in the first step, inhibited by weak CO2 adsorption/binding, and the release of CO from the surface in the last step, inhibited by strong CO binding, can be regarded as the rate-determining step; whereas the conversion of COOH* to CO* is usually easy during the second step. Compared with Pd nanoparticles, we think that the presence of Cu in the bimetallic PdCu nanoparticles significantly altered the nature of the electro-active species and thus adsorbate-binding energy [29, 37]. In fact, it has been confirmed that the heat of adsorption of CO could be lowered on the PdCu(111) surface compared with the Pd(111) due to the structural factor, i.e. alloy effect [39]. In order to further verify this, we investigated the desorption behavior of CO on the bimetallic PdCu and monometallic Pd nanoparticles by means of TPD-MS. Actually, it was generally found that many of the trends observed in activity and selectivity for CO2 reduction can be explained at least in part by CO binding energy on catalyst surface [34]. As shown in Fig. 4, comparing the TPD-MS spectra with those obtained from monometallic Pd nanoparticles, it was obvious to note that the maximum desorption temperature of CO from bimetallic PdCu nanoparticles was still significantly lower than that of CO from Pd due to the ligand effect with Cu on Pd-CO by modifying the electronic structure of the Pd constituent or lowering heat of CO adsorption in structural factor [40, 41]. Thus, alloying Pd with Cu results in lowering the adsorption affinity of CO-like intermediate compared to Pd during CO2 reduction [42]. In other words, the Pd nanoparticles that bind CO strongly are less active in CO2 reduction process because they are poisoned by CO or other

intermediates that form during CO2 reduction, and consequently, hydrogen evolved from the competing water reduction, may be the main product observed [34]. On the other hand, the bimetallic PdCu nanocatalysts that bind CO relatively weakly produce mostly CO, since the CO would release from the surface before it can be further reduced to products such as alcohols and hydrocarbons. Obviously, the proper amount of Cu would be the vital factor for the modulation of electronic structure of Pd atoms and binding energy with CO. The excessive Cu would result in the formation of copper oxide and palladium oxide (Fig. 2 and Tab. 1), which would depress the catalytic activity and selectivity of CO. Furthermore, the geometric effects would also play a key role for the activity enhancement. This is because the addition of Cu to Pd and enrichment of Cu on the particles surface would change the bond lengths of Pd-Pd and Cu-Cu bond, as confirmed by the EXAFS results. For hydrogen adsorption, it has been reported that the Pd-H bond is weaker in Pd-Cu alloy than in pure Pd because of a decreased binding energy of hydrogen with palladium on the alloy surface [43]. Moreover, for the low amount of adsorbed H on the Pd-Cu surface, it is also very difficult to discuss about a possible migration of dissociated hydrogen from Pd to Cu surface atoms [43], thereby further protonation of adsorbed CO was difficult during the CO2 reduction process. Hence, the bimetallic Pd85Cu15/C catalyst showed a high selectivity for CO formation derived from electrochemical reduction of CO2. Conclusion In summary, we have synthesized bimetallic PdCu/C catalysts with different compositions and investigated their electrocatalytic reduction of CO2 to CO. Pd85Cu15/C catalyst showed higher faradaic efficiency, current density and mass activity for CO production than Pd56Cu44/C and Pd/C catalysts. EXAFS and CO TPD-MS studies suggested that highly selective CO production over Pd85Cu15/C catalyst is due to the presence of an optimum ratio of the copper element and low-coordination sites. Thus, the bimetallic PdCu alloy nanocatalysts would suppress hydrogen evolution reaction and simultaneously promote the conversion of CO2, which is highly desired in the electrochemical recycling of CO2. Our study demonstrates great potentials of Pd-Cu alloy nanoparticles for the selective

electrochemical reduction of CO2 to CO. Further, with the recent advances in chemical synthesis of nanoparticles with controllable size, composition or morphology, various bimetallic nanoparticles catalysts can now be designed to control specific reaction pathways in order to achieve selective electrochemical reduction of CO2. Acknowledgements This work was supported by the National Natural Science Foundation of China (21173009, 21222306, 21303119 and 21573222), 973 Project (2013CB933100) and Tianjin Research Program of Application Foundation and Advanced Technology (No. 15JCQNJC05300). G. X. Wang also thanks the financial support from CAS Youth Innovation Promotion. The XAS experiments were conducted in Shanghai Synchrotron Radiation Facility (SSRF) for Pd K edge and Beijing Synchrotron Radiation Facility (BSRF) for Cu K edge.

Figure 1. Typical TEM images of the bimetallic Pd-Cu nanoparticles: (A) Pd85Cu15, (B) Pd56Cu44, (C-D) Pd85Cu15 on the carbon support after heat-treatment at different magnification.

Figure 2. Pd K edge (A) and Cu K edge (B) XANES spectrum of PdCu/C after H2 pretreatment; Fourier transform magnitudes of k3 - weighted EXAFS data and theoretical fits of bimetallic PdCu/C after heat-treatment under H2 atmosphere: C) Pd K edge for Pd85Cu15/C; D) Cu K edge for Pd85Cu15/C; E) Pd K edge for Pd56Cu44/C; F) Cu K edge for Pd56Cu44/C.

Figure 3. CO2 reduction activity over PdCu/C and Pd/C catalysts in CO2-saturated 0.1 M KHCO3 solution. A) FE of CO; (B) current density for CO production; (C) current density for H2production; (D) mass activity for CO production.

Figure 4. TPD-MS spectra of CO adsorbed on Pd and PdCu nanoparticles supported on the fumed silica.

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Vitae Zhen Yin received his Ph.D. in Physical Chemistry from the Dalian Institute of Chemical Physics (DICP), Chinese Academy of Science (CAS) in 2010, then worked as a postdoctoral fellow at the Nanyang Technology University (NTU), Singapore. In 2012, he joined the faculty as Assistant Professor at the Department of Chemical Engineering at the Tianjin Polytechnic University, China. His current research interests include the energy conversion and nanocatalysts, synthesis and application of nanostructured materials.

Dunfeng Gao received his BSc in materials chemistry from China University of Petroleum (East China) in 2009 and Ph. D degree in physical chemistry from DICP, CAS in 2015. He is now a postdoctoral researcher in Ruhr University Bochum, Germany. His research interest is focused on water and carbon dioxide electrolysis.

Siyu Yao recieved his Ph.D degree from Peking University in 2014, working on the XAFS characterization of nano catalysts. After graduation, he became the research assistant in Prof. Ding Ma’s group. His research interest is developing highly efficient catalysts for low temperature hydrogen production and purification.

Bo Zhao received his bachelor degree in Sichuan University, majored in applied chemistry. He is a PhD candidate in the College of Chemistry and Molecular Engineering of Peking University now, under the direction of Prof. Ma Ding. His current research is focused on catalysis chemistry, including syngas conversion and zeolite catalysis.

Fan Cai received his bachelor degree in chemistry from Nanjing University in 2011. He is currently a Ph.D. candidate in Prof. Xinhe Bao’s group at DICP, CAS. His research interest is focused on carbon dioxide electrolysis.

Lili Lin is currently doing her PhD under the supervision of Prof. Ding Ma in the College of Chemistry and Molecular Engineering of Peking University (PKU). She received her bachelor's degree in Applied Chemistry from Dalian University of Technology (DUT) in 2012. Her research interests focus on the aqueous phase reforming of alcohol, including the synthesis of noble metal nanoparticles as catalyst.

Pei Tang is a PhD candidate now under the supervision of Prof. Ding Ma in Peking University. And he graduated from Shandong University with a bachelor degree in 2011. He is mainly interested in the development of noble catalysis systems on methane conversion and carbon catalysis.

Peng Zhai is pursuing PhD in the Beijing National Laboratory for Molecular Sciences (BNLMS, China) at Peking University under the direction of Prof. Ding Ma. His current research is focused on the new process of syngas conversion and catalyst development.

Guoxiong Wang received his B. S. from Wuhan University in 2000 and Ph. D in Physical Chemistry from DICP, CAS in 2006. After working at Catalysis Research Center, Hokkaido University, Japan from 2007 to 2010 as postdoctoral researcher, he joined State Key Laboratory of Catalysis, DICP as an Associate Professor and was promoted as full professor in 2015. His research interests include highly efficient electrocatalytic materials and processes for electrochemical energy conversion and storage.

Ding Ma read chemistry at Sichuan University and graduated in 1996. He obtained his PhD from the State Key Laboratory of Catalysis at the Dalian Institute of Chemical Physics in 2001. After his postdoctoral positions at Oxford University and the University of Bristol, he started his research career at the Dalian Institute of Chemical Physics as Associate Professor in 2005. He was promoted to full Professor in 2007 and moved to Peking University in 2009. His research focuses on heterogeneous catalysis, particularly when applied to energy innovation, for example methane and syngas conversion. He also works on developing new reaction routes for sustainable chemistry and in situ spectroscopic methods which can be used to study reaction mechanisms.

Xinhe Bao received his Ph. D. in Physical Chemistry from Fudan University in 1987. He held an Alexander von Humboldt Research Fellow position in Fritz-Haber Institute between 1989 and 1995, hosted by Prof. Gerhard Ertl. Following that, he joined DICP as a full Professor. He became a member of the CAS in 2009. His research interest is nano and interfacial catalysis, focusing on the fundamental understanding of heterogeneous catalysis, including development of new catalysts and novel catalytic processes related to energy conversion and storage.

Highlights 

The high selectivity for CO production can be achieved in CO2 electrochemical reduction with bimetallic PdCu alloy nanoparticles.

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Controllable size and composition for the bimetallic nanoparticles are critical to the CO2 reduction activity enhancement with high CO Faradaic efficiency.

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The Pd85Cu15/C catalyst shows the highest Faradaic efficiency, current density and mass activity for CO production.