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The Tunable and Highly Selective Reduction Products on Ag@Cu Bimetallic Catalysts Towards CO2 Electrochemical Reduction Reaction Zhiyuan Chang, Sheng-Juan Huo, Wei Zhang, Jianhui Fang, and Hailiang Wang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017

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The Journal of Physical Chemistry

The Tunable and Highly Selective Reduction Products on Ag@Cu Bimetallic Catalysts Towards CO2 Electrochemical Reduction Reaction Zhiyuan Chang,[a] Shengjuan Huo,[a],[b],[c]* Wei Zhang,[a] Jianhui Fang,[a] Hailiang Wang[b],[c]* *

[a]

Department of Chemistry, Science Colleges, Shanghai University, 99 Shangda Road,

Shanghai, 200444, China [b] Department of Chemistry,Yale University, New Haven, Connecticut 06520, United State [c] Energy Sciences Institute, Yale University, New Haven, Connecticut 06516, United State

ACS Paragon Plus Environment

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ABSTRACT：Bimetallic electrocatalysts can improve the activity and selectivity over their monometallic counterparts by tuning the structure, morphology and composition. However, there scarecely was a systematic model to understand the structural effect relationship on CO2 electrochemical reduction reaction, especially for product tuning process by introduction of second metal to grow into outer layers. Herein, we report a structure-controlled model about growth process of Ag@Cu bimetallic nanoparticles are fabricated by a polyol method, that is, reducing mixtures of Ag+ and Cu2+ (excess amount) in ethylene glycol (reducing agent) in the presence of polyvinyl pyrrolidone. Structurual characterizations reveal that a series of Ag@Cu NPs are tuned from Ag core, Cu modified Ag, to Cu outer shell by controlling the heating time (0-25 min). Moreover, highly selective catalysts with the tuning reduction products from carbon monoxide to hydrocarbons can be realized. Different from the “dilution” effects between Ag and Cu, the volcanic curve for carbon monoxide production is detected for the introduction of Cu and the peak point is the Ag@Cu-7 electrocatalyst (heating time is 7 min). Similarly, interestingly, when Cu cladding layer continuously grows, the hydrocarbons are not a simple proportional additio and optimized at Ag@Cu-20 (heating time is 20 min). The geometric effects dominantly account for the synergistic effect of CO product and control the surface activity to hydrocarbons. This study serves as a good starting point to tune the energetics of the intermediate binding to achieve even higher selectivity and activity for core-shell structured catalysts


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INTRODUCTION Due to its ambient operating conditions and the possibility of direct integration with renewable resources, converting CO2 to useful fuels by electrocatalytic process is regarded as a promising path to reduce excessive greenhouse gas and generate a sustainable supply of chemical commodities.1-9 A wide variety of metal materials, such as Cu,10-16 Ag,17-24 Au,25-28 as well as other metals have attracted considerable attentions. In particular, Cu has gained the more interest because of its capability to reduce CO2 into hydrocarbon fuels. However, the activity, selectivity and durability are far from satisfaction.29 Therefore, it is critical to boost the performance of the catalysts with better selectivity that efficiently electroreduce CO2. To achieve this goal, the development of bimetallic electrocatalysts for CO2 reduction reaction (CO2RR) is an alternative method because two different metals can potentially offer better catalytic performances over their monometallic counterparts by tuning the structure, morphology and composition.30, 31 It is believed that the reaction kinetics for CO2 reduction can be enhanced by tuning the binding strength of intermediates on a bimetallic metal catalyst surface.32 In terms of the design and synthesis of bimetallic Cu based catalysts, Nørskov’s group has established a set of effective theories that described the adsorption energy and reaction activity as well as DFT-based screening methods, making far-reaching advances in electrocatalytic reactions.33-35 Under theoretical guidance, a series of bimetallic catalysts, such as Cu-Au,32, 36-38 Cu-Pd,39 Cu-In40, 41 and Cu-Sn42 have been introduced to exhibit the improved surface activities towards CO. Cu-Pt alloy43, 44 or Cu modified Pt electrocatalysts45 still could show the capability of Cu to reduce CO2 into hydrocarbon products. Especially, tuning d-band structure and geometric effect on Cu-Au,32 Cu-Pt43 alloy catalysts shed light on the influential factors on the catalytic performances.46, 47 Generally, the modification of the surface electronic and geometric


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structures of metals can be attributed to tuning of adsorption energy for adsorbed species. The former affects the bonding strength between adsorptive species and metal surface atoms, while the latter affects the numbers of bonds.34 Till now, it was still not an easy task to distinguish these effects and explain how the reactivity was tuned. It is urgent to find a well-defined system that can show the structural change from the presence of the dissimilar atoms to form the outer layer with the different interatomic distances. This can help understand the complexed tuning mechanism and design the catalysts towards CO2RR for scale up in the future. Unfortunately, systematic studies on core-shell structured bimetallic system which can tune the selectivity were rarely reported. Due to its higher electron conductivity and better surface enhanced Raman scattering effects, Ag-Cu bimetallic nanomaterials received extensive attention.48-51 Nevertheless, only a handful of such reports exsited for the Ag-Cu bimetallic nanoparticles (NPs) electrocatalysts toward eCO2RR. For instance, as for alloy materials, Watanabe’s group reported the synergistic effect for CO on the electroplating Ag-Cu alloy film for CO2RR.52 Nogami et al pointed that the AgCu bulk alloy to the CO2RR in a pulsed condition and found the selectivity to C2H5OH compounds.53 In terms of Cu modified Ag catalysts, Choi et al studied Ag-Cu dendrite film catalysts to obtain their catalytic activity and selectivity for CO2RR to CO.54 Unfortunately, the “dilution” effects between Ag and Cu were indicated and the highest FE for CO was still for pristine Ag by introduction of Cu.

In addition to the controversal opinions in terms of

synergistic or dilution effect between Cu and Ag, Ag@Cu core-shell structure for CO2RR, which was also a good structure model to study the function of exotic atoms, had not been addressed. Thus, it was meaningful to clarify or study these controversal opinions origined from the structure and activity.


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Luckily, great efforts were devoted by Tsuji’s group to the synthesis of bimetallic NPs and the investigation of their mechanisms.49, 55-58 By means of a one-step polyol method to fabricate the epitaxial growth of noble metal to core-shell (Au, Ag, Pd and Cu) structures by controlling reaction time. Their fabrication concept inspired us to seek a series of catalysts with different structures and compositions during the growth process to apply in CO2RR and tune or optimize the reduction products. Therefore, a strategy to design the selective surfaces in the formation process of core-shell structure was presented. In this article, efforts were mainly focused on the exploration of the introduction of Cu as a secondary metal in Ag@Cu formation process for a systematic model to study the tuning of selectivity and activity.The catalysts varying from Ag core, Cu modified Ag, Ag@Cu core-shell NPs in the growth process have been fabricated and the structure-activity relationship of the catalysts was discussed. Correspondingly, these samples showed different electrocatalytic results towards CO2RR from the optimized CO to hydrocarbons. It was worthy to be mentioned that different from the “dilution” effects between Ag and Cu, the volcanic curves for carbon monoxide and hydrocarbons products were detected. Ag@Cu-7 (7 min of heating time ) exhibited the extraordinary activity and synergistic reaction towards CO and Ag@Cu-20 (20 min of heating time) for ethylene over the monometallic counterpart. The increased electrocatalytic activities were mainly related to the surface geometric effect and electronic effect. EXPERIMENTAL SECTION Materials Cupric acetate monohydrate [AR, 99.99%], silver nitrate [AR, 99.99%], ethylene glycol [EG, AR, 99.99%] and polyvinyl pyrrolidone [PVP, 40000 in terms of monomer units, AR, 99.99%] were used in the synthesis of Ag@Cu bimetallic NPs. All aqueous solution and reactions were


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conducted in ultrapure water (>18 MΩ•cm). Nafion solution (5wt % in a mixture of alcohols and 45wt% water) was used in the ink formulation. High-purity CO2 (99.999%) and Ar (99.99%) gas were acquired from Pengpu gas limited company (Shanghai). All chemicals, unless stated otherwise, were supplied by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Sample Preparation and Characterization Ag@Cu NPs were fabricated according to the previous references by adjusting the temperature control method and reaction time.49 In brief, a one-step reduction method was used by reducing mixtures of AgNO3 and Cu(OAc)2•H2O in EG in the presence of PVP at 180℃ for 5~25 min. The temperature profile of solution is shown in Figure S1 (SI). A certain amount of AgNO3, Cu(OAc)2 and PVP in EG were mixed in a three necked flask and the final concentrations were 1.57, 6.36 and 191mM, respectively. After Ar gas bubbling, the reaction began when heating the mixed solution from a room temperature. The solution color started to change from yellow, green, brown, black, copper and reddish brown after heating for 4.5, 5, 7, 10, 15, 20 and 25 min, respectively. These color changes suggested that the contribution of Cu increased with the prolonging reaction time. Then, each sample solution was rapidly cooled in a water bath and then centrifuged at 12000 rpm for 15 min each time. The precipitates were re-dispersed in ethanol and ultrapure water, and then centrifuged at least five times to remove extra PVP. The Ag@Cu NPs prepared at different heating reaction time were named as Ag@Cu-5, Ag@Cu-7, Ag@Cu-10, Ag@Cu-15, Ag@Cu-20, Ag@Cu-25 respectively. For comparison, the pristine Ag NPs or Cu NPs were fabricated using the same amount of AgNO3 (1.57 mM) or Cu(OAc)2•H2O (6.36 mM) in EG in the presence of PVP (191 mM) at 180℃ for 2 h. For inductively coupled plasma optical emission spectrometry (ICP-OES), a certain amount of sample was first dissolved in aqua regia (3:1 V/V HCl/HNO3) for at least 48 h in a 50 ml


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volumetric flask, then analyzed with a ICAP 6300 (HONGKONG SINCERE TECHND LOGY). Before X-ray photoelectron spectroscopy (XPS) measurement, the catalysts were pressed into 0.5 mm thick and 2 mm diameter thin slice and placed under vacuum. For XPS test (ESCALAB 250Xi; Thermo Fisher Scientific China., Ltd, Al Kα source), the binding energy (B.E.) was scanned from 0 to 1380 eV and each spectrum was collected over B1000 scans. B. E. was corrected against C1s (284.6 eV) reference standard. Transmission electron microscopic (TEM) images were carried out in an instrument (JEM-2010F; JEOL Ltd.) at an accelerating voltage of 200K eV. The solution was diluted 100 times for ultraviolet–visible spectroscopy in the UV-Vis region (UV-3600; Shimadzu Corp.). Electrochemical Measurements Electrochemical measurements were carried out in a home-made H-type cell with a cathodic and an anodic compartment separated by a Nafion membrane (Nafion N117, Dupont, USA), using a three-electrode assembly at room temperature. The electrolyte was CO2-saturated 0.1 M potassium bicarbonate (KHCO3), pH=6.8, prepared by bubbling 0.1 M KHCO3 solution with CO2 for 1 h and electrochemically purified for more than 16 h to remove trace metal ions according to the reference.59 The Ar and CO2 saturated electrolytes were formed by purging the corresponding gas into electrolyte for at least an hour. An L-type glassy carbon electrode with a diameter of 5 mm as a working electrode on which 20 µL of the ink formulation solution (10 mg electrocatalyst was dissolved in the mixed 880 µL ethanol and 120 µL 5 wt % Nafion solution ) was dropped. Pt gauge and saturated calomel electrode (SCE) served as a counter electrode and a reference electrode, respectively. All potentials were reported versus the reversible hydrogen electrode (RHE) using the following equation (1):

E RHE ( V ) = ESCE ( V ) + 0.242 V+ 0.0591V∗ pHelectrolyte


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The resistances of the cell were determined before every experiment by electrochemical impedance spectroscopy (EIS) of the electrochemical station. The uncompensated solution resistance at AC frequency ranging from 10 M Hz to 1 Hz was measured at -0.5 V and the average value was used. Cyclic voltammetric measurements were conducted from 0 V to -1.35 V with the scanning rate of 50 mV·s-1. Detection and Quantification of Products In order to evaluate the selectivity of the CO2 reduction, we used the online electrochemical gas chromatograph (GC) technique to detect gas products and estimated their faradaic efficiencies (FEs). As illustrated schematically in Figure S2 (SI), the setup was consisted of a CHI 660D electrochemical workstation, H-type electrochemical cell and gas chromatography system (GC2060, Shanghai). The GC was equipped with a molecular sieve 13X and hayesep D column with Ar flowing as a carrier gas. H2 concentration was determined by a thermal conductivity detector, and CH4, CO, C2H4 were determined using a flame ionization detector. During the electrolysis, the flow of CO2 was maintained at a 20 sccm and the electrolyte was kept under constant stirring. FE data were calculated through the following equation (2) where m0 was the amount of standard gas, A0 was the peak area of standard gas, A was the peak area of detection, M was the relative molecular mass, i was the instantaneous current, ʋ was the gas flow rate, n was the amount of substance, Z was the electron transfer number, F was the Faraday’s constant.

FE % =

nZF ×100% = Q

m0

Α ⋅ v ⋅ ΖF Α0 ×100% Μ ⋅i

⑵.

RESULTS AND DISCUSSION Growth mechanism of Ag@Cu bimetallic NPs


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The time-dependent growth process was reported herein by simply reducing AgNO3 and excess amount of Cu(OAc)2 in EG. In the polyol reduction, some organic radicals and electrons were formed as intermediates in the thermal decomposition of EG, seen in the equations (3) and (4).60 At the first stage, Ag NPs were mainly formed. As we all know, in equations (5) and (6), the standard potential of Ag+/Ag0 (+0.78 eV) is much higher than that of Cu2+/Cu0 (+0.34 eV). Therefore, the reduction rate of Ag+ occurred more rapidly than that of Cu2+ when mixtures of AgNO3 and Cu(OAc)2•H2O were reduced in EG. Even less reduced Cu0 by EG in the first stage should be replaced or oxidized back to Cu2+ by Ag+ in the precursor. That was the reason why metallic Ag was dominant as the core at the beginning stage. Due to the excess of Cu2+ in the precursor mixture, Cu2+ ions were reduced and appeared with the reduced Ag as the second stage, we denoted them in the formation process of Cu modified Ag NPs as the second reaction stage. Thereafter, the presence of Ag and Cu would accelerate the following reducing reaction of the Cu formation. The continuously reduced Cu could not interpolate the surface Ag core and instead got deposited on the surface of core as Cu shells as the last stage. The thickness of Cu shells would increase with the prolonging reaction time until the complete reduction of Cu2+.57 The as-formed NPs in the three stages were denoted as Ag@Cu NPs. CH 2OH − CH 2OH → CH 3CHO + H 2O

⑶.

2CH 3CHO → CH 3COCOCH 3 + 2 H + + 2e −

⑷.

Ag + + e − → Ag 0

E = +0.78 V

⑸.

Cu 2+ + 2e − → Cu 0

E = +0.34 V

⑹.

TEM Characterization Figure 1


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TEM (Figure 1) was then utilized to further characterize the size, morphology and structure of the Ag@Cu bimetallic NPs together with the Ag and Cu NPs benchmarks. As seen from Figure 1(a), about Ag@Cu-5~Ag@Cu-25, different structures of NPs were fabricated, although it was difficult to distinguish between Ag and Cu components from the dark and bright contrast of TEM images. The series of NPs were largely spherical, with diameters of 180±20 nm, larger than Ag NPs (ca. 50 nm) or Cu NPs (ca. 150 nm). In the whole growing process, the core was formed, covered by small bumps. Then, the bumps grew, aggregated, emerged as a shell structured layer, finally formed a thicker shell, resulting the gradually increasing average diameters. It was estimated that the respective average shell thickness was about 15 nm for Ag@Cu-20 and 30 nm for Ag@Cu-25, respectively. As the important stages of crystal growth, the TEM-EDS and line analysis data about Ag and Cu components along the cross section lines, seen from Figure 1(b), were obtained for the catalysts of [email protected], Ag@Cu-7 and Ag@Cu-20. To better investigate if Ag core was a part structure of Ag@Cu NPs, [email protected] have been prepared and measured. As seen from Figure 1b, [email protected] were quasi-spherical to some extent with diameters of 185±25 nm, and the size and morphology were similar to those of the Ag@Cu-7 and Ag@Cu-20. It was worth noting that Cu components were not found in the [email protected] according to the data of EDS, line analysis and ICP-OES. All these results supported the fact that Ag core was a part structure of Ag@Cu NPs. These EDS mapping images results for Ag@Cu-7 and Ag@Cu-20 also gave the evidences about the inner Ag cores (red) and outer Cu shells (blue), respectively. Meanwhile, these line analysis data suggested that Cu modified Ag NPs clearly formed for Ag@Cu-7 NPs and 15±3 nm Cu shells were partially overgrown on Ag core for Ag@Cu-20. Hence, it coincided well with the aforementioned grown mechanism for the series of Ag@Cu


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NPs. All these results confirmed that the morphologies and components for the well distributed NPs (Ag@Cu-5~Ag@Cu-25) could be readily controlled by the heating time. UV-Vis absorption spectra Figure 2 UV-Vis absorption spectra were also measured to characterize optical properties and to examine time-dependent growth process of the series Ag@Cu bimetallic NPs, seen from Figure 2. The spectra for pristine Ag NPs and Cu NPs were also shown for comparison. It was clear that the surface plasmon resonance (SPR) bands for Ag (black line) and Cu NPs (red line) were at 430 nm and 606 nm, respectively. The SPR band for these bimetallic NPs appeared with one dominant peak between the band for pristine Ag NPs and that for pristine Cu NPs, rather than a simple mixture of them61, 62 with two SPR bands. The positions of the SPR bands red-shifted from 450 to 598 nm with the increasing Cu percentage.63, 64 These spectral changes indicated that Ag was the main component of the resulted particles in the initial stage. With the increase of heating time, SPR band of Cu shell grew and increased. Also, there was a better combination between Ag core and Cu shell, which was clearly evident in TEM images. X-ray photoelectron spectroscopy and X-ray diffraction (XRD) patterns Figure 3 In order to investigate the surface composition and chemical valence state of the electrocatalysts, the XPS measurements were performed. XPS survey spectra (SI, Figure S3) for the bimetallic NPs have been carried out. After carefully surveying the spectra, only the peaks related with Ag, Cu, O, and C elements were identified. The appearance of oxygen was understandable in the process of transferring the samples (surface absorbed oxygen). The higher resolution scans of the Ag 3d region and Cu 2p region were presented in Figure 3a and 3b. Ag 3d


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core levels can be split into two spin-orbit pairs at 367.2 eV and 373.2 eV, which can be attributed to the Ag 3d5/2 and Ag 3d3/2 of metallic state, respectively. Similarly, Cu 2p1/2 and Cu 2p3/2 for Cu 2p in metallic state can be found at the B. E. near 951.8 eV and 932.1 eV, which were consistent with the results reported elsewhere.65 This illustrated that EG was an effective reducing reagent in this case. The band positions about Ag 3d5/2 and Ag 3d3/2 shifted to the higher B. E. with longer reduction time, especially compared to those for pristine Ag NPs. On the contrary, the trends were clear that those about Cu 2p1/2 and Cu 2p3/2 shifted to the opposite direction. It was reasonable that Ag was richer in electron and had the ability to transfer electron to Cu, leading to its gradual shift towards higher B. E. with more Cu in the samples. On the contrary, Cu served as the electron acceptor and more interatomic charges transferred between Ag and Cu when they were in the modified or core-shell structure. This coincided well to the observation reported by Yang’s group, who reported that the similar B. E. changing trends for Cu-Au alloy NPs with the improved Au content.32 However, compared to those for pristine Cu NPs, those bands were in the higher B.E., mostly due to the surface oxidation to a certain extent. Furthermore, with regard to the intensities for the time-dependent samples, the band intensities about Ag decreased and those about Cu increased gradually. The surface atomic ratios of Ag and Cu were1:0.02, 1:0.6, 1:1.79, 1:2.2, 1:7.63, and 1:18.9 for the Ag@Cu-5, Ag@Cu-7, Ag@Cu-10, Ag@Cu-15, Ag@Cu-20 and Ag@Cu-25 samples, respectively. Higher Cu contents and the fewer Ag components (hence stronger Cu bands) as well as thicker Cu shells, which in turn more effectively decayed photoelectrons from the shielded Ag cores, greatly reducing the Ag bands in the XPS spectra. Because XPS can only assess the top nanometers of surfaces, the shielding effect of Cu shells on the photoelectron generated in the Ag core was significant; therefore, surface atomic composition data derived from XPS did not reﬂect the actual Ag and Cu contents


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especially for the core-shell structure. Consequently, ICP-OES results quantified the average bulk composition, where the specimen was completely digested and ions in solutions were tested. The analysis showed that the respective Ag to Cu ratios for the six bimetallic metal samples were 1:0.06, 1:0.2, 1:1.29, 1:1.86, 1:2.31, and 1:3.69. The Ag:Cu ratio in the last sample was very close to the that from the precursors. That indicated that all the metallic precursors were reduced in the end. Besides, XRD measurements (Figure S4, SI) were also performed to analyze both the structures and crystalline facets of as-prepared NPs. As seen in Ag@Cu-5~Ag@Cu-25, the diffraction peaks at 2θ at 38.2°, 44.3°, 64.6°, 77.4°, and 81.4° were due to reflections from (111), (200), (220), (331), (222) planes of face-centered cubic Ag (PDF-04-0783), and the distinct peaks at 2θ values of 43.4°, 50.5°, 74.2°, 89.9° corresponded to diffraction from (111), (200), (220) and (311) planes of the face-centered cubic structure of metallic Cu (PDF-04-0836), respectively. Moreover, XRD results also suggested Ag in the bimetallic NPs belonged to mixed phases with Cu in all the reaction stages. Besides, no obvious shift in Cu and Ag peak positions (Figure S4 (b) for enlargement of Figure S4 (a)) suggested that there was no lattice parameter change for these series electrocatalysts during the growth process. Simply, in the initial stage of reaction, the characteristic peaks of Ag were detected at Ag@Cu-5 and even no diffraction peaks of Cu can been detected for Ag@Cu-7, which clearly indicated that Ag was the dominant element. On the basis of TEM line analysis data and no peak shift in XRD, we herein considered that these bimetallic NPs did not belong to AgCu alloys, and temperorily denoted as Cu modified Ag NPs (the early stage of Ag@Cu) .57, 58 Subsequently, one weak peak typical of the Cu (111) at 38.1° appeared for Ag@Cu-10. Then, the more intensity of Cu (111) increased with the prolonged reaction time, even other characteristic peaks Cu (200), (220) and (311) were


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clearly observed in the last stage of growth process. These spectral changes evidenced that Cu component of the NPs increased with heating time. Interestingly, different from the changing trend of Cu, the intensity of Ag peaks didn’t increase continuously but decreased from Ag@Cu15. These phenomena were due to the progressive increase of the Cu thickness (shell) limited the access to the subjacent copper surface .66,67 In most bimetallic core-shell systems, the XRD peaks from the core metal were influenced to decrease and even not observed due to the core metal being in kinematic diffraction state. These evidences further supported the formation of coreshell structure obtained by TEM, UV-Vis and XPS spectra, which provided the model to study the resulted electrocatalytc properties in the CO2RR. Cyclic voltammetry measurements Inspired by the attractive nanostructured change of the as-prepared Ag@Cu electrocatalysts, we investigated their electrocatalytic activities towards CO2RR. Shown in Figure 4 were CVs of those electrocatalysts in the absence and presence of CO2 in 0.1 M KHCO3 solution. Generally, hydrogen evolution reaction (HER) was a mainly competing process of CO2 reduction on the electrode.12, 23 Therefore, it was crucial to suppress the byproduct formation. Figure 4 indicated that CO2RR dominated the reduction process at potentials negative of -0.5 V. For all these electrocatalysts, much higher reduction current densities were observed in CO2 saturated solution than those in Ar saturated solution. The current density of CO2RR outcompeting HER could be roughly calculated by subtracting the current density obtained at the black dash lines (HER current) from that recorded at the red lines (the overall current of HER and CO2RR), as inferred by the green arrow lines in those figures. The results clearly demonstrated that Ag@Cu-7 (Ag dominant) and Ag@Cu-20 (Cu dominant) bimetallic materials could achieve the better activities for CO2RR than other counterparts. In addition, as could be seen in Figure 4, the current


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densities ( either in Ar or in CO2 saturated solution) of Ag@Cu-10~Ag@Cu-25 and Cu NPs were larger than that of Ag NPs, Ag@Cu-5 and Ag@Cu-7, which illustrated that when Cu dominated as the active center in the NPs, HER was a mainly competing process, consistent with the improved faradaic efficiency of H2 obtained from the followed online product analysis. Figure 4

Electrocatalytic Activity and Selectivity Figure 5 Figure 6

The selectivity of products could be tuned by controlling the reaction time when designing and forming the electrocatalyst models. The GC analysis proved that the main carboneous gas products changing from CO to ethylene at the potential of -1.06 V. Figure 5 summarized the FE trends of methane, ethylene, H2 and CO dependent on the applied potentials for different electrocatalysts. From Figure 5, the pure Ag, Ag@Cu-5, Ag@Cu-7 and Ag@Cu-10 mainly exhibited the electrocatalytic properties of Ag with the higher CO selectivity.17, 18, 21, 66 On the contrary, the remainders showed the higher selectivity towards hydrocarbons.69 Figure 6 summerized the selectivities for CO and C2H4 at -1.06 V for various Ag@Cu NPs for CO2RR in Figure 5. As the different kinds of metals, Ag favored to electrocatalyze CO2 into CO and Cu for hydrocarbons. Some researchers thought the existing “dilution” effects between Ag and Cu, as the black (FE for CO) and red (FE for ethylene) dash lines shown in Figure 6. Instead, it was striking that either CO or ethylene endured a volcano-type correlation for the eight electrocatalysts. For pure Ag and [email protected] (Figure S5 ), the FEs for CO were close at this


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potential, which agreed with other reports about bulk Ag materials.17, 21 The electrocatalytic properties of Ag@Cu-5 NPs in the CO2RR were little different with those of pure Ag or [email protected] (Figure S5). The sample of Ag@Cu-7 gave the highest FE towards CO with a value of 82% and decreased dramatically to 20% for Ag@Cu-10. It was informative to compare the CO2 electroreduction activity for Ag@Cu-7 with that for its monometallic counterpart. The Ag@Cu-7 showed the FE was dramatically higher than the pristine Cu (8%) or Ag (72%) NPs fabricated following the same method. In addition, in Table SI, we listed that the reported Ag based bimetallic materials for CO2RR. In comparison to other Ag based CO-selective electrocatalysts, the Ag@Cu-7 catalyst was one of the best bimetallic material for CO2 reduction to CO in terms of the FE and applied potentials. Similarly, the sample of Ag@Cu-20 gave the peak activity for ethylene with a value of 28.6%, better than its monometallic counterparts, which was reported for the first time about the electrocatalytic property on the Ag@Cu coreshell structured surface. This was consistent with the concept that Cu outlayer kept the identity and gradually away from the Ag core. Moreover, not only CO but also ethylene showed the peak FEs at -1.06 V at which H2 byproduct was minimized.10, 67At the more positive potentials, HER was dominant. Partial current densities for the series of Ag@Cu electrocatalysts were calculated and listed in Figure S6. Similar with the trends in FEs, either CO or ethylene in partial current densities also endured a change with the volcano-like shape. By contrast, the partial current densities for H2 and CH4 showed the overall increasing trend with the increase of copper content in electrocatalysts mostly due to the more affility for Cu to H at these potentials. It was worthy to be pointed out that the partial current densities for H2 formed a “slump” at Ag@Cu-7 or Ag@Cu-20, respectively, which meant the HER got a good suppression for these two catalysts. (Figure S7, SI). Finally, results demonstrated that the Ag@Cu-7 and Ag@Cu-20 electrocatalysts


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displayed the superior activity and stability for 5 h at the potential of -1.06 V in 0.1M KHCO3 (Figure S8, SI). Discussion It was worthy to understand the composition and structure dependent activity and selectivity of the series Ag@Cu bimetallic catalyst models. The following part would discuss it from the electronic and geometric effects. According to Nørskov’s theoretical calculation,71 the activity of the CO2RR to CO on transition metal catalyst surface was usually limited by a formation of *COOH (* denoted an adsorbed site), in which the active catalysts should bind *COOH strongly enough to lower the energy requirement for this reaction. In addition, the catalysts should bind *CO weakly enough to desorb CO from the surface easily without undergoing further protonation to undesired products (Scheme S1, SI ). Also, according to the theory,33-35 d-band center in which the adsorption energy between metal and surface adsorbed species became weaker when εd decreases, vice versa. The adsorption energy between *CO and Cu active site was more intense than that on Ag due to the higher εd for Cu.10, 67Consequently, the binding strength, which mainly derived from the bond between the active site and that intermediate, should increase as Cu was introduced. Therefore, on the premise of the similar structure and size for the series of Cu modified Ag bimetallic NPs, accordingly, based on the results about electron transfer between Cu and Ag by XPS, pure Ag NPs were expected to be at the top of the volcano for CO2 to CO, and the activity of CO should decrease with the increase of the Cu composition. That was the “dilution effect” of Cu in the reported AgCu systems. Similarly, Jiao’s group reported that a slight introduction of Sn into AgSn system dramatically suppressed the production of CO.72 However, the facts were opposite to


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the premise. We observed that Ag@Cu-7 had the optimum binding strength for CO to reach peak activity among the series of NPs for CO2 to CO conversion, which suggested that the catalytic activity to CO was not solely determined by the electronic effects, but other influential factors. The geometric effect which can change the local atomic arrangement at the active sites, also had a larger effect on the binding strength of intermediates. These phenomena have been previously reported by Yang’s group, who found that Au3Cu exhibited the highest CO FE rather than pure Au NP catalyst.32 Yousung Jung’s group also theoretically studied the bifunctional interface of Au and Cu for improving CO2 electroreduction and pointed that the addition of Cu as oxygen affinitive metal into Au could stabilize the *COOH which accounted for the higher CO FE.73 This also indicated that functional Ag (modified) with more oxygen affinitive Cu via oxygenmetal interaction indeed helped to stabilize *COOH, which was a key point to lower the energy requirement for eCO2RR to gain higher FE towards CO. Additionally, the small amount of Cu for Ag@Cu-7 can not offer the possibility to stabilize of *CO to protonate successively to hydrocarbons. Thus, CO was the dominant product until the dramatic conversion to ethylene for the increased Cu content for Ag@Cu-10. In the recent work, Ma et al discussed that a range of bimetallic Cu−Pd catalysts with ordered, disordered, and phase-separated atomic arrangements were studied as the eCO2RR catalysts to determine key factors needed to achieve high selectivity. They also found that geometric effects rather than electronic effects seemed to be key factor in determining the selectivity of bimetallic Cu-Pd catalysts.74 Therefore, we can temporarily regarded the conflicts in the synergistic or dilution effect between Cu and Ag for these AgCu bimetallic electrocatalysts mainly derive from the mixing patterns of the components played an important role in determining each catalyst’s activity and selectivity. In our case, we suspected


18

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that the Cu modified Ag catalysts (Ag@Cu-7) had the better mixed AgCu active sites that can promote the CO production. Figure 7 To further explain the fact, tafel analysis was performed to gain mechanistic insights for the CO2 reduction on the catalyst’s surface. As shown in Figure 7, tafel plot at different overpotentials η as a function with the CO partial current density jCO, on Ag NPs, Ag@Cu-5 and Ag@Cu-7. It was commonly considered that the first step in the two-electron reduction of CO2 •to CO on most metal surfaces is one-electron transferred to a CO2 molecule and forming CO2ads

intermediate species adsorbed on the metal surface (step 7). Previous studies suggested that the first step proceeded at a much more negative potential (

0 ECO

•2 / CO2 ads

= −1.9V vs SHE

)17, 68 than the

following steps and is the rate-determining step for the whole process. In subsequent steps, the •intermediate CO2ads anion would take two protons and another electron, and formed a CO and a •H2O molecule (step 8,9,10).71 Step 8 was a chemical step involving protonation of CO2ads to

form

COOH ads

intermediate.12 Step 9 was an electrochemical step involving proton and electron

transfer, and step 10 was the desorption of CO from the metal surface. •CO2 + e - → CO2ads

⑺.

CO2•ads + HCO3- → COOH ads + CO32-

⑻.

COOH ads + HCO3- + e - → COads + H 2O + CO32-

⑼.

COads → CO

⑽.

As shown in Figure 7, the Ag NPs exhibited a tafel slope of 117.8 mV/dec-1 , which was close to the theoretical tafel slope of 133 mV dec-1 for bulk Ag electrode under similar experimental


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conditions.18 These results indicated that this electrocatalyst could have the same ratedetermining step with bulk Ag, which was assigned previously to the one-electron reduction of •-

CO2 to form CO2ads .18, 21 In contrast, Ag@Cu-5 exhibited a Tafel slope of 101.2 mV dec-1, indicating that the reaction kinetics was faster than that on Ag NPs and the rate-determining step was not pure electron transfer. As for the Ag@Cu-7 electrocatalyst, which displayed the lowest Tafel slope of 78.6 mV dec−1, a little higher than the theoretical value of 59 mV dec−1, suggested that the reaction kinetics was the fastest among these samples and a chemical step (such as step 8, 9) comparing with electron transfer step (step 7) was probably the main rate-determining, followed by another electron transfer.71,76 It appeared that the presence of modification on Ag promoted selectivity and activity for the CO2 reduction reaction. All of the above indicated that mainly influenced by the geometric effect, the reaction mechanism of CO2 to CO has changed when the less Cu coupling with the surface of Ag core. Turning to the hydrocarbon Faradaic efficiency, the pure Cu was highly selective. When the reaction time was prolonged, Cu cladding layer appeared for Ag@Cu surface and Cu was dominant as the electrocatalytic reaction center. The lattice mismatch between Ag and Cu atoms raised the interatomic distance of the Cu atoms relative to that of pure bulk Cu and gave rise to tensile surface lattice strain effects that altered the adsorption energy of reactive intermediates. That is, since the atomic radius of Ag was larger than that of Cu, Cu lattice space was stretched, and the lattice would expand. Thus its radius of d band became wider and d band center rose, causing the enhancement of *CO adsorption onto the Cu surface. It had more possibility for the adsorbed *CO to be further reduced to hydrocarbon products. That was why the Ag@Cu-20 showed the better activity towards ethylene. As we all knew, the influence of tension effects was short-range. When the cladding layer was further grown to shell metal to form a multi-layer


20

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coating structure, the impact for the Ag core metal on the lattice size of shell metal gradually weakened. The Cu coating shell would gradually exhibit the consistent electrochemical performance with the bulk Cu after tens of atomic layers, leading to a decline of hydrocarbon products’ FEs. CONCLUSIONS In summary, we accomplished the formation process in Ag@Cu core-shell system that could show the structural change, which could lead to the activity and selectivity change in CO2RR. The structures and compositions of NPs were controlled by tuning the heating reaction time in a simple polyol reduction method. TEM, UV-Vis, XRD and XPS proved that the NPs endured the process the presence of the dissimilar Cu atoms as surface modification to forming the outer layer and electron transferred from Ag to Cu. The binding energies between the metals and the intermediates could be tuned and optimized by assuming the geometric effects, and consequently different final chemical carboneous product profiles were observed. The synergistic reactions rather than pure “dilution” effects between Ag and Cu were pointed in Ag@Cu-5 or Ag@Cu-20 NPs, which played an important role in producing the higher CO or hydrocarbons. We expect our study to be particularly beneficial and intuitive as it enables a rational core-shell catalyst design to CO2RR.

ASSOCIATED CONTENT Supporting Information Details of the temperature profile of reagent solution, schematic of the online electrochemical gas chromatograph system and home-made H-type electrochemical cell; X-ray photoelectron survey spectra; XRD patterns for the series of electrocatalysts ; comparison table of various Ag


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based electrocatalyst for selectivity and activity; analysis in FE and current density; electrochemical stability measurements. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Shengjuan Huo: E-mail: [email protected]; *Hailiang Wang: E-mail:[email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was sponsored by the Natural Science Foundation of China (Grant No. 21103105). The corresponding authors gratefully acknowledged the financial support from China Scholarship Council for the research in Yale University and the Doctoral New Investigator grant from the ACS Petroleum Research Fund. REFERENCES (1)

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Using

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29

Three-step

Reduction.


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Figure 1. (a) TEM images of Ag NPs, Cu NPs and Ag@Cu NPs prepared at different heating time. (b) TEM-EDS data of Ag@Cu NPs fabricated at the heating time of 4.5 min, 7 min and 20 min.


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Figure 2. UV-Vis absorption spectra of Ag NPs, Cu NPs together with a series of Ag@Cu NPs prepared at different heating time.


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Figure 3. High-resolution XPS spectra of Ag 3d region (a) and Cu 2p region (b) for Ag NPs, Cu NPs and Ag@Cu NPs prepared at different heating time.


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Figure 4. The cyclic voltammograms (CVs) of electrocatalysts for pristine Ag NPs, Cu NPs, and Ag@Cu NPs prepared at different heating time were recorded in Ar- and CO2-saturated 0.1 M KHCO3 with a scan rate of 50 mV/s between 0 and -1.2V(vs. RHE).


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Figure 5. FEs of gaseous products in 0.1 M KHCO3 solution as a function of potential for the pristine Ag, Cu and the series of Ag@Cu electrocatalysts prepared at different heating time.


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Figure 6. Theoretical linear “dilution effects” and measurement values for FEs of CO (black) and C2H4 (red) for the introduction of Cu in the Ag NPs; The two turning points in Faradaic efficiency for CO and C2H4 were marked.


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Figure 7. Tafel plots at different overpotentials η (iR-corrected) as a function with the CO partial current density jCO, on Ag NPs (black square), Ag@Cu-5 (red sphere), and Ag@Cu-7 (blue triangle).


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Table of Contents


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The Tunable and Highly Selective Reduction ... - ACS Publications

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