CO2 reduction to acetate in mixtures of ultrasmall(Cu)n,(Ag)m bimetallic nanoparticlesYing Wanga, Degao Wanga, Christopher J. Daresb, Seth L. Marquarda, Matthew V. Sheridana, and Thomas J. Meyera,1

aDepartment of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; and bDepartment of Chemistry and Biochemistry, FloridaInternational University, Miami, FL 31199

Contributed by Thomas J. Meyer, November 26, 2017 (sent for review August 8, 2017; reviewed by Etsuko Fujita and Clifford P. Kubiak)

Monodispersed mixtures of 6-nm Cu and Ag nanoparticles wereprepared by electrochemical reduction on electrochemically poly-merized poly-Fe(vbpy)3(PF6)2 film electrodes on glassy carbon.Conversion of the complex to poly-Fe(vbpy)2(CN)2 followed bysurface binding of salts of the cations and electrochemical reduc-tion gave a mixture of chemically distinct clusters on the surface,(Cu)m,(Ag)n|polymer|glassy carbon electrode (GCE), as shown byX-ray photoelectron spectroscopy (XPS) measurements. A (Cu)2,(Ag)3|(80-monolayer-poly-Fe(vbpy)3

2+|GCE electrode at −1.33 Vvs. reversible hydrogen electrode (RHE) in 0.5 M KHCO3, with8 ppm added benzotriazole (BTA) at 0 °C, gave acetate with afaradaic efficiency of 21.2%.

CO2 reduction | electrocatalysis | electrodeposition | bimetallicnanoparticles | artificial photosynthesis

Efficient reduction of carbon dioxide to useful fuels and chem-icals is an important research goal of artificial photosynthesis

(1–7). Solar or electrochemical reduction of CO2 is complicated bythe large overpotentials required for one-electron transfer reductionto CO2

− [with E(CO2/CO2−) = −1.9 V vs. normal hydrogen

electrode (NHE) at pH 7] and a high reorganizational energy (8).Subsequent steps to give formate or CO are energetically morefavorable. A growing number of catalytic systems for CO2 reductionhave been identified that give C1 products (9–13) but obtainingmulticarbon (C2+) products has proven more difficult due to thehigh barrier for C–C bond formation on electrode surfaces (14).Current research has led to robust catalysts for CO2 re-

duction, including molecular (15, 16), metallic (10, 17, 18), andnonmetallic catalysts (9) with an extensive literature on metalcatalysts (19, 20). Cu has proven to be the most effective me-tallic CO2 reduction catalyst with an ability to form a variety ofhigh-energy-density products including high-value-added, mul-tiple carbon, C2+, products (20). At Cu electrodes, Jaramilloand coworkers (21) have identified 16 CO2 reduction products,including 12 multicarbon products. In follow-up experiments,the lack of selectivity by Cu has led to the synthesis of Cu-basedbimetallic catalysts (3, 22, 23) or to decreases in the size of theCu catalysts (24). In these experiments, difficulties arise fromcomplicated and time-consuming synthetic procedures by con-tamination by added surfactants and catalysts (25).Electrochemical deposition provides a fast and convenient ap-

proach for depositing nanoparticles on electrode surfaces (26).The procedure can be less time-consuming than wet chemicalsynthesis, but there are difficulties in obtaining small, confinednanoparticles and an extended range in size from hundreds ofnanometers to micrometers in diameter (27). Here we utilize aprocedure described earlier for surface binding and cluster for-mation (24) to prepare ∼6-nm clusters of Cu and Ag on thesurfaces of electropolymerized films. We also report that surface-dependent, electrochemical reduction of CO2 occurs on thesesurfaces with C–C coupling to give acetate as a significant product.

Results and DiscussionA general procedure for preparation of the electrodes has beendescribed previously (2, 28) and is shown in an adapted form

in Scheme 1. In the procedure, the vinyl-derivatized salt[Fe(vbpy)3][PF6]2 (vbpy is 4-methyl-4′-vinyl-2,2′-bipyridine) iselectropolymerized on the surface of a glassy carbon electrode(GCE). Subsequent addition of CN− to an external solutioncauses displacement of a –vbpy ligand by CN− by immersingthe polymerized electrode in 0.1 M [TBA][CN] for 20 min(28). The CN− ligands act as a bridge to added cations in so-lution and exposure of the films gives the ligand-bridged filmadducts FeII-CN-Mm+ (2, 28). Further reduction of the films atpotentials sufficient to reduce the coordinated metal ions re-sults in reduction and transfer of the reduced metals to thesurface of the films, Eq. 1, where nanoparticles form. For filmsloaded with multiple metal ions, composite nanoclusters formon the electrode surface providing a basis for multiple surfaceinteractions and cluster formation. On surfaces on whichmultiple nanoparticles are added, the ratio of available metalscan be tuned by the metal ion composition in the precursorsolution.

nFeII-CN-Mm+, bpy  �����!+me− ,−CN−

  nFeIIðbpyÞ2+, ðMnÞ0. [1]

Polymerization of [Fe(vbpy)3]2+ onto a 0.071 cm2 GCE in 1 mM

solution, in 0.1 M TBAPF6 MeCN, was monitored by cyclic vol-tammogram (CV) scanning at 250 mV s−1 (Fig. 1A). Well-defined redox waves for the couples poly-[Fe(vbpy)3]

2+/1+ andpoly-[Fe(vbpy)3]

1+/0 at E1/2 = −1.28 V and −1.45 V vs. Ag/Ag+

were observed. As shown, the peak currents increase with thenumber of cycles, consistent with the growth of poly-Fe(vbpy)3

2+

on the electrode surface. The latter occurs by reduction of the

Significance

Efficient reduction of carbon dioxide to useful fuels andchemicals is an important research goal in artificial photosyn-thesis. Significant progress has been made for the C1 products,CO and HCOO−. We report here a procedure based on the useof ultrasmall, monodispersed Cu and Ag bimetallic nanoparticleson thin, electrochemically polymerized poly-Fe(vbpy)3(PF6)2 films.They reduce CO2 to acetate at pH 7 in aqueous HCO3

− solutionsat relatively high efficiencies with significant rate enhancementswith added benzotriazole. In the sequence of clusters, the mostefficient results for acetate production were obtained in films of(Cu)2,(Ag)3 with a faradaic efficiency of 21.2% for acetate fromCO2 at −1.33 V vs. reversible hydrogen electrode in 0.5 M KHCO3

with 8 ppm of added benzotriazole at 0 °C.

Author contributions: Y.W. and T.J.M. designed research; Y.W. performed research; Y.W.,D.W., and S.L.M. contributed new reagents/analytic tools; Y.W., D.W., C.J.D., and M.V.S.analyzed data; and Y.W. and T.J.M. wrote the paper.

Reviewers: E.F., Brookhaven National Laboratory; and C.P.K., University of California,San Diego.

The authors declare no conflict of interest.

Published under the PNAS license.1To whom correspondence should be addressed. Email: tjmeyer@unc.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1713962115/-/DCSupplemental.

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vbpy ligands to vinyl radicals, initiating C–C bond formation (2,28, 29). Given the presence of multiple polymerizable vinyl sub-stituents, polymerization is accompanied by extensive cross-linkingas the polymer deposits on the electrode surface to form theporous open structure shown in Fig. 1E. The surface coverageof poly-Fe(vbpy)3

2+ was estimated to be 6.44 ± 0.48 nmol/cm2

from peak current integrations of the poly-[Fe(vbpy)3]3+/2+ wave

at E1/2 = 1.03 V vs. Ag/Ag+ (Fig. 1B). Based on this analysis, thecoverage used in this study was ∼80 molecular layers with a mono-layer surface coverage of 8 × 10−11 mol/cm2 (2, 28, 29). Thethickness of the polymer film on the electrode was ∼250 nm(Fig. S1).Following the −CN displacement step, the poly-[Fe(vbpy)3]

3+/2+

couple shifts cathodically by 0.51–0.52 V vs. Ag/Ag+, coinci-dent with −bpy replacement by –CN− (28). The change in filmstructure is also shown in UV-vis spectra (Fig. S2). There is ared shift in the visible absorption bands for the dπ(FeII) → π*(bpy) metal-to-ligand charge-transfer transitions following –

CN replacement. The metal ions were added by immersing poly[Fe(vbpy)3](CN)2jGCE in solutions containing different ratiosof Cu2+ and Ag+ in CH3CN which leads to the rapid incorporation

of Cu(II) and Ag(I). Addition of the metal ions was evident byshifts in UV-vis spectra with a blue shift observed upon addition ofthe ions. After binding to the metal ions, the cyano group becomesa better π-acceptor, stabilizing the dπ levels, decreasing the dπ →π*energy gap (28). Following addition of the ions, a single-pulseelectrodeposition at −1.7 V vs. Ag+ for 30 s was carried out whichled to rapid reduction of the cyano-bound Cu(II)/Ag(I) ions toCu(0)/Ag(0).Fig. 1C characterizes the surface morphology of the polymer-

ized electrode. The polymer film has an open porous structurewhich presumably enhances mass transport during electrochemicalreactions (30). The nanoparticulate structure is not obvious in thecross-sectional image in Fig. S1 because the particle sizes were toosmall to resolve by SEM. The average (Cu)2,(Ag)3 particle diam-eter was 6 ± 2 nm as characterized by the transmission electronmicroscope (TEM) images in Fig. 1D. This small particle size isdocumented in uniform distributions of particles prepared bydirect electrodeposition (Table 1). The X-ray photoelectronspectroscopy (XPS) spectrum in Fig. 1E shows the characteristicmetallic Ag 3d 2/5 band at 366.8 eV and a mix of metallic Cu andits oxide form by XPS in Fig. 1F (31, 32). The appearance of oxide

Scheme 1. Fabrication of Cu,Ag-polymer-GCE electrodes.

Table 1. Properties of electrodeposited of Cu and Ag nanoparticles

Material d/nm Condition Substrate Technique Ref.

Cu 25.1 0.029 g Cu(BF4)2·xH2O and 0.058 g of dodecylbenzenesulfonic acid sodium salt

Au Electrochemical cycling (41)

51 1 mM CuSO4 in 0.1 M H2SO4 HOPG Double-pulse deposition (42)20–80 2.0 mM CuSO4 in 0.1 M Na2SO4 GCE/CNTs Electrochemical cycling (43)50–80 10 g/L CuSO4·5H2O, 12 g/L NaCl and 20 g/L H3BO3, 50 °C TiO2-NT Direct electrodeposition (44)

Ag 50–150 1.0 mM AgNO3 in 0.1 M KNO3 GCE/ZnO Double-pulse deposition (45)50–80 0.43 mg/mL AgNO3 in 0.1 M NaNO3 Au Electrochemical cycling (46)16.6 5 mM AgNO3 in 0.1 M KNO3 with poly

(N-vinylpyrrolidone) as the additivePt Electrodeposition (47)

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in the Cu sample is not surprising because the Cu nanoparticlesare easily oxidized, especially when small (31, 33). Based on thebinding energies for Cu and Ag from XPS measurements, com-pared with their monometallic forms (Fig. S3), in the Cu–Agparticle mixtures, there are separate Cu and Ag nanoparticle

clusters rather than mixed-metal alloys. This in contrast to earlierresults described for clusters of CuxPdy where electrochemicalformation of clusters of the alloy was observed (2). The differencein behavior may be due to differences in the electrochemical de-position methods. Single-pulse electrodeposition provides a large

Fig. 1. (A) Room-temperature CVs showing the electropolymerization of 1 mM [Fe(vbpy)3](PF6)2 on a GCE in 0.1 M TBAPF6/MeCN at 250 mV s−1. (B) CV of thepoly-[Fe(vbpy)3]

3+/2+ couple before replacement of -vbpy (black solid line) and after replacement (gray solid line) at 100 mV s−1 in 0.1 M TBAPF6/MeCN, roomtemperature. (C) SEM image for a (Cu)m,(Ag)n/80-layer-poly[Fe(vbpy)3](PF6)2/GCE surface. (D) TEM image and size distribution (Inset) for (Cu)2,(Ag)3 nano-particles; and XPS spectra for Ag (E) and Cu (F) in (Cu)2,(Ag)3-polymer-GCE.

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driving force over a short period of time, initially with selectivedeposition of the more reducible metal. The surface atomic ratioof Cu and Ag was determined by XPS with at least three parallelmeasurements for each sample.During the electrodeposition process, nucleation and growth

from within the particle occur sufficiently rapidly to separatethe two effects completely (34). Under normal conditions, withhigh concentrations of metal ions in the external solution athigh applied potentials, large particle sizes with a broad sizedistribution would be expected (34). In the strategy adoptedhere, CN− coordination plays a critical role in dictating particlesize and distribution. Electrodeposition, under the same con-dition but without the CN− ligand, resulted in the appearanceof a film for Cu and a large flake for Ag as shown in Fig. S4.Presumably, the diluted concentration of metal ions in theelectrodes, by coordination to the CN− ligand, decreases localinteractions resulting in a decrease in particle size (34).

The electrocatalytic reactivities of the nanoparticles of thepolymer-stabilized, (Cu)m,(Ag)n/GCE electrodes for CO2 reductionwere evaluated in 0.5 M KHCO3 with 8 ppm of benzotriazole(BTA, Scheme 2) added. To increase the solubility of CO2, all ex-periments were performed at 0 °C (35). The products of CO2 re-duction were identified and quantified by 1H NMR in the liquidphase, and by GC for the headspace gaseous products. This (Cu)m,(Ag)n/polymer/GCE assembly showed a stable catalytic current overextended electrolysis periods as shown in Fig. S5.The 8e− product (Table S1), CH3COO− (Fig. 2A), is a highly

reduced product. Its appearance is unusual because of the difficultiesin overcoming high-energy barriers for C–C formation on surfacesat low temperatures. Its appearance as the major liquid product atlow temperatures is one of the few examples in CO2 reductionchemistry (21). Other products that appear include formate in theliquid phase and CO and methane in the gas phase. A controlexperiment was conducted using the identical parameters exceptN2 was used in place of CO2. Under this condition, no productpeaks were observed in the 1H NMR (Fig. S6). The major productin the background is hydrogen from reduction of the solvent.The appearance of acetate by the Cum,Agn clusters led to a

more detailed investigation. The results of a study in which thesurface metal atom composition was varied are shown in Fig. 2B.Data in the figure appear for acetate generation from the clustersCu, Cu3–Ag, Cu2–Ag, Cu2–Ag3, and Ag in CO2 saturated 0.5 MKHCO3 at 0 °C at E = −1.53 vs. RHE over periods of 1 h. Basedon the data in the figure, there was an increase in the appearanceof acetate as Ag was added to the nanoparticles with a maximumreached at 5.5% for Cu2–Ag3 clearly showing that surface metalcomposition plays an important role in acetate formation.The influence of BTA on CO2 reduction was also investi-

gated with the reactive Cu2–Ag3 cluster mixture. As shown inFig. 2C, CO2 reduction by the Cu2–Ag3/polymer/GCE electrode,at 0 C at −1.53 V vs. RHE for 1 h, gave acetate and methane at5.5% and 1.8% with 8 ppm added BTA. Without BTA thevalues were 0% acetate and 0.3% methane. Similar results were

Scheme 2. Structure of BTA.

Fig. 2. (A) NMR spectrum of acetate (δ = 1.83 ppm) with DMSO (δ = 2.5 ppm) as the internal standard. (B) Efficiency of acetate formation as a function of theatom ratios of Cu and Ag in (Cu)m,(Ag)n/polymer/GCE in CO2-saturated 0.5 M KHCO3, −1.53 V vs. RHE after 3,600 s, 0 °C. (C) Product distribution with (solid)without (empty) 8 ppm BTA on (Cu)2, (Ag)3j80-layer-poly[Fe(vbpy)3](PF6)2jGCE electrode, condition as above. (D) As above, influence of the applied potentialon acetate production for a (Cu)2,(Ag)3/polymer/GCE electrode.

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obtained for a Cu/polymer/GCE (Fig. S7) film. Without addedBTA, acetate 0.23% and methane appeared in 23% yields andwith acetate the yields were 0.7% with methane at 3.10%. Therole of BTA was selective. For example, there was no impact ofadded BTA on Ag/polymer/GCE nanoparticle reductions. Itsrole may arise from its adsorption on Cu with chemical changesat the electrode interface. BTA has been shown to adsorb onmetallic Cu surfaces by physical and chemical adsorption (36).The potential dependence of acetate generation by the (Cu)2,

(Ag)3 cluster was also investigated. Faradaic efficiencies wereevaluated as a function of applied potential with results shown inFig. S8. At −1.33 V vs. RHE, acetate was generated in 21.2%yield. Decrease of the surface potential to −0.93 V resulted in noacetate. The appearance of acetate increases with applied po-tential with the highest efficiency of 21.2% found at −1.33 V vs.RHE. Further decreases in potential resulted in decreased for-mation of acetate with no acetate formed by −1.73 V. A relatedbehavior has been reported by Jaramillo and coworkers at Cuelectrodes with CH3COO− maximized as a product at −1.1 V vs.RHE and H2 the dominant product at more negative potentials.The mechanism of C–C formation is still debatable. From

the data in Fig. 2B, addition of Ag clusters to the surface playsan important role with acetate maximized as a product for thecluster pair Cu2–Ag3. Ag is a known catalyst for CO2 re-duction to CO with insight on mechanism from calculationsby Peterson and Nørskov (37). Once formed by loss of CO2from the surface, CO(gas) is favored thermodynamically asshown in Scheme 3. Cu, on the other hand, has a moderateability to adsorb and desorb CO (37). A possible contributorto acetate production may be Ag-catalyzed formation of COfollowed by its capture on neighboring Cu sites for C–C bondformation, Scheme 3. A similar observation was made by Yeo andcoworkers (3) for phase-segregated CuZn nanoparticles.In the mechanism in the scheme, acetate could be produced

after a series of hydroxylation and electron transfer steps (21).In an alternate route, CO insertion (3, 38, 39) as observed byKenis and coworkers (38) on pure Cu surfaces could also occurin ethanol generation. Following CO2 reduction to –CH2 on thesurface of Cu, dimerization of the latter with free CO from Agwould occur to give the C–C bond on the surface. Once formedon the surface, further reduction of acetaldehyde would giveacetate as the final product (40).

ConclusionWe have introduced here the details of a well-defined electro-chemical procedure for preparing a family of (Cu)m,(Ag)n clusters

on an underlayer of poly[Fe(vbpy)3](PF6)2jGCE. Monodispersed,ultrasmall, ∼6-nm particles were obtained by electrodepositionwithin the films to give nanoparticles that are among the smallestparticle sizes available by this technique. In their reductionchemistry, a key factor is reduction of CO2 with an enhancementby added BTA. The ability of clusters to undergo CO2 reductionto acetate is highly dependent on the mole fraction ratio in the(Cu)m,(Ag)n/polymer/GCE mixtures. In the sequence of clusters,the most efficient results for acetate were obtained for (Cu)2,(Ag)3/polymer/GCE films with a faradaic efficiency of 21.2% foracetate at −1.3 V vs. RHE in 0.5 M KHCO3 with 8 ppm of BTAat 0 °C.

MethodsElectrochemical Measurements. All of the electrochemical measurementswere performed on a CHI601 D potentiostat station (CH Instruments, Inc.)with a three-electrode setup by using a Pt mash as the counterelectrode, aAg (for nonaqueous system) and a saturated calomel electrode (foraqueous system) as the reference electrode, and a GCE as the workingelectrode. The GCE was first polished with a decreasing size of 1.0- and0.05-μm alumina lapping compounds for 5 min each and followed bysonication in an ultrasound bath before use. The potential used for CO2

reduction reaction (CO2RR) is corrected to RHE to make comparison withliterature.

Electrode Fabrication. The polymerization process was carried out in anitrogen-saturated solution containing 1 mM [Fe(vbpy)3](PF6)2 in 0.1 MTBAPF6/CH3CN. The GCEwas then immersed into this well degassed solutionto perform consecutive scans to get poly-[Fe(vbpy)3]/GCE. This electrodewas then transferred into 0.1 M TBACN/MeCN solution and left for 20 min toallow full replacement of –CN with –bpy. Before being put into solutionwith Cu/Ag, the formed dicyano film/GCE was washed with MeCN anddried. After 20 min of incorporation in the metal ion precursor solution,the electrode was cleaned with MeCN and transferred into a nitrogen-saturated 0.1 M TBAPF6/MeCN to run electrodeposition under −1.7 Vvs. Ag for 30 s. Aggregation of particles were observed with longer elec-trodeposition time.

Detailed product analysis and CVs are presented in Supporting In-formation.

ACKNOWLEDGMENTS. We thank Dr. Carrie Donley and Dr. Amar Kumbharfor assistance with XPS and SEM measurements. This research was sup-ported solely by the UNC EFRC: Center for Solar Fuels, an Energy FrontierResearch Center funded by the US Department of Energy Office of Science,Office of Basic Energy Science under Award DE-SC0001011. This work madeuse of instrumentation at the Chapel Hill Analytical and NanofabricationLaboratory, a member of the North Carolina Research Triangle Nanotech-nology Network, which is supported by the National Science Foundation(Grant ECCS-1542015) as part of the National Nanotechnology CoordinatedInfrastructure.

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Scheme 3. Possible reaction pathways for CO2 reduction to acetate on (Cum),(Ag)n/polymer/GCE electrodes.

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