Tuned Chemical Bonding Ability of Au at Grain Boundaries forEnhanced Electrochemical CO2 ReductionKang-Sahn Kim,† Won June Kim,† Hyung-Kyu Lim,‡ Eok Kyun Lee,*,† and Hyungjun Kim*,‡

†Department of Chemistry and ‡Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea AdvancedInstitute of Science and Technology (KAIST), Yuseong-gu, Daejeon 305-701, Korea

*S Supporting Information

ABSTRACT: Electrochemical carbon dioxide (CO2) reduc-tion is an emerging technology for efficiently recycling CO2into fuel, and many studies of this reaction are focused ondeveloping advanced catalysts with high activity, selectivity,and durability. Of these catalysts, oxide-derived metalnanoparticles, which are prepared by reducing a metal oxide,have received considerable attention due to their catalyticproperties. However, the mechanism of the nanoparticles’activity enhancement is not well-understood. Recently, it wasdiscovered that the catalytic activity is quantitatively correlatedto the surface density of grain boundaries (GBs), implying thatGBs are mechanistically important in electrochemical CO2reduction. Here, using extensive density functional theory(DFT) calculations modeling the atomistic structure of GBs on the Au (111) surface, we suggest a mechanism of electrochemicalCO2 reduction to CO mediated by GBs; the broken local spatial symmetry near a GB tunes the Au metal-to-adsorbate π-backbonding ability, thereby stabilizing the key COOH intermediate. This stabilization leads to a decrease of ∼200 mV in theoverpotential and a change in the rate-determining step to the second reduction step, of which are consistent with previousexperimental observations. The atomistic and electronic details of the mechanistic role of GBs during electrochemical CO2reduction presented in this work demonstrate the structure−activity relationship of atomically disordered metastable structuresin catalytic applications.

KEYWORDS: grain boundary, CO2 reduction, electrochemical reduction, density functional theory, Au surface

■ INTRODUCTION

Over the past decades, human beings have relied on fossil fuelsfor over 80% of their total energy needs, and, at present, carbondioxide (CO2) accumulation in the atmosphere has led to thedemand for renewable energy sources to replace fossil fuels.1−3

Consequently, much research has focused on developingvarious chemistries to transform excess CO2 into variouscarbon frameworks with chemical energy. Among severalongoing attempts, electrochemical conversion technology isadvantageous because of its potentially high reactivity andefficiency at ambient conditions and its ease of scale-up tolarge-scale processes when combined with renewable energyharvesting technologies such as photovoltaics.4−7

In particular, the selective formation of carbon monoxide(CO) has received much attention because not only iselectrochemical CO2 reduction to CO, which is a two-electronprocess, the simplest reduction reaction pathway of the variousroutes, but CO is also an industrially important gas productwith broad applications in chemical manufacturing,8 as abioregulator in medicine9 and as a lasing medium in high-powered infrared lasers.10 However, no satisfactory solutionwith practical applicability has yet been found. A key obstacle isthe limited catalyst performance in terms of activity, selectivity,

and stability. Although the thermodynamic potential for CO2

reduction to CO requires only −0.11 VRHE, the overpotentialsof selective CO-forming catalysts, e.g., Au and Ag, are as high as610 mV and 840 mV, respectively.11

In 2012, Kanan et al. reported that electrochemicallyoxidizing and then reducing an Au catalyst, which was denotedan oxide-derived Au (OD-Au) catalyst, resulted in a dramaticimprovement in its catalytic activity and a substantial decreaseof ∼200 mV in the overpotential.12 Their follow-up study in2015 further revealed that the grain boundary (GB) density onthe Au surface and the electrochemical catalytic activity arestrongly correlated.13 It should be noted that atomic disorder atgrain boundaries has been proposed to enhance the catalyticactivity in several different reactions.14−22 For example, theabundance of grain boundaries and stacking faults wasdiscussed to lead to the high activity of silver-supportedcatalysts in ethylene epoxidation.22 However, an atomistic andelectronic level of mechanistic understanding of the enhancedcatalytic activity in CO2 reduction at grain boundaries has not

Received: February 9, 2016Revised: May 2, 2016Published: May 31, 2016

Research Article

pubs.acs.org/acscatalysis

© 2016 American Chemical Society 4443 DOI: 10.1021/acscatal.6b00412ACS Catal. 2016, 6, 4443−4448

yet been addressed. Therefore, fundamental questions aboutthe relationship between the metastable surface structure andthe catalytic activity remain.In this work, first-principles density functional theory (DFT)

calculations were performed to model the atomistic structure ofGBs on the Au (111) surface and to investigate electrochemicalCO2 reduction to CO at GBs. Based on a theoretical model forestimating the CO2 reduction potential on metal surfaces,23 thepotential variance along the GB surface was estimated, and theatomic and electronic structures were further analyzed todetermine the role of the grain boundary. It was found that thebroken local spatial symmetry at the GB tunes the adsorbate-to-metal σ-bonding and metal-to-adsorbate π-backbondingstrengths on the Au surface relative to those of the bulk Au.This change in chemical bonding ability at the GB helps tostabilize the key COOH intermediate, resulting in enhancedcatalytic activity in CO2 reduction.

■ RESULTS AND DISCUSSIONAn atomistic model of the Σ3 {112} high-angle grain boundary(HAGB) on the Au (111) surface was constructed as shown inFigure 1. Of the many possible HAGBs, the Σ3 {112}

symmetric tilt HAGB was chosen for this study because Σ3GBs are known to be the most abundant HAGB on Au surfacesfrom experimental analyses of the grain boundary characterdistribution (GBCD).24−27 Additionally, the Σ3 {112} GB isthe only Σ3 GB that can be constructed mathematically on aclean Au (111) surface. The Σ3 {112} HAGB was constructedby exposing the {112} face of a bulk face-centered cubic (FCC)

Au crystal and performing an inversion operation on theexposed {112} face as plane of symmetry.The model system consisted of 3 atomic layers containing a

total of 150 Au atoms. Due to the periodic boundary conditionsof the simulation cell, two domains in the model formed twoGB lines. It should be noted that the GB lines were notidentical; one (lower GB line in Figure 1b) was concave withthe atoms near the GB line slightly caved into the surface,whereas the other (upper GB line in Figure 1b) was convexwith the atoms near the GB line slightly protruding from thesurface. In the following discussion, the atop Au catalytic sitesare labeled from t1 to t9 in increasing order, with t1 locatednear the concave GB, t5 located in the middle, i.e., the bulk-likeregime, and t9 located near the convex GB.The SIESTA program28 was employed for the large-scale

DFT calculations using the Perdew−Burke−Ernzerhof (PBE)exchange-correlation functional29 and numerical atomic orbitalbasis sets. The free energies were calculated by diagonalizingthe partial Hessian of the adsorbate, and the solvation energieswere taken into account using the Delphi program,30,31 whichsolves the Poisson−Boltzmann equation numerically (furthersimulation details are fully described in the SupportingInformation).The electrochemical CO2 reduction pathway to CO includes

the following elementary steps23,32

+ + → *+ −CO (aq) [H e ] COOH (aq)2 (R1)

* + + → * ++ −COOH (aq) [H e ] CO (aq) H O (aq)2(R2)

* →CO (aq) CO (g) (R3)

where the asterisk (*) denotes adsorbed species on the catalystsurface. In our previous study, we decomposed the reaction freeenergy (ΔG°) of each elementary step ((R1), (R2), and (R3))into the COOH and CO binding free energies (ΔGb

COOH andΔGb

CO, respectively) under the bias potential U (VRHE):ΔG°(R1) = 2.02 + U + ΔGb

COOH, ΔG°(R2) = −1.07 + U +ΔGb

CO − ΔGbCOOH, and ΔG°(R3) = −0.50 − ΔGb

CO, in eVs.23

In this study, ΔGbCOOH and ΔGb

CO were calculated using anAu surface model with GBs and were then converted into ΔG°(U = 0). The results are listed in Table 1, in which the mostthermodynamically unfavorable step of the three steps at thezero-bias limit is highlighted. The COOH binding affinity forAu atoms near the concave GB (t1 and t2 sites) is markedlyenhanced by 270−300 meV compared to the clean Au (111)surface. The COOH binding affinity for Au atoms near theconvex GB (t7, t8, and t9 sites) is more profoundly enhancedby 800−1300 meV, causing the first reduction step to bespontaneous even under zero bias; however, the substantial

Figure 1. (a) Tilted view (2 × 1 × 1 supercell) and (b) side and topviews (1 × 1/2 × 1 supercell) of the atomistic model of the Σ3 {112}high-angle grain boundary (HAGB) on the Au (111) surface. Thethree atomic layers contain 150 atoms. Two bulk domains form twoGB lines (concave, convex), which are indicated by the brown andivory atoms, respectively. The 9 atop catalytic active sites are labeled ast1−t9 and shown in magenta.

Table 1. Binding Free Energies and Zero-Bias Reaction Free Energies at t1−t9 Sitesa

aThe most unfavorable step of the three elementary steps (R1, R2, R3) is highlighted in red.

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increase in the CO binding affinity near the convex GB makeseither the second reduction step or the final CO desorptionstep thermodynamically unfavorable.To illustrate the effects of these changes in the binding

affinity near the GBs, the reaction free energy profiles at fourdifferent active sites (t1, t7, and t9 near the GBs and t5 in themiddle) were constructed and are compared to that of the cleanAu (111) surface in Figure 2a. The first two steps in the energy

profile are reduction steps ((R1) and (R2)), and the last step isCO desorption (R3). The bias potential of U = −0.4 VRHE waschosen (cf. the operation potential of OD-Au was −0.35 VRHEin the experiment12). Under finite bias potential, CO2 reductionat the t1 site consists of two downhill processes followed by aslight uphill process requiring 45 meV for CO desorption.Considering the fact that the first reduction step on the cleansurface requires 260 meV, the decrease of ∼200 mV in theoverpotential observed in the experiments can be explained byreduction at the t1 site.12

Because it is difficult to disrupt the stable sp-hybridizationsymmetry of the carbon atom to bend the linear CO2molecule,33 the first reduction step is usually rate-determiningas observed on the clean Au (111) surface and also in the bulk-

like regime at the t5 site. At active sites near the GBs (t1, t7, t9sites), however, the first reduction step becomes spontaneous,whereas the second reduction or CO desorption step becomesuphill (has a low rate; is rate-determining). Assuming that thesecond electron transfer from the electrode to the molecule isnot finished before complete CO desorption from the catalystsurface, the rate-determining step (RDS) changes from the firstreduction step to the second reduction step at the active sitesnear the GBs, which is consistent with the experimentallyobserved change in the Tafel slope from ∼120 mV·dec−1 forpolycrystalline Au to ∼60 mV·dec−1 for OD-Au.12

Additional calculations on the feasibility of further reductionof CO to COH/CHO at t7 and t8 sites are shown in Figure S7.The significant downhill process followed by uphill withsubstantial energy cost (>0.3 eV) at catalytic active sites nearthe convex GB (t7, t8, and t9 sites) in Figure 2a and Figure S7implies that these sites will be immediately occupied byintermediate species. The bound species being neitherdesorbed nor further reduced, the sites near the convex GBare expected to hardly participate in the electrochemical activityduring catalytic cycles. On the other hand, the thermodynamicsinvolved in the turnover process of CO2 at sites near theconcave GB (e.g., t1 and t2 sites) are feasible and relevant tothe experimentally observed electrochemical activity.The reaction free energies of the CO2 reduction steps change

dramatically when a GB is introduced, because it alters thecorrelation between ΔGb

COOH and ΔGbCO on metal surfaces.

Au atoms near GBs can more effectively stabilize COOH overCO, resulting in a new correlation between ΔGb

COOH andΔGb

CO, which is compared to the preexisting correlation linefor various clean metal (i.e., Zn, Ag, Au, and Cu) surfaces inFigure 2b.To understand the origin of the efficient stabilization of

COOH over CO at Au atoms near the GBs, the electronicstructures were analyzed. The density of states (DOS) of theAu surface with GBs was compared to that of the clean Ausurface (Figure S1) in terms of peak positions, shapes, andlocation of the d-band centers.34 No obvious differences areobserved, suggesting that the interaction between the metalband and adsorbate (usually characterized using d-band theory)cannot fully explain the specific COOH binding affinity for theAu atoms near the GBs. Instead, the adsorbate-metal bondingmust be understood based on a more localized orbital−orbitalinteraction concept.35 Indeed, the adsorbate binding results intwo localized states (sharp peaks in the DOS) at E-Ef = −7.3/−6.3 eV for COOH binding (Figure 3a) and E-Ef = −8.1/−7.4eV for CO binding (Figure 3b). Projected density of states(PDOS) analysis further reveals that the lower-energy state isdue to the carbon pz orbital and Au dz2 orbital (Figure 3e andFigure 3f), whereas the higher-energy state is due to the carbonpx/py orbital and Au dxz/dyz orbital (Figure 3c and Figure 3d)(the surface normal direction was chosen as the z-direction).Based on the metal−ligand bonding theory of organometalliccatalysts, the lower-energy state can be assigned to theadsorbate-to-metal σ-bonding state, whereas the higher-energystate is the metal-to-ligand π-backbonding state. Theseassignments are further confirmed by the real-space visual-ization of these states in Figure S4.The occupations of the σ-bonding and π-backbonding states

when COOH and CO are adsorbed at each catalytic site of t1−t9 were calculated by integrating the DOS peaks. Figure 4a andFigure 4b show the variation of relative state occupations withthe state occupations of clean surface as zero-base. It should be

Figure 2. (a) Representative reaction free energy profiles for thefollowing catalytic sites under a finite bias potential of U = −0.4 VRHE:t1 (red) near the concave GB, t7 (blue) and t9 (green) near theconvex GB, and t5 (magenta) in the bulk regime. For comparison, thereaction free energy profile calculated for the clean Au (111) surfacewith no GB is also shown. (b) Correlation between the COOH andCO binding free energies (ΔGb

COOH and ΔGbCO, respectively). For

comparison, the correlation line for GB-free clean metal (Cu, Au, Ag,and Zn) surfaces is presented,23 showing that the COOH intermediateis more effectively stabilized relative to CO at GBs. The contour mapshows the theoretical reduction potential determined using ΔGb

COOH

and ΔGbCO following the model described in ref 23.

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noted that the state occupation is proportional to the strengthof each binding mode.35−37 The σ-bonding strength remainsnearly constant at all the binding sites from those near the GBsto that in the bulk regime (Figure 4a). In contrast, the π-backbonding strength increases significantly when the adsor-bate binds to Au sites near the GBs, particularly in the case ofCOOH binding (Figure 4b). Similar trends are also observedfor the site-dependent binding energies (Figure S5). Toconfirm that the π-backbonding affinity near the GBs isstronger, the change in the C−O bond length was calculatedand is shown in Figure 4c (a longer C−O bond corresponds tostronger metal-to-adsorbate π-backbonding). The results followa similar trend to that of changes in the π-backbonding stateoccupation.In highly symmetric crystalline structures, the atomic orbitals

of the metal atoms exhibit substantial overlap, resulting in anenergy band as described by the linear combination of atomicorbitals (LCAO) theory. To exploit the Au dz2 or dxz/dyzorbitals to form a σ- or π-bond, respectively, with an adsorbate,the atomic orbital from the band must be localized, whichrequires energy. Based on the orbital shapes due to the Auatom arrangement in the topmost layer of the (111) surface(hexagonal close-packed layer in the xy plane), it is expectedthat the in-plane orbital−orbital overlap of the dxz/dyz orbitals isgreater than that of the dz2 orbitals, suggesting that the energycost of ΔEloc. is larger for π-backbond formation. Theintroduction of a GB is expected to break the local symmetry,leading to a decrease in ΔEloc. for π-backbond formation at theGB and thus an increase in the π-backbonding affinity. Figure5a and Figure 5b show the average nearest-neighbor distancesfor the surface Au atoms and the corresponding standarddeviations, respectively. The results clearly demonstrate that thelocal symmetry of the close-packed surface is broken at the GB,which leads to stronger metal-to-adsorbate π-backbonding. Thelocal symmetry is distorted more substantially near the convexGB.

Figure 3. Au density of states (DOS) when (a, c, e) COOH and (b, d, f) CO are adsorbed at the t1 site located near the grain boundary. (a) and (b)Au DOS at the t1 site before (black) and after (red) adsorption. The DOS of the C atom is also shown in blue. As shown by the projected density ofstates (PDOS) in (c)-(f), the total Au DOS in this energy range is mostly due to the Au d-band, and the two sharp peaks that appear after adsorbatebinding are due to dz2-pz (lower-energy peak) and dxz/yz-px/y (higher-energy peak) interactions.

Figure 4. Relative state occupations of the (a) σ-bonding (lower-energy peak in the DOS) and (b) π-backbonding (higher-energy peakin the DOS) states. The relative state occupation of parts a and bmeans the state occupation of each catalytic site minus the stateoccupation of the clean surface. The π-backbonding strength increasesnoticeably when COOH binds near a GB. (c) The C−O bond length,another indicator of the π-backbonding strength, also demonstratesthe enhanced π-backbonding near the GBs.

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However, the presence of a GB has a greater effect on theAu-COOH π-backbonding strength than on the Au-CO π-backbonding strength (see Figure 4b and Figure 4c), due to thesymmetry of the π* orbital of the adsorbate species. The linearCO molecule, which has degenerate π*xx and π*yy orbitals,adsorbs on an Au atom along the surface normal z-direction. Incontrast, the COOH π* orbitals are restricted to be eitherparallel or normal to the COOH molecular plane. Metal-to-adsorbate bond formation requires that the adsorbate π* orbitalbe properly aligned with the metal dxz/dyz orbitals to maximizethe orbital−orbital overlap. In contrast to the dxz/dyz orbitals ofa single Au atom, the rotational invariance around the z-axis ofthe dxz/dyz orbitals of an Au atom at the surface is brokenbecause of the local environment. Consequently, while thedegenerate CO π* orbitals can readily achieve maximal overlapwith the dxz/dyz orbitals in the presence or absence of a GB byrotating the orbitals around the z-axis, the rotationally variantCOOH π* orbitals cannot. Therefore, the breaking of the localsymmetry near a GB is expected to have a more profound effecton COOH adsorption than on CO adsorption.

■ CONCLUSIONThe origin of the superior catalytic activity in electrochemicalCO2 reduction observed at the Σ3 {112} high-angle grainboundary on the Au (111) surface was elucidated using DFT.The simulation results were validated by their successfulreproduction of important experimental results, namely thelarge decrease of ∼200 mV in the overpotential and the changein the RDS from the first reduction step to the secondreduction step, which results in a decrease in the Tafel slopefrom ∼120 mV·dec−1 to ∼60 mV·dec−1. Further analyses of theatomic and electronic structures reveal that the broken localspatial symmetry near the GBs is the key factor leading to theincrease in the Au metal-to-ligand π-backbonding strength,which effectively stabilizes the COOH intermediate and thusenhances the CO2 reduction activity. Beyond the GB effect, the

possibility exists that the remaining oxygen affects the catalyticactivity of oxide-derived metal catalysts, though presumably notsignificant in the noble metal cases (e.g., Au, Ag). It will be ofinterest in future studies. We further anticipate that the resultsof this study provide new insight into the origin of the superior(electro)chemical catalytic activity at GBs based on chemicalbonding concepts, which can be further utilized to designadvanced catalysts with controlled grain boundaries.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acscatal.6b00412.

Computational details and method to construct anatomistic model of grain boundary; changes of densityof states before and after COOH/CO adsorption; wavefunction density maps of localized states (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: eklee@kaist.ac.kr (E.K.L.).*E-mail: linus16@kaist.ac.kr (H.K.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Global Frontier R&D Program(2013M3A6B1078884) of the Center for Hybrid InterfaceMaterials (HIM) funded by the Ministry of Science, ICT &Future Planning, and also by the New & Renewable EnergyCore Technology Program of the Korea Institute of EnergyTechnology Evaluation and Planning (KETEP) grantedfinancial resource from the Ministry of Trade, Industry &Energy, Republic of Korea (20153030031720).

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Figure 5. (a) Average nearest-neighbor distances and (b) correspond-ing standard deviations. The values (Å) are relative to those on theclean surface. The local symmetry of the surface, which is composed oftop-layer nearest neighbors and second-layer nearest neighbors, isbroken to a greater degree at the convex GB than at the concave GB,which leads to stronger COOH binding at the convex GB.

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