Suppression of Competing Reaction Channels by Pb AdatomDecoration of Catalytically Active Cu Surfaces During CO2ElectroreductionCheonghee Kim,† Tim Moller,† Johannes Schmidt,‡ Arne Thomas,‡ and Peter Strasser*,†

†Department of Chemistry, Chemical Engineering Division, Technical University Berlin, 10623 Berlin, Germany‡Department of Chemistry, Functional Materials, Technical University Berlin, 10623 Berlin, Germany

*S Supporting Information

ABSTRACT: The direct electrochemical conversion of carbon dioxide tochemicals and fuels is of fundamental scientific and technological interest. Thecontrol of the product selectivity, expressed in terms of the Faradaic efficiency,has remained a great challenge. Herein, we describe a surface-electrochemicalsynthetic strategy to tune the electrochemical CO2 reduction selectivity andyield by controlled suppression of the hydrogen evolution reaction (HER)reaction channel, resulting in increased Faradaic efficiencies for fuels andchemicals. We demonstrate that bimetallic catalysts consisting of only minutesubmonolayer amounts of Pb adatoms deposited on Cu surfaces exhibit andmaintain unusually high selectivities for formate (HCOO−) over a large range ofoverpotentials. The bimetallic adatom electrodes were prepared using under-potential electrodeposition (UPD), which is able to precisely control the adatomcoverage. While as little as 0.16 ML Pb surface adatoms on a polycrystalline Cusurface boosted the observed Faradaic HCOO− product selectivity 15 times, the0.78 ML Pb/Cu catalyst showed the most favorable ratio of HCOO−/H2 production rate thanks to the effective suppression ofthe HER combined with a partial (−1.0 to −1.1 V vs RHE) enhancement of the HCOO− production. We argue that thefavorable product efficiency is caused by selective adatom poisoning on the strongest binding hydrogen adsorption sites; inaddition, electronic effects of Pb adatoms change the chemisorption of reactive intermediates. Our study reveals synthetic accessto tailored selective bimetallic copper catalysts for the electrochemical CO2 reduction and demonstrates the enormous effect ofeven minute amounts of surface adatoms on the product spectrum.

KEYWORDS: lead, copper, bimetallic catalyst, CO2 reduction reaction, selectivity

■ INTRODUCTION

Carbon dioxide is a key greenhouse gas leading to globalclimate changes, so conversion of anthropogenic atmosphericCO2 to fuels and chemicals could be an important contributiontoward a sustainable global carbon cycle.1−4 ElectrochemicalCO2 reduction reaction to higher-value hydrocarbon moleculesis an attractive process since it can be coupled with renewableenergy resources such as solar and wind.5−7 However, to datehighly energy- and Faraday-efficient (chemically selective)electrocatalysts for CO2 reduction have been largely elusive,especially at high current densities, because of large kineticoverpotentionals and the competitive kinetically facile hydro-gen evolution reaction (HER) in aqueous environment.6

Therefore, developing efficient CO2 reduction catalysts withsuppressed HER and reduced overpotential has remained ascientific challenge of great urgency.A large number of metal catalysts most prominently copper

(Cu), gold (Au), silver (Ag), tin (Sn), and others have beeninvestigated for the electrocatalytic CO2 reduction.

8−14 Amongthem, Cu has been identified as promising catalyst to producehydrocarbons such as methane (CH4), ethylene (C2H4), and

alcohols. More recently, for Cu metal catalysts, most studiesfocused on investigating recently emerged reactivity-control-ling factors and hypotheses such as grain boundaries, Cu+

species, subsurface oxygen atoms, local pH effects, size effects,and shape effects and their influence on the electocatalytic CO2reduction activity and selectivity.15−24 For example, Mistry etal. reported that the presence of Cu+ species in preoxidized Cucatalysts are key factor for high activity and enhanced ethyleneselectivity in electrocatalytic CO2 reduction.

18 Although theydeveloped new plasma-treated Cu catalysts with improvedactivity and selectivity, they still produced large amounts ofhydrogen gas.This is why strategies to tune CO2 reduction activity and

selectivity by suppressed HER are so important. Metals knownto have a naturally low HER efficiency are tin (Sn), indium(In), and lead (Pb). As main group metals, they have beenknown to mechanistically favor formic acid, while hydrogen

Received: July 19, 2018Revised: December 5, 2018Published: December 28, 2018

Research Article

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production is largely suppressed.9,25,26 The reason for that hasbeen uncovered: Density functional theory (DFT) calculationscompared and classified metal catalysts according to theirmajor product during electrochemical CO2 reduction. Thatway it became possible to understand the origin of theirreactivity in terms of correlated binding energies ofintermediates, explaining their mechanistic reactivity.27 Thatstudy demonstrated that formic-acid-producing metals havenone or only negligible amounts of atomic hydrogen adsorbedat the relevant potentials. In particular, oxide-derived lead(OD-Pb) has exhibited 700-fold lower H+ reduction activity.28

Having said that, a plausible strategy toward Cu-basedcatalyst with suppressed HER involves the combination ofmain group metal properties and Cu catalysts in form ofbimetallic surface catalyst systems. These differ from bulk Cualloy systems (ordered or disordered) in that they are notsubject to preferential surface segregation effects and allow theevaluation of intrinsic bimetallic synergies. Some scattered Cu-based surface bimetallic approaches have been reported.29−34

Cu−In and Cu−Sn bimetallic catalysts prepared by electro-chemical reduction on oxide-derived copper (OD-Cu) showedunusually high CO Faradaic efficiency. These studiesdemonstrated that use of a second metal like core−shellstructure has the capacity to control the entire set of CO2reaction pathways in favor of one single product. Still, beyondthese two examples, few to none studies have consideredbifunctional and electronic structure effects between Cu andsubmonolayer adatoms of a second metal for CO2 reduction.This study changes this.Here, we demonstrated that submonolayer adatoms of a

dissimilar metal electrodeposited on a polycrystalline Cusurface strongly enhances the catalytic selectivity (Faradaicefficiency) of the electrochemical CO2 reduction reaction dueto the coverage-controlled suppression of the HER. As a modelsystem, we considered the Pb/Cu system and deposited Pbadatoms in the form of sub- to one-single monolayer on Cufoils using an underpotential deposition method (UPD). Whileas little as 0.16 ML Pb surface adatoms on a polycrystalline Cusurface boosted the observed Faradaic HCOO− productselectivity 15 times, the 0.78 ML Pb/Cu catalyst exhibitedunusually high selectivity for electrochemical CO2 reduction toHCOO−, and showed previously unachieved HCOO−/H2production ratios. We propose that neighboring Pb adatomswere decorated on the HER active site to weaken theadsorption of atomic hydrogen, and at the same time servedas active sites for the CO2 reduction, thereby stabilizingreactive intermediates of the formic acid molecule.

■ EXPERIMENTAL SECTION

Preparation and characterization of Pb electro-deposited Cu foil. The Cu foil (Alfa Aesar, 1.0 mmthickness, 99.9999%) was cut to the desired electrode size (1× 2.5 cm) and electropolished in 85% phosphoric acid (Sigma-Aldrich, ≥ 85%) at 4 V versus titanium wire (Ti; Alfa Aesar,1.0 mm dia, 99.99%) for 5 min. The Cu electrode was rinsedwith ultrapure water and dried with nitrogen. Pb waselectrodeposited on Cu electrodes in a aqueous solution of0.01 M Pb(ClO4)2·xH2O (Aldrich, ≥ 99.995%) with 0.1 MHClO4 (Aldrich, 70%) by adjusting the applied potential tocontrol the extent of layer deposition. The Pb foil (Alfa Aesar,1.0 mm thickness, 99.9995%) was cut to the desired electrodesize (1 × 2.5 cm) and mechanically polished with alumina

slurries, and then thoroughly rinsed with ultrapure water anddried with nitrogen.The prepared electrodes were analyzed by X-ray photo-

electron spectroscopy (XPS; Thermo Scientific K-Alpha) andatomic force microscopy (AFM; Cypher S).

Electrochemical measurements. Platinum mesh 100(Sigma-Aldrich 99.9%) and Ag/AgCl (3 M NaCl) were used asa counter electrode and a reference electrode, respectively.Electrochemical measurements were performed by using EC-Lab SP-300 potentiostat in a custom-made two-compartmentelectrochemical cell, and Nafion 212, a proton exchangemembrane, was used to separate the catholyte and the anolyte.The electrolyte solution (0.1 M KHCO3; Sigma-Aldrich, ≥99.95%) was purged with high purity CO2 (99.999%) gaswhich average flow rate (Q) was 30 cc/min, and the pH of theelectrolyte was 6.8 after saturation. The CO2 reductionreaction was measured by using chronoamperometry at eachfixed potential and gaseous products were quantified by a gaschromatography (GC; Shimadzu GC 2014) equipped with athermal conductivity detector (TCD) and a flame ionizationdetector (FID). Argon (air liquid 5.0) was used as the carriergas. To quantify formic acid concentration, the electrolyte wasanalyzed by high-performance liquid chromatograph (Agilent1200 series) equipped with an organic acid resin from Ziemerchromatographie column, a reflection index detector (RID)and a UV detector.Solution resistance was measured with Potential Electro-

chemical Impedance Spectroscopy (PEIS) to correct iR loss.The measured potentials for CO2 reduction reaction werecompensated for iR loss and were reported versus thereversible hydrogen electrode (RHE) by using the followingequation. The potential of the reference electrode for Ag/AgCl(3 M NaCl) is 0.21 V.

E E(vs RHE) (vs Ag/AgCl) 0.21 V 0.0591 V pH= + + ×

Calculations of the production rate and Faradaicefficiency of products. The production rate and Faradaicefficiency were calculated from GC chromatogram peak areaswhere V is volume concentration of gas products based oncalibration of the GC, Q is flow rate, itotal is a measured currentby the potentiostat, F is Faradaic constant, P0 is pressure, T istemperature, and R is ideal gas constant, 83.314 mL·bar·mol−1·K−1.

V QP

RTproduction rate 0= × ×

F Ei

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production rate100partial

total total= × =

××

For liquid product, the production rate and Faradaicefficiency were calculated from HPLC chromatogram peakareas where Cliquid is the concentration of liquid products basedon calibration of the HPLC, v is the volume of the electrolyte, tis the electrolysis time, Qtotal is a measured total charge.

C v

tproduction rate liquid=

×

F EQ

Q

nF C v

Q. . 100 100partial

total

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total

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×

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■ RESULTS AND DISCUSSION

Pb adatom-modified polycrystalline Cu surfaces were preparedusing an underpotential deposition method (UPD)35,36 thatwas performed in the solution of 0.1 M HClO4 and 0.01 MPb(ClO4)2·xH2O with electropolished Cu foil as substrates.The coverage of Pb on the Cu foil was precisely controlledfrom 1 monolayer (ML) down to submonolayer adatoms byadjusting the applied electrode potential as shown in Figure 1a.The peak at −0.26 V (vs Ag/AgCl) indicates that a Pbmonolayer is formed on the Cu foil surface. Through the UPDcharge, it can be estimated the amount of charge required toform the monolayer on the exposed Cu surface. Furthermore,by comparing the ratio of the applied charge to the total leadUPD charge, the deposited amounts were controlled to 0.16,0.54, 0.78, and 1 ML Pb/Cu. The number, shown as Figure 1ainset, indicates four distinct applied potentials (vertical yellowlines numbered one to four) and their related Pb adatomcoverage of 0.16, 0.54, 0.78, and 1 ML Pb, respectively. Thesamples are henceforth referred to as xML Pb/Cu.To confirm the Pb deposition, X-ray photoelectron spec-

troscopy (XPS) and atomic force microscopy (AFM) wereconducted on reference electropolished Cu foils and xML Pb/

Cu bimetallic samples. Figure S1a shows the Cu 2p peaks forboth 0.78 ML Pb/Cu and Cu foil samples. Furthermore, onlythe 0.78 ML Pb/Cu displayed Pb 4f peaks (Figure S1b) whichindicate Pb was well-deposited on the Cu surface. In addition,the amount of Pb on the Cu foil was estimated by using Cu 2pand Pb 4f peaks in XPS. The atomic ratio of Pb was 1.09, 1.37,3.12, and 1.31% (Figure 1b), which was in good agreementwith the charge-based coverage evaluations, given thedifficulties to accurately detect and quantify individual adatomcoverages by methods with multinanometer probing depth.Even though 1 ML Pb/Cu amount seemed like an outlier, itwas in accordance with expectations as addressed below.Atomic force microscopy (AFM) topographical images inFigure 1c, 1d, and Figure S2 evidence that Pb adatoms weredeposited two-dimensionally in a single-layer morphology onthe Cu substrates and did not result in 3D Pb clusters ornanoparticles under the chosen underpotential conditions. Wefound that, from AFM images, the surface roughness wasdecreased during the Pb deposition. It is noteworthy that the 1ML Pb/Cu electrode was not uniform due to 37% of Pb−Culattice mismatch and the resulting thermodynamic instability.37

This is why the 1 ML Pb/Cu XPS result deviated from the

Figure 1. (a) Cyclic voltmmograms of Pb UPD on electropolished Cu in 0.1 M HClO4 and 0.01 M Pb(ClO4)2·xH2O. (b) Atomic % of Pb and Cufollowing XPS measurements. AFM topographical images for (c) Cu foil and (d) 0.78 ML Pb/Cu. (e) Scheme of Cu and Pb modified Cu system(0.16 and 0.78 ML Pb/Cu). Pb adatoms are decorated preferentially on HER and CO2RR sites, i.e. on edge and step sites of Cu.

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atomic ratio trend. Figure 1e shows a schematic illustration ofwhere the Pb adatoms are located on the copper surface.Next, the electrochemical CO2 reduction reaction of the

prepared Pb/Cu electrodes was explored. It is noteworthy thatregardless of the Pb coverage, the Pb adatom modified Cuelectrodes mainly produced formic acid (Figure S3 and FigureS4). Figure 2 shows the iR-corrected potential dependentproduction rate and Faradaic efficiency profiles of H2 andHCOO− measured at the steady-state current density in CO2-saturated 0.1 M KHCO3 for Cu foil, 0.78 ML Pb/Cu and Pbfoil. The 0.78 ML Pb/Cu showed an exceptionally low H2Faradaic efficiency given the high overpotential compared toboth pure Cu and pure Pb foils (Figure 2a), while it hadcomparable HCOO− Faradaic efficiency with a solid pure Pbfoil (Figure 2b). This is important and underlines the massiveeffect of adatom decoration. The maximum HCOO− Faradaicefficiency was 74.2 ± 2.3% @ −1.14 V, 25.3 ± 4.9% @ −1.04V, and 81.4 ± 5.2% @ −1.17 V for 0.78 ML Pb/Cu, Cu foil,and Pb foil, respectively. Although Pb foil showed highermaximum HCOO− Faradaic efficiency, it required a moreanodic onset overpotential than the 0.78 ML Pb/Cu (FigureS5a) in the lower cathodic potential region (−1.0 to −1.1 V vsRHE).In order to investigate the variations in the Faradic

competition between hydrogen evolution and CO2 reduction,we compared absolute product yields for HER (Figure 2c), andHCOO− production by CO2 reduction reaction (Figure 2d),respectively. We emphasize that the competitive H2 productionrate at 0.78 ML Pb surface coverage was extremely suppressed,in particular as much as 652 times lower, compared with a pureCu foil. Even pure Pb, commonly known as the most efficient

material to suppress HER, showed a larger H2 evolution rate(Figure S5b) than the bimetallic 0.78 ML Pb/Cu catalysts.Thus, 0.78 ML Pb/Cu not only displayed increased absoluteHCOO− production rates compared to Pb at overpotentials inthe range of −1.0 to −1.1 V vs RHE but also significantlydecreased H2 production rates over a broad overpotentialrange of −1.1 to −1.3 V. Clearly, the 0.78 ML Pb/Cu has asomewhat lower rate of HCOO− formation than Cu foil due tothe blocking effect of Pb adatoms of sites (Figure S5c) thatpreviously served for the formation of H2 and HCOO−.To demonstrate the exceptional product selectivity of the

0.78 ML Pb/Cu surface, we then considered the ratio of theHCOO−/H2 production rates in Figure 3. For the Faraday

Figure 2. Faradaic efficiency of (a) H2 and (b) HCOO− for Cu foil, 0.78 ML Pb/Cu and Pb foil. Production rate of (c) H2 and (d) HCOO− forCu foil, 0.78 ML Pb/Cu, and Pb foil. CO2 electrochemical reduction was performed in CO2-saturated 0.1 M KHCO3.

Figure 3. Trends in HCOO−/H2 production ratio for Cu foil, 0.78ML Pb/Cu, and Pb foil.

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efficiency, it is the percentage of the partial current to totalcurrent for gas products (in the case of liquid products, it isreplaced by the charge), which may be relatively low due to thevarious formation of products. Therefore, in order to comparethe actual quantitative analysis, selectivity was shown bycomparing only hydrogen production rate (Figure 2c) andformate production rate (Figure 2d). In the introduction of Pbadatoms on the Cu surface, suppression of H2 production wasoptimized such that the 0.78 ML significantly outperformedCu and Pb metals individually by having greater HCOO−/H2ratios over the entire applied potential range up to high currentdensities. The Pb/Cu system represents an impressive exampleof the dramatic effect of an incomplete adatom overlayer of adissimilar metal on the electronic and structural properties, andultimately catalytic properties of a metal surface.38−40 Minuteamounts of Pb adatoms caused the emergence of newbeneficial catalytic properties for the electrochemical CO2reduction reaction.In order to gain insight in the effects of Pb adatoms over the

entire coverage range, the production rate and Faradaicefficiency of all samples were compared at fixed potentials of−1.05 and −1.25 V vs RHE (Figure 4). All of Pb/Cuelectrodes showed lower H2 production rate than Cu foil, andthe 0.78 ML Pb/Cu showed the lowest H2 and CH4production rate compared with 0.16, 0.54, and 1 ML Pb/Cu(Figure 4a). On the other hand, all Pb modified Cu electrodesdisplayed similar HCOO− and CO production rates. Thesetrends remained similar even at −1.25 V vs RHE (Figure 4b)suggesting that absolute HCOO−, CO, and CH4 productionrates were quite independent of the detailed Pb amount. Morespecifically, the 1 ML Pb/Cu sample ceased to produce C2H4just like Pb foils at −1.25 V vs RHE. Pb foil exhibited loweroverall activity at −1.05 V vs RHE, yet higher H2 productionrate at −1.25 V vs RHE compare to the bimetallic electrodes.Comparing HCOO− Faradaic efficiencies of all samples at afixed applied potential of −1.05 V vs RHE (Figure 4c),

Faradaic efficiency of 0.78 ML Pb/Cu revealed the highestselectivity thanks to suppressed HER. The Faradaic efficiencyof HCOO− was 44.0 ± 1.5%, 50.9 ± 11.8%, 70.5 ± 0.7%, 52.2± 5.6%, 14.9 ± 4.9%, 50.3 ± 17.3% for 0.16 ML, 0.54 ML, 0.78ML, 1 ML Pb/Cu, Cu foil and Pb foil, respectively. At morenegative potentials (−1.25 V vs RHE), the favorabledifferences between HCOO− and H2 Faradaic efficiencieswidened for the bimetallic Pb adatom Cu electrodes. Figure 4dshows a comparative synopsis of products and their Faradaicefficiency at −1.25 V vs RHE. The Faradaic efficiency of Cufoil exhibited 4.0 ± 3.0%, 30.7 ± 1.3%, and 51.6 ± 2.2% forHCOO−, H2, and CH4, respectively. Noteworthy, the HCOO

−

Faradaic efficiencies for 0.16, 0.54, and 0.78 ML Pb/Cu wereabout 10 times higher than those of H2 Faradaic efficiency. Incontrast, the HCOO− Faradaic efficiencies of 1 ML Pb/Cu andPb foil ranged only about 6 times and 3 times higher than theH2 Faradaic efficiency, respectively. In brief, the Pb/Cu systemshowed an efficient selectivity (ratio of desired product/H2product) over the entire potential.Considering the electrocatalytic properties of Pb-deposited

Cu catalysts and density functional theory (DFT) calculationof Cu and Pb metals reported in literature,27,41−45 we proposea plausible mechanism for the selective formation of formicacid: Cu mainly produces methane, ethylene, hydrogen, andalcohols depending on the applied potential during CO2reduction. Pb, in contrast, primarily generates formic acidand hydrogen. The product selectivity of the CO2 reductionreaction is controlled by the chemisorption of reactiveintermediates such as *OCHO or *COOH. HCOO− isthought to form by a one proton-one electron transfer to*OCHO. While *COOH leads to the formation of *CO,which subsequently desorbs to form gaseous CO(g) if and onlyif the CO chemisorption is sufficiently weak, *CO can befurther protonated and reduced to CH4 or undergo C−Ccoupling with another *CO to form C2H4 and C2H5OH if the*CO chemisorption is strong enough.27,46,47 Cu was shown to

Figure 4. Production rate and Faradaic efficiency at fixed potential of (a,c) −1.05 V and (b,d) −1.25 V (vs RHE), respectively.

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support both pathways, while Pb is thought to stabilize onlythe *OCHO intermediate. In addition, it has also beenreported that the formation of selective products in electro-chemical CO2 reduction is affected by the morphology androughness of Cu. The high index surfaces of copper such asCu(110) or Cu(211) are known as the active sites forproducing C1 hydrocarbons, while the selectivity of C2H4 andCH4 is controlled by the ratio of Cu(100) to Cu(111) facets.Given that, even at small coverages, Pb adatoms is able tostrongly enhance the HCOO− formation, we conclude that Pbadatoms must be decorated preferentially on shared active sitesfor the HER and the CO2 reduction on Cu surface (Figure 1e),resulting in substantial suppression of HER and drastic changein the carbonous product spectrum, mainly producingHCOO−. Any other sites on the residual Cu surface must berelated to the formation of products other than HCOO−.Following Taylor’s concept of a small fraction of catalyticallyactive surface sites within a large total population of surfacesites, the primary reaction product HCOO− is likely producedon that minute amount of under potentially deposited Pbadatoms or Pb adatom pairs, where only the *OCHOintermediate is stabilized. We concluded that Pb/Cu systemhas both geometry and electronic structure effect due to itshighly suppressed H2 production and comparatively accel-erated HCOO− production relative to Pb.

■ CONCLUSIONSWe have prepared (sub) monolayer Pb modified Cu catalystsusing electrochemical underpotential deposition (UPD).Minute coverages of Pb demonstrated massive effects on thecatalytic activity and selectivity of the electrochemical CO2reduction reaction. Pb adatom modified Cu electrodes areexceptionally effective HCOO− producers and displayunprecedented sustained suppression of the competitivehydrogen evolution resulting in high product efficiencies.Especially, the 0.78 ML Pb/Cu showed a high ratio ofHCOO−/H2 production rate due to effective suppression ofHER which was suppressed as much as 652 times comparedwith Cu foil. We found that the Pb adatom, regardless of theamount, could be placed on the active site of CO2 reductionand the active site of HER, thereby stabilizing the *OCHOintermediate and weakening the *H adsorption to stronglyaffect the Cu electrode activity. It is expected that the highselectivity and activity in electrochemical CO2 reduction canbe tuned with a second metal, which may have effects inchanging the binding energy strength and selective affinity fordetermining intermediates. If it can maintain efficientselectivity, especially at high overpotential, this will be one ofthe key factors in the development of future flow cells.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acscatal.8b02846.

AFM topographical images and further electrochemicalCO2 reduction data (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: pstrasser@tu-berlin.de.ORCIDCheonghee Kim: 0000-0002-9502-3429

Tim Moller: 0000-0002-4353-0197Arne Thomas: 0000-0002-2130-4930Peter Strasser: 0000-0002-3884-436XNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported German Federal Ministry ofEducation and Research (Bundesministerium fur Bildungund Forschung, BMBF) under Grant #03SF0523 -“CO2E-KAT”. We thank Jana Lutzki in the Technische UniversitatBerlin for her support with AFM measurements and TaehoYoon for his great help with the 3D figure.

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ACS Catalysis Research Article

DOI: 10.1021/acscatal.8b02846ACS Catal. 2019, 9, 1482−1488

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