Subsurface plays a critical role in CO2 activation by Cu(111) surfaces to form chemisorbed CO2, the first step in reduction of CO2 Marco Favaroa,b,c,1, Hai Xiaod,e,1, Tao Chengd,e, William A. Goddard IIId,e,2, Junko Yanoa,f,2, and Ethan J. Crumlinb,2

aJoint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, CA 94720; bAdvanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720; cChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720; dJoint Center for Artificial Photosynthesis, California Institute of Technology, Pasadena CA 91125; eMaterials and Process Simulation Center, California Institute of Technology, Pasadena CA 91125; and fMolecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Contributed by William A. Goddard III, May 9, 2017 (sent for review January 26, 2017; reviewed by Charles T. Campbell and Bruce E. Koel)

A national priority is to convert CO2 into high-value chemical sorbed form of CO2 was stabilized by a partial negative charge products such as liquid fuels. Because current electrocatalysts are δ− induced by electron capture (CO2 ) (Fig. 1A) (7, 8). The same not adequate, we aim to discover new catalysts by obtaining a experiments showed that no physisorption is observed upon detailed understanding of the initial steps of CO2 electroreduc- increasing the temperature of the Cu substrate to room tempera- tion on copper surfaces, the best current catalysts. Using ambient ture (r.t.) (298 K) (Fig. 1B). Previous ex situ studies performed pressure X-ray photoelectron spectroscopy interpreted with quan- in ultrahigh vacuum (UHV) (about 10−9 Torr) after relatively tum mechanical prediction of the structures and free energies, we low CO2 exposures [from a few to hundreds of Langmuir (L)] show that the presence of a thin suboxide structure below the at temperatures between 100 K and 250 K did not reveal CO2 copper surface is essential to bind the CO2 in the physisorbed adsorption or dissociation on clean Cu(100) (9), Cu(110) (10), configuration at 298 K, and we show that this suboxide is essen- and Cu(111) (11). However, Nakamura et al. (12) showed that tial for converting to the chemisorbed CO2 in the presence of when the exposure is increased to sensibly higher values (pres- as the first step toward CO2 reduction products such as sures ranging between 65 Torr and 1,300 Torr for hundreds of formate and CO. This optimum suboxide leads to both neutral seconds), a nearly first-order dissociative adsorption of CO2 on and charged Cu surface sites, providing fresh insights into how clean Cu(110) can be detected between 400 K and 600 K (with an to design improved dioxide reduction catalysts. activation energy of about 67 kJ·mol−1), according to the reaction CO2,g → COg + Oads (where Oads stands for surface adsorbed oxy- CO2 reduction | suboxide copper | ambient pressure XPS | gen). A similar phenomenology was also observed by Rasmussen density functional theory | M06L et al. (9) on clean Cu(100) for CO2 pressures of about 740 Torr and temperatures in the range of 475–550 K (finding an activation he discovery of new electrocatalysts that can efficiently con- energy of about 93 kJ·mol−1). On the other hand, a recent study vert (CO2) into liquid fuels and feedstock T by Eren et al. (13) performed at much lower CO2 partial pres- chemicals would provide a clear path to creating a sustain- sures (between 0.05 Torr and 10 Torr) revealed that CO2 can dis- able hydrocarbon-based energy cycle (1). However, because CO2 sociatively adsorb on Cu(100) and Cu(111) with the consequent is highly inert, the CO2 reduction reaction (CO2RR) is quite formation of surface as well. Indeed it has been suggested unfavorable thermodynamically. This makes identification of a that the CO2 might dissociate more easily on preoxidized Cu sur- suitable and scalable catalyst an important challenge for sus- faces (3), but there is little evidence to support this important con- tainable production of hydrocarbons. We consider that discov- cept. Activation of CO2 via assumed chemisorbed CO2 species ering such a catalyst will require the development of a complete was reported also on Cu stepped surfaces (11, 14), but direct in atomistic understanding of the adsorption and activation mech- situ proof of the existence of such species on Cu(111) is lacking. anisms involved. Here the first step is to promote initiation of These uncertainties and discrepancies indicate the importance of reaction steps. Copper (Cu) is the most promising CO2RR candidate among pure metals, with the unique ability to catalyze formation of valu- Significance able hydrocarbons (e.g., methane, ethylene, and ethanol) (2). However, Cu also produces hydrogen, requires too high an over- Combining ambient pressure X-ray photoelectron spectros- potential (>1 V) to reduce CO2, and is not selective for desirable copy experiments and quantum mechanical density functional hydrocarbon and alcohol CO2RR products (2). Despite numer- theory calculations, this work reveals the essential first step for ous experimental and theoretical studies, there remain consider- activating CO2 on a Cu surface, in particular, highlighting the able uncertainties in understanding the role of Cu surface struc- importance of copper suboxide and the critical role of water. ture and chemistry on the initial steps of CO2RR activity and These findings provide the quintessential information needed selectivity (3, 4). To reduce CO2 to valuable hydrocarbons, a to guide the future design of improved catalysts. source of protons is needed in the same reaction environment (2), with water (H2O) the favorite choice. Thus, H2O is often the Author contributions: M.F., H.X., T.C., W.A.G., J.Y., and E.J.C. designed research, per- formed research, analyzed data, and wrote the paper. solvent for CO2RR, representing a sustainable pathway toward solar energy storage (1). However, we lack a comprehensive Reviewers: C.T.C., University of Washington; and B.E.K., Princeton University. understanding of how CO2 and H2O molecules adsorb on the The authors declare no conflict of interest. Cu surface and interact to first dissociate the CO2 (5, 6). An Freely available online through the PNAS open access option. overview of the various surface reactions of CO2 on Cu(111) is 1M.F. and H.X. contributed equally to this work. reported in Fig. 1, illustrating the transient carbon-based inter- 2To whom correspondence may be addressed. Email: [email protected], JYano@ mediate species that may initiate reactions. lbl.gov, or [email protected]. Previous studies using electron-based spectroscopies observed This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. physisorption of gas-phase g-CO2 at 75 K, whereas a chemi- 1073/pnas.1701405114/-/DCSupplemental.

6706–6711 | PNAS | June 27, 2017 | vol. 114 | no. 26 www.pnas.org/cgi/doi/10.1073/pnas.1701405114 Downloaded by guest on September 26, 2021 Downloaded by guest on September 26, 2021 fH of CO of steps initial realistic to CO exposed of while pressures formed gas species initial the determining CO formate. of adsorbed step cates reduction first the to leading ( Cu. CO of responsi- codosed layers is two ML top the 0.08 between about oxygen of coverage stabilizing oxide H for subsurface ble additional a without that but observe we Cu(111) into incorporated ately (C CO CO absorbed of l forms The B) h rsneo usraeoye ed oaseicinteraction specific a to CO leads gas-phase with oxygen subsurface of CO presence of the the H activation below of species and presence suboxide adsorption the of and presence allows surface the calculations Cu that theory and conclude experiments functional to of us density combination of This bar- reaction (16) riers. and clusters reaction molecular level for optimized the was M06L that of (DFT) the energies at free with binding products complemented and are eV) studies structures These (200–1,200 molecular interface. X-rays solid/gas soft the with at reac- performed and spec- (APXPS) photoelectron surface X-ray troscopy the pressure of ambient using structure products, electronic tion the of probing situ CO (physisorbed) ear the Here conditions. situ 1. Fig. aaoe al. et Favaro a obtain to min), 60 (for K 1,100 at annealing Torr) and (0.15 min) hydrogen 45 in keV, cycles 2 argon incidence, repeated (normal by sputtering sample, (Ar) polycrystalline a from situ in prepared CO of reduction initial a the motes to state physisorbed (b linear species chemisorbed the bent from transition the through tion (g (l figuration -CO D C B A h dopino CO of adsorption The ) -H oavneti nesadn,w netgtdi ealthe detail in investigated we understanding, this advance To h usraeepsn anyteC(1)oinainwas orientation Cu(111) the mainly exposing surface Cu The 2 δ − 2 2 2 n CO and sosre t28Kfrpesrsrnigfo 10 from ranging pressures for K 298 at observed is nC(1)adsbxd ufcs(Cu surfaces suboxide and Cu(111) on O ) ie ysalaonso uoie rvsCO drives suboxide, of amounts small by aided O), vriwo ufc ecin fCO of reactions surface of Overview 2 2 δ n H and − -CO r bevda 5Kfrpesrsu o10 to up pressures for K 75 at observed are l -CO 2 2 nC(1)cmoe f00 Lo usraeoxide, subsurface of ML 0.08 of composed Cu(111) on O 2 .I diin H addition, In 1C). Fig. , and , 2 2 2 2 htsaiie hssre ierCO linear physisorbed a stabilizes that t28Kad07Tr.Hr O Here Torr. 0.7 and K 298 at dopinbt ln n ntepresence the in and alone both adsorption g-CO n H and 2 b-CO hnasbufc xd tutr sdeliber- is structure oxide subsurface a when 2 2 niae a-hs CO gas-phase indicates -CO 2 ohphysisorbed Both (A) Cu(111). pristine on 2 niae et(hmsre)CO (chemisorbed) bent indicates 1,15). (13, O 2 2 ,wihwt h i fH of aid the with which ), i.1 D). Fig. (HCOO−, formate to 2 2 2 yasre H adsorbed by nC Fg ) Specifically, 1). (Fig. Cu on lyacuilrl nthe in role crucial a play O h oprtv neato of interaction cooperative The D) 2 nC(1)udrvrosin various under Cu(111) on 2 sub ntegsphase gas the in O 2 x niae subsurface indicates , 2 =1.5,2 −6 l O -CO 2 ads −6 .I hswork this In O. ort . Torr. 0.1 to Torr 2 HCOO− ; Only (B) Torr. niae lin- indicates 2 )vain via O) adsorp- 2 2 (A . pro- O 2 con- ind- and sso h ecat (O reactants the of ysis nlsscabr(n prtn taprilpesr fabout of pressure However, partial a the at on 10 operating mounted (and (QMS) chamber spectrometer analysis mass quadrupole tional xeietlcniin n netgtdsrae a .. 9 ) (exper- K): CO 298 B) CO r.t., condition pure (at A) surfaces condition various investigated imental the for and C ( obtained conditions results highlighted direction. experimental fitting the (111) multipeak with and the measurements peaks along APXPS photoelectron beam (3 λ) the primary of the volume detect- of Schematic probed by energy (B) kinetic obtained keV. of a cycles 5 micrograph with of annealing SEM (SE) electrons and and secondary eV sputtering the 110 ing after of surface energy Cu kinetic the electron an at obtained 2. Fig. H codosing by (p Torr pressure 0.7 total at the stant whereas Torr 0.35 at kept 2 Fig. in pattern (LEED) diffraction 1 typical Cu Cu on Torr 0.7 CO C) or pro- C- cleaning in evident the shown no as after and cedure, observed clean were was contaminations surface O-based sample The (18). Cu faces Cu(111), metallic on CO Cu dosing and while performed were and 1 Table tion (see conditions surfaces experimental investigated other and the For surface. Cu(111) metallic CO K), (298 study. this of and focus spectra the the constitute in not their captured do Therefore, not even- limit. were detection an features the to physical/chemical below mea- is led overall which the area of signal, 1% probed sured than less large boundaries, contribution the boundary grain grain over tual of data presence the the averaging from beam contributions a possible over of averaged APXPS size were spot the spectra collected throughout The sample unchanged experiments. characterized remained The sizes. location regions mean surface micrometers crystalline of to tens leads with procedure annealing 2A and Fig. sputtering measurements (SEM) microscopy CD A

292 normalized intensity (arb. units) x −6 efre tr.t. at performed 2B) (Fig. experiments APXPS the During =1.5 A B C D E o ute eal) h CO the details), further for 2 or,ddntrva Ocoscnaiain ftegases. the of cross-contaminations CO reveal not did Torr),

H + LEED (110 eV) .Teeprmna odtosaesmaie nTbe1. Table in summarized are conditions experimental The O. 290 EDpattern LEED (A) APXPS. using surfaces Cu various of Investigation x l- =1.5 × 2 -CO CO binding energy(eV) . oro Cu on Torr 0.7 O i.S2 Fig. 2 3 eosrcina hw ytelweeg electron low-energy the by shown as reconstruction 1 x . mi imtr lhuhw antexclude cannot we Although diameter. in mm ∼0.8 b- ufcs hra CO whereas surfaces, O =1.5 288 2 2 H + CO a rtitoue t07Tr ntepristine the on Torr 0.7 at introduced first was HCOO 2 ) n eprmna odto )pr CO pure E) condition (experimental and O); 2 C-O(H) . oro ealcC(1) eprmna condition (experimental Cu(111); metallic on Torr 0.7 O hw ht ocmtnl ihtegsdosing gas the with concomitantly that, shows 286 PNAS i.S1 Fig. C-C SEM (SE, 5 keV) h 284 x v =387eV =2.5 C=C 2 2 x | CO , =1.5 C 1s . oro ealcC(1) (experimental Cu(111); metallic on Torr 0.7 ue2,2017 27, June nadto,tei iums anal- mass situ in the addition, In . ;(xeietlcniinD CO D) condition (experimental O; 282 50 ,adCu and O, μ 2 2 2 m n H and , .TeAXSmeasurements APXPS The O. normalized intensity (arb. units) (p pressure partial B 2

A B C D E HEA aperture nbt ealcCu(111) metallic both on A 2 534 n H and | 1) cnigelectron Scanning (17). (p <1Torr) gas phase h 2 v o.114 vol. x e binding energy(eV) H O ) sn conven- a using O), - =2.5 2ads uprigInforma- Supporting eemn htthis that determine and 532 -CO 2 tot l- uoiesur- suboxide O eecodosed were O 3 CO sadO1s O and 1s C D) a etcon- kept was ) 2 3 | λ Cu O o 26 no. 530 x Cu-O (CO 2 Cu O-O . oron Torr 0.7 sub C-O(H) /HCOO b- x CO h 2 v Cu <111> 2 )was )) ads 2 =632eV | 528 H + x 0.5 O 1s 6707 x 3 2 O

CHEMISTRY Table 1. Various Cu surface structures and experimental To disentangle the role of oxygen on the surface and subsur- conditions explored with APXPS face regions, we carried out a similar analysis on O 1s core-level Total spectra (Fig. 2D). The analysis performed on C 1s was used to Experimental Surface Gas pressure, Temperature, help the interpretation of the O 1s spectral envelope while also condition structure environment Torr K accounting for the different relative abundances. As with C 1s, we partition the O 1s spectral window into three regions. At low A Metallic Cu(111) CO2 0.7 298 BEs we identify the states of O bonded as follows: (i) surface B Metallic Cu(111) CO2 + H2O (1:1) 0.7 298 adsorbed O (Cu-Oads)on metallic Cu and on suboxidic Cux O C Cux=2.5O CO2 + H2O (1:1) 0.7 298 structures (Cux O-Oads) at 531.0 eV and 529.6 eV, respectively D Cux=1.5O CO2 + H2O (1:1) 0.7 298 (15, 25–27); (ii) subsurface adsorbed O (Osub) on metal Cu (Cu- E Cux=1.5O CO2 0.7 298 Osub) at 529.8 eV (27) (as we discuss in a later section, such a presence of suboxide plays an important role in stabilizing the l-CO2); and (iii) for Cux>1O the O 1s is centered at 530.3 eV −6 (for pressures exceeding 10 Torr), uptake of carbon contami- (15, 18, 25). It is noteworthy that Oads groups on the Cu surface nations readily occurred [the corresponding binding energy (BE) can serve as nucleation sites for hydroxylation when in the pres- being centered at 285.1 eV]. Therefore, we cannot completely ence of H2O. However, the detection of eventual Cu-OH groups exclude eventual side reactions and interplay between carbon via photoelectron chemical shift identification is complicated by contaminations and the copper surface. the fact that in the same spectral range (530.6–530.8 eV) sev- To understand how interactions between the catalyst surface eral oxygen-based species overlap (such as formate, C-(OH), and and CO2 determine the mechanisms of the initial CO2 reduction O-R species with R = −CH3, −CH2CH3). On the other hand, steps, we established the experimental conditions under which a the presence of the C 1s spectral counterpart of formate and chemisorbed CO2 state can be stabilized. This provides the basis C-(OH) (well discriminated in BE) allowed us to build up a con- for tailoring novel catalysts with improved electrochemical per- sistent O 1s fitting. Therefore, although we cannot completely formance toward the CO2RR. exclude the presence of surface Cu-OH, its concentration is Previously it was difficult to probe these early steps experimen- most likely below the detection limit of the technique (about tally because r.t. studies require pressures of CO2 high enough to 0.02 ML). C-O bonds fall instead in the middle region, namely stabilize a physisorbed configuration sufficiently to allow detailed between 530.8 eV and 532.0 eV. Within this range, from lower to investigations of various adsorption dynamics, but this high- higher BE, we identify chemisorbed CO2, C-O(H), and formate pressure gas makes it difficult to use electron-based spectro- (HCOO−) overlapping at 530.8 eV; l-CO2 at 531.4 eV; and car- scopies. Our use of APXPS overcomes this difficulty (19–21). bonates at 531.8 eV (15, 25). Finally, at high BE we observed To discriminate between physisorbed and chemisorbed CO2, we adsorbed H2O (H2Oads) at 532.4 eV (15, 25). monitor the spectral BE shifts of the corresponding C 1s and The difficulty in discriminating between Cu0 and Cu+ using O 1s photoelectron peaks as a function of the different sur- Cu core levels has been well established and is clearly evidenced faces and experimental conditions. Physisorption mediated by from Fig. S5A, reporting the Cu 3p photoelectron spectra. To weak van der Waals (vdW) interactions [surface binding ener- overcome this limitation, the various Cu surfaces were charac- gies of a few millielectronvolts, comparable to kB T = 25.7 meV terized by means of the Cu Auger L3M4,5M4,5 transition and the at 298 K (7)] generally leaves the adsorbate electronic structure valence band (VB) as described in Discussion and as reported in unchanged compared with its gas-phase configuration (7, 22–24). Figs. S5B and S6. In contrast, the chemical bonding needed to form chemisorbed It is important to note that the BE of the aforementioned CO2 on the Cu surface redistributes the electronic density in the chemically shifted components for C 1s and O 1s do not change adsorbate, leading to appreciable BE shifts compared with the with the experimental conditions (within the spectral resolution, physisorbed state (8). ∼0.15 eV), with an exception only for the adsorbed CO2, where The adsorption state of CO2 and the overall surface chemistry the adsorption configuration (b-CO2 vs. l-CO2) depends on the of the various systems were monitored by multipeak deconvo- experimental conditions. In particular, we observe an important lution on both the C 1s and O 1s photoelectron spectra (Fig. 2 decrease by ∼0.50 eV (Fig. 2B) of the C 1s BE when CO2 is C and D), using chemically shifted components sensitive to the codosed with H2O on the metallic Cu(111) surface (Fig. S2). This initial state effects. Fig. S3 A and B reports the integrated peak work was inspired by similar experiments previously reported areas of the chemically shifted components for C 1s and O 1s by Deng et al. (15), where they dosed CO2 and H2O sepa- deconvolution, respectively, normalized by the total area under- rately and together on a polycrystalline (nonoriented) Cu sam- neath the spectra. C 1s and O 1s photoelectron spectra were ple. The authors observed the presence of an adsorbed CO2 acquired under APXPS conditions at photon energies of 387 eV species at r.t. (with the corresponding C 1s centered at BE = and 632 eV, respectively. Because the kinetic energy of the escap- 288.4 eV), which they labeled as a negatively charged adsorbed δ− ing C 1s and O 1s photoelectrons is about 100 eV, the probed “CO2 .” We believe their adsorbed CO2 species could actually depth, 3λ (λ is the electron mean free path) is about 1.2 nm, be attributed to the l-CO2 configuration observed and computed from the topmost layer (Fig. 2B). in this work. Interestingly, however, the authors did not observe The deconvoluted C 1s spectra (see Supporting Information for a new component in the adsorbed CO2 spectral region (287.9– further details) exhibit two main spectral regions: (i) At low BE 288.5 eV, i.e., the b-CO2), passing from the exposure to pure we see chemical species that can be assigned as graphitic car- CO2 to CO2+ H2O. In addition, we observe only a weak pres- 3 bon (284.5 eV), sp (C-C) carbon (285.2 eV), and C-O(H) bonds ence of reaction products between CO2 and H2O codosed at (286.3 eV), based on the literature values (15). (ii) At higher BE r.t. (Fig. 2 C and D), whereas Deng et al. (15) observed the sig- we see spectral fingerprints of higher oxidized carbon structures nificant development of the methoxy group spectral component and adsorbed CO2, where deconvolution of the spectra indicates (−OCH3, BE = 285.2 eV) when codosing CO2 and H2O. These the presence of formate (HCOO−) (287.3 eV), chemisorbed differences might be addressed by the higher experimental gas (denoted b-CO2 for bent), and physisorbed CO2 (denoted l-CO2 pressures used in this study, as well as potentially different inves- for linear) (287.9 eV and 288.4 eV, respectively) and carbonate tigated surface structures formed by different surface cleaning (−CO3) (289.4 eV) (15). Finally, a sharp peak centered at about and annealing procedures. Overall, these differences can poten- 293.3 eV corresponds to the photoelectron emission of g-CO2 tially lead to a different surface reactivity. The results reported by (Fig. S4). Deng et al. (15) have been obtained on a polycrystalline surface

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Supporting ∆ p rw3o al 2), Table of 3 (row thresh (9 )= K) H(298 2 t;(C atm; 3 = 2 rdce tutrsfr14M fphysisorbed of ML 1/4 for structures Predicted suhl by uphill is i hssre ierCO linear physisorbed (i) : 2 = u11 ih14M O ML 1/4 with Cu(111) ) l sub -CO × .2e,wihwudsaiiephysisorbed stabilize would which eV, −0.12 10 ntefrainof formation the on l u11 ih14M O ML 1/4 with Cu(111) ) -CO −7 2 2 2 npitn ealcC(1)(i.2 (Fig. Cu(111) metallic pristine on t28Kwt rsueo . orCO Torr 0.7 of pressure a with K 298 at .0e atricuigzr-on energy zero-point including [after eV −0.30 (b u11 ih14M fbt O both of ML 1/4 with Cu(111) (D) and Torr; = ∆E sub 2 (9 ,07Tr)= Torr) 0.7 K, ∆G(298 . Vcmae with compared eV ∼4.9 -CO = ∆G ob bevda 9 npr metallic pure on K 298 at observed be to . 2 1) neetnl,ee fCO if even Interestingly, (13). 2 .6e n netap fbinding of enthalpy an and eV −0.36 .3eV, −0.13 hti omdol fe digH adding after only formed is that ) rsieCu(111), pristine A) 2 .8M)o ufc suboxide surface of ML) ∼0.08 ads oeue h reeeg for energy free the molecule, rw1o al 2) Table of 1 (row p sub 2 thresh l -CO rw2o al 2) Table of 2 (row , (l -CO or Both Torr. 7 = 2 sn aiu levels various using , sub 2 ∆ bv .5 Torr 0.150 above ) smre norange). in marked is 02 eV, +0.27 = G .7e.These eV. +0.27 g 5MTr)for Torr) M ∼25 l -CO -CO 2 02 eV, +0.21 = ∆G 2 (9 K, ∆G(298 nvrosCu various on sub C l -CO = ∆G ( n case and C i.S4). Fig. 2 n (ii) and sub p and l l ads sstill is thresh 2 -CO -CO −0.39 has ) was 2 and O, D, 2 D = 2 2 . F / LO ML O 1/4 ML 0.06 APXPS DFT epciey speeti u rsieC(1) oprdwith Compared Cu(111). pristine our in present O is (denoted respectively) O amount adsorbed small subsurface a and/or indicate surface shifts of 1s O observed experimentally The CO observe Physisorbed to atm 33 of pressure K. a 298 in require at would decrease which eV, large +0.27 the However, CO CO physisorbed free heat). the specific from entropy and ZPE mechanical is ing quantum surface the The to with interactions. energy bond vdW electronic (QM) weak of teristic 179 of 3.11 angles O-C-O 1.164 and of distance physisorbed The Cu(111). ae M)euvlns(L)o CO of (MLE) equivalents (ML) layer 3A Fig. CO Physisorbed Structure Method experimental with BE 1s of O distributions results calculated various APXPS of with and Cu(111) atoms of O models surface DFT 2. Table npitn Cu(111), pristine on (A) structures predicted O of tration of levels p different with Cu(111) Further vdW (16). tions, in known energies presented of bonding are benchmark known details large accurately a with to clusters functions comparing correlation that by exchange DFT of and optimized version energy vdW M06L kinetic empirical the both the use includes we for Instead basis Cu. rigorous for corrections no correction empirical is there with However, included (29). usually London is for which account dispersion, not do (LDA)] approximation local-density DFT DFT i.4. Fig. u11 ih14M O O ML 1/4 with Cu(111) .5Tr o H for Torr 0.35 = sub , 02 eV. +0.28 = ∆G ∆ rmteCao fCO of atom C the from A ˚ aae S1 Dataset (9 )= K) H(298 0Lpeitdsrcue o chemisorbed for structures predicted M06L hw h rdce ufc tutr o / mono- 1/4 for structure surface predicted the shows 2 / LO ML 1/4 / LO ML 1/4 n CO and O 2 2 2 and , ads ads ihOo Cu(111) on O with nCu(111) on hti naoal by unfavorable is that .0e nhlyo odn atrinclud- (after bonding of enthalpy eV −0.30 sub oprdwt 1.163 with compared A PNAS ˚ .8M O ML 0.08 + O ML 1/4 + , alsS1 Tables = ∆G sub ads 2 opttoa eal fDTCalcula- DFT of Details Computational (A–C . | 2 ◦ .6e;ad(C and eV; −0.06 l ue2,2017 27, June oeuelast reeeg for energy free a to leads molecule -CO iha qiiru itneof distance equilibrium an with , sub o n ieveswt hmclillus- chemical with views side and Top ) sub sub and 2 . 2 δ δ oeue aeaCObond C-O a have molecules srpre o 9 ,and K, 298 for reported is ∆G O O oteC ufc,charac- surface, Cu the to S2. ads ads 2 = = | ∆ nmtli (O metallic on . eV; −0.4 eV; −0.3 δ o.114 vol. δ Predicted nC(1)wt / ML 1/2 with Cu(111) on ) (9 ,07Tr)= Torr) 0.7 K, G(298 O O sub ads and 10 V (B eV; +1.07 = ∆G b-CO ngsphase gas in A ˚ = = ∆ δ . eV −2.2 . eV −1.3 δ δ | O = E O O 2 sub δ o 26 no. ads sub sub O ihH with ads .6eV −0.36 n O and = = sub . eV −1.6 eV −1.5 | -free) 2 on O 6709 on ) sub ,

CHEMISTRY the O 1s BE of O in the l-CO2 configuration, we observe an by ∆G(298 K, 0.7 Torr) = 0.26 eV [it is 0.43 eV endother- experimental shift (δOads) of −0.4 eV for Oads and an experi- mic for Cu(111)Osub,x=0.5]. On the other hand, our DFT cal- mental shift (δOsub) for Osub of −1.6 eV. To deduce the nature culations predict that this formate can extract an H from of this Oads, we consider the three cases reported in Table 2. the −OH to form formic plus Oads, which is stable with For computational convenience we assumed a 2 × 2 surface ∆G(298 K, 0.7 Torr) = −0.05. cell, but the experimental Osub coverage is about 0.08 MLE. For We expect that learning how to tune the character of the sur- the 2 × 2 unit cell, our DFT calculations find two cases with O face atoms (Cu0 vs. Cu+ in this case) to manipulate these relative 1s BE consistent with experiment. Fig. 3C with one Osub per cell energetics of l-CO2 plus H2O, b-CO2 plus H2O, formate plus leads to a BE = −1.35 eV whereas Fig. 3D with one Osub and one OH, and formic acid plus Oads may allow us to design modified Oads leads to BE = 0.31 and 1.54 eV. Referencing to gas-phase systems aimed at accelerating these reaction steps. For exam- O2 (standard conditions), Fig. 3D is ∆G = −2.34 eV more stable ple, we hypothesize that other subsurface anions such as S or Cl than Fig. 3C. For case Fig. 3C we predict ∆G = −0.39 eV bond- might favorably modify the energetics by changing the charges −7 ing for l-CO2 (a pressure threshold of 2 × 10 Torr), whereas and character of the surface atoms and/or replacing some Cu Fig. 3D leads to ∆G = −0.13 eV with a pressure threshold of with Ag, Au, or Ni with different redox properties. 7 Torr, both consistent with experiment. Activation of the inert linear l-CO2 molecule requires enforc- b Simultaneous dosing of CO2 in the presence of H2O leads ing a bent -CO2 configuration (30) with great chemical stabi- lization, but pristine Cu(111) and corresponding derivatives with to a dramatic change in the character of the surface CO2, showing clearly the adsorption characteristics for chemisorbed Osub and/or Oads do not deliver sufficient stabilization, as shown in our calculations. Thus, forcing CO2 to have the necessary angle b-CO2. For a Cu(111) surface that includes some surface sub- ◦ ◦ oxide, the DFT calculations lead to several local minima (Fig. (120 ∼140 ) and appropriate distance (∼2 A)˚ to the pristine Cu 4): (i) physisorbed l-CO2 plus H2Oad,(ii) chemisorbed b-CO2 surface, we find no stable local minimum; all of the initial bent plus H2Oads (Fig. 4 A–C), (iii) reacted COOHads plus OHads CO2 structures relax into the stable l-CO2 physisorption state. (Fig. S7A), and (iv) HCOOH plus surface O (Fig. S7B). However, the presence of modest amounts of Osub generates a ads + 0 In the case of Cu(111) without Osub (Fig. 4A), the C atom mixture of surface Cu and Cu atoms that combines with H2Oad 0 of b-CO2 is chemically bonded to a surface Cu , whereas the to stabilize the b-CO2 structure reported in Fig. 4B. We conclude two O atoms accommodate the partial negative charge trans- that this configuration of surface atoms and H2O is responsible ferred from the Cu surface, with one stabilized by hydrogen for stabilizing b-CO2 and opening up the possibility of forming bonding to H2Oad. However, this b-CO2 leads to a QM binding formate, formic acid, etc. This elucidates the first reduction step energy of ∆E = −0.23 eV, but including vibrational and entropy of CO2. contributions we find b-CO2 is unstable, with ∆G(298 K, 0.7 This combination of APXPS experiments and DFT calcula- Torr) = 1.07 eV, which agrees with our experiments. tions enabled us to obtain a detailed understanding of the initial When the Osub is increased to 1/4 ML (Fig. 4B), we find that steps of CO2 activation by H2O on a Cu surface. We find that a 0 the C atom is chemically bonded to two surface Cu , one O modest level of Osub between the top two Cu layers is essential atom is chemically bonded to one Cu0 center, and the other O for stabilizing physisorbed l-CO2. + atom is stabilized by the surface Cu pulled up by H2Oad. This This unexpected finding may explain a general observation b-CO2 leads to ∆G(298 K, 0.7 Torr) = −0.06 eV, which is stable empirically derived in the literature from the catalytic perfor- in agreement with our experiments. mance of Cu for CO2RR: It is known that Cu cata- However, increasing the Osub to 1/2 ML, we predict that lysts previously treated to generate surface oxides generally show ∆G(298 K, 0.7 Torr) = +0.28 eV, which is unstable. Here the improved activity compared with the pristine metallic surface (3, C atom is chemically bonded to a surface Cu+ that shares an O 6, 31). From our experimental results and theoretical predictions, atom bearing a partial charge (stabilized by a hydrogen bond- we conclude that the topmost layer needs to expose metallic cen- + ing to H2Oad on surface Cu ). Our experiments also show that ters, because CO2 can efficiently chemisorb only on such centers increased levels of Osub decrease the binding of b-CO2. Thus, we (Fig. 4B), to form the activated molecular substrate for subse- find that chemisorbed b-CO2 is stable only for the case in Fig. 4B quent reduction to formate and other products. However, we find with 1/4 ML O . Having more O or none at all destabilizes that the presence of a subsurface oxide structure is also needed sub sub + b-CO2. We explain this in terms of the distinct interactions of to promote H2O chemisorption onto a Cu center. This enables 0 + Cu and Cu induced by the Cu(111)Osub,x=0.25. the electronic communication between adsorbed CO2 and H2O, This result of an optimum Osub for b-CO2 is in agreement with favoring the transition from a linearly physisorbed l-CO2 to a bent our experiments for CO2 and H2O codosing on the Cux=2.5O chemisorbed b-CO2. From Fig. 4B, reactions to form formate and and Cux=1.5O suboxide structures, which shows both b-CO2 and formic acid are possible but not favored under our conditions. l-CO2, but with a l-CO2/b-CO2 ratio of 3.8 and 5.3 for Cux=2.5O These results provide the insight that subsurface oxide plays to the Cux=1.5O structure, respectively (the ratio was deter- a critical role in the initial steps for activating CO2, provid- mined from both C 1s and O 1s spectra) (Fig. S3 A and B). In ing a foundation for the rational development of unique active addition, Fig. S8 reports the experimental results of exposing electrocatalysts. the Cux=1.5O structure to 0.7 Torr of 1:1 CO2 and O2. In this ACKNOWLEDGMENTS. This work was supported through the Office of Sci- case we do not observe chemisorbed b-CO2, but only physisorbed ence, Office of Basic Energy Science (BES), of the US Department of Energy l-CO2 and its conversion to surface −CO3 (carbonate). (DOE) under Award DE-SC0004993 to the Joint Center for Artificial Photo- synthesis and as part of the Joint Center for Energy Storage Research, DOE The DFT calculations predict that on Cu(111)Osub,x=0.25, b Energy Innovation Hubs. The Advanced Light Source is supported by the -CO2 can react with H2Oad to form formate plus OHads, but Director, Office of Science, Office of BES, of the US DOE under Contract the product is unstable in our conditions, with ∆G(298 K, 0.7 DE-AC02-05CH11231. The QM calculations were carried out on the Zwicky Torr) = +0.20 eV, making it endothermic from b-CO2 in Fig. 4B supercomputer at Caltech.

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