Spin Uncoupling in Chemisorbed OCCO and CO2 â Two

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Spin Uncoupling in Chemisorbed OCCO and CO – Two High-Energy Intermediates in Catalytic CO Reduction 2

Svante Hedström, Egon Campos Dos Santos, Chang Liu, Karen Chan, Frank Abild-Pedersen, and Lars G.M. Pettersson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02165 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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The Journal of Physical Chemistry

Spin Uncoupling in Chemisorbed OCCO and CO2 – Two High-energy Intermediates in Catalytic CO2 Reduction Svante Hedström1, Egon Campos dos Santos2,1, Chang Liu1, Karen Chan3, Frank AbildPedersen3, Lars G. M. Pettersson1* 1

Department of Physics, Albanova University Center, Stockholm University, 10691

Stockholm, Sweden 2

GPQIT, Departamento de Química, ICEx, Universidade Federal de Minas Gerais, Belo

Horizonte − MG, Brazil, 31.270−901 3

SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator

Laboratory, Menlo Park, CA 94025, USA

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Abstract The production of useful compounds via the electrochemical carbon dioxide reduction reaction (CO2RR) is a matter of intense research. While the thermodynamics and kinetic barriers of CO2RR are reported in previous computational studies, the electronic-structure details are often overlooked. We study two important CO2RR intermediates: ethylenedione (OCCO) and CO2 covalently bound to cluster and slab models of the Cu(100) surface. Both molecules exhibit a near-unity negative charge as chemisorbed, but otherwise they behave quite differently, as explained by a spin-uncoupling perspective. OCCO adopts a high-spin, quartet-like geometry, allowing two covalent bonds to the surface with an average gross interaction energy of −1.82 eV/bond. The energy cost for electronically exciting OCCO− to the quartet state is 1.5 eV which is readily repaid via the formation of its two surface bonds. CO2 conversely, retains a low-spin, doublet-like structure upon chemisorption, and its single unpaired electron forms a single covalent surface bond of −2.07 eV. The 5.0 eV excitation energy to the CO2− quartet state is prohibitively costly and cannot be compensated for by an additional surface bond.


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Introduction The electrochemical CO2 reduction reaction (CO2RR) is projected as an important part of a sustainable, carbon-neutral energy economy.1–3 Many different products can be formed, depending on the element4 and structure of the metal catalyst,5,6 the applied potential, solvent pH,7 and other reaction conditions.8,9 Product molecules with more than one carbon atom are highly sought after since they have a higher energy density and industrial demand. Despite the rich chemical landscape of catalytic CO2RR, the pathways to many products share a limited number of important intermediates. Examples of such intermediates, in order of increasing degree of reduction, include chemisorbed *CO2, *CO, and *OCCO,10–12 where * denotes a surface-adsorbed state. In the gas phase, neutral singlet ethylenedione, or OCCO, is unstable with respect to dissociation into 2 CO,13,14 and the triplet state is metastable but readily crosses to the dissociative singlet state under experimental conditions. However, the cationic, anionic, and dianionic redox states are stable and have been studied in significant detail by ab initio15 and experimental14,16–18 methods. On metal catalyst surfaces, *OCCO is produced by dimerization of *CO and has been identified as a crucial, early intermediate toward multi-carbon products in CO2 reduction.10–12 Formation of a double C=C bond entails a high kinetic barrier, making *OCCO formation a potentially rate-limiting step in the route to CO2RR products such as ethanol and ethylene. Another high-energy intermediate is chemisorbed *CO2 since CO2 in either the gas or aqueous phase is thermodynamically highly stable, and so its chemisorption and concomitant reduction has been suggested to be a rate-determining step in CO2RR under certain conditions,19–25 particularly on weakly binding metals such as Au.26 Despite the importance of *CO2 and *OCCO, their electronic



structures under electrocatalysis-like conditions have so far not been reported in detail, with previous studies rather focusing on larger-scale energetics and kinetics. We focus here on the electronic structure of *CO2 and *OCCO on the Cu(100) facet since its geometry favors *OCCO adsorption and shows a higher activity for CO2 reduction into multicarbon products,27–29 and moreover has been suggested to be the eventual predominant facet for polycrystalline Cu under modest-potential CO2RR conditions.30 We use cluster models which permit a more detailed analysis of orbitals and different spin states, are facile to chemically reduce, and can benefit from well-developed implicit solvent models. Cluster models are also advantageous since one can easily increase their size until the effective coverage is so low that the change in potential upon chemical reaction is reduced to a minimum. Key properties, such as chemisorption energies and geometries, are validated by comparison to slab models. Explicit or implicit water is included throughout, since solvation is crucial for the stabilization of polar adsorbates such as CO2 which adopts an anion-like state when surface-bound.31,32

Spin Uncoupling As an extension of the “linear combination of atomic orbitals” (LCAO) model, the electronic state of a chemisorbed system can be described as a combination of the available electronic states of the two parts, i.e. the surface and the adsorbate.33 When a molecule forms covalent bonds to a surface, its internal spin-paired electronic structure needs to be broken, being instead replaced by new bonds to the surface. This process can be decomposed into two steps: first an intramolecular spin uncoupling, which for closed-shell molecules corresponds to an excitation to the triplet state,34,35 and then the coupling of the unpaired electrons of the adsorbate with those of the surface, recovering an overall closed-


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shell system. Analogously, the surface electronic structure also adapts to facilitate bonding.36,37 This spin-uncoupling–bond-preparation perspective is illustrated in Figure 1.

Figure 1. Schematic depiction of spin uncoupling in the chemisorption of a molecule (each circle representing e.g. a methylene in C2H4 or carbonyl group in OCCO) to a surface (ellipse). The intramolecular excitation is in reality concerted with the surface bond formation so the intermediate state is never actually observed, but it is a useful concept to understand the electronic structure and to decompose the energy contributions of covalent chemisorption. The chemisorbed state results from combining the spin-uncoupled molecular and surface states, making the geometry of the chemisorbed species similar to its vacuum high-spin state. This state typically shows an elongated intramolecular bond due to the decrease in bond order when exciting an electron from a bonding to an antibonding orbital. A molecule that is already high-spin, e.g. O2, does not need to rehybridize to chemisorb. A singlet-to-triplet spin uncoupling is shown here, but the same principle holds for doublet-to-quartet uncoupling for radical molecules, with the only difference that doublet adsorbates can form a single covalent bond without electronic excitation, whereas an excitation to the quartet state permits formation of up to three covalent bonds. According to this perspective, the observed adsorption energy can be decomposed into three contributing terms. First the cost of unpairing the electrons in the adsorbate, second the cost of exciting the electrons in the metal surface (this cost approaches zero for

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extended metal surfaces), and third the favorable energy of forming the adsorbate–surface bond(s) which becomes a measure of how strongly the two species actually interact. We think of this as “capitalistic chemistry”; in analogy with economic terms the molecule (and sometimes surface) invests energy Eex in excitation to the high-spin state, but receives a return on investment in terms of the covalent bonds, i.e. the gross interaction energy Eint, and the net profit is the final adsorption energy ∆Eads.38 ∆E = E (ads) + E (surf) + E

(1)

A small net adsorption energy could imply a weak chemical interaction, but not necessarily; it could also simply mean that the rehybridization cost is large. The surface spin-uncoupling energy Eex(surf) vanishes for extended metal surfaces but is non-zero for clusters where it generally decreases with increasing cluster size, albeit slowly.39 Even so, the variation of Eex(surf) with the number of atoms is not linear or even monotonic due to the sensitivity of the band development to the exact cluster shape and structure. Furthermore, clusters with an even number of Cu atoms tend to be closed-shell, with larger HOMO–LUMO gaps than uneven-numbered Cu clusters which cannot pair all its electrons.40,41 These two effects explain why specific cluster sizes display enhanced properties, including chemical stability and adsorption energies.40,42–45 When using clusters to represent an extended surface, Eex(surf) becomes a finite-size artifact which leads to binding strengths that are underestimated and overly sensitive to the specific cluster size and shape. Reliable cluster ∆Eads estimates are nevertheless feasible by explicitly calculating the magnitude of this artificial Eex(surf) and subtracting it from ∆Eads as a systematic bond-preparation correction.37,39


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Computational Methods A Cu66 cluster model was prepared as outlined in the Supporting Information (SI). It underwent structural relaxation with density functional theory (DFT), using the RPBE exchange–correlation functional46 and the modest TZVP-GGA47[Cu]/TZVP48[C,O,H] basis set as implemented in the deMon2k49 software, see SI for example input file. Clusters with a bound *CO2 or *OCCO adsorbate and 0–3 explicit waters nH2O were optimized at the same level of theory with 49 edge Cu atoms frozen to emulate the rigidity of an extended surface, whereas the water, adsorbate, and the 17 adjacent surface and core Cu atoms were structurally relaxed. The adsorbate was then removed, and the resulting water-decorated reference clusters were optimized at the same level of theory. All systems then underwent single-point calculations with the larger Def2-TZVP basis set;50,51 all reported energies are obtained with this basis set unless otherwise noted. The nH2O = 2 case was also studied with 1 additional electron ne, employing spin-unrestricted DFT. Molecular systems were studied at the same level of theory. The effect of implicit solvation on clusters and molecules was studied using the ORCA 4.0.1.2 software52,53 and the SMD polarizable continuum model (PCM) of water,54 which has previously been used to study surface adsorption.55,56 Structures were based on the deMon2k-optimized geometries, but further optimization of the central 17 Cu atoms and adsorbates were performed with the 49 edge Cu atoms frozen. The RPBE46/Def2-TZVP(f)//Def2-SVP50 level of theory was employed, see SI for example input file. The effect of dispersion interactions on the clusters was also investigated, in the form of the DFT-D3 model57 using Becke–Johnson damping58 (D3BJ). Molecular systems as well as a CO2Cu2− model system were calculated at the same level of theory as above, without dispersion. The



ORCA electron density was analyzed with the Bader Charge Analysis code.59 The Chimera software was used for cluster and molecule visualization.60 Periodic slab calculations were performed with the GPAW software61 relying on the gridbased projector-augmented wave method62 in finite-difference mode using the RPBE46 functional, a grid spacing of 0.2 Å, and a 3×4×1 k-point sampling, see SI for example input. The slabs have 4×3×4 copper atoms in the x, y, and z directions respectively, using an optimized lattice constant of 3.67 Å and a vacuum layer of 15 Å above the exposed (100) surface to avoid inter-slab interactions in the z direction. Slabs were optimized with the two bottom Cu layers frozen and a single explicit water, with and without a CO2 or OCCO adsorbate. Molecular systems were studied at the same level of theory without periodic boundary conditions, where the 3OCCO or CO2 were first optimized in presence of H2O followed by removal of the adsorbate to optimize the water only. Chemically reduced slabs are often prepared by including an alkali atom in the water layer which spontaneously self-ionizes, reducing the metal.11,63 However, in our slabs there is no water layer to hydrate the alkali atom so instead we include a Li atom hydrated by 4 H2O in the vacuum region while increasing the vacuum layer by 7 Å to a 22 Å total. It spontaneously ionizes to a solvated Li+ and an electron at the surface. The potential vs vacuum Φ for clusters and slab models is defined as per Eq. 2: Φ(cluster) = −E , Φ(slab) = Φ!" − Φ#$""% (2) The potential Φ is nearly constant in the cluster models whose large size affords changes in Fermi level ∆EF < 0.10 eV in all cases, despite the highly polarizing adsorbates studied. Our computational approach captures the electronic-structure energetics of chemisorption, whereas thermal sampling, non-local correlation, and constant-potential corrections are beyond the scope of this study. 8 Plus Environment ACS Paragon

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Results and Discussion Ethylenedione Chemisorption The dimerization of carbon monoxide yields ethylenedione, OCCO, a molecule which is thermodynamically unstable in the neutral but stable in the electronically reduced form.13,15 Thus, it is not surprising that on copper surfaces, chemisorbed *OCCO exhibits a negative charge,10 here around −1 e according to a Bader charge analysis, see Table 1. In the slab model, the Li(H2O)4 approach for chemically reducing the surface induces an electric field in addition to donating an electron, which increases the *OCCO charge qOCCO by 0.13 e. Conversely, qOCCO in the cluster is quite insensitive to the addition of an electron since it mainly populates the copper bulk. qOCCO also becomes more negative with more complete solvation, even going beyond −1 e in the PCM solvent due to its strong capability for stabilization of charged and polar states. No charging-up of the explicit water is observed in any system (|qH2O| < 0.08 e). Without any water present, the configuration where *OCCO binds with two C atoms spontaneously dissociates into 2 *CO. A higher-energy configuration where OCCO binds with O and C has been shown to be stable also without water.10 The adsorption energy ∆Eads of OCCO is defined as the reaction energy of Eq. 3. Accordingly, a more negative ∆Eads means stronger binding: ∗

[ ()* H, O.]12 3 + 5OCCO • ()* H, O. → 8 ∗OCCO • ()* H, O.9

12 3

+ )* H, O (3)

Here the meta-stable 3OCCO triplet state is used as the reactant reference, making the chemisorption reaction charge-balanced. The same number of water molecules nH2O are included in all systems entering in Eq. 3 because the adsorbate molecule is stabilized



significantly by water and the stabilization should be equal for reactant and product states, to get a fair reaction energy estimate. The nH2O H2O without adsorbate are optimized starting from the joint adsorbate–(nH2O H2O) geometry, removing the adsorbate, and then optimizing the water without letting it adopt an entirely different geometry. We believe this is the fairest way of estimating Eads. The calculated Eex(surf) due to finite-size effects in the clusters, amounting to 0.09 eV for ne = 0 and 0.18 eV for ne = 1, was subtracted as a bondpreparation correction to ∆Eads,37,39 as described in the final paragraph in the Spin Uncoupling section above. The resulting ∆Eads are shown in Table 1. Overall, the cluster and slab show very similar binding, in agreement with an earlier report that *OCCO strongly prefers to bind to four Cu atoms in a Cu(100)-like square but is quite insensitive to the coordination number of those Cu atoms.64 The cluster ∆Eads show only a weak dependence on ne, whereas the slab models are more stabilized by chemical reduction, again explained by the electric field caused by the Li atom. The effect of this electric field is qualitatively comparable to the effect of the PCM, which also has a clear electrostatic stabilizing influence on the highly polar chemisorbed species. Since we conduct no thermal sampling, ∆Eads is naturally sensitive to the exact water configuration and thus varies somewhat from nH2O = 1–3. However, the overall effect of solvation becomes evident when replacing the few explicit waters with PCM, which clearly stabilizes the chemisorbed state while avoiding sensitivity to specific water configurations. The quite large Cu66 cluster models facilitate near-constant electrostatic potentials Φ (defined by Eq. 2) throughout the chemisorption reaction, for both neutral and singly reduced systems. Compared to the clusters, the smaller unit cell of the slabs results in larger changes in Φ due to chemisorption, although less so in the reduced ne = 1 system


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due to the Li-induced electric field. We stress that these ∆Eads estimates should not be seen as quantitative due to lack of entropy contributions and constant potentials, but that the converged electronic states nevertheless permit a reliable analysis of the electronic structure. Table 1. The electronic properties of chemisorbed *OCCO on cluster and slab models of Cu(100). On Cu66(100) cluster ne {nH2O}

qOCCO [e]

∆Eads [eV]

Φ r/p[eV]a

0 {1}

−0.78

−1.68

4.07/4.00

0 {2}

−0.77

−1.54

4.07/3.96

1 {2}

−0.78

−1.53

1.95/1.85

0 {3}

−0.83

−1.68

4.08/4.01

0 {PCM}

−1.13

−1.99

3.70/3.61

0 {PCM} D3b

−1.25

−3.15

3.69/3.60

On 4×3×4 Cu(100) slab 0 {1}

−0.79

−1.74

4.13/4.67

1 {1}

−0.92

−1.86

2.31/2.46

a

The potential of the reactant/product of Eq. 3, respectively.

b

With D3BJ dispersion.



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Figure 2. A) Optimized *OCCO chemisorbed on a Cu66 cluster as stabilized by three solvating H2O molecules or B) by implicit PCM H2O. C) Gas-phase optimized geometries of the ethylenedione molecule in the respective electronic states: 3OCCO, 2OCCO−, and 4

OCCO−.

The geometry of *OCCO chemisorbed on the Cu66(100) cluster is shown in Figure 2. A net of two covalent surface–adsorbate bonds are formed between the two C atoms and four Cu atoms. The formation of these two bonds require two electrons each from the surface and the adsorbate. According to a spin-uncoupling argument as described in Figure 1 it is thus expected that *OCCO should adopt a structure similar to its vacuum high-spin state. As evident from Table 2 and Figure 2, the chemisorbed geometry indeed closely matches the gas-phase quartet which is the lowest-energy OCCO− state having at least two unpaired electrons available for covalent bonding. The calculated reorganization energy of 4OCCO− between its gas-phase equilibrium and surface-bound geometries is 0.35 eV. Table 2. Geometric coordinates of OCCO as chemisorbed on a Cu66(100) cluster or a Cu(100) slab while solvated by 1 H2O, compared to its anionic states in vacuum. *OCCO-slab

*OCCO-Cu66

2

OCCO−


4

OCCO−

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RCC [Å]

1.53

1.57

1.33

1.43

RCO [Å]

1.26/1.28

1.25

1.26

1.26

AOCC [°]

121/123

122

152.0

134

DOCCO [°]

−1.4

0.3

180.0

0.0

RCCu [Å]

2.09–2.10

2.11–2.12

–

–

The calculated energy cost of exciting 2OCCO− to the high-spin 4OCCO− in the gas phase and in implicit water is 1.44 eV and 1.49 eV, respectively, which corresponds to the spinuncoupling energy Eex(ads) according to the spin-uncoupling picture of Figure 1 and Eq. 1. The quartet state energy can be used to estimate a gross interaction energy Eint, which for acetylene and ethylene on Cu(110) and Cu(100) was previously reported to be about −4.60 eV (53 kcal/mol per Cu–C bond) using the TZVP basis set.34 We use a similar definition, with the following charge-balanced reaction in which the copper cluster remains roughly uncharged throughout the chemisorption reaction (qCu66 = 0.0 and −0.2 e in reactant and product state, respectively) and the adsorbate electronic state is constant: [ ∗(2 H, O)] + ;OCCO3 • (2 H, O) → [ ∗OCCO • (2 H, O)]3 + 2 H, O (4) As per this reaction and Eq. 1, the total interaction energy Eint amounts to −4.42 eV with the TZVP basis set, comparing very closely to the ethylene value above. This agreement is expected since *OCCO bonding to a Cu surface is analogous to that of *C2H4. However, single-point calculations with the significantly larger Def2-TZVP basis set show a smaller Eint of −3.65 eV due to reduced basis-set superposition error (BSSE), mainly thanks to fpolarizing functions on the Cu atoms. A spin-uncoupling reasoning, similar to the one here presented for *OCCO surface adsorption, has previously been used by Zhao et al. to explain the inherent instability of



neutral 1OCCO in the gas phase,65 as follows. Two closed-shell CO molecules must both be electronically excited in order to produce unpaired electrons which are prepared for C=C bond formation. Exciting two CO molecules costs 11.3 eV according to our spinunrestricted DFT calculations, which cannot be compensated for by the formation of a C=C double bond whose strength only amounts to about 6.5 eV.66

Carbon Dioxide Chemisorption Chemisorption of CO2 is the first step in electrocatalytic CO2RR, and it is energetically demanding due to the exceptional thermodynamic stability of 1CO2(g) or 1CO2(aq). The calculated Bader charges of *CO2 in Table 3 indicate that the chemisorbed state corresponds to the anion, analogously to the ethylenedione discussed above. The anionic character is consistent with the experimental need for negative potentials to thermodynamically drive CO2 activation in electrocatalysis. Compared to *OCCO, Table 3 shows that *CO2 is more sensitive to the degree of reduction of the system. The slightly less negative qCO2 with ne = 1 is unintuitive and here attributed to the approximate nature of Bader charges and the fact that this system is open-shell and calculated with unrestricted DFT. The following reaction is used to define ∆Eads(CO2): ∗

[ ()* H, O.]12 3 + CO, • ()* H, O. → 8 ∗CO, • ()* H, O.9

12 3

+ )* H, O (5)

The slab and cluster ∆Eads values with explicit water reported in Table 3 are in broad agreement with a recent study from Cheng, Xiao, and Goddard who employed ab initio metadynamics and found that CO2(physisorbed) to CO2(chemisorbed) was endothermic by 0.39 eV.25 Examining the effect of explicit dispersion in form of the D3BJ model in


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conjunction with PCM solvation, we find chemisorption to be exothermic by a few hundred meV. Similar results were recently found by Garza and Head-Gordon who argued that appropriate descriptions of non-local correlation including dispersion interactions is crucial for exothermic chemisorption.32 Like in the *OCCO case, the cluster ∆Eads fluctuates somewhat as a function of nH2O due to the specific water configurations, but when moving to the more fully solvating PCM, ∆Eads shows a distinct decrease of about 0.4 eV. This can be explained by the strong capacity of PCM to stabilize charged species; our calculations show that CO2 is readily reduced in PCM water with an electron affinity of 3.0 eV whereas the corresponding value in vacuum is −0.8 eV, i.e. endothermic reduction. Although we find positive chemisorption energies without explicit dispersion, all calculations converge to the same stable (local) minimum structure of *CO2 and the electronic structure is analyzed in this configuration. Table 3. The electronic properties of chemisorbed *CO2 on cluster and slab models of Cu(100). *CO2 On Cu66(100) cluster ne {nH2O}

qCO2 [e]

∆Eads [eV]

Φ r/p [eV]a

0 {1}

−0.59

0.62

4.09/4.10

0 {2}

−0.75

0.79

4.09/4.09

1 {2}

−0.68

0.57

1.94/2.00

0 {3}

−0.79

0.59

4.14/4.10

0 {PCM}

−0.95

0.25

3.70/3.72

0 {PCM} D3b

−1.05

−0.26

3.69/3.68

*CO2 On 4×3×4 Cu(100) slab 0 {1}

−0.60

0.56

4.13/4.53

1 {1}

−0.74

0.42

2.31/2.42



a

The potential of the reactant/product, respectively.

b

With D3BJ dispersion.

The optimized structure of *CO2 on the Cu66 cluster is shown in Figure 3. The optimized geometry is practically independent of the type and degree of water solvation as long as some water is present. Without any water conversely, CO2 spontaneously breaks the surface bonds and becomes linear, physisorbed. The bent, chemisorbed geometry has direct analogs in gas-phase CO2, albeit not the neutral ground state but rather the 3CO2, 2CO2−, and 4CO2−, states, as seen in Figure 3C. In the *OCCO case above, we found two covalent bonds to the surface, and a resulting chemisorbed geometry corresponding to the high-spin quartet state. For *CO2 on the other hand, the geometry resembles most closely that of the gas-phase doublet, see Table 4. The reason why *OCCO adopts a quartet-like geometry and *CO2 a doublet-like structure is readily explained with a spin-uncoupling argument. The spin-uncoupling energy cost Eex(ads) is 1.49 eV for OCCO−, a modest energy investment which pays off since Eint is −1.82 eV/bond. For CO2− conversely, Eex(ads) is calculated at 4.94 and 4.95 eV in vacuum and PCM water, respectively (4.27 and 4.32 eV for neutral CO2 singlet-to-triplet excitation), and this energy cost cannot be compensated by an additional surface bond. Adopting the high-spin structure is not an economically sound investment for *CO2!


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Figure 3. A) Optimized *CO2 chemisorbed on a Cu66 cluster as stabilized by three solvating H2O molecules or B) by PCM H2O. C) Gas-phase optimized geometries of the carbon dioxide molecule in the respective electronic states: 1CO2, 3CO2, 2CO2−, and 4CO2−. Table 4. Geometric coordinates of CO2 as chemisorbed on a Cu(100) slab solvated by explicit water, on a Cu66(100) cluster in PCM water, or as singly reduced on a minimal model surface of 2 Cu in PCM, as compared to various electronic states in vacuum. CO2slab

CO2Cu66

CO2Cu2

2

CO2−

4

CO2−

1

RC–O [Å]

1.27/1.29

1.25/1.29

1.26/1.28

1.26

1.36

1.18

AO–C–C [°]

124.2

128.0

126.6

133

111

180

RC–Cu [Å]

2.01

2.07

1.98

–

–

–

RO–Cu [Å]

2.18

2.17

2.22

–

–

–

CO2

We estimate the interaction strength as the energy of the following reaction in which the cluster remains approximately neutral and the CO2 electronic state is constant: ∗

(2 H, O) + CO,3 • (2 H, O) → [ ∗CO, • (2 H, O)]3 + 2 H, O (6)

The total Eint amounts to −2.51 or −2.07 eV with TZVP or Def2-TZVP, respectively. Given that *CO2 effectively binds with a single covalent bond, this is quite comparable to 17 Plus Environment ACS Paragon


the value per Cu–C bond for *OCCO as discussed above. The slight enhancement beyond the typical strength of C–Cu σ-bonds is explained by an additional weaker electronic interaction between the surface-near O and its adjacent Cu which is supported by an elongation of that O=C bond, see Table 4. This interaction was investigated in further detail by calculations on a small model system of CO2Cu2−. Its Bader charge (qCO2 = −1.04 e) and optimized geometry show a noteworthy agreement with the cluster-bound *CO2, see Table 4, indicating that the nature of the bonding is analogous. Figure S3 in the SI depicts those occupied orbitals of CO2Cu2− which are bonding with respect to Cu–C and Cu–O, demonstrating that the overall chemisorption is dominated by Cu–C bonding. The similar interaction energy per Cu–C bond found here for *OCCO and *CO2, as well as reported previously for C2H2 and C2H4,34 is consistent with the “scaling-relations” trend that the strength of a single bond between an atom of a certain element and a metal surface is largely insensitive to what molecule the atom pertains to.67–69 This derives from the fact that chemical bonds are typically highly local in nature whereas the energy cost of attaining the bond-prepared state can depend on properties of much longer range, for example cluster excitation energies.

Conclusions Slab models are commonly favored over clusters for extended surface calculations, despite the convenience of the latter with respect to e.g. implicit solvent models and chemical reduction. Our comparisons of key properties such as adsorbate charges and adsorption energies show slab-vs-cluster differences as small as 0.02 e and 0.06 eV, respectively, as long as conditions are otherwise equal, the clusters are sufficiently large, and finite-size artifacts are systematically corrected for. This supports the reliability of cluster models for heterogeneous catalysis calculations. 18 Plus Environment ACS Paragon

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Our computational investigation shows that chemisorbed ethylenedione and carbon dioxide both correspond to a fully anionic state when solvated by water. This explains the potential-dependent activity and selectivity for these species observed in experiments. Furthermore, the highly polar nature of these adsorbates leads to a strong dependence on the degree of solvation which has been one aspect of a recent debate regarding the endo/exothermicity of CO2 chemisorption,31,32 other aspects being subsurface oxygen,70–72 entropic effects, and non-local correlation. Although similar with respect to charge state, the two chemisorbed species differ in which intramolecular spin state becomes involved when the electrons of the isolated molecule undergo covalent bonding to the metal. Whereas *CO2 exhibits character of the gas-phase doublet, *OCCO geometry clearly corresponds to the quartet spin state. This is explained by the much larger electronic excitation energy of CO2− compared to OCCO−; only for OCCO can the excitation energy investment be compensated for by formation of an additional covalent bond to the surface. 4OCCO− thus forms two covalent Cu–C bonds whereas 2CO2− can only form a single effective covalent bond to the surface with its single unpaired electron. The gross per-bond interaction strength is similar between the two species, and also compares closely to ethylene and acetylene reported previously,34 which is consistent with the traditional scaling-relations concept. Nevertheless, since *OCCO binds with two covalent bonds, its total interaction strength is larger which results in a strong exothermicity of its chemisorption. The spin-uncoupling concept is a useful tool to understand surface bonding in terms of the involved electronic states, and gives a simple prediction of bonding structures and intuitive understanding why e.g. CO typically chemisorbs standing up while C2H2 and O2 lie down on the surface. It further informs on



why some intermediates such as *OCCO are strongly surface-bound on Cu(100) and never observed as free molecules during electrocatalysis. This effect could be exploited by designing catalysts with sites that promote a spin-uncoupled, multiple-bond chemisorbed structure for intermediates which are too weakly bound, or vice versa for too strongly adsorbed species.

Supporting Information Cluster design details, electronic-structure details, figures with optimized slabs, CO2Cu2− model system orbitals, example input files, and optimized xyz coordinates.

Acknowledgement Funding from the Knut and Alice Wallenberg foundation (Grants No. KAW-2013.0020 and KAW- 2016.0042), from the Swedish Research Council through the Swedish Research Links program (Grant No. 348-2013-6723) and from the Swedish Energy Agency (Project 42024-1) is gratefully acknowledged. The calculations were performed using resources provided by the Swedish National Infrastructure for Computing (SNIC) at the HP2CN center.

Author Information *Corresponding Author: [email protected]

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