CO and CO2 Electrochemical Reduction to Methane on Cu, Ni, and

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CO and CO Electrochemical Reduction to Methane on Cu, Ni and CuNi (211) Surfaces 3

Tuhina Adit Maark, and Birabar Ranjit Kumar Nanda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01665 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 5, 2016

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The Journal of Physical Chemistry

CO and CO2 Electrochemical Reduction to Methane on Cu, Ni, and Cu3Ni (211) Surfaces Tuhina Adit Maark* and B. R. K. Nanda Condensed Matter Theory and Computational Lab, Department of Physics, IIT Madras, Chennai 600036, India.

ABSTRACT: The electrocatalytic properties of Cu, Ni and Cu 0.75Ni0.25 alloy are investigated for CO and CO2 reduction to methane by density functional calculations. We show that as Ni content increases in Cu(1-x)Nix (211) surfaces (x = 0, 0.25 and 1) the binding energies (ΔE) of the adsorbates involved in the reaction mechanism decrease. Linear scaling relations are known to exist between ΔEs of adsorbates binding via C (O) atom over pure transition metal surfaces. However, we find that alloying Cu and Ni has the potential for breaking these relations for certain pairs of adsorbates. The decrease in the repulsive coulombic interaction between the adsorbate and the charges induced on the Cu-Ni alloy surface explains the adsorption site preference. The ΔE shift with respect to pure Cu is larger for species binding through C than O. Various trends exhibited by the binding energies are understood by analyzing the chemical bonding through local density of states and charge density isosurfaces of the bare and adsorbed surfaces. The free energy profile for CO and CO 2 reduction to CH4 on the alloy surface is a mix of its behavior on Cu and Ni (211). Our calculations predict that CH4 generation directly from CO reduction on Cu0.75Ni0.25 (211) can

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occur at an earlier applied potential than required for Cu and Ni (211) surfaces. However, it will be the opposite case for CO2 reduction to CH4.

1. INTRODUCTION Electrochemical reduction is an attractive strategy for decreasing carbon CO 2 emissions by converting it to a hydrocarbon based fuel over a heterogeneous catalyst. Among the known electrocatalysts, pure copper electrodes are by far the best for this reaction due to their high yields and selectivity of hydrocarbon production.1 The overall CO2 reduction reaction to methane (CH4) in gas phase: CO2 + 4 H2 → CH4 + 2 H2O, ΔG = -132.4 kJ mol-1

(1)

should be observed at +0.17 V with respect to the reversible hydrogen electrode (RHE) as the reference electrode.2 However, experimentally on an electrocatalyst surface such as Cu the potential at which CH4 starts getting produced (known as the onset potential) occurs much later at ~ -0.8 V with respect to RHE. 3 The overpotential, which is the difference between the equilibrium and onset potentials, is thus quite significant for Cu (~ 1 V) and extra energy needs to be supplied to drive the reaction. 4 In a novel study employing density functional theory (DFT) Peterson et al. 5 examined a network involving 41 different intermediate steps to identify the lowest energy pathway for the electrochemical conversion of CO2 to CH4 on Cu (211) surface as: CO2 + * + (H+/e-) → COOH* (2) COOH*+ (H+/e-) → CO* + H2O (3) CO*+ (H+/e-) → CHO* (4) CHO*+ (H+/e-) → CH2O* (5) 2


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CH2O*+ (H+/e-) → CH3O* (6) CH3O*+ (H+/e-) → O* + CH4 (7) O*+ (H+/e-) → OH* (8) OH*+ (H+/e-) → * + H2O (9)

where * represents an active site on the electrode surface and CaHbOc* (where a = 0 or 1, b = 0-4, and c = 0-2) is a species adsorbed on this site. The authors further determined that the large positive free energy change of equation (4) i.e. the protonation of CO* to CHO* was responsible for the experimentally observed high overpotential for CO 2 reduction. In another DFT based work various transition metals were investigated as CO2 reduction electrocatalysts.6 The authors found that the binding energies (∆E) of intermediates in the reaction mechanism binding via C (O) were linearly related to each other. For instance, ∆ECHO vs. ∆ECO plot was a straight line with a slope of 0.88. As a result the CO2 reduction electrocatalytic activity, which would otherwise have been a function of ∆E of eight intermediates, can be described as a function of ∆ECO and ∆EOH. However, due to these 'linear scaling relations' an increase or decrease in ∆ECO (∆EOH) is likely to be accompanied by a simultaneous increase or decrease in ∆E of all other C- (O-) centered molecules, so that any enhancement of the activity of metal catalysts gets nullified. Thus, a pressing need is to find a strategy that independently modifies the binding energies of key intermediates. In this regard using transition metal alloys as catalysts is an interesting route. Kim et al. synthesized and examined Au (1-x)Cux (x = 0.25, 0.5 and 0.75) alloys and found that the fraction x governed the product selectivity in CO 2 reduction.7 Increasing x shifted the preference from CO to CH4 generation. In another new experimental study Cu-Pt alloy nanocrystals composed 3



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of various Cu:Pt ratios (1:2. 1:1, 2:1. 3:1 and 5:1) were synthesized. 8 As concentration of Pt was decreased and of Cu increased in these nanocrystals, competition from H 2 production decreased and the selectivity for CH 4 generation increased until a Cu:Pt ratio of 3:1 was reached. Cu3Ag and Cu3Au (211) surfaces in which Cu atoms present at the edge in the top layer are substituted by Ag and Au atoms have also been studied theoretically. 9 Though a slightly lower overpotential for CO2 electroreduction to CH 4 was evaluated for Cu 3Ag vs. Cu (211), it was also predicted to have a greater preference for producing H 2. On Cu3Au (211) a much higher overpotential was calculated and it was found unsuitable as an electrocatalyst for the same reaction. Recently, a Cu-In alloy surface has been reported to display high selectivity for the electrochemical conversion of CO 2 to CO with a low overpotential. 10 DFT calculations performed in combination showed that the In atom preferred to substitute a Cu edge atom of the (211) surface. In their seminal work Hori et al. have experimentally shown that Cu and Ni differ in their selectivity for CO2 reduction products: at sufficiently negative applied potentials (~ -1 V vs. RHE) CH4 formation is favored on Cu and H 2 generation takes place preferentially on Ni. 1 In this work we have compared the electrocatalytic properties of Cu (1-x)Nix (x = 0, 0.25 and 1) (211) surfaces (also referred to as Cu, Cu 3Ni and Ni (211)) for the production of CH 4 by carrying out DFT calculations. The main objectives are to develop a fundamental understanding of the effect of alloying on (a) the preference of adsorption sites by different adsorbates, (b) binding energies of species participating in the reaction mechanism, and (c) free energy profile behavior when using CO and CO 2 as individual starting reactants for CH 4 production. 4


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We show that alloying results in the formation of charged centers of the type Cu δm+ and Niδnon Cu3Ni (211). The change in adsorption site preference on alloying is explained in terms of reduction of the repulsive interaction between the adsorbates and the induced charges on the surface. The strengthening of binding energies with increase in Ni content in Cu (1-x)Nix is correlated to the shifting up of the d-band center of the surface. The larger shift in ΔEs of Ccentered adsorbates than O-centered adsorbates on Cu 3Ni (211) relative to Cu (211) surface is understood from the increase in number of empty antibonding states. Based on the free energy profiles CO reduction to CH 4 on Cu3Ni (211) is predicted to have a much earlier onset potential (vs. RHE) of -0.54 V than Cu (211) at -0.68 V and Ni (211) at -0.84 V. However, we expect it to be associated with a higher overpotential than Cu (211) for CO 2 reduction and have a selectivity for H2 production similar to Ni (211).

2. COMPUTATIONAL AND THEORETICAL DETAILS: All calculations were performed taking into account spin polarization using the Quantum Espresso program package11 based on DFT, planewaves and pseudopotentials. PerdewBurke-Ernzerhof (PBE) exchange-correlation functional was employed along with ultrasoft pseudopotentials. The PBE-lattice constants were determined to be 3.672, 3.624 and 3.509 Å for Cu, Cu3Ni and Ni, respectively via variable cell relaxation at 8 × 8 × 8 k-points. Pure metal and alloy fcc (211) surfaces were constructed as slabs of (3 × 3) unit cells with six layers of metal atoms and 20 Å vacuum between neighboring slabs. Of these six layers, the bottom four layers were constrained and only the top two layers along with the adsorbates were allowed to relax to take into account surface disorder. Figure 1(a) depicts the side view of a Cu (211) surface. In the top layer, atoms belonging to column '1' form the edge while the 5



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atoms belonging to columns '2' and '3' are located at steadily lower heights such that the surface resembles a step. Geometry optimizations of all the surfaces were carried out at 4 × 8 × 1 k-points, planewave cutoff of 33 Ry, density cutoff of 264 Ry/Bohr3, electronic minimization criterion of 1.0e -6 Ry, and convergence threshold for forces during ionic minimization of 0.004 Ry/Bohr.

Figure 1. Side view of (a) Cu (211), (b) Cu 3Ni (211) surface-(i), and (c) Cu 3Ni (211) surface(ii). Atoms belonging to column '1' in the top layer form the edge in each (211) surface. In(b) Ni atoms are located in column '2' while in (c) Ni atoms are present in columns '1' and '3' of the top layer. A schematic of the five unique adsorption sites of Cu (211) is provided in (d), where X represents an arbitrary adsorbate. Brown and gray spheres are Cu and Ni atoms, respectively.

As mentioned in the Introduction section, binding energies of C-centered intermediates (COOH, CO, CHO, and CH2O) have been found to be linearly related to each other on transition metals.4 Due to these linear scaling relations ∆ECOOH for instance can be expressed in terms of ∆ECO. As a consequence ∆ECO can serve as a descriptor of the observed CO 2 to CH4 reduction activity trends on pure transition metals. This results in a “volcano” activity plot,

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with the peak in activity occurring at a particular ∆ECO and the activity decreasing on each of its sides for stronger and weaker values. Cu is known to be close to the volcano peak, while Ni has a comparatively lower (i.e. more negative) binding energy. It is to be noted that two (211) surfaces can be generated for Cu 3Ni. In surface-(i) (see Fig. 1(b)) Ni atoms are located in column '2' and in surface-(ii) (see Fig. 1(c)) they are present in columns '1' and '3' of the top layer. We found that CO adsorption on surface-(ii) (-0.15 eV) was stronger than on surface-(i) (0.06 eV) and Ni (211) (-0.08 eV). Thus, we expect it to have an even poorer CO 2 reduction activity than Ni and therefore, for all further analyses we have modeled Cu 3Ni (211) only as surface-(i). All the ∆Es were calculated using total energies of ½H 2 molecule, ½graphene (unit cell of graphene contained two C atoms) and (H 2O – H2) to reference each H, C, and O atom, respectively. A pathway alternative to the one outlined in equations (2)-(9) in which CO* is protonated to COH*, which subsequently leads to formation of C* and then CH y* (y=1-3) has been proposed for Cu (111) for CO2 electroreduction to CH4 by Nie et al.12 based on evaluation of activation energy barriers of the elementary steps. However, binding strength and preferred adsorption sites of intermediates differ with the surface morphology. ∆ECOH < ∆ECHO on Cu (111) by 0.12 eV12 and > ∆ECHO on Cu (211) by 0.81 eV4. Therefore, we do not expect the magnitudes and orders of activation barriers to be the same on (111) and (211) surfaces of pure metals and their alloys. Furthermore, Peterson et al. 5 have shown than CO* to COH* step would be unfavorable compared to CO* to CHO* step for surfaces with ∆ECO between that of Cu and Ni(211). Therefore, in this work the reaction mechanism as outlined in equations (2)-(9) has been considered for CO2 reduction to CH4 on Cu(1-x)Nix (211) surfaces.

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The computational hydrogen electrode 13 was employed to compute the reaction free energies (∆Greac) of each electrochemical step outlined in equations (2)-(9) as follows: ∆Greac = ∆Ereac + ∆EZPE + ∆∫CPdT - T∆S + ∆GU

(10)

In equation (10) ∆Ereac is the reaction energy and ∆EZPE, ∆∫CPdT, and T∆S are the net zeropoint energy, heat capacity and entropic corrections, respectively. In this work ∆Ereac also includes correction for solvation effects. The values of the above mentioned corrections for all the systems (gas molecules and adsorbed surfaces) were taken from Refs. 4 and 8. The term ∆GU = eU introduces the dependence on the electrical potential U. Herein, we have taken U = 0 V and so this term does not have any effect in our study. Within the computational hydrogen electrode, U is the potential with respect to RHE and therefore, correction for pH is not required. Further, this model also allows each (H +/e-) pair to be referenced as ½H2 (g). Induced atomic charges on individual atoms in the bare and adsorbed surfaces considered herein were calculated using a program developed by Henkelman et al. 14,15 based on the Bader partitioning scheme16. Local density of states were computed using the tetrahedron method17 at denser 8 × 12 × 1 k-points mesh. Iso-surface charge density plots were visualized using VESTA 3.3.218,19 and the geometrical structures were illustrated using XCrysDen 1.6 20,21

.

3. RESULTS AND DISCUSSION: 3.1 Adsorption sites (a) Cu and Ni (211) surfaces: The stepped Cu (211) surface has five unique adsorption sites, namely, ontop, bridge, threefold bridge, three-fold behind and four-fold. These are schematically illustrated in Fig. 1(d). 8


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Adsorption of every intermediate involved in the reaction mechanism of CO 2 to CH4 reduction outlined in equations (2-9) was considered at each of these sites on Cu (211) in order to determine the corresponding minimum energy sites which are displayed in Fig. 2(a). All the species have been assumed to have the same minimum energy sites over Ni (211).

(b) Cu3Ni (211) surface: The presence of Ni atoms in column '2' of the top layer of Cu 3Ni (211) (surface-(i) in Fig. 1(b)) creates additional sites at which adsorption can take place. That is why, for instance, we considered a vertical adsorption of CO through C both at a Cu atom located in column '1' and over a Ni atom in column '2' of the top layer (for labeling refer to Fig. 1(b)). The sites yielding the minimum energies for each adsorbate displayed in Fig. 2(b) were finally considered for the calculation of binding energies for Cu 3Ni (211). Comparing Figs. 2(a) and (b), it can be seen that the most preferred binding sites for CH 3O (bridge), O (three-fold bridge) and OH (bridge) are the same on Cu and Cu 3Ni (211). Each of these species bind to the (211) surfaces only through O. We have also studied adsorption of a H atom in regard to the competing hydrogen evolution reaction. H atom also remains positioned at the three-fold bridge site on both the surfaces. Interestingly, those adsorbates which adsorb only through C (e.g. CO) or simultaneously through C and O (COOH, CHO and CH2O) undergo a change in their most favored adsorption sites on Cu 3Ni (211) and in general prefer to adsorb on the Ni atom present in column '2' of the top layer (for labeling refer to Fig. 1(b)). The above adsorption site preference is explained in detail in Section 3.3 in terms of charges induced over Ni and Cu atoms (Ni δm-, Cuδn+) in the Cu3Ni (211) surface.

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Figure 2. Reaction mechanism of CO2 to CH4 reduction on (a) Cu and Ni (211) and Cu 3Ni (211) with the different species adsorbed at their minimum energy sites. Brown, gray, yellow, red and blue spheres are Cu, Ni, C, O and H atoms, respectively. 10


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3.2 Binding energies Figure 3 illustrates the variation of binding energies of the adsorbates positioned at their minimum energy sites as a function of the fraction of Ni content in Cu (1-x)Nix (211) surfaces. The PBE ∆Es of all the species on Cu (211) calculated from this work showed a good agreement with those reported in Ref.

8

obtained using the same exchange-correlation

functional but with the Projector Augmented Wave method, 5 × 5 × 1 k-points and 30 Ry cutoff. This lends confidence to our calculations.

Figure 3. Binding energies of adsorbates participating in the reaction mechanism of CO 2 to

CH4 reduction as a function of fraction of Ni content in Cu (1-x)Nix (211) surfaces. PBE calculated ∆Es on Cu (211) reported in Ref.

8

are as follows: COOH* = 1.04 eV, CO* = 0.93

eV, CHO* = 1.43 eV, CH2O* = 0.83 eV, CH3O* = -0.27 eV, O* = 0.77 eV, and OH* = -0.28 eV.

In general, on all the three surfaces ∆Es of (a) H, OH and CH3O are negative and the strongest, (b) CHO are positive, large and the weakest and (c) O, CH 2O, CO and COOH are positive but of medium strength. Specifically, in a set of related adsorbates as percentage of H in the adsorbate decreases ∆E weakens i.e. shifts from negative to positive values. e.g. ∆E

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of OH < O and CH3O < CH2O < CHO. Overall the ∆Es of all adsorbates on Cu3Ni (211) are less than on Cu (211) but greater than on Ni (211), however, the trend is non-linear. It is apparent that certain adsorbates are more affected and others less by alloying than what is expected based on a linear variation with respect to the fraction of Ni. Quantitatively, the shift (in eV) in ∆Es of Cu3Ni (211) relative to Cu (211) is as follows: CO (-0.81) > CHO (-0.63) > CH2O (-0.50) > COOH (-0.36) > O, H (-0.29) > OH (-0.12) > CH 3O (-0.02). This trend indicates that binding energies of adsorbates binding through C or C and O are more strongly modified by alloying than of those binding via only O. Furthermore it is interesting to note that the magnitudes of ∆E shift of the set of adsorbates binding via C or O atoms are not all the same. For instance, the change in ∆ECOOH is only ~ 0.4 times the change in ∆ECO. This is significantly different from the ∆ECOOH vs. ∆ECO slope of 0.73 established from the well-known linear scaling relationships in Ref. 4. ∆ECH3O exhibits a shift that is only 0.17 times the shift of ∆EOH, while the ∆ECH3O vs. ∆EOH slope of 0.974 predicted from the linear scaling relations is much higher. Thus, alloying Cu and Ni to form Cu 3Ni (211) has the potential for getting off these linear scaling relations between binding energies of key adsorbates for pure transition metals.

3.3 Understanding adsorption site preference and binding energy trends (a) Adsorption site preference: We have highlighted in Section 3.1(b) that species binding through C or C and O atoms undergo a change in adsorption site preference on Cu 3Ni (211) compared to Cu (211), while adsorbates binding via O remain adsorbed at the same position on both the surfaces. In order to understand this we calculated the atomic charges in the bare surfaces from Bader analysis. 12


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These are listed in Table 1 for selected centers in the top layer of the surfaces. It can be seen that in both Cu and Ni (211) the Cu and Ni atoms are neutral. Alloying in Cu 3Ni (211) induces marginal charges on the atoms: in the top layer Ni (2-a) acquires a negative charge of -0.082 e and the Cu atoms in its vicinity develop a positive charge (~ +0.040 e) implying a charge transfer from Cu to Ni.

Table 1. Bader atomic charges at selected centers over Cu, Ni and Cu 3Ni (211) surfaces. Negative and positive charges are induced over Cu and Ni atoms, respectively in Cu 3Ni (211) surfaces, which affect the binding energies depicted in Fig. 3. It is to be noted that the listed centers belong to the top layer alone and that overall Cu 3Ni (211) is a neutral surface. Structure

Atom label

Bader atomic charges Cu (211)

Ni (211)

Cu3Ni (211)

1-a

0.010 (Cu)

0.010 (Ni)

0.035 (Cu)

1-b

0.010 (Cu)

0.010 (Ni)

0.037 (Cu)

2-a

0.007 (Cu)

0.008 (Ni)

-0.082 (Ni)

2-b

0.007 (Cu)

0.008 (Ni)

0.037 (Cu)

3-a

-0.009 (Cu)

-0.008 (Ni)

0.004 (Cu)

3-b

-0.009 (Cu)

-0.010 (Ni)

0.006 (Cu)

Based on the electronegativity values we expect the electron density in a bond to be shifted towards O and away from C, such that the central O in O, OH and CH 3O will have a negative charge while the C end in COOH, CO, CHO, and CH 2O will be positive. Thus, it is 13



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understandable that adsorbates binding through O prefer to remain located at the sites (threefold bridge, bridge and bridge, respectively) which are primarily formed by the positively charged Cu centers in Cu 3Ni (211). In contrast C-centered molecules (e.g. CO, COOH, etc) would want to be located at sites involving the negatively charged Ni center to minimize the repulsive interaction between charges having the same sign.

Figure 4. Sum of local density of states (LDOS) for the up and down spin for the d-band of selected atoms of Cu, Cu 3Ni and Ni (211) surfaces. The d-band shifts up as x increases from 0 to 0.25 to 1 in the Cu (1-x)Nix systems. For labeling of atoms refer to Table 1. The energies are referenced to the Fermi energy (Ef) for each surface.

(b) Surface-dependent binding energy trend: It has been illustrated in Fig. 3 that in general ∆Es of all adsorbates decrease on increase of fraction of Ni from 0 to 1 in the Cu (1-x)Nix (211) surfaces. In this section we understand this trend based on the d-band theory22,23 for transition metals. According to this theory the order of adsorption energies of molecules across transition metals is dictated by the interaction of the adsorbate bonding and antibonding states with the metal d electrons: specifically as the d14


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band shifts up in energy, the binding energy becomes stronger. Figure 4 displays the sum of local density of states (LDOS) for up and down spins for the d-band of selected Cu and Ni atoms in the top layer of Cu, Cu3Ni and Ni (211) surfaces (for labeling refer to Table 2). The shifting up of the d-band closer to the Fermi level on going from Cu to Cu 3Ni to Ni (211) is evident from the figure, which based on the d-band theory explains the overall binding energy trend with respect to increasing fraction of Ni in the underlying substrate.

Figure 5. Charge density surfaces of (a) CHO/Cu (211), (b) CH 2O/Cu (211) and (c) CH3O/Cu (211) plotted at iso-value = 0.09 Ry/Bohr 3, which is ~1/15th of the maximum charge density of each surface. A decrease in covalent character of the C-Cu bond occurs on going from (a)(c).

(c) Binding energy trend in related adsorbates: A previous DFT study has shown that for a particular adsorbate, such as O or CO, as average coordination number of metal atoms at a binding site increases with the change in surface morphology from (111) to (211) to a 13-atom cluster, the binding energy of the adsorbate increases.24 We had pointed out in Section 3.2 that irrespective of the surface the ∆Es of the adsorbates increase (i.e. become positive) as number of H atoms decrease. For instance, ∆ E

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of OH < O and CH3O < CH2O < CHO. The minimum energy sites for OH and O are the bridge and three-fold bridge sites, respectively corresponding to a coordination number of two and three metal atoms. As this coordination number is maintained on all the three surfaces examined herein, ∆E of OH is consistently less than of O.

Table 2. Atomic charges induced over selected species in the CH nO/Cu (211) (n=1-3) systems. For labeling used for Cu atoms refer to Table 1. O binds to Cu (1-b) in CHO* and CH2O* and both to Cu (1-a) and (1-b) in CH 3O*. Centers through which the adsorbate is binding are in bold. Value in {} is the charge on O available in each adsorbate for interaction with the metal surface. This available charge increases from CHO* to CH 3O* implying an increased ionic interaction. Bader atomic charges (e) CHO/Cu (211)

CH2O/Cu (211)

CH3O/Cu (211)

Cu (1-a): 0.137

Cu (1-a): 0.070

Cu (1-a): 0.285

Cu (1-b): 0.240

Cu (1-b): 0.258

Cu (1-b): 0.274

O: -1.657 {-0.319}

O: -1.765 {-0.345}

O: -1.382 {-0.508}

C: 1.216

C: 1.281

C: 0.699

H: 0.122

H: 0.129

H: 0.068

H: 0.01

H: 0.050 H: 0.057

On Cu (211) overall two metal atoms coordinate to each CH nO (n = 1-3) adsorbate at its most favored site. Figure 5 depicts the charge density surfaces of the analogous CH nO/Cu (211) 16


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systems at the same iso-value. It is evident that there is a decrease in covalent character in the bonding between C and Cu atoms on going from CHO* to CH 2O* to CH3O*. This is accompanied by an increased negative charge available over the O centers for an enhanced ionic interaction with the Cu centers (see Table 2). As typically ionic bonds are more difficult to break than covalent bonds the ∆E order exhibited by this series of adsorbates is understandable. The same electronic properties were depicted by these adsorbates on Cu 3Ni and Ni (211) surfaces and so are not shown here.

(d) Adsorbate-dependent binding energy trends: Figure 3 showcases that ∆Es of O (C)-centered adsorbates are generally stronger (weaker) and shift less (more) on alloying relative to pure Cu. To understand these two trends we have chosen to analyze LDOS of representative cases: CH 3O* and CO*. On Cu (211) CH 3O and CO adsorb at the bridge site, but the former binds via O and the latter through C. It can be seen from Fig. 6 that just below the Fermi level O p states of the adsorbate hybridize with the Cu (1-a) d states of the metal atoms forming the bridge site in CH 3O/Cu (211). In comparison there is little interaction between the C p states of CO and Cu (1-a) d states of the substrate. As a result we expect CH 3O to adsorb more strongly than CO on Cu (211) and have a more negative ∆E. A similar LDOS picture was found in case of CH 3O and CO adsorption of Cu 3Ni and Ni (211) and so the corresponding figures are not shown here.

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Figure 6. Sum of LDOS for up and down spin states of Cu d, O p and C p states of CH3O/Cu (211) and CO/Cu (211). Both species are adsorbed at the bridge site. For labeling on atoms refer to Table 1. The energies are referenced to the Fermi energy (E f) for each surface. In the valence band O p states overlap with Cu d states in CH3O/Cu (211). No such interaction occurs between C p and Cu d states.

Figure 7 depicts the LDOS plots of O p and metal d states of CH3O/Cu(1-x)Nix (211) and of C p and metal d states of CO/Cu(1-x)Nix (211). The metal atoms correspond to the sites at which the species are adsorbed. Recalling from Figs. 2(a) and (b), CH 3O on all the three surfaces and CO on Cu and Ni (211) adsorb at the bridge site between (1-a) and (1-b) atoms, while CO on Cu3Ni (211) binds to the Ni (2-a) atom. It can be seen from the figure that the LDOS of O p and metal (1-a) d states of CH3O/Cu (211) and CH3O/Cu3Ni (211) are similar. In comparison the same are shifted up for CH 3O/Ni (211) such that there is a slight increase in 18


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the number of antibonding states which lie above the Fermi level. The figure also shows that in contrast the LDOS of C p and metal d states of CO/Cu3Ni (211) are similar to those of CO/Ni (211). They are also up-shifted compared to the analogous LDOS plots for CO/Cu (211) leading to a significant increase in the number of empty antibonding states. According to the d-band theory22, 23 interaction of the adsorbate states with d electrons of the transition metals leads to generation of new bonding and antibonding states in the adsorbate-metal surface system. Furthermore, the bonding is stronger if the antibonding states are shifted above the Fermi level and they become more empty. This thereby explains why binding energy of CH3O/Cu3Ni (211) shows a smaller shift relative to CH 3O/Cu (211) while that of CO/Cu3Ni (211) exhibits a larger shift with respect to CO/Cu (211).

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Figure 7. Comparing sum of LDOS of up and down spins of O p and metal d states of CH3O/Cu(1-x)Nix (211) (x = 0, 0.25, 1) with C p and metal d states of CO/Cu(1-x)Nix (211) (x = 0, 0.25, 1). The energies are referenced to the Fermi energy (E f) for each surface. For labeling on atoms refer to Table 1. LDOS of CH 3O/Cu3Ni (211) are similar to CH3O/Cu (211) and of CH3O/Ni (211) are slightly up[shifted. LDOS of CO/Cu 3Ni (211) are similar to CO/Ni (211) and are up-shifted relative to LDOS of CO/Cu (211). The up-shift leads to greater number of empty of antibonding states and therefore stronger binding energies.

3.4 Free energy profile for CO2 reduction to CH4 The experimental onset potential can be estimated theoretically as -ΔGmax/e, where ΔGmax is the largest positive or uphill free energy change among all the steps in the reaction mechanism. The corresponding step is thus called the potential limiting step and the predicted onset potential is referred to as the limiting potential. The free energy profiles at U = 0 V vs. RHE for CO2 reduction to CH4 on Cu, Cu3Ni and Ni (211) surfaces are illustrated in Fig. 8(a). It can be seen from the figure that on Cu (211) the first step i.e. the adsorption of COOH itself is uphill. The only other uphill steps are (a) CHO* formation on protonation of CO*, (b) protonation of CH3O* to yield O* and CH4 and (c) OH* protonation. Of all the four steps, CO* to CHO* step is the potential limiting step with a ΔGmax of 0.68 eV, so that the limiting potential is -0.68 V vs. RHE. This is in reasonable agreement with the experimental onset potential of ~ -0.8 V vs. RHE reported for CH4 formation from CO2 on a Cu electrode.1 Compared to Cu (211), on Ni (211) (i) the first step (equation (1)) is predicted to occur with a ΔG of ~ 0 V, (ii) the protonation of CH3O* becomes downhill in free energy, (iii) protonation of CHO* to CH 2O*

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is associated with a positive ΔG, and (iv) protonation of COOH* to form CO* and H 2O is very strongly downhill (ΔG of -0.76 eV vs. -0.31 eV on Cu (211)). However, ΔGmax corresponds to the final step i.e. protonation of OH* so that the calculated limiting potential is 0.84 V vs. RHE. Interestingly the behavior of the free energy profile of Cu 3Ni (211) is a direct reflection of the differences in binding energy responses of the adsorbates on alloying Cu and Ni as discussed in the earlier sections. As the species binding through C or C and O atom are more affected, the ΔGs of the equations (2-7) involving these adsorbates change considerably and are closer in magnitudes and of similar sign for Ni (211). Since the binding energies of adsorbates interacting with the surface through O atom alone do not change significantly, the ΔGs of the final two steps (equations (8-9)) over Cu 3Ni (211) are similar to that of Cu (211). Consequently, the step with ΔGmax on Cu3Ni (211) is the same as on Cu (211): protonation of CO* to CHO*. However, due to the stronger stabilization of CO* with respect to CHO* the onset of CO2 to CH4 reduction is predicted to occur later at -0.86 V vs. RHE.

3.5 Competition from hydrogen evolution reaction Hydrogen evolution reaction is known to be a competing reaction for CO 2 reduction to CH4 and in electrochemical experiments H 2 is the first product on Cu and Ni. The HER mechanism is as follows: * + (H+/e-) → H* (11) H* + (H+/e-) → H2 + * (12) Our calculated ∆G(equation (11)) at U = 0 V vs. RHE are: 0.05 eV for Cu (211), -0.34 eV for Cu3Ni (211) and -0.35 eV for Ni (211). Thus, the subsequent release of H 2 will be downhill in free energy for Cu (211) and uphill for Cu 3Ni and Ni (211) by the same magnitudes. 21



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Therefore, hydrogen production will get activated on Cu, Cu 3Ni and Ni (211) on application of U = -0.05, -0.34 and -0.35 V vs. RHE, respectively. The experimental work of Hori et al. 1 has demonstrated that H2 evolution preferentially occurs on Ni instead of CH 4 formation. The similarity between the ∆G(equation (11)) and closeness of the binding energies of CO* and CHO* on Cu3Ni (211) (refer to Fig. 3) with the corresponding values on Ni (211) suggests that Cu3Ni (211) will behave like Ni (211) and similarly exhibit a higher selectivity for H 2 generation over CO2 reduction to CH4 even at reasonably large negative potentials.

Figure 8. Free energy profile at U = 0 V vs. RHE for (a) CO2 to CH4 reduction and (b) CO to CH4 reduction on Cu, Cu 3Ni and Ni (211) surfaces. In (b) Dashed line corresponds to the pathway in which CO first adsorbs as CO* which then undergoes six (H +/e-) transfer steps and

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the continuous line corresponds to the pathway in which the first step itself involves a (H +/e-) transfer to form CHO*.

3.6 Free energy profile for CO reduction to CH4 In this section we investigate the direct reduction of carbon monoxide to methane to examine whether CO* protonation to form CHO* can be circumvented as it is the most uphill step for Cu and Cu3Ni (211) and the next most uphill step for Ni (211) in the CO 2 reduction mechanism. Furthermore, as CO is a poisonous gas, converting it into a fuel would also be a useful reaction. We consider two pathways for the reaction mechanism for CO reduction to CH4: (1) The first step could be adsorption of CO to form CO* without any (H +/e-) transfer, which is followed by equations (3-8) of the CO 2 reduction mechanism. (2) Alternatively, the first step may involve a (H+/e-) transfer such that gaseous CO directly forms CHO*. The subsequent steps are the same as equations (4-8). The corresponding free energy profiles at U = 0 V vs. RHE are plotted together in Fig. 8(b). It is clear from the figure, in the first pathway CO* formation is strongly downhill in free energy on all the three surfaces. In this scenario CO* to CHO* step will remain as the most uphill step in free energy on Cu and Cu 3Ni (211) and OH* protonation will be the same on Ni (211). In the second pathway, the protonation of CO to CHO* is uphill in free energy by 0.22 eV on Cu (211) and downhill by -0.41 and -0.67 eV on Cu3Ni and Ni (211), respectively. We therefore, expect CO reduction on Cu (211) to take place perhaps only via the first pathway and the limiting potential (-0.68 V) to be unchanged compared to CO2 reduction. The downhillness of CO to CHO* step on Cu 3Ni and Ni (211) suggests that CHO* could be a meta-stable state on these surfaces. In this case, CO

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reduction may proceed via the second pathway and OH* protonation would become the potential limiting step on Cu3Ni as well. Though the limiting potential on Ni (211) would remain unchanged (-0.84 V), on Cu 3Ni (211) it would increase to -0.54 V vs. RHE compared to -0.68 V on Cu (211) and lead to enhanced catalysis. However, we note that the adsorption of CO is more favorable on Cu3Ni (211) than direct formation of CHO* and so it may be susceptible to CO poisoning similar to Ni (211).

4. CONCLUSIONS: Conversion of CO and CO2 to a usable fuel such as methane is of tremendous interest. In this work we computationally studied the effect of alloying Cu and Ni in 3:1 ratio on the electrocatalytic activity for the above mentioned reduction reactions in comparison to the constituent pure transition metals. We found that presence of this small fraction of Ni was sufficient to shift the adsorption site preference and modify the binding energies such that the well known linear scaling relationships linking adsorbates binding through C or O centers were broken. We developed an understanding of the nature of adsorption and ΔE trends via analysis of the electronic structure in terms of atomic charges, charge densities and local density of states of the surfaces without and with adsorbates. The variation in binding energies of key adsorbates due to alloying was reflected in the free energy profile behavior of the reaction mechanisms. Based on the modification in the free energy change associated with the potential limiting step, we predicted that the Cu0.75Ni0.25 alloy will exhibit enhanced electrocatalytic properties for CO reduction to CH 4 due to its reduced overpotential compared to pure Cu and Ni. However, the opposite will occur for CO 2 reduction to CH4.

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AUTHOR INFORMATION Corresponding Author *Email: [email protected] Phone: +91-44-22574887 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT TAM would like to acknowledge IIT Madras for her Institute Postdoctoral Fellowship. The authors acknowledge the Computer Center, IIT Madras for providing computational facilities.

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3. Hori, Y.; Murata, A., Takahashi, R. Formation of Hydrocarbons in the Electrochemical Reduction of Carbon Dioxide at a Copper Electrode in Aqueous Solution. Faraday Trans. 1 1989, 85, 2309-2326. 4. Hori, Y. Electrochemical CO2 Reduction on Metal Electrodes. In Modern Aspects of Electrochemistry; Springer: New York, 2008; Vol. 42, Chapter 3, pp 89−189. 5. Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Norskov, J. K. How Copper Catalyzes the Electroreduction of Carbon Dioxide into Hydrocarbon Fuels. Energy Environ. Sci. 2010, 3, 1311-1315. 6. Peterson, A. A.; Norskov, J. K. Activity Descriptors for CO 2 Electroreduction to Methane on Transition-Metal Catalysts. J. Phys. Chem. Lett. 2012, 3, 251-258. 7. Kim, D.; Resasco, J.; Yu, Y.; Asiri, A. M.; Yang, P. Synergistic Geometric and Electronic Effects for Electrochemical Reduction of Carbon Dioxide using Gold–Copper Bimetallic Nanoparticles. Nature Commun. 2014, 5. 8. Guo, X.; Zhang, Y.; Deng, C.; Li, X.; Xue, Y.; Yan, Y. M.; Sun, K. Composition Dependent Activity of Cu–Pt Nanocrystals for Electrochemical Reduction of CO 2. Chem. Commun. 2015, 51, 1345-1348. 9. Hirunsit, P. Electroreduction of Carbon Dioxide to Methane on Copper, Copper–Silver, and Copper–Gold Catalysts: A DFT Study. J. Phys. Chem. C 2013, 117, 8262-8268.

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10. Rasul, S.; Anjum, D.H.; Jedidi, A.; Minenkov, Y.; Cavallo, L.; Takanabe, K. A Highly Selective Copper–Indium Bimetallic Electrocatalyst for the Electrochemical Reduction of Aqueous CO2 to CO. Angew. Chem. 2015, 127, 2174-2178. 11. Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I. et al. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys: Condens. Matter 2009, 21, 395502. 12. Nie X, Esopi MR, Janik MJ, Asthagiri A. Selectivity of CO 2 Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps. Angew. Chem. Intl. Ed. 2013, 52, 2459-2462. 13. Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L. R. K. J.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a FuelCell Cathode. J. Phys. Chem. B 2004, 108, 17886-17892. 14. Tang, W.; Sanville, E.; Henkelman, G. A Grid-Based Bader Analysis Algorithm without Lattice Bias. J. Phys.: Condens. Matter 2009, 21, 084204. 15. Bader

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17. Blochl, P. E.; Jepsen, O.; Andersen, O. K. Improved Tetrahedron Method for Brillouin Zone Integrations. Phys. Rev. B 1994, 49, 16223. 18. Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272-1276. 19. VESTA can be downloaded at http://jp-minerals.org/vesta/en/download.html. 20. Kokalj, A. Computer Graphics and Graphical User Interfaces as Tools in Simulations of Matter at the Atomic Scale. Comp. Mater. Sci. 2003, 28, 155-168. 21. XCrysDen is available for free download at www.xcrysden.org. 22. Hammer, B.; Norskov, J. K. Electronic Factors Determining the Reactivity of Metal Surfaces. Surf. Sci. 1995, 343, 211-220. 23. Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Norskov, J. K. Surface Electronic Structure and Reactivity of Transition and Noble Metals. J. Mol. Catal. A: Chem. 1997, 115, 421-429. 24. Peterson, A. A.; Grabow, L. C.; Brennan, T. P.; Shong, B.; Ooi, C.; Wu, D. M.; Li, C. W., Kushwaha, A.; Medford, A. J.; Mbuga, F. et al. Finite-Size Effects in O and CO Adsorption for the Late Transition Metals. Top. Catal. 2012, 55, 1276-1282.

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CO and CO2 Electrochemical Reduction to Methane on Cu, Ni, and

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