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Catalytic Activity and Product Selectivity Trends for Carbon Dioxide Electroreduction on Transition Metal-Coated Tungsten Carbides Sippakorn Wannakao, Nongnuch Artrith, Jumras Limtrakul, and Alexie M Kolpak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05741 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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The Journal of Physical Chemistry

Catalytic Activity and Product Selectivity Trends for Carbon Dioxide Electroreduction on Transition Metal-Coated Tungsten Carbides Sippakorn Wannakao1,2,*, Nongnuch Artrith1, Jumras Limtrakul2, and Alexie M. Kolpak1* 1 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 2 Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210 Thailand *corresponding author email: [email protected], [email protected]

Abstract Electrochemical reduction of CO2 is a promising technology to produce hydrocarbon and alcohol fuels in a carbon neutral cycle. To utilize the technology on a commercial scale, inexpensive and earth abundant catalysts are needed. Here, we employ density functional theory calculations to investigate activity and product selectivity trends for CO2 electroreduction reaction on transition metal monolayer coated tungsten carbides, M/WC (M = Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, Pt and Au). Such core-shell systems have the potential to reduce the loading of precious metals, such as, Ag, and Au while the catalytic properties for CO2-to-CO reduction of metal surfaces remain. We find that, at low potentials (< 0.5 V), Cu/WC and Pd/WC core-shell particles catalyze CO2 reduction to CO as the main product due to low COOH formation and CO desorption free energies. Further, we show that the binding energies of the carbo- and oxo-intermediates on the M/WC catalyst surface obey scaling relations with the binding energies of CO and O, respectively. The binding energies of these two key intermediates exhibit linear relationships with the total d-band center of the surface metals, and thus the d-band center can be utilized as a unified electronic-structurebased descriptor to predict the product selectivity. Such a unified descriptor is useful for efficient screening for improved core-shell catalysts. This work illustrates the potential of core-shell metal/metal-carbide particles as a platform for the design of inexpensive CO2 electroreduction catalysts.

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Introduction Electrocatalytic carbon dioxide (CO2) reduction is a promising pathway to reduce carbon emission, as the reaction is possible under ambient and aqueous condition.1-2 Further, CO2 can be electrocatalytically converted to industrially important feedstock products and synthetic fuels such as CO, CH3OH, HCOOH, and light hydrocarbons3-7. However, in order to utilize CO2 reduction in large-scale industrial and commercial applications, inexpensive and earth abundant catalyst materials need to be discovered. Transition metal carbides (TMCs) have recently attracted interest as catalysts and support materials to reduce the loading of precious metals in catalysis applications.4, 8-11 The metal-covalent properties of TMCs enable them to be simultaneously more chemically stable, durable, catalytically active, and selective compared with their parent metals. Consequently, TMCs have been applied for many thermal and electrochemical catalysis applications.4, 12-15 Among those TMCs, tungsten carbide (WC) exhibits similar electronic properties to platinum-group metals, is stable over a wide range of pH and external potential conditions, and strongly binds most metals and adsorbed molecular species.9-11,

16-17

Owing to these

attractive properties, WC has been utilized as a catalyst and support material for metal monolayers to reduce the loading amount of transition metals in various thermo- and electrochemical processes.17 For example, Hunt et al. demonstrated that non-sintered, metalterminated WC nanoparticles are efficient catalysts for the hydrogen evolution reaction (HER).18 Recently, several studies have shown that metal-covalent systems also offer an avenue to perturb the binding energy scaling relations that are known from metallic surfaces, which in principle makes it possible to achieve greater catalytic activities than for metallic catalysts.14,

19-24

In particular, metal carbides break the scaling relations because of their

strong oxygen affinity.14, 21, 25 However, in the case of WC in aqueous solutions, the strong oxygen affinity of the WC surface leads to the irreversible adsorption of OH and O species originating from water splitting reactions, which results in surface blockage by those species. Previously, we proposed based on first principles calculations that core-shell systems with metal monolayers or sub-monolayers on WC cores would make it possible to benefit from the carbide stability while the strong metal-WC bonding prevents surface oxidation under electrocatalytical reaction conditions.21 An experimental study has also confirmed that alloying WC nanoparticles with doped atoms enhances the oxidation resistance of the particles while maintaining their electrochemical reactivity.26


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The thickness of metal coatings on WC surfaces can be controlled by thermal evaporation.11,

27-29

Recently, the synthesis of metal-carbide nanoparticles with adsorbed

noble metal monolayers and sub-monolayers has been achieved.30-31 These core-shell materials show high activity for methanol electrooxidation and are stable against CO poisoning30,

32

. Such properties make core-shell materials attractive as potentially cost

effective catalysts for CO2 electroreduction with minimal amount of noble transition metal loadings.31 In this article, we present an in-depth computational investigation of the catalytic mechanism and surface structures of such materials to advance our understanding of their catalytic properties and to potentially facilitate the rational design of catalysts that are tailored for specific reactions. Traditionally, most transition metals were believed to catalyze CO2 reduction only to the products H2 and CO, and only Cu catalysts were known to support further reduction of CO to hydrocarbons or alcohols.1 However, Kuhl et al.6 demonstrated that methane and methanol can also be produced by a broad group of transition metal surfaces, including Au, Ag, Zn, Cu, Ni, Pt and Fe. These results promise the opportunity to achieve CO2 reduction to synthetic fuels in a single process and, therefore, have inspired many theoretical studies investigating the details of such a further reduction of CO to methane and methanol.14, 22, 33-35 To access the entire reduction pathway, complex catalysts materials beyond bulk transition metals are required.19, 21, 23, 33 Calculations of the free energy of reaction intermediates have been an cost-effective way to understand the trends for CO2 electroreduction on metal surfaces36-38 and alloys,33,

35, 39

and have provided a model for numerous theoretical

mechanistic investigations and for computational screening for new electrochemical catalysts.20-21, 24, 33, 38, 40-42 Here we employ density functional theory (DFT) calculations and the computational standard hydrogen electrode (CHE) approach to determine the trends for CO2 electroreduction on core-shell metal/WC surfaces. The relationship between binding energies of reaction intermediates is used to identify the catalytic activity and product selectivity for the reaction. In particular, the CO* and O* (asterisk indicates that the intermediate is adsorbed on the surface) are key intermediates to predict product selectivity (CO, CH3OH or CH4) and surface blockage from water splitting products on the metal coated WC surfaces.



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Figure 1. Periodic surface slab model used for the simulation of the monolayer of 13 transition metals on the WC surface. As an example, adsorbed CO molecules on the metal monolayer are shown. Transition metal atoms are colored dark blue, carbon atoms are gray, tungsten light blue, and oxygen red. Models and Methods For efficient screening, we modeled the WC(0001) surface using a 6-layer slab with a (2×2) in-plane supercell, as we found no significant difference between binding energies for a (3×2) and a (2×2) supercell in our previous report21. A vacuum region of 15 Å along the direction perpendicular to the surface was used for all calculations. Monolayers (M/WC) and sub-monolayers (1/4M/WC) of the metal species Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, Pt and Au were added by decorating the hcp sites, which are known to be the most favorable adsorption sites on the W-terminated WC surface21, 30, 43, as shown in Figure 1. All calculations were performed using density functional theory (DFT) with the revised Perdew-Burke-Ernzerhof (RPBE) functional44 as implemented in the all-electron FHI-aims code.45 Relativistic treatment was included using the Atomic ZORA approximation.46 Geometry optimizations were done with the predefined light basis set (i.e., using a 4th order expansion of the Hartree potential, radial integration grids with 302 points in the outer shell, and a tier 1 basis set) with a 2×2×1 k-point mesh. Energies and density of states (DOS) were subsequently computed with the tight basis set (6th order expansion, 434 grid points, and a tier 2 basis) and a 6×6×1 k-point mesh (6×6×1 for (2×2) supercells) and a k-grid factor of 10×10×1 for DOS calculations. The metal monolayer or sub-monolayer and the top W and C layers were allowed to relax during the geometry optimization, while the rest of the slab was kept fixed in the bulk geometry to model a bulk-like WC substrate (see Figure 1.). All binding energies reported are determined at the most energetically favorable binding site of each intermediate on the given surface. We determined the mechanism for electrocatalytic CO2 reduction by constructing free energy diagrams (Figure 2) based on the computational standard hydrogen electrode (CHE)


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approach.37, 47 The free energy of each electrochemical step A* + [e- + H+] AH* along the reaction pathway can be related to the external potential by the equation:

∆GA*AH*(U) = GAH* - GA* - ½ GH2 + neU, where U is the applied potential and n is the number of electron-proton pairs involved in the elementary step. The highest potential U for which every elementary step is exergonic (i.e., ∆GA*AH*(U) ≤ 0) is considered as the limiting potential, UL. The value of UL has previously been found to be generally consistent with the experimental onset potential (within 0.1-0.2V).37, 47 Free energies were obtained from DFT total energies (EDFT) corrected by zeropoint energy (ZPE), heat capacity (Cp), and entropy (-TS) contributions: GDFT = EDFT + ZPE + ʃCpdT –TS As variations in these correction terms for different structures are small, they can be well approximated as constant for all materials. Thus we applied free energy corrections to the electronic energies based on previously determined values37 (Table S1). Using these wellestablished corrections to RPBE electronic energies, computed equilibrium potentials for CO, HCOOH, CH4 and CH3OH reduction agree well with experimental values (Table 1). Only the thermodynamics of the reaction were explicitly considered because kinetic barriers for electrochemical H-transfer are usually small and surmountable at room temperature.48-49 In addition, such small kinetic barriers are diminished at the high applied voltages that occur at the conditions of the electrochemical reduction.50-51 To estimate the importance of magnetic effects on the reaction free energies, spinpolarized DFT calculations for selected systems were carried out using the Vienna Ab initio Simulation Package (VASP)52-54 using also the RPBE functional and equivalent convergence parameters (Table S2). As seen in Table S2, the free energy trends obtained from the spinpolarized calculations are close to those from non-spin-polarized calculations for every elementary step. Therefore, we did not consider spin polarization for further analysis. The binding energy of a reaction intermediate is defined as the difference between the energy of the slab with adsorbed intermediate (ECxHyOz@slab) and the sum of the energy of the bare slab (Eslab) and the formation energies of C, H, and O with respect to H2O(g), H2(g) and CO(g):



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∆Ebinding = ECxHyOz@slab – (Eslab + xEC + yEH + zEO). To obtain the d-PDOS, we modified the DOS output file format of the FHI-aims code to include the individual orbital components. The average d-band (d-band center) shifts were calculated for the surface metal atoms for both total d-PDOS and orbital-resolved d-PDOS. The d-band center is calculated as = / . Interface energies of metal coatings on the WC surface were computed relative to the isolated WC surface and to the metallic bulk systems and are summarized in Table S3. As seen in the table, most of the metals adsorb strongly on the WC surface with negative interface energy. The only exception is Mn which thus might form islands on the WC surface. Generally, the interface energies indicate strong M-WC bonding compared to the MM bonds in the bulk metals.30 Based on the computed interface energies and on experimental success in preparing controlled metal coatings on WC surfaces30-31, it is plausible that transition-metal monolayers can be stabilized as coatings on WC surfaces. In addition to the monolayer (1ML) coatings, we determined the reaction free energies for CO2 reduction over Cu/WC and Au/WC core-shell systems with two monolayer (2ML) thick metal coatings to evaluate the effect of higher metal coverage (see Table S4). We find that the 2ML metal coatings exhibit reduced binding free energies of the COOH* and CO* intermediates by ~0.3 eV when compared to those of the 1ML systems. This indicates that the interaction with the WC substrate rapidly diminishes with the thickness of the metal coating, approaching the properties of the bulk transition metals38. However, at 2ML coating the potential limiting step remains the same as for the 1ML systems, and therefore we consider only 1ML coatings, considering that WC-core TM-shell nanoparticles can be prepared through tailored coating approaches that allow fine-grained control of the coating thickness for sub-monolayer up to monolayer coverages.10-11, 30-31, 55

Table 1. Experimental and calculated equilibrium potentials for CO2 reduction to HCOOH, CO, CH3OH and CH4. Reaction 1. CO2 + 2(H+ + e-) HCOOH 2. CO2 + 2(H+ + e-) CO + H2O 3. CO2 + 6(H+ + e-) CH3OH 4. CO2 + 8(H+ + e-) CH4

Equilibrium Potential, U (eV, vs RHE) Experiment Theory -0.20 -0.20 -0.12 -0.10 0.03 0.01 0.17 0.15


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Results and Discussion We employed the methodology outlined in the previous section to investigate the elementary reaction steps of the CO2 electroreduction comprising the following singleelectron transfer steps, (1)-(9): * + CO2 + (H+ + e-) COOH*

(1)

COOH* + (H+ + e-) CO* + H2O (2) CO* CO + *

(3)

CO* + (H+ + e-) CHO*

(4)

CO* + (H+ + e-) COH*

(5)

CHO* + 2(H+ + e-) CH3O*

(6)

CH3O* + (H+ + e-) CH3OH + *

(7)

CH3O* + (H+ + e-) CH4 + O*

(8)

O* + 2(H+ + e-) H2O

(9)

while the free energy of the HER is represented by the Volmer reaction, * + (H+ + e-) H*. Table 2 Free energy for each elementary step along the CO2 reduction pathway for monolayer M/WC, and also included is free energy for HER, all energies are in units of eV. Step

1 2 3 4 5 6 7 8 9 HER

Reaction free energy on M/WC surface Mn

Fe

Co

Ni

Cu

Zn

Ru

Rh

Pd

-1.49

-0.70

-0.34

-0.27

0.21

0.95

0.22

0.14

0.19

0.10

-0.55

-0.77

-0.74

-0.63

-0.28

-1.10

-0.86

1.43

1.28

1.15

1.05

0.46

-0.64

0.91

-0.25

0.42

0.77

0.92

0.87

0.57

0.35

0.73

1.09

1.15

1.28

0.67

0.20

-0.02

-0.28

0.78

0.44

0.18

-2.61

-2.43

2.31 -0.69

Ag

Ir

Pt

0.88

-0.02

-0.04

0.68

-0.70

-0.66

-0.99

-0.67

-0.59

0.76

0.55

-0.18

1.05

0.75

-0.06

1.15

1.01

0.89

0.77

1.11

0.83

0.57

1.60

1.21

1.72

1.62

1.94

1.74

1.67

1.80

-0.32

-1.10

-0.02

-0.02

-0.04

-0.10

0.50

0.52

0.59

0.19

-0.32

-0.32

-0.44

-0.46

-0.52

-1.07

-0.78

-0.82

-1.43

-1.85

-0.99

-0.83

-0.82

-2.07

-0.66

-0.17

-0.12

-0.50

-0.41

-0.08

1.79

0.94

0.10

-0.57

-0.59

0.54

-0.88

-1.44

-2.03

-1.37

-1.50

-2.43

-0.45

-0.32

-0.35

-0.21

0.22

0.60

0.31

-0.05

0.17

-0.02

-0.18

0.15


Au


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Table 3 Selectivity of product/(limiting step), and limiting potential, UL, for CO2 electroreduction on metal coated WC surfaces . M/WC surface

Product/Limiting step

UL (V)

Mn

CH4/(9)

-1.18

Fe

CH4/(9)

-0.92

Co

CH4/(4)

-0.77

Ni

CH4/(4)

-0.92

Cu

CO/(1), CH4(4)

-0.21, -0.87

Zn

CO/(1)

-0.95

Ru

CH4/(4)

-1.15

Rh

CH4/(4)

-1.01

Pd

CO/(1), CH3OH(4)

-0.19, -0.89

Ag

CO/(1)

-0.88

Ir

CH3OH/(4)

-1.11

Pt

CH3OH/(4)

-0.83

Au

CO/(1)

-0.68

Figure 2. Free energy diagram for CO2 reduction on different metal monolayers on the WC surface for pH = 0 and U = 0 V. Figure 2 shows the calculated Gibbs free energy diagram (at pH =0 and U=0 V) for electrochemical reduction of CO2 according to the reaction steps (1)-(9). All free energies corresponding to the diagram are also summarized in Table 2. Product selectivity, potential limiting step, and limiting potential are reported in Table 3. Similar to pure metal catalysts,


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HER can compete with the CO2 reduction as it is thermodynamically more favorable. Under CO2 reduction conditions, however, CO adsorption is dominant at a high negative potential promoting CO2 reduction.56 CO is the first product that can be obtained by two (H+ + e-) transfers along the reaction pathway. The limiting potential for CO production depends on how strongly the COOH* and CO* intermediates are bound to the surfaces. The selectivity of CO as product depends on the binding strength of the surface to the CO molecule. In particular, metals that interact strongly with the CO molecule, such as Ni, Pt, and Fe, suffer from poisoning by CO or later reduction intermediates. As a consequence, those metals produce hydrogen from the HER as a major product. The weakly interacting metals, on the other hand, release CO before it is reduced to further reduction products. Consistent with experiments for metal surfaces, Zn, Ag, and Au exhibit a potential for CO production, as the CO desorption free energies are negative on these metals and CO can be released spontaneously. The calculated limiting potentials for CO2 reduction to CO are -0.95, -0.88, and -0.68 V for Zn/WC, Ag/WC, and Au/WC, respectively. These values are in reasonable agreement with previous experimental57 and theoretical40 reports of reduction potentials for CO2 electroreduction to CO on the pure metals Zn, Ag, and Au (-1.12, -0.95 and -0.72 V, respectively). The first H transfer to CO2 to form the COOH* intermediate is the potential limiting step for these weakly binding metals. The second electron-proton transfer occurs exergonically with negative free energy to form CO* and H2O. CO is energetically favored to desorb from the surface, and therefore CO is likely to be the main product for these weakly interacting metal core-shell M/WC systems. Since the catalytic properties of the M/WC remains close to that of the pure Ag and Au metals, core-shell M/WCs would be useful to reduce the amount of precious metals used in those catalysts. On the other hand, due to their strong carbophilic properties, metals such as Mn, Fe, Co, and Ni, can spontaneously reduce CO2 to COOH* with a negative free energy. However, these metals suffer from a strongly bound CO*, and the CO desorption energies for M/WC with M = Mn, Fe, Co, and Ni are 1.43, 1.28, 1.15, and 1.05 eV, respectively.

As a

consequence of the strong CO* binding, these systems promote HER as a major product, while a consecutive 6(H+ + e-) or 8(H+ + e-) transfer of CO2 reduction at a higher electrode potential provides either methane or methanol as minor product. Moreover, recent experiments demonstrated that methane and methanol can be obtained even from catalysis over weakly interacting surfaces, such as, Au, Ag, and Zn, if the CO production rate overcomes the rate of CO desorption at higher potential ranges.6 Hence, in the present work



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we investigate the further reduction of CO* on all monolayer M/WC surfaces to evaluate the potential of the different metal coatings to support one-step reduction of CO2 to synthetic fuels. The CO* protonation step requires the highest potential for most of the systems with the exception of Mn/WC, and Fe/WC which interact strongly with the intermediates. In principle, the reaction can proceed via two different pathways depending on whether the protonation occurs at the C or the O atom, resulting in the formation of CHO* (4) or COH* (5), respectively. However, for all of the metals considered here, the reaction via the CHO*, for which both the C and O atom can interact with the surface, is energetically favored over COH*, which can only coordinate with the C atom on the surface. The reduction of the CO* intermediate has been shown to be the potential limiting step on transition metal surfaces.38 This is confirmed by the high surface coverage of CO* found during CO2 reduction on Cu5859

, Ni60-61, Pt62, and Fe61. Further reduction of the CHO* via two (H+ + e-) transfers results in the formation of

CH3O*. This step is generally more facile and occurs at lower limiting free energy compared to the CO* protonation (Figure 2). CH3O* can, in turn, either be reduced to methanol (7) or methane (8), where the selectivity of methane and methanol production is governed by the oxophilicity and carbophilicity of the surface.6,

63

In particular, less oxophilic surfaces for

which ∆G(7) < ∆G(8) tend to release oxygenated products (i.e., methanol), whereas more strongly oxophilic (∆G(7) > ∆G(8)) surfaces stabilize the O* bound to the surface so that methane is released. However, too strong oxygen affinity also leads to O* surface blockage and thus catalyst poisoning either from H2O dissociation or CO2 reduction. Reaction step (9) is therefore directly related to the oxophilicity and selectivity of the products at higher electroreduction of the catalyst surfaces.


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Figure 3. The relationship between a) the binding energy of carbo-species (COOH, CHO, and COH) relative to that of CO and b) the binding energy of oxy-species (OH and CH3O) relative to the O binding energy on monolayer metal on WC surfaces.

Since the carbophilicity and oxophilicity of the surface are essential to control the product selectivity, we investigated the relationship among binding energies of adsorbed species with respect to the binding atoms. Linear scaling relations are commonly found for metal surfaces.64 We use the CO* binding energy as a key descriptor for the carbo-species COOH*, CHO* and COH* that coordinate with the C atom to the surface. The binding energy of CO* has previously been used to describe reactivity trends of pure metal surfaces.36 For the oxo-species OH* and CH3O* (O coordinates to the surface), the binding energy on metal surfaces commonly scales with the O* binding. The trends of the reaction intermediate binding energies on the various M/WC systems are shown in Figure 3. As seen in the figure, the binding energies of the carbospecies on most M/WC surfaces follow the scaling relation. Exception is Mn/WC for which the binding energies deviate from the trend due to the strong interaction with both the C and O atoms of the molecules. This deviation might be a result of the instability of the Mn monolayer coating on the WC surface (see discussion of interface energies above). Fe/WC also interacts strongly with carbon and oxygen leading to stronger binding energies for the COOH* and CHO* intermediates. This behavior is in agreement with the literature, as surfaces with strong oxygen affinity have previously been shown to break the scaling relations for COOH*, CHO* and CO*.14, 25 Based on the linear regressions in Figure 3, the binding energy of carbo-species on M/WC surfaces can be approximated in terms of the CO binding energy Eb(CO) (the mean squared error (MSE) is given as an error estimate) as



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Eb(COOH) = 0.9485Eb(CO) + 0.3743

(MSE = 0.0838 eV) a)

Eb(CHO) = 1.0675Eb(CO) + 0.5625

(MSE = 0.1075 eV) b)

Eb(COH) = 1.5102Eb(CO) +1.6802

(MSE = 0.2644 eV) c)

The CO binding energy of -1.08 eV, which is the point at which the desorption free energy ∆G(3) equals +0.50 eV, is shown as the dotted black line in Figure 3a to indicate an approximate threshold for CO gas production. For stronger CO binding, further reduction of the surface CO* intermediate is likely. Furthermore, we consider a COOH* binding energy of -0.32 eV (dotted blue line in Figure 3a) as a threshold for low limiting potential for CO selective production, as below this binding energy the COOH* intermediate can be obtained with a limiting potential below 0.5 V. Using these criteria, we find that Cu/WC and Pd/WC could produce CO as the main product at low potential ranges. The limiting potential (UL) is ~0.2 V for both metal monolayer systems. As mentioned before, weakly carbophilic surfaces, including Ag/WC, Au/WC, and Zn/WC are highly selective for CO production but require higher electrode potentials to overcome the first H transfer to CO2. In case of the oxy-species (Figure 3b), the O* binding energy correlates well with both the OH* and the CH3O* binding energies. The corresponding scaling relations are Eb(OH) = 0.4137Eb(O) – 0.1103

(MSE = 0.0498 eV) d)

Eb(CH3O) = 0.4031Eb(O) – 1.7159

(MSE = 0.0393 eV) e)

The dotted red line in Figure 3b indicates a potential of U = -0.5 V, the point at which the reaction might become limited by OH* (Eb(OH) = -0.77 eV) and O* species (Eb(O) = -0.97 eV). The strong binding energy of those intermediates may result in surface blockage in a low negative potential range. For example, Mn/WC and Fe/WC require 1.18 and 0.92 eV, respectively, to overcome the potential barrier. To assess the selectivity for methane and methanol production, we consider the O* binding energy of 1.12 eV (dotted black line in Figure 3b) where ∆G(7) = ∆G(8). Those surfaces that interact weakly with oxo-species, including Pd/WC, Ag/WC, Ir/WC, Pt/WC, and Au/WC, tend to produce methanol as the product rather than methane. Our results agree well with the experimental observation that Au, a weakly O-binding metal, produces only methanol while Fe, a strongly O-binding metal, produce only methane.6 Hence, the O affinity is the key for selectivity of further reduction


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products. Combining these insights, the catalytic activity and selectivity can both be well described by the binding energies of just two reaction intermediates, CO* and O*.

Figure 4. Relationship of the CO and O binding energies with the center of the d-band. For Zn/WC the sp-band center is also shown for illustration. In order to find a single descriptor for the reaction trend, we further investigate the relationship between the binding energies of the key intermediates CO* and O* with the electronic structure of the surface in terms of the mean of the d-pDOS (the d-band center)65. As seen in Figure 4, the d-band center correlates linearly with both CO* and O* binding energies. The only exception from this trend is the Zn/WC system, for which the d-band is completely filled and energetically far below the Fermi level. For Zn/WC, the d-band center is therefore not an appropriate descriptor of the valence band and the sp-band center should be considered instead (shown by the blue triangle in Figure 4). The trend lines in Figure 4 that approximate the relationship between the d-band center (εd) and the binding energies of the key intermediates are given by Eb(O) = -1.2052 -1.8276

(MSE = 0.2410 eV) f)

Eb(CO) = -0.3507 – 2.0035 (MSE = 0.0270 eV) g)



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Figure 5 d-PDOS and its components of the surface metal before (black lines) and after (red lines) CO* and O* adsorption. The p-PDOS of the adsorbed atoms is shown in blue lines. The d-PDOS can be further resolved into contributions from individual d orbitals. Choosing the Cartesian z axis as the direction of the surface normal, there are three principle groups of d orbitals: 1) orbitals within the surface plane (dxy and dx2-y2), 2) orbitals perpendicular to the surface plane (dz2), and 3) orbitals diagonal to the surface (dxz and dyz). We previously showed that the PDOS projected onto those d orbitals with contributions in the direction of the surface normal, i.e., the dxz, dyz, and dz2 components, are correlated with the binding energy and adsorption site selectivity on metal (sub-)monolayer M/WC surfaces.21 In essence, the dxz and dyz components govern the adsorption on the surface hollow (hcp) sites and affect the O* binding energy while the dz2 component determines the binding on the ontop sites that are favored by the CO* intermediates. Note that the difference between the component-resolved d-PDOS and the total d-PDOS is minimal for pure metal Pt(111) surfaces but becomes important for Pt/WC surfaces owing to their partially covalent character.21 The changes in the total d-PDOS and in its dz2 and dyz components caused by the adsorption of different reaction intermediates on the pristine WC surface and on the Pt/WC surface are shown in Figure 5. The variation of the remaining d components is shown in Figure S1 and Figure S2 in the Supporting Information. As seen in Figure 5, the dz2-band


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dramatically changes after CO* adsorption. This strong response is due to the adsorption of the CO* intermediate on top of the metal atom, as the adsorbed CO shifts the metal conduction band to higher energies. For O* adsorption, where the intermediate favors the hcp site of the metal surfaces, the dxz or dyz contributions of the metal valence bands are shifted to lower energy. Note that the electron density close to the Fermi level is affected the most after intermediate adsorption. These results agree with experiments by Gorzkowski et al.66, which revealed the correlation of the DOS near the Fermi level with chemisorption strength and catalytic activity for non-uniform core-shell structures of Pd/Pt. We previously showed for the case of sub-monolayer M/WC core-shell particles that the center of the d-components including dxz, dyz, and dz2 components correlates with the binding energies of reaction intermediates while the total d-band center is not a good descriptor.21 The reason for this discrepancy is the strong binding affinity of the (0001)WC surface that causes adsorption on the W atoms regardless of the metal species M. Owing to covalent interactions, the M metal atoms modulate the electron density around neighboring W atoms in a directed fashion and thus differently for different d-band components, which in turn affects the adsorption of reaction intermediates.

Figure 6. Relationship of the total d-band center and the center of the dxz and dz2 d-band components of the surface metals and W atoms for metal monolayers and ¼ monolayers on WC, respectively. In the case of complete metal monolayers on WC, the M-W interaction is screened and the total d-band center predicts binding energy trends well.21 As such, the d-band components of the metal monolayer (M atom) behave completely analogous to the total dband, as shown in Figure 6 for the dxz and dz2 centers. Therefore, for core-shell M/WC



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particles with full M monolayer coverage, the total d-band center derived from the total dPDOS is a good descriptor for predicting the trends of the CO2 electroreduction. Combining the scaling relations (a)-(g), the selectivity of CO2 electroreduction on monolayer metal/WCs can also be expressed in terms of the d-band center. Figure 7 shows a volcano plot predicting the reaction selectivity purely based on the d-band center. The volcano plot illustrates the selectivity between methanol and methane product determined by the free energy difference of reaction steps (7) and (8). The d-band center describes excellently the selectivity between both products, and this relationship could therefore be used for the screening for efficient core-shell electrocatalysts solely by considering the electronic structure of the catalyst surfaces.

Figure 7. Volcano plot predicting the free energy change for the selectivity determining steps based on the total d-band center. This relationship can be used to predict the methanol and methane product selectivity purely based on the electronic structure of the catalyst surface.

Conclusion In this work, we employed density functional theory calculations to investigate activity and selectivity trends for CO2 electroreduction over core-shell of metal/WC surfaces with 13 different metal species. By comparison of adsorption energies, and electronic d-band centers, we find that metals that weakly interact with the reaction intermediates, i.e., Zn/WC, Ag/WC, Au/WC, produce CO as a major product and with similar limiting potential as in their pristine metal surfaces. Therefore, these core-shell M/WC systems are potentially useful for reducing


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the amount of precious metals (Au and Ag) in existing catalysts. Further, we find that the Cu/WC and Pd/WC surfaces exhibit a low limiting potential of UL ~ 0.2 V for CO2 reduction to CO. Further reduction of CO leads to the production of methane and methanol, which was also found experimentally for transition metals. Selectivity between both products depends on the oxygen binding strength of the surfaces. The surfaces that strongly bind O produce methane while the weakly interacting surfaces produce methanol. In addition, we show that the binding energy of the key intermediates CO* and O* is a good descriptor for both catalytic activity and selectivity, as they correlate strongly with the binding energies of the other intermediates along the reaction pathway. The CO* and O* binding energies also correlate linearly with the d-band center of the metal shell on the WC core, and as such the dband center is a good single-parameter descriptor for CO2 electroreduction. This insight provides us with a simple qualitative relationship that can be used for high-throughput screening for efficient core-shell electrocatalysts. Finally, we show that considering the orbital components of the d-band near the Fermi level might be key for the construction of quantitative models for the prediction of catalytic activity. Further investigation will focus on refining our understanding of the interplay of the d-component-resolved electronic density of states and molecular adsorption energies for the description of complex surfaces. Supporting Information Additional tables and figures for free energy corrections, free energies obtained from spin polarized calculations, interface energies of M/WCs, free energy of reaction on 2ML of Cu/WC and Au/WC, and changes in d-band components caused by adsorptions of reaction intermediates.

Acknowledgement S.W. acknowledges the Postdoctoral Fellowship from Vidyasirimedhi Institute of Science and Technology. N.A. thanks the Schlumberger Foundation for a Faculty for the Future fellowship. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562.



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Table of Contents


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Figure 1. Periodic surface slab model used for the simulation of the monolayer of 13 transition metals on the WC surface. As an example, adsorbed CO molecules on the metal monolayer are shown. Transition metal atoms are colored dark blue, carbon atoms are gray, tungsten light blue, and oxygen red. 200x118mm (300 x 300 DPI)



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Figure 2. Free energy diagram for CO2 reduction on different metal monolayers on the WC surface for pH = 0 and U = 0 V. 561x326mm (95 x 95 DPI)


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Figure 3. The relationship between a) the binding energy of carbo-species (COOH, CHO, and COH) relative to that of CO and b) the binding energy of oxy-species (OH and CH3O) relative to the O binding energy on monolayer metal on WC surfaces. 685x304mm (95 x 95 DPI)



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Figure 4. Relationship of the CO and O binding energies with the center of the d-band. For Zn/WC the spband center is also shown for illustration. 314x276mm (96 x 96 DPI)


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Figure 5 d-PDOS and its components of the surface metal before (black lines) and after (red lines) CO* and O* adsorption. The p-PDOS of the adsorbed atoms is shown in blue lines. 175x287mm (300 x 300 DPI)



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Figure 6. Relationship of the total d-band center and the center of the dxz and dz2 ¬d-band components of the surface metals and W atoms for metal monolayers and ¼ monolayers on WC, respectively. 304x264mm (96 x 96 DPI)


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Figure 7. Volcano plot predicting the free energy change for the selectivity determining steps based on the total d-band center. This relationship can be used to predict the methanol and methane product selectivity purely based on the electronic structure of the catalyst surface. 92x71mm (300 x 300 DPI)



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Table of Contents 82x44mm (300 x 300 DPI)


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Catalytic Activity and Product Selectivity Trends for ... - ACS Publications

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