Tungsten Carbide Surfaces

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

CO, CO2 and H2 Interaction with (0001) and (001) Tungsten Carbide Surfaces: Importance of Carbon and Metal Sites Andrey A. Koverga, Elizabeth Florez, Ludovic Dorkis, and Jose A. Rodriguez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11840 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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The Journal of Physical Chemistry

CO, CO2 and H2 Interaction with (0001) and (001) Tungsten Carbide Surfaces: Importance of Carbon and Metal Sites Andrey A. Kovergaa,b, Elizabeth Flórezb*, Ludovic Dorkisa, José A. Rodriguezc

aUniversidad

Nacional de Colombia sede Medellín, Facultad de Minas, Departamento de Materiales y

Minerales, Grupo de Investigación en Catálisis y Nanomateriales, Medellín 050041, Colombia bUniversidad

de Medellín, Facultad de Ciencias Básicas, Grupo de Investigación Mat&mpac, Medellín 050026, Colombia

cChemistry

Department, Brookhaven National Laboratory, Upton, NY 11973-5000, United States

*Corresponding

author. Elizabeth Florez, Phone: +57 4 340 54 79, E-mail: [email protected]

Abstract In this work, a systematic study on the adsorption of atomic and molecular hydrogen and carbon oxides on cubic (001) and hexagonal (0001) WC surfaces by periodical density functional theory is reported. Calculations have been performed by employing the Perdew-Burke-Ernzerhof exchange correlation functional with Van der Waals corrections to account for the dispersive force term. In addition, dipole corrections were applied for W- and C-terminated hexagonal WC(0001) surfaces . Good agreement is found between calculated and reported data for representative bulk properties. Regarding surface properties, our results indicate that atomic hydrogen adsorbs quite strongly while H2 does it, in general, dissociatively on the studied surfaces, with very small energy barriers (< 0.35 eV) for the cleavage of the H-H bonds. The C sites of the carbide play an essential role in the binding of H atoms and the cleavage of H-H bonds. Studies examining the interaction of tungsten carbide with CO and CO2 also evidence the importance of C sites. The reactivity of a C- and Wterminated (0001) hexagonal WC surfaces significantly differs. Atomic hydrogen, carbon monoxide and CO2 are more stable on a C- than on a W-terminated surface, and only this latter termination is able to cleave spontaneously a C-O bond of the CO2 molecule. This difference in reactivity may open a number of possibilities for fine-tuning of the selectivity of the resulting material or designing compounds catalytically

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active for specific reactions by carefully adjusting the proportion of C-, W- and mixed-terminations during the synthesis procedure.

1. Introduction

Since Levy and Boudart1 reported on the platinum-like catalytic properties of tungsten carbide for selected hydrogenation reactions, transition metals carbides (TMC) became a subject of numerous studies.2-15 Such an interest was caused mainly by the unique characteristics of the TMC that combine properties of ceramic materials and those of metals16 including electric and thermal conductivities.17 Over the years, various metal carbides have proved to be catalytically active materials for the hydrogen evolution reaction (HER),18-21 alkane hydrogenolysis,2 benzene hydrogenation,3 the water-gas-shift,12 CO2 hydrogenation22 and its reduction to CO,15 hydrogen production from organic compounds,23 and other chemical transformations where hydrogen is a reactant.1,8,21 Attempts have been made to tie catalytic activity to characteristics of the carbide such as its ability to release adsorbed oxygen and the metal:carbon ratio in the compound.8,22,24 The TMCs also have shown to be excellent as supports for late transition and noble metals.20,25 From the plethora of transition metals carbides, the catalytic properties of molybdenum carbide are the most studied.2,3,12,15,21-23 In the case of tungsten carbide, it has been demonstrated that the material can hydrogenate unsaturated hydrocarbons1 and, under certain conditions, the compound can show a HER activity comparable to Pt, the best pure metal known for catalyzing this reaction.26-28 Additionally, it has been shown that tungsten carbide, similarly to carbides of niobium and molybdenum, is capable to catalyze the CO2 conversion to CO in gas phase, demonstrating certain resemblance in its catalytic properties to molybdenum carbide.24 However, while there is an extensive amount of theoretical data on systems comprised of molybdenum carbide and adsorbates of interest, such as carbon monoxide, carbon dioxide or hydrogen, the same cannot be said about tungsten carbide. Some studies examining the WC interaction with hydrogen, water and CO are available,29-33 but they either did not report the whole range of the adsorbates mentioned above, or 2 ACS Paragon Plus Environment

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did not consider the role of the surface termination. Besides, these studies did not examine the chemical behavior of the cubic phase of tungsten carbide. In this sense, a study of the H atom, H2, CO and CO2 adsorption properties on (0001) hexagonal and (001) rock salt type cubic WC surfaces could shed light on the molecular basis for its reactivity. In addition, by comparing these results with analogous systems already well-studied by means of theoretical methods, like (001) molybdenum carbide surfaces,22,25,34,35 it would be possible to clarify whether experimental similarities on the reactivity of these surfaces have root on a more fundamental level. Another important aspect is to evaluate the role of the surface composition on the catalytic properties of these materials. Note that while the (001) surface of a cubic WC exposes a 50:50 mixture of C and W sites, a hexagonal WC can be C- or Wterminated. Thus, by working with these two different phases, it is possible to examine the relative reactivity of C- and W- sites in the carbide. The outline of the present article is as follows: first surface and key bulk properties of WC will be computed. Next, interactions of the (0001) hexagonal and (001) cubic WC surfaces with H, H2, CO and CO2 will be analyzed and compared to available data for other TMC surfaces. Additionally, the nature of surfaceadsorbate interactions will be studied by analyzing changes in charge distribution for the studied systems. Finally, energy barriers for H2 dissociation and C-O bond cleavage in carbon dioxide would be evaluated to be able to compare the likeliness of their occurrence on tungsten carbide and molybdenum carbide in selected cases. This would allow drawing initial conclusions on the possibility of H2 and CO2 catalytic reactions on WC and defining paths for future research in this area.

2. Computational details

Spin-polarized calculations were carried out using Vienna ab initio simulation package (VASP).36,37 Cubic and both W-termination and C-termination of hexagonal WC(0001) surfaces were considered. Hexagonal tungsten carbide surfaces were represented by (2×2) supercell consisting of two W and two C atomic layers, 3 ACS Paragon Plus Environment


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with 10 Å of vacuum separating repeating images in z direction containing 18 atoms of each kind. For structure optimization, adsorbed species and two outer layers were allowed to relax simultaneously, with forces converged to 0.01 eV Å-1, while the two bottom layers were frozen to their bulk geometry, thus, comprising the (2+2) approach. Cubic carbide (001) surface was modeled in a similar fashion, using (2×2) unit cell with three mixed atomic layers, containing 24 atoms of each kind, and 10 Å of vacuum along z direction between repeating images. Here one upper layer was allowed to relax together with adsorbed moieties while two bottom layers represented the bulk, i.e. (1+2) approach. To assure that selected number of atomic layers in each supercell were sufficient to obtain correct energy values, calculations for CO adsorption on hexagonal (0001) and cubic (001) surfaces were repeated using thicker slabs comprising 6 layers and 4 layers respectively. The (2+4) and the (2+2) approaches were used for hexagonal (0001) and the cubic (001) surfaces. Obtained values for adsorption energies vary only slightly: 3.71eV vs. 3.64 eV, 2.43 eV vs. 2.39 eV and 2.86 eV vs. 2.78 eV for C-terminated (0001), W-terminated (0001) and cubic (001) surfaces respectively, confirming the adequacy of selected models. A similar method was used to confirm that there was no need to use a vacuum layer bigger than 10 Å. Dipole corrections were applied to systems with W- and C-terminated (0001) polar surfaces to avoid the effect of dipole interaction on total energy, similarly to works on polar Mo2C surfaces.22,34 The geometry search was performed using a cut-off for the plane-wave base set of 400 eV. 5×5×1 Monkhorst-Pack scheme38 was used to describe reciprocal space for all surfaces. Fermi level smearing was done by means of the Methfessel-Paxton approach39 with a Gaussian width of 0.2 eV. The Projector Augmented Wave core potentials40 coupled with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation potential41 approximation to the exchange-correlation energy were used. The choice of the functional is motivated by its adequate description of model systems of transition metal carbides surfaces22,42,43 and its low absolute error in estimated adsorption energies. The van der Waals corrections as implemented by Grimme44 were used for all systems. With the aim to verify the adequacy of the selected model several bulk properties of tungsten carbide were analyzed, such as the bulk modulus that was obtained from the expression:

( )𝑇,𝑉

𝐵0 = ― 𝑉 0

∂𝑃

∂𝑉

(1) 0


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The relative stability of each phase was assessed by calculating the cohesive energy, calculated as: 𝐸𝑐 =

(𝑛𝑊𝐸𝑊 + 𝑛𝐶𝐸𝐶) ― 𝐸𝑏𝑟𝑒𝑙 𝑁𝑊𝐶

(2)

Here 𝐸𝑏𝑟𝑒𝑙 is the total energy of relaxed bulk system, 𝐸𝑊 and 𝐸𝐶 are the energies of the isolated W and C atoms in vacuum, 𝑛𝑊 and 𝑛𝐶 the number of atoms of each kind in bulk system and 𝑁𝑊𝐶 is number of formula units WC in the unit cell. From this, positive 𝐸𝑐 values would indicate an exothermic formation process of a phase. Lattice constants for each phase were elucidated as well. Bulk calculations were performed by using the same set of parameters for the surface relaxation, with the exception of 11×11×11 k-point grid used to sample the Brillouin zone. Theoretical values for both phases of tungsten carbide were evaluated as well and compared to available experimental data. The adsorption energy values were obtained from the following expression: Eads = ESurf+Ad – (Esurf + Ead)

(3)

Here ESurf+Ad is the total energy of the tungsten carbide surface with the adsorbed moiety on it, ESurf and EAd stand for the energy of the bare surface and of the isolated adsorbate in vacuum, respectively. EAd values were obtained with a set of parameters identical to the ones used for bulk calculations. Figure 1 shows schematic models for the tungsten carbide surfaces examined in our study. The following adsorption sites have been considered in this work: top, bridge and hollow sites for hexagonal tungsten carbide surfaces and various types of top and bridge sites on a cubic (001) surface (Figure 1). On both C- and Wterminated surfaces two types of three fold hollow sites could be distinguished: one, with no atoms in the layer directly below it, denominated in this work as hollow-e, and two with an atom below, denominated in this work as hollow-W and hollow-C, depending on the atom located beneath the hollow site.



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Figure 1. Adsorption sites available on hexagonal C-terminated (0001) WC surface, hexagonal Wterminated (0001) surface and cubic (001) WC surface. Tungsten and carbon atoms are represented by silver and black spheres, respectively. Adsorption sites are highlighted with blue.

On the cubic (001) surface, the adsorption atop is possible above a carbon atom of the surface (top-C) or above a tungsten atom (top-W). The adsorbate could be also located on a site between either one C and one W atom, referred in this work as C-W bridge, or two Tungsten and two Carbon atoms, in our work denoted as W-W bridge. Since there were nine atoms exposed to vacuum in the supercells, representing both terminations of hexagonal (0001) surfaces, the total coverage θ is equal to 1/9 for each adsorbate. On cubic (001) surface the total coverage was 1/16. These values were used to minimize the probability of lateral interactions between adsorbate moieties in repeating images, especially in the case of CO2 and Hydrogen molecule. A charge distribution analysis was performed for providing information on electron distribution on the system and its subparts, and it gives important insights regarding the interaction of adsorbed molecules with the surface. In this work the scheme proposed by Bader was used45 as implemented in VASP by Henkelman, Arnaldsson and Jónsson 46 By knowing the charge distribution for a carbide surface with a molecule adsorbed on it and the isolated molecule in gas phase and on the bare surface, it is possible to obtain the charge density difference (CDD), defined as: ∆𝜌𝑧 = 𝜌𝑧𝑠𝑢𝑟𝑓 + 𝑎𝑑𝑠 ― 𝜌𝑧𝑠𝑢𝑟𝑓 ― 𝜌𝑧𝑎𝑑𝑠 6 ACS Paragon Plus Environment

(4)

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This CDD can be further visualized by using additional software tools, like VESTA.47 Charge distributions in the bare surface and in the adsorbed molecule are obtained by assuming that these partial systems have the same geometry that in the composite system (comprised of the surface and the adsorbate), as described elsewhere.48,49 For transition state search and energy barrier evaluation for CO2 and H2 dissociation the climbing image Nudged Elastic Band (CI-NEB) method was employed, as implemented by Henkelman, Uberuaga, and Jónsson50 and Henkelman and Jónsson.51 Initial images were created using the geometries corresponding to the most stable adsorption of each moiety on the WC surfaces. Final images were obtained by simultaneously relaxing the surfaces with dissociation products placed on them and selecting the ones with the lowest energy. In total four intermediate images were used to connect the initial and final images. The calculations were considered as converged as soon as the total forces were smaller than 0.01 eV/Å. Since true transition states are characterized by the presence of exactly one imaginary frequency, they can be confirmed by performing vibration analysis. Vibrational frequencies were obtained at the harmonic level with a numerical calculation and diagonalization of the force constant matrix by considering all the degrees of freedom for the adsorbed moieties on the surface. The corresponding elements of the Hessian matrix were obtained as finite differences of 0.03 Å.

3. Results and discussion 3.1

Bulk properties of hexagonal and cubic tungsten carbide

Table 1 summarizes the lattice constant, cell volume, density, bulk modulus, the cohesive energy and the Bader charge on W atoms calculated for the cubic and hexagonal phases of WC and experimental values given in the literature. As it can be appreciated from this table, calculated structural parameters show a good agreement with previously reported results.52-55 This could be attributed to the fact that bulk properties obtained by using the PBE functional usually are closer to experimental values, compared to results obtained 7 ACS Paragon Plus Environment


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with other functionals.56,57. Any existing differences can be attributed to the presence of impurities or defects in actual tungsten carbide phases, not included in the calculations.

Table 1. Calculated lattice constants, cell volume {Vcell}, density {ρ}, bulk moduli {B}, cohesive energy {Ec} and Bader charges of W atoms {Q} compared to available literature data

Phase

Lattice constant / Å

Hexagonal WC

Cubic WC

a

b

c

Calc.

2.92

2.92

2.85

Lit.

2.9152

2.91 52

2.84 52

Calc.

4.38

4.38

4.38

Lit.

4.3253

4.32 53

4.32 53

Vcell / Å3

ρ / g•cm-3

B / MBar

Ec / eV

Q/e

20.99

15.49

4.11

16.74

+1.18

15.6554

4.1355

16.7055

15.52

4.4

15.85

4.3253

17.053

20.95

+1.47

On the other hand, by analyzing the cohesive energy values it is realized that the hexagonal phase is more stable than the cubic one. As seen in molybdenum carbide systems,56 the charge distribution analysis shows a charge transfer from metal atoms to carbon, similarly to previously reported results32,53,55 These findings suggest an ionic contribution to W-C bonding. In tungsten carbide the bonding has been reported to have a combined metallic, covalent and ionic nature.55,59 Overall, our results confirm both the adequacy of the chosen model and similarities in bulk properties between tungsten and molybdenum carbides.

3.2 3.2.1

WC surfaces interactions with H, H2, CO and CO2 H adsorption on tungsten carbide surfaces

Figure 2 displays the different adsorption structures examined for H atom adsorption in this study. Initially the H atom was placed on a given surface site at a distance of approximately 1.5 Å from the surface, and the 8 ACS Paragon Plus Environment

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system was allowed to relax. Table 2 summarizes adsorption energies and adsorption geometries for all the investigated sites, calculated after relaxation of the system. In addition, preferred adsorption sites for each surface are highlighted in bold letters, and Bader charges for these most stable sites are given. As it can be seen, calculated values for adsorption energies and preferred adsorption sites on hexagonal WC(0001) are in a good agreement with those reported before.29,30

Figure 2. Stable geometries for the H atom on the studied surfaces obtained after the relaxation. The legend below each panel indicates the initial site. Tungsten, carbon and hydrogen atoms are represented by silver, black and blue spheres, respectively.

Table 2. Adsorption energies {E}, hydrogen –nearest surface atom distance {d(H-WC)}, perpendicular distance of hydrogen to the surface plane {d⟂ (H-WC)} and Bader charge values {Q} for H atom on WC surfacesa.

Surface

Initial site

Final site

E / eV

Hexagonal C-

Top

Top

-3.92 (-1.50)

Bridge

Top

Hollow-E

termination

Hexagonal Wtermination

d (H-WC)b / Å

d⟂ (H-WC) b / Å

Q/ e

1.09

1.09

+0.16

-3.92

1.09

1.09

Top

-3.92

1.09

1.09

Hollow-W

Hollow-W

-1.18

1.79

0.62

Top

Top

-1.89

1.80

1.80

Bridge

Hollow-E

-3.54

1.97

1.00

Hollow-E

Hollow-E

-3.54 (-0.84)

1.97

1.00

Hollow-C

Hollow-C

-3.42

2.02

1.10

c

c


-0.48


Cubic

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Top-C

Top-C

-3.87 (-0.14)c

1.11

1.10

Top-W

Top-W

-2.59

1.74

1.74

C-W bridge

Top-W/C-W bridge

-3.28

1.74

1.70

W-W bridge

W-W bridge

-2.11

2.05.

1.29

+0.12

Most stable adsorption sites are hinted with bold font

a

d corresponds to the distance between the adsorbate and the closest surface atom; d⟂–is the shortest perpendicular distance between the adsorbate and

b

the surface c

Calculated values were obtained following the same methodology reported by Michalsky, Zhang and Peterson.21

On hexagonal (0001) C-terminated surfaces, hydrogen tends to occupy an adsorption site located directly atop on a C atom of the surface, adopting a typical C-H bond length of 1.09 Å. At such a low coverage (1/9 ML), the shift from the initial position to final atop configuration is taking place for all adsorption configurations, except for hollow-W where the hydrogen atom movement is hindered by a tungsten atom underneath. On this particular adsorption site, hollow-W, the distance between the H and the W atom located in the subsurface layer of the slab is comparable to the distance between these two atoms on the top adsorption site on a W-terminated hexagonal (0001) surface,. However, the difference in adsorption energy between these two sites on C- and W -terminated surfaces is almost 0.70 eV, evidencing the greater stability of H atoms on C-terminated surface caused by the presence of surface C atoms. On a W-terminated hexagonal (0001) WC surface more stable adsorption sites can be distinguished than on C-termination. In contrast to the latter surface, although adsorption on atop of a W atom is also possible, it is the least favorable. The most stable configuration for H atom on W-terminated surface is achieved by its adsorption on hollow-e (see Figure 2). Indeed, to the same geometry hydrogen atom initially placed on bridge site converges. On the cubic (001) surface, a hydrogen atom can be adsorbed on a C, a W or a C-W coordination (see Figure 2). After examining all these possible configurations, it has been found that bonding on top of a C site is the most stable configuration. In this case, a difference as large as ~1.3 eV on the adsorption energy is calculated relative to the H adsorption energy on top of a W site (Table 2). Thus, H atom preferably occupies a site atop of a C atom on the surface, forming a C-H bond. As it can be appreciated in Table 2, adsorption energies for all binding sites on WC materials are rather large, similar to what has been reported for adsorption of H atom on cubic carbides of titanium, vanadium, 10 ACS Paragon Plus Environment

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tantalum and niobium as well.60 Nevertheless, the calculated values of adsorption energies strongly depend on the choice of gas phase reference, DFT method used to compute them, i.e. model parameters, functional describing the exchange correlation energy among others. For example, if adsorption energies of hydrogen are calculated by using half of the H2 molecular energy in the gas phase as the reference energy instead of using the energy of atomic H used in this work, and employing revised Perdew-Burke-Ernzerhof pseudopotential instead of PBE as reported by Michalsky, Zhang and Peterson21, lower energy values are obtained (Table 2). Within this framework, the H adsorption energy on a hollow-e site of the W-terminated (0001) surface is 0.84 eV, as reported by Michalsky, Zhang and Peterson,21 while energies on the C-termination and on the cubic (001) surface are -1.50 and -0.14 eV, respectively. Overall, an H atom atop of a C atom on the C-terminated hexagonal and atop of C atom of the mixed cubic (001) surfaces are equally stable, while H atoms adsorption on top of W atom of W-terminated hexagonal is weaker than on the mixed cubic (001) surface. This may suggest that the interaction of H and C is rather localized and not affected by the presence of W atoms in the same layer. However, this it is not the case for the interaction of H with W. In order to get a better understanding of the nature of the adsorbate-surface interaction, identify directly involved atoms of the system and elucidate the role of the carbon and metal sites, a charge transfer analysis was performed. Analyzing charge density difference plots, given in Figure 3, it can be seen that on a Cterminated (0001) surface, an H atom solely interacts with C atoms of the surface in the proximity, and the charge transfer, although small, occurs from the adsorbate to the surface. On the other hand, on a metalterminated surface, the adsorbate charge transfer takes place not only from the W atoms closest to H atom but also from other surface W atoms in the adsorbate vicinity. On a cubic (001) surface, similar to H atoms adsorbed atop on the C-terminated (0001) surface, charge is transferred from the adsorbate to the carbide surface through a localized interaction between the H atom and a C surface atom, confirming the similar adsorption dynamics between these two surfaces.



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Figure 3. Charge density difference (CDD) plots for a H atom adsorbed on hexagonal C-terminated (0001), W-terminated (0001) and cubic (001) WC surfaces. Here green and red regions correspond to charge loss and charge accumulation, respectively.

When comparing to reported values on H adsorption on molybdenum carbide, it can be seen that adsorption sites for H atom on molybdenum and tungsten carbides often coincide for analogous terminations.34,35 In these two materials, the most stable sites are located atop of C atoms of cubic and C-terminated hexagonal surfaces and in case of metal terminations – on hollow with C atom of the sublayer located below. On both carbides, adsorption of H is rather strong and most stable adsorption sites are related to available carbon atoms.35. Nevertheless, it can be said that H atom adsorption on tungsten is more stable than on molybdenum carbides: -3.92 eV vs. -3.17 eV on C-terminations, -3.54 eV vs. -3.31 eV on metal terminations and -3.87 eV vs. -2.91 eV on cubic carbides.35 Considering the Bell-Evans-Polanyi (BEP) principle and the experimental volcano type curve for the HER activity as a function of H adsorption energy for reactions involving hydrogen atom, 21,61 weaker adsorption energies for molybdenum carbide relative to tungsten carbide materials may indicate a lower catalyst activity of tungsten carbide with respect to MoxC. Nevertheless, in both cases the strong adsorption could prevent the removal of H atoms and eventually block available adsorption sites. The adsorption energy for hydrogen on Pt(111), the best known metal electrocatalyst for HER, obtained within a similar theoretical framework as employed here is -2.66 eV.60 This implies that both molybdenum and tungsten carbides would be located at the left side of the volcano curve, the strong adsorbing side,21,62indicating that the surface would be saturated by atomic H. If the values for H adsorption energies (Table 2), calculated as in [21] are used, the free energy of the adsorbed H would be -1.26, -0.60 and 0.10 eV for C-terminated, W-terminated hexagonal and cubic


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WC respectively, applying the procedure reported by Nørskov et al.62, clearly evidencing the HER activity of WC materials, compared to a ΔGH on Pt(111) of -0.13 eV. On the other hand, the role of exposed surface W atoms may be critical for CO2 hydrogenation on this surface. Adsorption via formation of C-H bonds on WC surfaces with exposed C atoms would imply a reduced H mobility. Thus even at the low H coverage CO2 hydrogenation would be hindered. In contrast, on the possible sites of W-termination the closeness of adsorption energies would facilitate the mobility of H atom making hydrogenation reaction possible if unoccupied adsorption sites are available for CO2. This, however, is only an approximation, as in practical systems the effects of temperature, pressure, other species available for adsorption, and the medium (in case of electrochemical experiments), among other factors, play and important role on determining the catalysts’ activity.

3.2.2

H2 adsorption on tungsten carbide surfaces

As mentioned above, the pioneer work of Levy and Boudart showed that tungsten carbide exhibits a Ptlike behavior when dealing with hydrogenation reactions.1,8 In this work, we performed the first systematic study of the adsorption and dissociation of H2 on (0001) hexagonal and (001) cubic WC surfaces. The diatomic molecule can adsorb either parallel or oriented perpendicularly to the surface. In our work, H2 was placed in an intermediate orientation, slightly tilted to an angle of approximately 140o to the surface, facilitating the geometry relaxation of the molecule. Figure 4 summarizes H2 and H (in case of H2 dissociative adsorption) stable geometries in the proximity to WC( surfaces. In general, we observed an extreme tendency towards dissociation of the H2 molecule, evidenced by barrierless dissociations on all the studied surfaces with barriers which were equal to zero, in agreement to what has been already reported from experimental studies of WC.1 No cleavage of the H-H bond was observed only for an adsorbed atop molecule on a C-terminated hexagonal WC, and above a top-W and bridge W-W sites in cubic WC. Nevertheless, on all these latter cases small dissociation energy barriers are calculated (< 0.35 eV). A compilation of these results is given on Table 3, 13 ACS Paragon Plus Environment


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where the dissociation energy barrier and Bader charges were calculated only for the most stable site after relaxation of the system.

Table 3. Adsorption energies {E}, hydrogen – nearest surface atom distance {d(H-WC)}, perpendicular distance of hydrogen to the surface plane {d⟂ (H-WC}, H-H bond length {dH-H}, dissociation energy barrier {Eb} and Bader charge values {Q} for H2 molecule on WC surfacesa.

Surface

Initial site

Final site

E / eV

d (H-WC)b / Å

d⟂ (H-WC)b / Å

dH-H / Å

Eb/ eV

Q/ e

Hexagonal C-

Top

Top

-0.14

2.13

2.13

0.76

0.35

+0.01

0.81

~0

+0.01

termination

Hexagonal Wtermination

Cubic

Bridge

Dissociates

Hollow-E

Dissociates

Hollow-W

Dissociates

Top

Dissociates

Bridge

Dissociates

Hollow-E

Dissociates

Hollow-C

Dissociates

Top-C

Dissociates

Top-W

Top-W

-0.43

1.96

C-W bridge W-W bridge

1.84 Dissociates

Top-W

-0.40

2.04

1.86

0.79

The most stable adsorption sites are hinted with bold font

a


b

the surface


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Figure 4. Stable geometries for H2 molecules on the studied surfaces. The legend below each panel indicates

the initial site. The colored spheres represent the same atoms as in Fig. 2.

From data in Figure 4 and Table 3, we can conclude that H2 dissociates on the studied WC surfaces, similar to the dissociative adsorption reported on molybdenum carbide surfaces.34,35 Both resulting adsorbed hydrogen atoms adopt the stable geometries, described above. Adsorption energies per H atom in this case were -3.93 eV, -3.54 eV and -3.89 eV for C-termination, W-termination and cubic WC, respectively. H2 adsorption on top of a C-termination and top-W site of the cubic (001) surface is characterized by a large distance between the hydrogen molecule and the surface, however a weak surface-adsorbate interaction is evidenced by elongated H-H bond. Observed intact adsorption may occur as the result of the structural distance of these sites from the stable sites for H atom adsorption as well as the significant distance between the surface and H2. In case of the cubic bridge W-W site, the initial orientation of the adsorbate molecule plays a significant role. If the H2 molecule is initially placed with H-H bond axis along surface W-W axis, it adopts a top-W site geometry; while if it is placed with the H-H bond axis oriented along surface C-C axis, the hydrogen dissociative adsorption occurs. An analysis of the CDD plots for these systems does not give evidence for a significant charge transfer between the carbide surfaces and the H2 molecule, as it can be observed in Figure 5.



Page 16 of 38

Figure 5. Charge density difference plots for H2 adsorbed on C-terminated (0001) hexagonal WC and (0001) cubic WC surfaces. Colors represent the same as in Fig. 3.

In order to get a deep insight on the dissociation process of the H2 molecule, Nudged Elastic Band (NEB) calculations were performed and results are shown in Figure 6. Beginning from a stable top site, NEB calculations show that on C-terminated hexagonal WC(0001), the energy barrier for H2 dissociation in Table 3 (0.35 eV) appears because of an H2 migration from the initial site to other site where barrierless dissociation takes place (in this case bridge), and not due to the molecule dissociation itself (Figure 6.a). This is in agreement with the performed frequency analysis which found no imaginary frequencies associated with this geometrical configuration. Regarding the W-terminated surface, H2, initially placed in close proximity to it dissociates without a barrier (Figure 6.b). These results indicate that H2 dissociation could occur readily on both terminations of the hexagonal WC(0001) surface. Our observations for H2 dissociation on W-terminated surface are in good agreement with those reported by Xi et. al who essentially found no barrier (0.02 eV) for the dissociation process.31

a. C-termination


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b. W-termination

c. Cubic Figure 6. Minimal energy path for H2 → 2H on the studied WC surfaces. Each diagram indicates the energy of the final configuration relative to the initial system and, where possible, the energy barriers associated with the dissociation, marked with triangles on the plot. The colored spheres represent the same atoms as in Fig. 2.

On a cubic (001) surface, NEB calculations evidence that H2 is weakly stabilized on top of a W atom and has no barrier associated with H-H bond cleavage (Figure 6.c). In contrast to the adsorption of the H atom discussed in the previous section, this fact bears an undeniable resemblance to how the hydrogen molecule interacts with a W-terminated hexagonal surface (Figure 6.b), discussed above, indicating the crucial role of surface tungsten atoms for hydrogen dissociation on tungsten carbide materials. As it can be appreciated in Fig. 6, in all cases the reverse process for the molecular hydrogen formation from adsorbed H atom would be occurring with a high-energy barrier, due to the formation of very stable CH bonds.



Page 18 of 38

From the practical point of view conditions in which hydrogenation reactions can be conducted usually imply a rich amount of H231,63,64 Therefore, the available adsorption sites are likely to be occupied by hydrogen atoms. This fact may suggest a low activity of these materials for catalytic hydrogenation reactions including CO2 hydrogenation. Similar observations were made for orthorhombic β-Mo2C, cubic δ-MoC35 and hexagonal α-Mo2C34 for which H2 dissociation was found to be exothermic, and associated with very low energy barriers. This property could explain the catalytic activity of WC for dehydrogenation and isomerization reactions of alkenes and cycloalkanes.1,65 Nevertheless, an H atom could be still transferred into C=C or C≡C bonds during the selective hydrogenation of olefins or alkynes.66 In contrast, it is a well-established fact that molybdenum carbide can be an efficient catalyst for hydrogen evolution reaction in acidic medium,67 whose activity could be tuned by doping, incorporating other transition metals into the material’s lattice.68 Similarities between tungsten and molybdenum carbides in their interaction with H2 molecule found in the present study would suggest comparable properties towards HER at similar pH values for these materials. However, due to the stronger adsorption of H atoms on tungsten relative to molybdenum carbide a lower HER activity of WC compared to MoxC is expected, because the higher blockage of surface atoms by H on WC. In fact, experimental electrochemical studies have already reported enhanced HER activity on cubic WC, although lower than on β-Mo2C,69 which would be in agreement to our theoretical results. Besides, since the hexagonal phase of molybdenum carbide have demonstrated a superior HER performance compared to the cubic one,70 it could be suggested that hexagonal WC could be a more promising HER electrocatalyst candidate than cubic WC. Unfortunately, to our knowledge, the electrochemical HER activity of hexagonal carbide has not been reported yet. In this regard, and taking into account the reported energy barrier for H2 dissociation on cubic δ-MoC, results reported here also suggest that differences on HER activities between hexagonal and cubic WC would be lower than among β-Mo2C and δ-MoC, because of the barrierless H2dissociation on both cubic and hexagonal WC surfaces.

3.2.3

CO adsorption on tungsten carbide surfaces 18 ACS Paragon Plus Environment

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In similar fashion to H2, a CO molecule initially was slightly tilted to adopt an intermediate position between being perpendicular and parallel to the surface. The C atom of the molecule was directed towards the surface, as it has been reported that adsorption of CO molecule normally occurs through the carbon atom on metallic surfaces.71,72 Table 4 summarizes adsorption energies and adsorption geometries for all studied surface sites, and Bader charges for the most stable site and Bader charges, calculated for CO adsorption, while Figure 7 depicts stable bonding geometries of CO molecule on the studied surfaces after relaxation of the system.

Table 4. Adsorption energies {E}, carbon – nearest surface atom distance {d(C-WC)}, perpendicular distance of carbon to the surface plane {d⟂ (C-WC} C-O bond length {dC-O}, angle between the surface and CO {∠ (WC-C-O} and Bader charge values {Q} for CO molecule on WC surfacesa

Surface

Initial site

Final site

E /eV

d (C-WC)b /

d⟂ (C-WC)b /

Å

Å

dC-O / Å

∠ (WC-C-

Hexagonal C-

Top

Top

-2.92

1.33

1.33

1.17

179.8

termination

Bridge

Bridge

-1.95

1.56

0.92

1.22

179.7

Hollow-E

Top

-2.92

1.32

1.33

1.16

179.8

Hollow-W

Hollow-W

-3.64

1.47

0.63

1.46

102.2

Hexagonal W-

Top

Top

-2.12

2.04

2.04

1.17

180.2

termination

Bridge

Bridge

-2.39

2.15

1.40

1.28

109.9

Hollow-E

Bridge

-2.39

2.14

1.41

1.28

109.8

Hollow-C

Bridge

-2.28

2.14

1.41

1.28

109.5

Top-C

Top-C

-2.78

1.33

1.33

1.19

180.1

Top-W

Top-W

-2.36

2.03

1.95

1.16

178.4

C-W bridge

Top-W

-2.38

2.03

1.98

1.16

177.8

W-W bridge

Top-C

-2.78

1.33

1.33

1.19

180.0

Cubic

Q /e

O)o -0.22

-0.55

-1.06

-0.48


a


b

the surface



Page 20 of 38

Figure 7. CO molecule stable bonding geometries on the studied surfaces. The legend below each panel indicates the initial site. CO carbon atoms are represented by orange spheres, oxygen atoms — by red spheres. Other atoms are represented as in Fig. 2.

From the data in Table 4 and Figure 7, it is seen that CO is more stable on C- than on W- terminated WC(0001) surfaces. In the first case on a C-terminated surface, CO adsorbed on hollow-W site is stabilized through two single (sp2-sp2) C-C bonds formed by the C atom of CO and two surface atoms, in addition to the C-O bond formed by the surface C atom and the oxygen atom of the CO molecule. As it can be appreciated, in this site there is a strong adsorbate-surface interaction, evidenced on the significant departure of the C-O bond length from the gas phase value (1.16 Å). The second most stable adsorption site for CO adsorption on a C-terminated surface is atop of a surface C atom, forming a very stable structure perpendicular to the surface with a double C=C bond, judging from the calculated C-C distance of 1.33 Å. Interestingly, the CO molecule initially placed on a hollow-e site adsorbs on top site, indicating that the molecule migrates to closest most stable site. Finally CO adsorbed on bridge appears to be the least stable on this surface. Like in the case of adsorption on a hollow-W site, discussed above, a C-O bond elongation of 1.22 Å was observed, indicating the interactions between the adsorbate and the surface. These interactions are weaker than on hollow-W site as evidenced by lesser CO geometry deformation and significantly lower adsorption energy. 20 ACS Paragon Plus Environment

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On the W-terminated (0001) hexagonal surface, two adsorption sites for CO can be distinguished: top and bridge. It can be seen that on the most stable bridge site the CO molecule adopts an orientation almost parallel to the surface. Compared to the gas phase, the C-O bond on W-terminated (0001) surfaces is elongated, although not as long as in the case of hollow-W adsorption on a C-terminated surface, indicating a weaker surface-adsorbate interaction. Additionally, the final geometry analysis indicates that while the C atom is located equidistantly from two W atoms, the oxygen atom in the CO molecule tends to adopt a position in which it would be located on another bridge site, which suggests an additional adsorbate stabilization of the CO molecule through W-O interactions. CO molecule initially placed on either of three-fold hollow sites also converged to the stable bridge site, indicating a higher mobility of CO molecule on this surface than on Cterminated. Adsorption atop of a W atom of W-terminated surface is not as stable as on a bridge site, although, the difference in energy between these two sites is less than 0.27 eV. The geometry of the resulting system is distinct from the stable geometry of CO on a bridge site, as it can be appreciated from bond distances values listed in Table 4. The biggest difference between these two adsorption geometries is the perpendicular orientation of the CO molecule in relation to the surface for the adsorption on top of a W atom. Our results show values of adsorption energies that are different from the ones previously reported for this termination.29 It is quite difficult to pin the exact reason behind this discrepancy. One can argue, however, that the choice of pseudopotential and convergence criterion may cause significant variations in the exact value of adsorption energies.73 Moreover, energy trends observed for CO adsorption on WC surfaces in our work are in a fair agreement with those observed for MoxC surfaces,73,75 suggesting consistency between these works and our model.

CO adsorption on cubic WC(001) surface appears to be an intermediate case between adsorption on Cand W-terminated hexagonal (0001) surfaces. Similar to the hydrogen atom adsorption, CO adsorption energies on top-C site and atop of a C atom of the C-terminated (0001) surface were found to be almost identical, while CO adsorbs stronger atop of a W atom of the cubic (001) surface than on a top site of the Wterminated (0001) surface. The distance between a surface carbon atom and the adsorbate also suggests formation of a double C=C bond, as discussed above for the adsorption on top of the hexagonal C-terminated 21 ACS Paragon Plus Environment


Page 22 of 38

(0001) surface. Stable configurations obtained after relaxation of CO molecule initially placed on a C-W bridge and a W-W bridge sites are identical to CO stable geometries on top-W and top-C sites, respectively. As it is seen from Table 4 no geometry deformation has been observed for CO adsorbed on cubic WC. Figure 8 shows the CDD analysis for the CO adsorption on WC surfaces. From this figure and from Table 4, it can be seen that for CO adsorbed on a top site of the C-terminated surface, the total charge acquired by the molecule is much lower than in the adsorption on the hollow-W site, and the charge loss is uniquely experienced by the surface C atom directly in contact with the adsorbate. CO adsorbed on the most stable hollow-W site of the C terminated surface receives charge through surface C atoms in the proximity of the O atom, while the loss of charge for surface C atoms closest to the C atom of the CO molecule was lower (left, Fig. 8). These results show that CO interaction with a C-terminated surface is quite localized, as it is limited to surface C atoms directly in contact with the adsorbate. The charge transfer from the metal terminated WC(0001) hexagonal surface to the adsorbate was significantly higher relative to the C-terminated surfaces. As it can be seen from Figure 8, here the charge transfer on W-termination additionally involves surface W atoms other than those in direct contact with the adsorbate, indicating that the electronic properties of surface atoms on this termination are strongly affected by the CO molecule. In this sense, one of the effects of the high amount of transferred charge is the almost parallel orientation to the carbide surface of the CO molecule, as depicted on Fig. 8 On cubic surfaces, CDD plots evidence a charge transfer comparable to the one on C-terminated hexagonal WC surfaces (right, Fig. 8), although the total net charge was not as small as in case of adsorption on top. Then, in agreement to what was discussed above, CDD plots also support the resemblance between these two surfaces.


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Figure 8. Charge density difference plots for CO molecule adsorbed on hollow-W site of C-terminated (0001), atop of C-terminated (0001), W-terminated (0001) and (001) cubic WC surfaces. Colors represent the same charge variations as in Fig. 3. . In the case of molybdenum carbides reported theoretical results on CO adsorption are only available for hexagonal surfaces.73,74,75 However calculated adsorption energies significantly differ. While for a metal termination, Ren et al. found the most stable threefold hollow site with -2.23 eV adsorption energy73, Liu and Rodriguez reported an adsorption energy of -1.75 eV for the most stable top site,74 and Tominaga and Nagai calculated a value of -3.33 eV for the most stable adsorption on threefold hollow site.75 Therefore, no clear comparison between the data in Table 4 and results on CO adsorption on metal termination of molybdenum carbides can be performed. Trends observed for CO adsorption on the C –termination of WC(0001) are in a good agreement with results reported for a C-termination of α-Mo2C(0001),73 although it is more stable on WC (-3.64 eV on hollow-W vs. -1.74 eV on top site). Overall, the discussed results indicate that CO strongly adsorbs on the studied tungsten carbide surfaces, which may lead to the blockage of catalytically active sites, making them unavailable for other adsorbates and, thus decreasing any catalytic performance in presence of this specie. Furthermore, adsorbed CO modifies the WC charge distribution, especially on metallic terminations on which the interaction of the adsorbate with the surface additionally involves other W surface atoms not in direct contact with the CO molecule. This shift on the electronic density would modify catalytic properties of CO covered relative to bare surfaces. In this sense, the adsorption of CO on metal surfaces is a common process employed to modify catalytic properties, as in the case of alkyne hydrogenation over Pd-based catalysts.76 Thus, results reported above would also suggest the resemblance between the catalytic properties of WC and those one of Pt-group metals.

3.2.4

CO2 adsorption on tungsten carbide surfaces



Page 24 of 38

As mentioned in the introduction, carbides can be useful materials for the hydrogenation of CO2, a stable non-polar molecule.15,22,77 This conversion involves first activation by bonding and then cleavage of one or two C-O bonds. The elongation of these bonds is essential in the activation process. In our calculations, initial geometry of the molecule involved a C-O bond length of 1.16 Å, an 180o O-C-O bond angle, and a molecular orientation parallel to the surface. Figure 9 depicts stable geometries, while Table 5 summarizes adsorption energies and geometries for all studied surface sites, and the dissociation energy barrier and Bader charges for the most stable sites after relaxation of the system.

Table 5. Adsorption energies {E}, carbon – nearest surface atom distance {d(C-WC)}, perpendicular distance of carbon to the surface plane {d⟂ C-WC} C-O bond length {dC-O}, O-C-O angle (∠ {O-C-O}, dissociation energy barrier {Eb} and Bader charge values {Q} for CO2 molecule on WC surfacesa

Surface

Initial site

Final site

E /eV

d (C-WC)b / Å

d⟂ (C-WC)b

d (C-O) / Å

∠(O-C-O)o

Hexagonal C-

Top

Desorbed

-

3.01

2.68

1.18

179.2

Bridge

Desorbed

-

3.09

2.72

1.16

180.1

Hollow-E

Hollow-E

-1.06

1.39

1.16

1.40

105.0

Hollow-W

Hollow-W

-2.31

1.42

1.00

1.46

Eb / eV

Q/e

109.9

0.71

-1.08

termination

Hexagonal W-

Top

Dissociated

Bridge

Dissociated

Hollow-E

Dissociated

terminated

Cubic

Hollow-C

Hollow-C

-1.56

2.12

1.54

1.33

131.3

0.08

-1.13

Top-C

Top-C

-2.44

1.44

1.44

1.30

129.8

1.15

-0.66

Top-W

Desorbed

-

3.45

3.45

1.11

179.6

C-W bridge

Desorbed

-

3.09

3.01

1.16

178.2

W-W bridge

W-W-bridge

-2.01

2.07

1.30

1.39

108.2


a


b

the surface


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Figure 9. Final stable geometries for the CO2 molecule on the studied surfaces. Legend below the each panel indicates the initial site. The colored spheres represent the same atoms as in Fig. 7.

CO2 adsorption on three-fold hollow sites of (0001) C-terminated WC is energetically the most favorable bonding configurations on hexagonal surfaces. The geometry on these sites is characterized by a CO2 molecule at a distance from the surface closer to the value found on a double C=C than on a single C-C bond. Another important feature, evident from Table 5, is the significant deformation of the internal geometry of the CO2 molecule, typical of activated states, which are characterized by a significant change on the value of the OC-O bond angle and on the C-O bond length. These deformed molecular geometries are commonly explained because of a net charge transfer from the carbide surface to the adsorbed molecule.22,77 On both adsorption sites, there is an additional stabilization of the CO2 molecule through the interaction of the oxygen atoms with the surface C atoms, as it can be appreciated in the upper part in Fig. 9. Adsorption atop of a C atom and on a bridge site on the C-terminated surface leads to CO2 desorption, as it is realized from the significant value of the C surface distance (> 3.0 Å). In both cases, the CO2 molecule acquired a geometry indistinguishable from its geometry in vacuum. In the case of Mo2C (001) orthorhombic surface, Posada-Perez et al. reported an easy C-O bond cleavage for CO2 adsorbed on top of a Mo atom.22 From our calculations, similarly to Mo2C, the W-terminated hexagonal WC(0001) surface is able to dissociate spontaneously CO2 on most of the adsorption sites, excepting the hollow-C site, where CO2 molecule adopts a geometry characteristic for an activated state, as 25 ACS Paragon Plus Environment


Page 26 of 38

discussed above. On the other available sites on this surface, CO2 splits to CO and O moieties. At the end of this process, CO occupies a bridge surface site, with a geometry identical to the most stable adsorption site for CO on a W-terminated surface (Table 4). The oxygen atom in all cases adopts a stable geometry on a hollow-C site, suggesting it to be energetically the most stable (Figure 9), similarly to what it has been reported for the O adsorption on metal-terminated molybdenum carbide surfaces.78 For the same reasons as in case of CO adsorption on W-termination our results differ from the ones reported by Tong, Wu and Chen.29 However, similarities in the way how CO2 interacts with molybdenum and tungsten carbide surfaces and in dissociation barriers as it can be appreciated below, suggest that our model describes CO2 adsorption on WC similarly to models used for molybdenum carbide.22,25 Finally, CO2 adsorption on a top-C site of cubic WC(001) surfaces results in carbon dioxide activation. In contrast to what has been discussed above in the case of CO adsorption, changes in the adsorbate geometry on this adsorption site are similar to those ones found for CO2 adsorption on a hollow-C site on the W-terminated, while there is no CO2 adsorption on atop of a C atom of C-terminated surface, Table 5. Moreover, these two geometries are quite close to the reported ones for CO2 adsorption on molybdenum carbides, on those surface sites where molecular activation can occur.22 CO2 adsorbed on a bridge W-W site is very stable as well, due to interactions of its O atoms with C atoms of the surface. The geometry it adopts indicates CO2 activation on this site.

Figure 10. Charge density difference plots for CO2 activated on C-terminated (0001), activated on hollow-C site of the W-terminated (0001) and on (001) cubic WC surfaces.


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Changes in the geometry of the CO2 adsorbed on C-terminated (0001) WC surface are in agreement with the observed charge transfer of ~1.08 e from the surface to the adsorbate. It can be appreciated from Figure 10, that the main loss of charge was observed for C atoms of the surface directly in the contact with oxygen atoms of the CO2. On the W-terminated (0001) surface, a charge transfer to the CO2 molecule of ~1.13 e is occurring only via the C atom in the molecule and three nearby W atoms of the surface. The facility with which metal atoms are able to cede the charge leads to significant changes in the CO2 geometry from the linear one. On the cubic (001) surface, the total charge transfer from the surface to the adsorbed moiety was the smallest among the studied cases. Here, it takes place almost exclusively through the surface C atom in the contact with the adsorbate indicating the localized nature of this interaction. Calculations performed to find energy barriers associated with carbon dioxide dissociation on each surface using the most stable CO2 configuration on each surface as the initial snapshot are given in Figure 11. On Cterminated surface, the transition state found corresponds to a CO2 molecule with an elongated C-O bond, and a simultaneous shift of the CO moiety to a C-C bridge site. Found transition state was further confirmed by a frequency analysis, which evidences only one imaginary frequency mode (271.73 cm-1) corresponding to the dissociation. The final stable configuration involves an O atom adsorbed on a top site of the surface and a CO adsorbed on a hollow-W site. Significant positive energies are calculated for the oxygen on a surface site different to top, due to the stability of the surface C-O bond on this latter configuration. For this system, an energy barrier of 0.71 eV is calculated, Table 5. As of now there are no data on energy barriers for CO2 dissociation on C-terminated hexagonal or orthorhombic (001) molybdenum carbides, thus it is not possible to assess whether C-O bond splitting occurs more readily on tungsten or molybdenum carbide with exposed carbon atoms.

a.

C-termination 27 ACS Paragon Plus Environment


b.

c.

Page 28 of 38

W-termination

Cubic

Figure 11 Minimal energy path for a CO2 → CO + O dissociation on the studied WC surfaces. Each diagram indicates the energy of the final configuration relative to the initial system and the barrier associated with the dissociation. The colored spheres represent the same atoms as in Fig. 7.

On the W-terminated surface, a small barrier was found for CO2 dissociation from the stable hollow-C site. When analyzing Fig. 11b, it can be seen that this small barrier is occasioned by the diffusion of the CO2 molecule from the hollow-C site toward the bridge site where dissociation takes place barrierless. This is corroborated by frequency analysis where no imaginary frequencies were found. If any other geometry is chosen as the initial snapshot, the dissociation is barrierless, as compiled in Table 5. Thus, CO2 dissociation will occur easier on the metallic termination of tungsten hexagonal carbide than on the metallic termination of orthorhombic Mo2C, where an energy barrier of ~0.7 eV was found.22 On the case of the cubic (001) surface, carbon dioxide dissociation has the highest energy barrier among the studied surfaces. The whole process is comprised of the C-O bond splitting and the diffusion of the O atom from the initial position to a stable top-W site. The estimated energy barrier is appreciably lower than the one 28 ACS Paragon Plus Environment

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found for CO2 dissociation on cubic δ-MoC (1.41 eV),25 indicating that this process would occur easier on cubic tungsten carbide. Conducted frequency analysis evidences only one imaginary frequency mode (430.79 cm-1) corresponding to the movement of CO and O moieties along the reaction path, confirming the TS. In contrast to calculated lower dissociation barriers on WC, it has been reported that Mo2C is more active than WC for CO2 reduction in gas phase.24 This apparent contradictory result could be explained by considering a stronger CO adsorption on tungsten than molybdenum, in agreement to the work of Liu and Rodriguez.74 This stronger adsorption, besides the stronger adsorption of H atoms could poison the WC surface and reduce its global catalytic activity. It would be of interest to investigate the coadsorption of CO2 and hydrogenating agents and evaluate possible reduction mechanisms that could lead to formation of hydrocarbons, since CO2 adsorption leaves some sites unoccupied, in order to validate or reject the latter explanation. Nevertheless, from results in Table 5, it is clear that WC is a viable candidate for the activation and hydrogenation of the CO2 molecule, as experimentally it has been found.24 In this sense, CO2 adsorbed on tungsten carbide surfaces comprises an extremely interesting system for further studies.

Conclusions Periodic DFT calculations have been carried out to investigate interactions between (001) surfaces of hexagonal and cubic tungsten carbide with atomic H, H2, CO and CO2. Our results show that the studied WC surfaces bind atomic hydrogen quite strongly and are able to easily dissociate H2, then, it is conceivable that tungsten carbide is a viable material for catalyzing hydrogenation reactions and HER. Arguably, the catalytic activity towards HER and CO2 reduction of WC is expected to be lower, compared to MoxC due to higher adsorption energies of the products of H2 and CO2 dissociation. In addition, DFT results also indicate that atomic hydrogen and carbon monoxide are more stable on C- than on W-terminated surfaces, while these latter surfaces are capable of barrierless breaking H-H and C-O bonds within the H2 and CO2 molecules, respectively. On the other hand, cubic WC showed adsorption properties for H atoms, H2 and CO molecules similar to the ones of a C-terminated surface. However this is not true regarding CO2 adsorption, for which the highest dissociation energy barrier is calculated.



Page 30 of 38

When comparing to reported properties for molybdenum carbides, similar adsorption energies and dissociation barriers trends were found, although differences on the absolute magnitude of these parameters make tungsten carbide unique. In overall, the discussed findings clearly indicate that WC is a promising material for the field of heterogeneous catalysis and the results suggest that it can be useful for tailoring catalytic systems active for reactions involving hydrogen, carbon monoxide and carbon dioxide.

Acknowledgements The authors would like to thank Universidad de Medellín (UdeM) for supporting their work. The authors acknowledge the financial support of the Facultad de Minas de la Universidad Nacional de Colombia. The research carried out at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Science and Office of Basic Energy Sciences under contract No. DE-SC0012704.

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