Theoretical Model for CO Adsorption and Dissociation on Clean and K

Article pubs.acs.org/JPCC

Theoretical Model for CO Adsorption and Dissociation on Clean and K‑Doped β‑Mo2C Surfaces Carolina Pistonesi,† María Estela Pronsato,† László Bugyi,‡ and Alfredo Juan*,† †

Departamento de Física, Universidad Nacional del Sur e IFISUR (UNS-CONICET), Av. Alem 1253, (8000) Bahía Blanca, Argentina Hungarian Academy of Sciences, Reaction Kinetics and Surface Chemistry Research Group, Szeged H-6701, Dóm sqr 7. POB 168, Hungary

‡

ABSTRACT: We studied the effect of K on the adsorption and dissociation of CO on the β-Mo2C (001) surface by density functional theory calculations. Molecular CO adsorbs more strongly on Mo-terminated surfaces than on Cterminated ones. Adsorption is energetically more favorable in the presence of preadsorbed potassium. The CO molecule withdraws electron density from the surface, being more extended on the K-doped surface. The CO dissociation was also evaluated, and reaction pathways were modeled, revealing that the C-terminated surface is energetically less favorable than the Mo-terminated one. For both surfaces, the activation energy barrier for dissociation increases with the K content. C−O vibrational frequencies were also computed on K-modified surfaces.

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INTRODUCTION The investigation of the effect of surface additives on the binding of admolecules is very important, because their presence can alter the activity of heterogeneous catalysts toward complete inhibition or considerable promotion.1 Alkali metals, being good electron donors, are known as excellent promoters in numerous surface reactions.2 In addition, one must keep in mind that gradual enhancement in the surface concentration of admolecules can strongly modify the electronic and geometric properties of a surface, leading to greatly different behavior from that of a clean surface. For this reason, during the adsorption process, even a well-defined single crystal surface may appear quite heterogeneous, showing a continuous change of adsorption properties, such as adsorption energy, see for example, alkali metal adsorption.2 Obviously, to understand the adsorption properties appearing at different surface coverages, it is important to reveal the surface modifying effect of the adsorbate itself and its reaction products. The modification of a catalyst following adsorption is possible not only on its surface but also in its subsurface or bulk region through migration of different species, changing especially the properties of nanosized particles. In this way, the diffusion processes and related energy changes can make the chemistry of nanoparticles used in real catalysis rather complex and dependent sensitively on experimental conditions. For complex systems, in the absence of appropriate experimental tools, elementary steps of surface processes can often be approached only by computer simulation. It is well documented that the inexpensive interstitial carbides of groups 4−6 early transition metals show similar or superb catalytic efficiency to the rare and expensive Ptmetals in a range of chemical reactions, such as hydrodesulphurization (HDS), hydrodenitrogenation (HDN), iso© 2012 American Chemical Society

merization of alkanes, CO hydrogenation, and the partial oxidation of methane to syngas.3,4 Moreover, it was found that ZSM-5 supported Mo2C catalyzed the aromatization of methane,5,6 ethanol,7 methanol,8 and dimethyl ether.9 The chemistry of different hydrocarbon fragments forming in these catalytic processes was successfully studied under UHV conditions.10,11 The activation of CO2 by potassium on Mo2C drew also attention12 because of the utmost importance of elimination of this harmful greenhouse gas from the atmosphere. In trying to clarify the elementary steps in the decomposition process of CO2 molecule and in the surface reactions of CO with hydrogen,13 the effect of an alkali additive on CO interaction with Mo2C surfaces was also investigated.14 Over the past years, quantum chemical calculations have emerged as a new tool to understand the structure of active surfaces and to determine reaction mechanisms. Kitchin et al.15 presented density functional theory (DFT) investigations of the physical, chemical, and electronic structure properties of several close-packed surfaces of early transition metal carbides. Their results indicate that hydrogen binds more weakly on carbonterminated β-Mo2C(0001) surfaces than on Mo(110). These results suggest that C termination may be required to obtain less reactivity than the parent metal and that the C atoms have a passivating role. On all C-terminated surfaces considered, stable C−H species are formed, suggesting that these carbon atoms may be chemically active. Adsorption of oxygen and CO on Mo- and C-terminated αMo2C (0001) was also investigated by DFT methods by Ren et al.16 The analysis of the partial density of states showed the Received: July 12, 2012 Revised: September 28, 2012 Published: October 22, 2012 24573

dx.doi.org/10.1021/jp3069317 | J. Phys. Chem. C 2012, 116, 24573−24581

The Journal of Physical Chemistry C

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the first two surface layers were allowed to relax. We used a kinetic energy cutoff of 750 eV for all calculations, which converges the total energy to ∼1 meV/atom for the primitive cell of bulk β-Mo2C. We have performed several tests increasing the number of relaxing layers (and the number of total layers in the slab), and no further improvement in energies was found. A detailed discussion on the impact of the number of layers and relaxation on transition metal slabs was recently published by de Morais et al.32 The CO adsorption energy was computed by subtracting the energies of the gas-phase and surface species from the energy of the adsorbed system as follows:

metallic character of the clean surfaces, for which the valence band region is dominated by the contribution of the Mo 4d band. They also found that CO occupies 3-fold sites on the Mo-terminated surface. On the C-terminated surface, however, CO prefers the top sites with the formation of CCO species. Adsorption and dissociation of CO on β-Mo2C surfaces was theoretically studied by Shi et al.17 using cluster models, finding stable CO for both Mo- and C-terminated surfaces. They found that on a C-terminated surface, the calculated adsorption energies are overall smaller, resulting in a weaker chemisorptive bonding due to the carbon substrate atoms at the surface getting involved in the adsorbate bond. Further calculations showed that dissociative CO adsorption for both terminations leads to quite stable geometries, where adsorption at the Cterminated surface is energetically less favorable than at the Mo-terminated surface. Calculation of reaction pathways for dissociative CO adsorption on Mo-terminated surfaces yielded a barrier energy of 0.89 eV. In order to provide fundamental information on how the adsorption of alkali metals can affect Mo2C catalysts, Woo Han et al.18 have studied the stability of several low-index surfaces of α-Mo2C and K adsorption using DFT calculations, finding that at low coverages, K atoms adsorb more strongly on the (001) surface. Although the reactions of different molecules on K-doped Mo2C surfaces were successfully modeled by our group with DFT calculations, resulting in reasonably well coincidence with experimental findings,19−22 the detailed modeling of alkali modifier for CO interaction with Mo2C surfaces is still lacking. In the present work, the reaction pathway of CO decomposition in the presence of potassium on molybdenum- and carbon-terminated β-Mo2C single crystals is modeled using DFT. Comparison with earlier23 and more recent14 experimental findings is also provided.

Eads(CO) = E(CO/slab) − E(COgas ) − E(slab)

With this definition, negative adsorption energy corresponds to an energetically favorable adsorption site on the surface, and more negative values correspond to stronger adsorption bonds, that is, to higher adsorption energies. Preadsorbed potassium was also considered for both molybdenum- and carbon-terminated surfaces. Two different potassium coverages were modeled. One and three potassium atoms were included on the surface, resulting in the K−Mo2C surfaces (1 K/8 Mo surface coverage) and the 3K−Mo2C surfaces (3 K/8 Mo). It has been experimentally estimated that a potassium coverage value, related to the underlying unit cell, of ∼0.33 corresponds to a completion of a monolayer (ΘK = 1 ML).12,14 In that sense, our models correspond to approximately 0.4 and 1 ML. The use of atoms to model the effect of alkali additives is a common approach in DFT calculations and has been discussed in detail.8,33−36 The adsorption of the CO molecule was investigated on different sites of Mo- and C-terminated surfaces, with CO coordinated via the C atom to the substrate. Clean and potassium-doped surfaces were considered. During optimization for both clean and K-doped surfaces, the adsorbate, the first two surface layers, and K were allowed to fully relax, while the other two layers were kept frozen. Details of bulk and surface optimization are reported in a previous paper.19 The electronic charges on atoms were computed using Bader analysis.37 The charge of the CO molecule was calculated adding the individual charges of the C and O atoms. The variation of electronic charge of specific surface atoms (close to CO) before and after CO adsorption was reported as follows:

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COMPUTATIONAL METHODS AND SURFACE MODELS DFT calculations were performed as implemented by the Vienna ab initio simulation package (VASP 5.2 version), which uses a plane-wave basis set and a periodic supercell method.23−27 Potentials within the projector augmented wave method (PAW)28 and the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) functional were used.29,30 Geometry optimizations were obtained by minimizing the total energy of the unit cell using a conjugatedgradient algorithm to relax the ions.31 The β-Mo2C phase has a crystal structure with Mo atoms slightly distorted from their positions in closed-packed planes and carbon atoms occupying one-half of the octahedral interstitial sites. The structure of the β-Mo2C(001) surface includes a series of alternating Mo and C layers. The calculated DFT lattice parameters for the bulk β-Mo2C are a = 5.273 Å, b = 6.029 Å, and c = 4.775 Å, which were obtained by bulk optimization in a previous study.19 We modeled molybdenumterminated and carbon-terminated surfaces with slabs of four layer thickness (two layers of Mo atoms and two layers of C atoms), and each slab has two formula unit cells width, resulting in 6.0 Å by 10.8 Å slabs with a thickness of 4.8 Å. The vacuum spacing between two repeated slabs was 11.8 Å. During optimization, the first two layers were allowed to relax, and a set of 3 × 3 × 1 Monkhorst−Pack k-points was used to sample the Brillouin Zone. For adsorption calculations, the adsorbates and

ΔqX = qX (CO/slab) − qX (slab)

where qX represents the electronic charge of the surface X atom (Mo, C, or K atom). CO dissociation on both surfaces was also investigated by considering first the adsorbed molecule and finally the adsorbed carbon and oxygen atoms individually at equilibrium positions. After that, we modeled possible reaction pathways using the nudged elastic band method (NEB)38 considering 12 geometry images for each cycle. The nature of transition states on the potential energy surface has been tested based on the analysis porposed by Henkelman et al.39

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RESULTS CO Adsorption on Clean Surfaces. CO adsorption on the Mo-terminated surface was analyzed on several adsorption sites. According to our calculations, the CO molecule 24574



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Table 1. Calculated Adsorption Energies (Eads) and Distances for CO on Three-Fold (3F) and Top (T) Sitesa Mo-terminated

C-terminated

distance, Å Eads, eV

CO

b

Mo−C

distance, Å K−C

K−O

Eads, eV

b

CO

Mo−C, C−Csurfc

K−C

K−O

2.37−2.34−2.37 1.39 1.31 2.29−2.36−2.24 1.39 2.31−2.53 2.94 1.29 2.25−2.50−2.54 1.44 2.11−3.24−3.57 1.40 1.28

−

−

− 3.26 3.58

− 2.57 3.36

3.01 2.85 2.85

3.03 2.51 2.53

3.02

2.70

Mo2C

3F

−2.07

1.20

2.03−2.51−2.61

−

−

−1.49

1.26

K−Mo2C

T 3F1 3F2

−2.03 −2.40 −2.33

1.17 1.24 1.22

2.01 2.03 2.01−2.58

− 2.93 3.44

− 2.73 3.71

−1.64 −1.93 −1.12

1.17 1.30 1.20

3K−Mo2C

T 3F 3F2

−2.20 −2.82 −2.64

1.20 1.28 1.29

1.98 2.02−2.30−2.48 2.04−2.27−2.43

3.63 3.02 3.26 3.06

3.46 2..68 2.65 2.94

−1.87 −2.65 −2.42

1.19 1.34 1.28

T

−2.56

1.24

1.93

2.92

2.68

−2.09

1.23

3F1 and 3F2 on K-doped surfaces are 3F sites first and second neighbors to K atoms, respectively. C refers to a carbon atom from the CO molecule, while Csurf refers to a surface carbon atom. bC−O gas phase bond length: 1.14 Å (VASP), experimental 1.1342. cThe distances between surface Mo atoms and the C atom of the CO molecule (Mo−C) and distances between the C atom of the CO molecule and the surface C atom (C−Csurf) are indicated in normal and italic font, respectively. a

Figure 1. Surface structure of adsorbed CO on a clean Mo-terminated Mo2C surface on a 3-fold site (a, b), on a top site (c), and on a C-terminated surface on a 3-fold site (d, e) and on a top site (f).

preferentially adsorbs on carbon-deficient 3-fold (3F) and on top (T) sites on the β-Mo2C (001) surface. Table 1 (top and left) shows the calculated adsorption energies and selected equilibrium distances for the lowest energy adsorption sites (details on other sites were previously reported22). For both sites (3F and T), the adsorption energy is similar (−2.07 and −2.03 eV). Figure 1a, b shows schematic views of the 3F CO adsorption site on the Mo-terminated surface. This is a tilted configuration; the angle formed by the

CO with the surface is 60.4°, and the molecule is shifted toward one Mo atom. In the case of CO adsorbed on top of Mo, Figure 1c shows that the configuration is not tilted. The shortest distance between the C atom of CO molecule and Mo atoms (indicated as Mo−C in Table 1) are similar for the 3F and ontop configurations resulting in amounts of 2.03 and 2.01 Å, respectively. In the case of the C-terminated surface, the CO adsorption on top sites (Eads = −1.64 eV) is energetically more favorable 24575



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Table 2. Net Charge of CO Molecule (qCO) and Charge Variation of Surface Mo (ΔqMo) and K (ΔqK) Atoms as a Result of CO Adsorption on Mo-Terminated Mo2C Surfacesa Mo-terminated Mo2C qCO ΔqMo

ΔqK

a

K−Mo2C

3K−Mo2C

3F

T

3F1

3F2

T

3F1

3F2

T

−0.720 ↓0.154 ↓0.265 ↓0.220 −

−0.407 ↓0.118

−1.050 ↓0.236 ↓0.262 ↓0.364 ↓0.120

−0.941 ↓0.166 ↓0.247 ↓0.350 ↓0.050

−0.556 ↓0.237

−1.305 ↓0.284 ↓0.435 ↓0.293 ↓0.129 ↓0.013 ↓0.097

−1.296 ↓0.254 ↓0.388 ↓0.337 ↓0.001 ↓0.118 ↓0.092

−1.058 ↓0.458

−

↓0.036

↓0.033 ↓0.093 ↓0.144

All charges in electron units e. The arrow pointing up (down) indicates that the charge is increasing (decreasing).

Table 3. Net Charge of CO Molecule (qCO) and Charge Variation of Surface Mo (ΔqMo), C (ΔqCsurf), and K (ΔqK) Atoms as a Result of CO Adsorption on C-Terminated Mo2C Surfacesa C-terminated Mo2C qCO ΔqMo

ΔqCsurf ΔqK

a

K−Mo2C

3K−Mo2C

3F

T

3F1

3F2

T

3F1

3F2

T

−0.524 ↓0.016 ↓0.025 ↓0.023 ↓0.208

−1.486 ↑0.062 ↑0.068 ↑0.077 ↓1.720

−0.931 ↓0.333

↓0.396

−0.608 ↓0.085 ↓0.114 ↓0.023 ↓0.030

−1.855 ↑0.077 ↑0.063 ↑0.114 ↓2.015

−1.128 ↓0.230 ↓0.008 ↓0.080 ↓0.368

−0.828 ↓0.111 ↓0.039 ↑0.058 ↓0.436

−2.363 ↑0.138 ↑0.072 ↑0.055 ↓2.383

↓0.040

↓0.013

↓0.011

↓0.140 ↓0.166 ↓0.061

↓0.158 ↓0.105 ↓0.064

↓0.142 ↓0.100 ↓0.002

All charges in electron units e. The arrow pointing up (down) indicates that the charge is increasing (decreasing).

than on 3F sites (Eads = −1.49 eV); the same preference for top sites was found by Ren et al.16 According to this, the adsorption on this surface is less stable than on Mo-terminated surface. A similar theoretical result was mentioned before by Kitchin et al.;15 their data indicated that H adsorbs more strongly to the metal-terminated carbide surfaces than to the closest-packed pure metal surfaces or to the carbon-terminated ones. For both surfaces, the C−O distance increases after adsorption being larger for the 3F case. On the C-terminated surface, we refer to a 3-fold site relative to the geometry of the Mo layer (see Figure 1d). But, we have calculated that the C atom of CO is closer to a surface C atom than to any other of the three closest surface Mo atoms and that the distances between the C atom of a CO molecule and Mo atoms are similar on both surfaces (see Table 1, in which C is referred to a carbon atom from the CO molecule, while Csurf is referred to a surface carbon atom). For this configuration, the C−Csurf distance between the C atom of CO and the surface C atom is 1.39 Å while C−Mo distances are 2.34 and 2.37 Å. The CO molecule is tilted with an angle of 44.6° and therefore is more directed toward the surface than on the Mo-terminated surface (compare Figure 1e and b). On this surface, top sites correspond to CO adsorption on top of a surface C atom (see Figure 1f) forming CCO species. The C−Csurf distance is 1.31 Å, which is shorter than the C−Mo distances for the 3F sites. In all cases, the CO molecule is closer to the surface than on the Mo-terminated configuration (compare side views of Figure 1b and c with e and f). Similar adsorption energies and preferential sites to those listed in Table 1 were found for CO on Mo- and C-terminated surfaces by Shi et al.17 using DFT with cluster models.

Table 2 presents the computed electronic charges for the CO molecule. This table also includes the variation of electronic charge of specific surface atoms (close to CO) before and after CO adsorption. In all cases, the surface transfers charge to the CO molecule making it negatively charged, which is accompanied by a decrease in the electron density of the surface atoms. On the Mo-terminated surface (first two columns, in Table 2), when CO is adsorbed on top, the electron charge of one Mo decreases by 0.118e, while when CO is adsorbed on the 3F site, the increase in its electron density originates mainly from three (instead of one) Mo atoms. In the case of a CO adsorption on a 3F site, the net charge transfer to the molecule is higher (−0.720e) than on the top site (−0.407e). As mentioned before, on the C-terminated surface, CO is closer to surface C than to Mo atoms, and the largest charge transfer is via surface C atoms. In this situation, when CO bonds on top of a C atom, only bonded by a surface Csurf atom, this atom shows a noticeable reduction of 1.720e on its electronic charge (see Table 3); the electron charge of the CO molecule is higher (−1.486e) for this on top site than for the 3F site (−0.524e). The direction of calculated charge transfer values on Mo- and C-terminated surfaces are in harmony with a former experimental finding that shows work function increase of 0.8 eV on a Mo2C surface as a result of saturation with CO at 150 K.10 CO Adsorption on K-Doped Surfaces. For the two potassium coverages on the surface considered, the preferential adsorption sites for potassium are the 3-fold positions. In the case of the 1 K/8 Mo coverage (∼0.4 ML) on the Moterminated surface, the potassium adsorption energy is −2.19 24576



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this site, the CO is tilted with an angle of 69.2°, and the K atom is moved away 0.65 Å from its original location. The adsorption energy is −2.40 eV and corresponds to a stronger adsorption when compared to the clean surface. The C−O distance increases from 1.20 to 1.24 Å, corresponding to a larger stretch of the molecule compared to the clean surface (see Table 1). The equilibrium K−O distance is 2.73 Å, which is close to the sum of K+ and O2− atomic radii.41 As a reference, the K−O distance in crystalline K2O obtained by DFT calculations is 2.787 Å.42 Xu et al. found that K has a tendency to abstract oxygen preadsorbed on Rh(110) froming K−O units.40 The computed K−C distance was 2.93 Å. The next site in order of preference is a 3F site second neighbor to K (3F2, Figure 2c, top view), having an adsorption energy of −2.33 eV (C−O, K−C, and K−O distances are 1.22, 3.44, and 3.71 Å, respectively, see Table 1). Finally, the third place in energy corresponds to a top site close to K, with adsorption energy of −2.20 eV (the C−O, K−C, and K−O distances are 1.20, 3.63, and 3.46 Å, respectively). Sites further away from the K atom correspond to a less favorable adsorption energies and are not reported here. On the C-terminated surface, the most favorable adsorption site for carbon monoxide is the 3F first neighbor to K as shown in Figure 2d, e. CO is tilted with an angle of 44.4°, the K atom is moved away by 0.76 Å, and the adsorption energy is −1.93 eV. This value is higher than that of the K-free surface but lower than that calculated for the Mo-terminated surface covered with the same amount of potassium. The C−O

eV (see Table 4). We will refer to this surface as the K−Mo2C slab. A similar value was reported by Xu et al. using DFT Table 4. Potassium Adsorption Energy (Eads) and Electronic Charge (q) for Two Coverages on Mo2C Surfaces Mo-terminated 1K/8Mo 3K/8Mo a

C-terminated

Eads, eV

qK

Eads, eV

qK

−2.19 −1.21

0.774 0.624a

−2.37 −1.79

0.836 0.671a

Average value.

calculations.40 An experimental finding14 also indicated an adsorption energy close to this value as suggested by the high temperature desorption peak of ∼900 K for potassium at the zero coverage limit. On the C-terminated surface, the potassium adsorption energy is −2.37 eV. A higher adsorption energy on C-terminated (compared to Mo-terminated) surfaces was also computed by Woo Han et al.18 using DFT calculations. For the 3 K/8 Mo coverage (∼1 ML, designated as 3K−Mo2C slab), the adsorption energy reduces to −1.20 and −1.79 eV on Mo- and C-terminated surfaces, respectively. A decrease in the adsorption energy with K coverage was also reported by Woo Han et al.18 Let us start with the CO adsorption on a K-doped Moterminated surface (K−Mo2C) at different adsorption sites. The most favorable adsorption site corresponds to a 3F site first neighbor to the potassium atom (3F1), (see Figure 2a, b). On

Figure 2. Surface structure of adsorbed CO on a Mo-terminated K−Mo2C surface on a 3-fold site first neighbor to K (a, b), on a 3-fold site 2nd neighbor to K (c), and on a C-terminated surface on a 3-fold site (d, e) and on a top site (f). 24577



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distance increases to 1.30 Å. The next favorable site corresponds to a top site close to a K atom with an adsorption energy of −1.87 eV (see Figure 2f). Following in energy, there is a 3F site second neighbor to K with adsorption energy of −1.12 eV, which is not a tilted configuration. In all cases, CO is closer to the surface at C-terminated configurations (compare side views of Figure 2b, e). Regarding charge transfer on the Mo-terminated surface, we have reported in a previous paper22 that, on surface Mo atoms, the electron density increases due to charge transfer from K mainly to its first neighbors, while K becomes positively charged. On the C-terminated surface, a similar charge was found, directed predominantly to a surface C atom and to a less extent to Mo atoms. Electron transfer from K to the surface is reduced with the increase in K coverage (see Table 4). This calculation is in agreement with experimental results on work function (WF) changes that suggest a considerable charge transfer from potassium to Mo2C at lower coverages and a gradual neutralization at and above one monolayer; correspondingly, the potassium adlayer is mainly ionic at low coverages while metallic at high coverages.12 Coming back to the Mo-terminated surface, when CO is adsorbed on top, surface Mo atoms reduce their charge by 0.237e (see Table 2). This is a higher reduction than on the clean surface without K. After adsorption, K also shows a small reduction (0.036e) in its charge. CO becomes more negatively charged on the K-doped surface with a charge of −0.556 (instead of −0.407 on the clean surface). When CO is adsorbed on the 3F site closer to the K atom, the three Mo atoms close to CO reduce their charge by 0.364e, 0.236e, and 0.262e, and also, K reduces its charge by 0.120e; consequently, the electron transfer to CO is even larger, resulting in a net charge on the carbon monoxide molecule of −1.050e (instead of −0.720e on the clean surface). In the case of the 3F site second neighbor to K, the CO charge is −0.941e. On the C-terminated surface, when CO is adsorbed on a C top site, the surface C atom reduces its charge by 2.015e (see Table 3), which is higher than the value of 1.720e on the clean surface, and again, CO becomes more negatively charged on the K-doped surface, with a charge of −1.855e (instead of −1.486e on the clean surface). When CO is adsorbed on the 3F site closer to the K atom, the surface C atom reduces its charge by 0.396e (instead of 0.208e on the clean surface), and the CO charge is −0.931e. In the case of the 3F site second neighbor to K, the CO charge is −0.608e. The calculated increase of electron density on the CO molecule in the presence of potassium is in agreement with the experimental observation that the WF of Mo2C at 0.4 ML K coverage increased by 1.5 eV due to saturation with CO,10 indicating an extended charge transfer toward CO on a K-covered surface as compared with the K-free one (where CO adsorption resulted in WF enhancement of 0.8 eV, see above). Finally, we have also analyzed the effect of a higher K coverage (3K−Mo2C surface, see Figure 3) for both Mo- and C-terminated surfaces. Table 1 shows that the CO adsorption energies increase even more with a higher K coverage being −2.82, −2.64, and −2.56 and −2.65, −2.42, and −2.09 eV for the 3F adsorption sites first and second neighbor to K and for the top sites on Mo- and C-terminated surfaces, respectively. The charge transfer to the CO molecule is also higher (see Tables 2 and 3). For the Mo-terminated surface, CO charges are −1.305e and −1.296e for 3F sites first and second neighbors to K and −1.058e for the on top site. Both Mo and K atoms

Figure 3. Surface structure of adsorbed CO on a Mo-terminated 3K− Mo2C surface on a 3-fold site (a, b) and on a C-terminated surface on a 3-fold site (c, d).

show more reduction in their charge. In the case of the Cterminated surface, CO charges are −1.128e and −0.828e for 3F sites first and second neighbors to K and −2.363e for the on top site. Surface C atoms show more reduction in their charge and also K atoms. CO Dissociation on Clean Surfaces. We also studied the dissociation of the adsorbed CO molecule to C and O atoms adsorbed on the surface. We investigated the dissociation process and the effect of K by first analyzing the stability of the adsorbed species on the surface and then modeling a plausible reaction pathway while O is moved away from the C atom. The preferential adsorption sites for the O atom were found by mapping the energy at different locations on the surface keeping the C atom near to its original position and performing a full geometry optimization. In all cases, the hollow sites were the energetically more favorable for oxygen location. Similar results were reported by Zhou et al.43 using DFT calculations for O adsorption on Mo(110) surfaces. The dissociation pathway connecting the energetically most favorable geometries of molecular and dissociated CO was then examined by NEB calculations. Let us start with the Mo-terminated surface with CO adsorbed on a 3F site. On the clean Mo2C surface, several configurations for dissociative CO with C and O atoms coadsorbed on the surface were examined. Configurations with C and O positioned on 3F sites first neighbor results in a repulsive interaction. Configurations of C and O atoms positioned on 3F sites second neighbors with an internuclear distance of 3.04 Å (close to the Mo−Mo distance of 3.01 Å) are favored. Similar result for the dissociation of CO on Rh surfaces was found by de Koster and van Saten.44 24578



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Mo2C) reported by Sremainak and Whitten35 presents similar results. The most favorable pathway leading to dissociation requires that CO2 be adsorbed at a site that does not share Pt atoms with the K adsorption site, that is, at next-nearest neighbor sites (as 3F2 site in our work). They also report an increase in the activation energy for CO2 dissociation when K is present (see Figure 4, cited paper). On C-terminated surfaces, the energy barriers for CO dissociation are even higher (see Figure 5). A small number of

Figure 4 shows the energy versus reaction coordinate of the explored pathways. During this process on the clean surface

Figure 4. Energy changes along the CO dissociation reaction pathway for the clean and K-doped Mo-terminated β Mo2C surface.

(black line), the energy of the system decreases by 1.23 eV with respect to the energy of the molecular adsorbed state with an activation energy barrier of 0.85 eV (see Table 5). A similar Table 5. CO Dissociation Pathways: Adsorption Energy for the Initial State (Eads), Activation Energy (Eact), Decrease on Energy (ΔEdiso), and C−O Distance for the Final Dissociated State

Moterminated

Mo2C K−Mo2C 3K−Mo2C

C-terminated

Mo2C K−Mo2C 3K−Mo2C

Eads, eV

Eact, eV

ΔEdiso, eV

C−O distance, Å

−2.07 −2.33a −2.40b −2.64a −2.82b −1.49 −1.93 −2.65

0.85 0.96 1.62 1.35 1.97 1.21 1.70 1.83

−1.23 −1.19 −1.49 −1.22 −1.22 −1.04 −0.56 +0.12

3.04 3.06 4.35 3.16 3.15 3.62 2.96 3.5

Figure 5. Energy changes along the CO dissociation reaction pathway for the clean and K-doped C-terminated β Mo2C surface.

energetically favorable configurations for C and O atoms were found. For the K−Mo2C surface, the decrease in the energy of system during the dissociation process is lowered, while a dissociated state on 3K−Mo2C surface even represents an increase in the energy of the system by 0.12 eV. These results are qualitatively similar to those obtained on the Moterminated surface: the presence of K on the surface hinders the CO dissociation. For both surfaces, the dissociation starting from CO on top resulted in much higher energy barriers (around twice the value of 3F); so according to our calculations, dissociation on these sites is more difficult. As shown in Figures 4 and 5, on both Mo- and C-terminated surfaces, the energetically most favorable site for CO dissociation is K−Mo2C (3F2). Note that a former study on a K-covered Mo(110) surface23 revealed that the presence of potassium cannot promote considerably the decomposition of CO, since the K-free surface was too active. Similarly, the Mo2C surface is quite reactive with CO, leading to its complete dissociation at 300−355 K.14 The driving force for this is the high affinity of Mo2C toward oxygen. This can explain that, in contrary to that found for platinum metals,1,2 on Mo2C, the O atoms are bonded not in the close vicinity of K adatoms but are coordinated to Mo atoms instead (Figure 5, inset). Even if the dissociation barrier of CO is enhanced in the presence of K, experimental findings prove that, provided the temperature is high enough to ensure the activation, the extent of CO decomposition at and above 1 ML K coverage is increased up to ∼100%, as shown by the development of TDS peaks characteristic for recombinative CO desorption.14 By comparing the calculated energy values listed in Table 5 to previous experimental data,14 it can be said that they are in

3F2, 3F site second neighbor to K atom. b3F1, 3F site first neighbor to K atom.

a

energy barrier of about 0.89 eV for CO dissociation on β-Mo2C was reported by Shi et al.17 In the case of the K-doped surface (K−Mo2C), the dissociation of the CO molecule on 3F sites located at first and second neighbors to the K atom was considered. For CO second neighbor to K (pink dashed line), we observed similar dissociation energy on the pathway and a small increase in the activation energy barrier up to 0.96 eV. In the case of CO first neighbor to K (red dotted line), which is the most favorable adsorption site energetically, the activation energy increases even more up to 1.62 eV. Considering higher K coverages (3K−Mo2C), the activation energy barrier is even higher: 1.35 eV for CO second neighbor to K (light blue dashdotted line) and 1.97 eV for CO first neighbor to K. On the Kdoped surfaces, the stronger adsorption sites, those closer to K, present higher energy barriers for dissociation, making dissociation more difficult than on the clean surface. Theoretical treatment of CO2 adsorbed on K-modified Pt(111) (a surface with an electronic structure similar to 24579



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Table 6. CO Stretching Frequencies, cm−1 (2119 cm−1 for Isolated CO) Mo2C 3F T

K−Mo2C

3K−Mo2C

Mo-terminated

C-terminated

Mo-terminated

C-terminated

Mo-terminated

C-terminated

1747 1926

1582 2188

1512 1847

1462 2097

1340 1282

1204 1990

On K-doped surfaces, the most favorable adsorption sites are those closer to K, making the dissociation more difficult. Also, the activation energy barrier for dissociation increases with the K content. The computed shift in the CO vibrational frequency agrees with previous HREEL data.

qualitative agreement. At higher K coverages, an increase in the CO adsorption energy was reported, see TPD spectra for the “K-CO complex” decomposition with Tp of 615 K (Figures 1B and 2A14) as compared with Tp of 450 K for CO desorption from the K-free surface. It is interesting that, although the adsorption energy of CO is higher on all K-covered surfaces than on K-free ones, its sticking coefficient is strongly reduced at higher K coverages (see Figure 1B14), suggesting that the activation energy of CO adsorption is enhanced at ΘK > ∼1 ML. Another interesting point is that the calculated activation energy for CO dissociation is lower at any potassium coverage than Eads (Table 5), so CO dissociation could easily occur at lower temperatures than CO desorption. Yet, it was observed that some surface K−CO complex characterized by 1485 cm−1 loss feature was stable at 450−505 K,14 indicating the absence of CO decomposition in this temperature range. The reason for this may be that the dissociation of part of adsorbed CO is hindered due to the lack of bonding sites for the dissociation products on the surface crowded with K, C, and O atoms and CO molecules. Increasing the temperature to 505−620 K, the desorption of CO (see Figures 1B and 6C14) frees up bonding sites for the dissociation products of CO, that is, for C and O atoms, and the low enough activation energy for CO dissociation (Table 5) makes possible a desorption controlled decomposition of CO. Vibrational Frequencies. The calculated stretching frequencies for the CO molecule are shown in Table 6. The stretching vibration mode for the isolated molecule is 2119 cm−1. Preadsorbed K produces a shift to lower frequency values. The CO bond is elongated (see Table 1) and possibly weakened, giving rise to lower stretching frequencies. Comparison of present calculated frequencies with experimental HREELS data for CO adsorbed on a K-modified Mo2C surface can be preformed (Figure 5A14); a peak of 2100 cm−1 for a K-free surface can be assigned to CO molecularly adsorbed on top sites. This peak is gradually shifted to lower frequencies with increasing K coverage. At higher coverages, peaks at 1305−1375 cm−1 were measured, which can be associated with our calculated frequencies for CO on 3F sites, shown in Table 6.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +54-291-4595101 ext 2800-2811; fax: 54-02914595142; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from PICT 1770, MINCyT-ARC/11/01, and SGCyT-UNS PGI 24/F048. A.J., C.P, and M.E.P. are members of CONICET. We also thank A.P. Farkas and F. Solymosi (from Hungarian Academy of Sciences) for their fruitful discussions and support by Hungarian Scientific Research Fund through OTKA K81660.

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REFERENCES

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CONCLUSIONS CO adsorption and dissociation on clean and K-doped Mo2C surfaces were computed using periodic DFT calculations. CO adsorbs more strongly on Mo-terminated surfaces than on Cterminated ones. The adsorption of CO is energetically more favorable in the presence of K preadsorbed on the surface. The lowest energy adsorption sites are those closer to the K atom. In all cases, the CO molecule withdraws charge from the surface, being more extended on the K-doped surface. With the increase of K coverage, CO binds more strongly to the surface, C−O distance increases, and the calculated stretching frequencies decrease. On the clean surface, CO dissociation leads to stable geometries, being the C-terminated surface energetically less favorable than the Mo-terminated surface. 24580



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Theoretical Model for CO Adsorption and Dissociation on Clean and K

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