Adsorption and Dissociation of CO x (x= 1, 2) on W (111) Surface: A

J. Phys. Chem. C 2008, 112, 3341-3348

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Adsorption and Dissociation of COx (x ) 1, 2) on W(111) Surface : A Computational Study Hsin-Tsung Chen, Djamaladdin G. Musaev,* and M. C. Lin* Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322 ReceiVed: October 1, 2007; In Final Form: NoVember 8, 2007

The mechanisms of adsorption and dissociation of COx (x ) 1, 2) molecules on the W(111) surface have been investigated at the periodic density functional theory (DFT) level in conjunction with the projectoraugmented wave (PAW) approach. The molecular structures, vibrational frequencies, and binding energies of W(111)/CO2, W(111)/CO, W(111)/C, and W(111)/O systems were analyzed. It was found that the most favorable adsorption configuration of W(111)/CO2 is the WCO2(IV-µ3-C2, O1, O1) configuration, where the bent CO2 molecule is located at the threefold-shallow site of the surface. The W(111)-CO2 bonding energy is calculated to be -37.6 kcal/mol. For the W(111)/CO, the β-CO configuration, WCO(II-µ2-C1, O1), with CO at the bridge site, is energetically the most favorable one. The calculated W(111)-CO adsorption energy is -37.9 kcal/mol. The C and O atoms are bound preferentially at the bridge and top sites, respectively. Potential energy surface for the decomposition of CO2 on W(111) has been constructed using the nudged elastic band (NEB) method. It was shown that the overall reaction CO2(g) + W(111) f LM1 f LM2 f P1 is exothermic by 61.0 kcal/mol and does not require thermal activation energy. Therefore, we expect the CO2 molecule to be in its dissociative adsorption state (into atomic C and O) on the W(111) surface. We also have predicted the rate constants for CO2 and CO dissociation on the W(111) surface.

I. Introduction Understanding the mechanism and factors governing the reaction of small molecules (such as NOx (x ) 1, 2), N2, COx (x ) 1, 2), O2, H2O, hydrocarbons, etc.) with transition metal surfaces is essential for designing novel and more efficient catalysts for important chemical processes, as well as new materials with unconventional physicochemical properties. In this aspect, elucidating the mechanism of the reaction of combustion gases with tungsten and tungsten alloys, which are important materials and widely used in high-temperature environments,1 is highly desirable. Previously, we have reported the mechanism and energy for the reaction of the W(111) surface with water molecules.2 In the present paper, we investigate the mechanism for the reaction of the W(111) surface with COx (x ) 1, 2) molecules. The interactions of CO2 with metal surfaces have been extensively studied using various techniques such as TPD, LEED, EELS, HREELS, XPS, and UPS.3-10 These studies have demonstrated that the CO2 molecule adsorbs only weakly on Pt,3 Pd,4 Cu,5 and Ag6 metal surfaces, but it dissociates to CO and oxygen atoms on Fe,7 Ni,8 Rh,9 and Re10 single-crystal surfaces. However, to our knowledge, the reaction of the W(111) surface with CO2 has not been a subject of previous studies, while the interaction of the CO molecule with W surfaces has been a subject of several experimental11-15 and theoretical16,17 papers. In the present paper, we study the adsorption and dissociation of both CO2 and CO on the W(111) surface using the density functional theory (DFT) with the projectoraugmented wave (PAW) approximation. We analyze the mechanisms of the reactions W(111) + COx, where x ) 1, 2, as well * Corresponding authors. E-mail: [email protected] (M.C.L.) and [email protected] (D.G.M.).

as structures, energies, and vibrational frequencies of all intermediates, transition states, and products of these reactions. II. Computational Procedures Reactants, intermediates, transition states, and products of the reactions W(111) + COx were calculated at the spin restricted DFT plane-wave level utilizing Vienna ab initio simulation package (VASP)18-20 with the projector-augmented wave method (PAW).21,22 In these calculations, we used the PerdewWang (PW91)23 and the revised Perdew-Burke-Ernzerhof (rPBE)24,25 functionals. The Brillouin zone was sampled with the Monkhorst-Pack grid. The calculations were carried out using (4 × 4 × 4) and (4 × 4 × 1) Monkhorst-Pack mesh26 k points for bulk and surface calculations, respectively, and 400 eV cutoff energy. The p(2 × 2) cell of W(111) surface was modeled as periodically repeated slabs with 6 layers (Figure 1a). In our calculations, the bottom three layers were frozen at the estimated bulk parameters, while the remaining layers were fully optimized. All slabs were separated by a vacuum spacing g15 Å, which guarantees no interaction between the slabs. Reported adsorption energies in this paper were calculated on the basis of the following equation:

∆Eads ) E[slab + adsorbate] - (E[slab] + E[adsorbate]) (1) where E[slab + adsorbate], E[slab], and E[adsorbate] are the calculated electronic energies of adsorbed species on the W(111) surface, clean (from the adsorbates) surface, and gas-phase molecule, respectively. Reported vibrational frequencies were obtained by diagonalizing the Hessian matrix of selected atoms within the VASP approach. The nudged elastic band (NEB) method27,28 was

10.1021/jp709575r CCC: $40.75 © 2008 American Chemical Society Published on Web 02/08/2008

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Figure 1. Slabs models for W(111) surface: (a) side view and (b) top view.

TABLE 1: Geometrical Parameters and Vibrational Frequencies of Gas-Phase CO2 and CO Molecules Calculated at the rPBE and PW91 (in Parentheses) Levels of Theory molecule symmetry

R(C-O) (Å) θ(O-C-O-) (deg) Vasym (cm-1) Vsym (cm-1) Vbend (cm-1) a

CO2 D∞h

CO C∞V

calcd

expta

calcd

exptb

1.176 (1.175) 180.0 (180.0) 2370 (2377)c 1321 (1321) 640 (644)

1.193 180.0 2349 1333 667

1.141 (1.142)

1.128

2147 (2136)

2170

From ref 33. b From ref 34. c Unscaled vibrational frequencies.

applied to locate transition states. The NEB method is an efficient method for finding the minimum energy path (MEP) between a given initial state and final state. The NEB method is a chain-of-states method, where a set of images between the initial and the final states must be created to achieve a smooth curve. At least eight images were used for each TS calculation. The rate constants for the dissociation reaction of CO2 on the W(111) surface were calculated using the variational RRKM theory as implemented in the Variflex code.29 III. Results and Discussions Computed lattice constants for bulk tungsten are 3.183 and 3.181 Å, at the PW91 and rPBE levels of theory, respectively, which are in good agreement with the experimental value of 3.165 Å.30 The calculated W-W bond distance vary within 2.754-2.750 Å, which also is consistent with the experimental value of 2.741 Å.30 As summarized in Table 1, the predicted geometrical parameters and vibrational frequencies of gas-phase CO2 and CO in a 15 Å cubic box using the VASP are in good agreement with available experimental data, too. In order to locate possible W(111)-adsorbate intermediates, the CO2 molecule, as well as its derivatives, CO, C, and O were placed on various sites of the W(111) surface, as shown in Figure 1b. For this purpose, we have considered four different adsorption sites on the surface, such as top, I (on one W atom of the first layer); bridge, II, (on the W-W bond, including W1-W1, W1-W2, and W2-W3 bonds); threefold-deep, III,

(on the top of the third layer W atom), and threefold-shallow, IV (on the top of the second layer W atom) sites of the surface. The notation (Cm, On), used below, indicates that C and O atoms are coordinated to the mth and nth layer W atoms, respectively. All localized intermediates and products of the reaction W(111) + COx are given in Figures 2-4, while the calculated adsorption energies of CO2, CO, C, and O on W(111) are summarized in Tables 2-4. Cartesian coordinates of all reported structures are given in Supporting Information. III.1. Geometry and Adsorption Properties of the W(111)/ CO2 System. As it should be expected, the first intermediate of the reaction of W(111) with CO2 is W(111)/CO2, which could have several isomers, WCO2(I-η1-C1), WCO2(I-η1-O1), WCO2(I-η2-O1, O1), WCO2(I-η2-C1, O1), WCO2(II-µ2-O1), WCO2(II-µ2-O1, O1), WCO2(II-µ2-O1, O1-b), WCO2(III-µ3-C3, O1, O1), and WCO2(IV-µ3-C2, O1, O1), as shown in Figure 2. (Here and in Figure 2, O1-b stands for O atom coordinated to two W atoms from the first layer.) Previously, it has been shown that the PW91 functional overestimates the calculated chemisorption energies of small molecules on metal surfaces compared with rPBE calculated and experimental values.31 Compared with the experimental values, the PW91 functional gives too large chemisorption energies numerically by about 14 kcal/mol, while the rPBE functional proves to be rather accurate with less than 5 kcal/ mol for CO on Ni(111) and Pd(111) surfaces.31 This finding is consistent with the conclusion of Schreiner,32 showing that PBE functional provides better agreement with experiments than BLYP and B3LYP functional. A similar trend was found in this paper (see Table 2); therefore, below, we discuss only rPBEcalculated data. Our data show that the isomer WCO2(IV-µ3C2, O1, O1) is energetically the most favorable one among all calculated structures of W(111)/CO2 and has an adsorption energy of -37.6 kcal/mol (see Table 2). As seen in Figure 2 and Table 2, the WCO2(IV-µ3-C2, O1, O1), WCO2(III-µ3-C3, O1, O1), WCO2(II-η2-O1, O1-b), and WCO2(I-η2-C1, O1) isomers of W(111)/CO2 have short W-C and W-O bond distances and relatively long C-O distances and are the chemisorbed species. The calculated W(111)-CO2 adsorption energies for these isomers are -37.6, -16.1, -4.9, and -6.5

COx (x ) 1, 2) on W(111) Surface

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Figure 2. Located isomers of adsorbed CO2 on W(111) surface and their important geometry parameters calculated at the rPBE and PW91 (in parentheses) levels of theory. Distances are in angstroms, and angles are in degrees. Their full geometries are given in Supporting Information.

kcal/mol, respectively. Meanwhile, structures WCO2(II-µ2-O1, O1), WCO2(I-η1-O1), WCO2(II-µ2-O1), and WCO2(I-η1-C1) can be characterized as physisorbed species with short C-O and long W-C/W-O bond distances. One should note that a structure like WCO2(II-µ2-C1, O1) is not stable and converges to a WCO2(IV-µ3-C2, O1, O1) structure upon optimization. We have also performed calculations for CO2 adsorption on (1 × 1), (2 × 2), and (3 × 3) ad-layer surfaces, corresponding to coverage of 1ML, 1/4 ML, and 1/9 ML, respectively, using the most stable WCO2(IV-µ3-C2, O1, O1) configuration. These studies show that the coverage effect on the calculated stability of this specie is negligible (smaller than 0.1 eV). III.2. Geometry and Adsorption Properties of the W(111)/ CO System. The coordination of the CO molecule to the W(111) surface leads to formation of W(111)/CO intermediate, which may exist in several isomeric forms: (a) the R states such as WCO(I-η1-C1), WCO(I-η1-O1), WCO(III-η1-C2), and WCO(IV-η1-C3); and (b) the β states (as being in inclined with respect to the surface) WCO(II-µ2-C1, C1), WCO(II-µ2-C1, O1), WCO(II-µ2-C1, O2), and WCO(II-µ2-C2, O1), presented in Figure 3. As our calculations show (see Table 3), the isomer WCO(II-µ2-C1, O1) of β state is energetically the most favorable one among all calculated structures with an adsorption energy of -37.9 kcal/mol, which is consistent with -41.3 kcal/mol reported by Johnson and co-workers.17 The WCO(II-µ2-C2, O1) and WCO(II-µ2-C2, O3) isomers of β state are only slightly (0.2 and 1.0 kcal/mol, respectively) higher in energy. The WCO-

(IV-η1-C2) structure is the most favorable isomer of R state with adsorption energy of -33.1 kcal/mol. Its WCO(I-η1-C1) isomer is only 3.8 kcal/mol higher in energy. These bonding energies are somewhat larger than experimental values of -18.2 kcal/mol reported by Hwu et al.14 and -26.1 kcal/mol reported by Ryu et al.16 but are in agreement with a recent theoretical value of -31.6 kcal/mol reported by Johnson and co-workers.17 The difference between the experimental and the theoretical results may be due to limitations of the Redhead analysis or to the over binding at the DFT calculations. The linear isomer WCO(I-η1-O1) where CO coordinated to W with its O atom is less stable than isomer WCO(I-η1-C1) where CO coordinated to W with its C atom. III.3. Geometry and Adsorption Properties of W(111)/C and W(111)/O Systems. The coordination of C and O atoms to W(111) leads to W(111)/C and W(111)/O intermediates, respectively. The resulted species may have four different isomers: WX(I-η1-X1), WX(II-µ2-X1, X1), WX(II-µ2-X1, X2), and WX(IV-η1-X2), where X ) C, O, shown in Figure 4. One should note that optimization of threefold-deep site structures converges to the bridge site WX(II-µ2-X1, X1) species for both X ) C and O. Furthermore, optimization of threefold-shallow site structures converges to the bridge site WX(II-µ2-X1, X2) structure for X ) C. As seen in Table 4, the radical adsorbates, atomic C and O, adsorb strongly to the W(111) surface. Similar to the recent theoretical result of Johnson and co-workers,17 the C and O atoms adsorb preferentially on the bridge site (II) and top site

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Figure 3. Located isomers of adsorbed CO on W(111) surface and their important geometry parameters calculated at the rPBE and PW91 (in parentheses) levels of theory. Distances are in angstroms, and angles are in degrees. Their full geometries are given in Supporting Information.

Figure 4. Located isomers of adsorbed X [X ) O, C (in italic)] atoms on W(111) surface and their important geometry parameters calculated at the rPBE and PW91 (in parentheses) levels of theory. Distances are in angstroms, and angles are in degrees. Their full geometries are given in Supporting Information.

(I), respectively. The adsorption energies of WC(II-µ2-C1, C1) and WO(I-η1-O1) are calculated to be -188.3 and -142.3 kcal/mol, which are in reasonable agreement with -170.9 and -145.0 kcal/mol, respectively, reported by Johnson and coworkers.17 According to our findings, carbon atoms are most likely to occupy the bridge sites with large adsorption energy. Since the carbon atom preferentially occupies the bridge sites of the surface, it may inhibit dissociative adsorption of the β-state CO. This finding is consistent with the observation of Hwu et al.14 who have showed that the carbon modified W(111) surface is not reactive with CO. III.4. Dissociation of CO2 on the W(111) Surface. To characterize possible reaction pathways of CO2 adsorption/ dissociation on the W(111) surface, we have selected energetically the most stable configurations of above presented intermediates and products (such as WCO2(IV-µ3-C2, O1, O1), WCO(II-µ2-C1, C1), WCO(II-µ2-C2, O1), WC(II-µ2-C1, C1),

TABLE 2: Adsorption Energies (kcal/mol) of CO2 Molecule on W(111) Surface Calculated at the rPBE and PW91 Levels of Theory site On-top (I) I-η1-C1 I-η1-O1 I-η2-O1, O1 I-η2-C1, O1 Bridge (II) II-µ2-O1, O1 II-µ2-O1, O1-b II-µ2-O1 Threefold-deep (III) III-µ3-C3, O1, O1 Threefold-shallow (IV) IV-µ3-C2, O1, O1

adsorption energy (pw91)

adsorption energy (rPBE)

7.6 -2.3 12.9 -12.5

12.1 1.4 20.9 -6.5

-2.3 -14.9 3.8

3.3 -4.9 5.7

-28.8

-16.1

-48.4

-37.6

COx (x ) 1, 2) on W(111) Surface

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TABLE 3: Adsorption Energies (kcal/mol) of CO molecule on W(111) Surface Calculated at the rPBE and PW91 Levels of Theory site On-top (I) I-η1-C1 I-η1-O1 Bridge (II) II-µ2-C1, O1 II-µ2-C1, C1 II-µ2-C1, O2 II-µ2-C2, O1 II-µ2-C2, O3 Threefold-deep (III) III-η1-C3 Threefold-shallow (IV) IV-η1-C2

state

adsorption energy (PW91)

adsorption energy (rPBE)

R R

-34.1 -1.1

-29.3 -2.6

β β β β β

-46.4 -40.0 -39.6 -45.3 -45.5

-37.9 -33.6 -31.4 -37.7 -36.9

R

-17.7

-10.3

R

-38.6

-33.1

TABLE 4: Adsorption Energies (kcal/mol) of Atomic O and C on W(111) Surface Calculated at the rPBE and PW91 Levels of Theory configuration O atom on-top (I-η1-O1) bridge (II-µ2-O1, O1) bridge (II-µ2-O1, O2) threefold-shallow(IV-η1-O2) C atom on-top (I-η1-C1) bridge (II-µ2-C1, C1) bridge (II-µ2-C1, C2) threefold-shallow(IV-η1-C2)

adsorption energy(PW91)

adsorption energy(rPBE)

-155.6 -152.4 -149.1 -133.6

-142.3 -136.9 -131.3 -119.3

-125.5 -195.5 -174.9

-120.0 -188.3 -168.4

and WO(I-η1-O1)) to construct the minimum energy paths (MEPs) using the NEB method. The proposed elementary steps to analyze for this purpose are the followings:

CO2(g) + W(111) f W(111)/CO2(ads)

(2)

W(111)/CO2(ads) f CO(ads)/W(111)/O(ads)

(3)

CO(ads)/W(111)/O(ads) f C(ads)/W(111)/O(ads)/O(ads) (4) The optimized structures of intermediates, transition states, and products of these processes are presented in Figure 5 with their important geometrical parameters. Full geometries of these structures are given in Supporting Information. In Figure 6, we schematically represent the potential energy profile of the overall reaction

CO2(g) + W(111) f C(ads)/W(111)/O(ads)/O(ads)

(5)

As seen in Figure 6 and discussed above, the adsorption of CO2(g) on the W(111) surface is exothermic by 37.5 kcal/mol and leads to WCO2(IV-µ3-C2, O1, O1) (LM1) intermediate. From the resulted intermediate LM1, reaction proceeds via the C-O bond activation at the transition state TS1 and formation of intermediate LM2 with the adsorbed CO and O in WCO(II-µ2-C2, O1) and W(I-η1-O1) manner, respectively. As seen in Figure 5, in the transition state TS1, the breaking C-O bond length is about 1.946 Å, which is a 0.596 Å larger than that in the molecularly adsorbed CO2 (in LM1). The formed W-CO(II-µ2-C2, O1) and W-(I-η1-O1) bond distances are 2.055 and 1.810 Å, respectively, which are closer to those, 1.986 and 1.738 Å, in product LM2 than 2.162 and 2.068 Å in reactant LM1. In other words, transition state TS1 is mostly late transition

state (more product-type than reactant-type). As seen from Figure 6, the process LM1 f TS1 f LM2 occurs with a 12.7 kcal/mol barrier and is exothermic by 11.6 kcal/mol. The second deoxygenation starts from the LM2 intermediate and leads to the final product, P1, with coadsorbed WC(II-µ2C1, C1) and two WO(I-η1-O1) fragments. This process occurs with a relatively large (almost twice larger than the first C-O bond activation barrier) activation barrier, 23.8 kcal/mol, at the transition state TS2, which is somewhat lower than the energy barriers of 27.4 and 32.5 kcal/mol reported by Johnson and coworkers.17 As seen in Figure 5, in the transition state TS2, the O atom of activating CO moves toward an adjacent W on-top site and its C atom eventually ends up in a twofold (bridge) site. The calculated C-O distance in the breaking C-O bond is 1.493 Å indicating that TS2 is a relatively early transition state. This is consistent with the calculated exothermic character of the process: reaction LM2 f P1 is found to be exothermic by 11.9 kcal/mol. Thus, overall reaction CO2(g) + W(111) f LM1 f LM2 f P1 is found to be exothermic by 61.0 kcal/mol and does not require thermal activation energy because all calculated transition states are energetically lower than the reactants CO2(g) + W(111). In summary, the CO2 molecule is expected to be in its dissociative adsorption state (into atomic C and O) on the W(111) surface. As it could be expected, intermediate LM2 as well as product P1 have several isomers lying higher in the energy. Indeed, the enegetically closest isomer of LM2, structure LM3, lies only 8.4 kcal/mol higher in energy. Similarly, the isomer of P1, structure P2, lies 8.7 kcal/mol higher in energy. Here, we will not discuss in details all of these isomers, which separate from their energetically most favorable counterparts with a small barrier and are not expected to make a significant contribution to the overall energy and mechanism of the entire reaction. However, we have presented some of those isomers in Figure 5, as well as in Supporting Information. In addition, we have investigated the diffusion of CO2 and CO molecules, and C and O atoms on the W(111) surface. For this purpose, we have selected the two energetically most favorable bonding sites as initial and final states of the reaction. Diffusion of the bent CO2, starting from the configuration WCO2(IV-µ3-C2, O1, O1) to give structure WCO2(II-µ3-C3, O1, O1), occurs by 26.7 kcal/mol barrier, which is much higher than the first C-O bond activation barrier, 12.7 kcal/mol, initiated from the same WCO2(IV-µ3-C2, O1, O1) reactant. Diffusion of CO molecule from its β-state structure WCO(IIµ2-C1, O1) to its R-state structure WCO(I-η1-C2) occurs with a 35.1 kcal/mol barrier, while the barrier of the reverse process is significantly smaller and is only 16.8 kcal/mol. An oxygen atom diffusion, that is, reaction WO(I-η1-O1) f WO(II-µ2-O1, O1), occurs by 15.8 kcal/mol energy barrier. The calculated diffusion barrier of the carbon atom, that is, reaction WC(II-µ2-C1, C1) f WC(III-µ2-C1, C2) requires a very large energy barrier, 50.0 kcal/mol. In other words, the strong binding and high diffusion barrier make the diffusion of C atoms on W(111) unlikely at the modest temperature and pressure conditions. III.5. Frequency Calculations. Although vibrational frequencies of W(111)/CO2 have not been reported previously, the vibrational frequencies of W(111)/CO have been studied experimentally by Hwu et al.14 who have measured HREEL spectra of W(111)/CO. Two major peaks, 400 and 2060 cm-1, and a broad band in the range of 1000-1600 cm-1 were detected at T e 230 K. The 400 cm-1 peak is assigned to the W-CO

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Figure 5. Various intermediates, products, and transition states of the reaction CO2 + W(111), and their important geometry parameters calculated at the rPBE level of theory. Their full geometries are given in Supporting Information.

Figure 6. Schematic presentation of the potential energy profiles of the reaction CO2 + W(111).

vibrational mode, while the 2060 cm-1 and 1000-1600 cm-1 peaks were assigned to the C-O bond stretching of the R-state CO and the C-O stretching mode of the β-state CO, respectively. The calculated (unscaled) frequencies of W(111)/CO2, W(111)/ CO, W(111)/C, and W(111)/O systems are given in Table 5. As seen from this table, the calculated asymmetric V(CO), symmetric V(CO), bent V(OCO), and V(W-CO2) stretching frequencies of the W(111)/CO2 system are in 978-1199, 9531073, 687-718, and 478-494 cm-1 ranges, respectively, for the several CO2-chemisorbed intermediates. These values are similar to the experimental values of 1370, 1072-1170, 766, and 403 cm-1 of the CO2-chemisorbed states on the Fe(111)

surface.7f For the CO2 adsorption intermediates, the calculated frequencies of CO2 are 2268-2385, 1249-1312, and 518-575 cm-1 for the asymmetric V(CO), symmetry V(CO), and bent V(OCO), respectively, which are similar to the experimental values of 2340, 1250, and 645 cm-1 of the adsorbed CO2 on the Fe(111) surface.7f The large red shifts (relative to C-O frequency of the CO2 molecule) are due to the weakening of the C-O bond. The calculated V(CO) frequencies of β states of W(111)/CO are 1329-1776 cm-1, which are consistent with the previously reported experimental values14 of 1000-1600 cm-1 and the theoretical values17 of 1318∼1645 cm-1. Again, the lower frequencies are due to the weakening of the C-O bond. The calculated C-O frequencies for R states are 1829-

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TABLE 5: Vibrational Frequencies of CO2, CO, C, and O Adsorbed on the W(111) Surface Calculated at the rPBE Level frequency (cm-1) Vsym(CO) Vbend(OCO)

site CO2

configuration

Vasym(CO)

I-η1-O1 I-η2-C1, O1 I-µ2-C1, O1 II-µ2-O1, O1 IV-µ3-C2, O1, O1 III-µ3-C3, O1, O1

linear bent bent linear bent bent

2385 1751 1160 2265 1199 978

1312 984 1009 1249 1073 953

575 697 678 518 718 687

CO

configuration

V(C-O)

V(W-CO)

V(W-O)

I-η1-C1 IV-η1-C2 III-η1-C3 II-µ2-C1, O1 II-µ2-C1, C1 II-µ2-C1, O2 II-µ2-C2, O1 II-µ2-C2, O3

R R R β β β β β

1938 1829 1665 1329 1776 1910 1468 1495

352 404 356 429 393 407 418 480

378 374 322 520 398

X ) O or C atom I-η1

-X1 II-µ2-X1, X1 III-µ2-X1, X2 IV-η1-X2

V (W-O)

V(W-C)

874 607 521 750

792 548 658 -

1938 cm-1, which are considerably lower than the experimentally reported value14 of 2060 cm-1 but close to the theoretical value17 of 1722-1930 cm-1. The calculated V(W-CO) frequencies are 352-429 cm-1, which are consistent with the experimental value of ∼ 400 cm-1. Finally, the V(W-C) and V(W-O) frequencies of W(111)/C and W(111)/O systems are 548-792 and 521-874 cm-1, which are in good agreement with experimental14 reported values (676 and 400-800 cm-1, respectively). III.6. Rate Constant Calculations. On the basis of the aforementioned PESs for the dissociation of CO2 on the W(111) surface, we have computed the rate constants of the following two reactions:

CO2(g) + W(111) f CO2(ads) (LM1) f CO(ads) + O(ads) (LM2) f C(ads) + 2O(ads) (P1) CO(g) + W(111) f CO (ads) f C(ads) + O(ads) For the reaction rate constant calculations, the stretching potential energy surfaces representing the barrierless association processes CO2(g)+ W(111) f CO2(ads) (LM1) and CO(g) + W(111) f CO (ads) which are the rate determining pathway were calculated along the reaction coordinate M-C, which is stretched from its equilibrium value to 4.5 Å with the step size of 0.15 Å. At each fixed M-C distance, the geometries of the bottom three atomic layers of the W(111) surface were frozen, while the atoms of remaining layers CO2 and CO were fully optimized at the rPBE level. On the basis of the VTST reported by Wardlaw and Marcus,35 the obtained stretching (M-C separation distance) potential energy surface is approximated with a Morse potential, V(r) ) De{1 - exp[-β(R - R0)]}2, where R is the reaction coordinate, R0 is the equilibrium M-C bond distance, and De is the bond energy without zero-point energy corrections. The parameters used by fitting the Morse potential to the stretching potential energy surface are R0 ) 2.156, β ) 1.76 Å-1, and De ) 39.91 kcal/mol; and R0 ) 2.033, β ) 1.71 Å-2, and De ) 38.91 kcal/mol for CO2(g)+ W(111) and CO(g) + W(111) reactions, respectively. Our calculations have been carried out for the temperature range of 200∼3000 K. The predicted rate constant (in molecular units, cm3/s) in

V(W-CO2) 568 425 314 478 494

the broad temperature range can be represented as

kCO2 ) 1.21 × 10-10 T-0.0028 exp(0.64 kcal mol-1/RT) kCO ) 1.72 × 10-12 T-0.0021 exp(2.88 kcal mol-1/RT) The rate constants for these dissociative adsorption processes are defined by36 the equation

d[X]surf/dt ) k(θ/As)[X]g , which has the unit of a flux, molecule cm-2 s-1. In this equation θ, As, and [X]g represent the fraction of available surface sites, the surface area, and the concentration of CO2 and CO gases in molecules/cm3, respectively. IV. Conclusions The mechanisms for the adsorption and dissociation of COx (x ) 1, 2) molecules on the W(111) surface have been studied at the density functional theory (DFT) level in conjunction with the PAW approach. It was found that the favorable adsorption configuration of W(111)/CO2 is a bent-CO2 WCO2(IV-µ3-C2, O1, O1) configuration with an adsorption energy of -37.6 kcal/mol. For the W(111)/CO, the β-CO configuration, WCO(II-µ2-C1, O1), with CO at the bridge site, is energetically the most favorable one. The calculated W(111)-CO adsorption energy is -37.9 kcal/mol. The C and O atoms bound preferentially at bridge and top sites, respectively. A potential energy surface for the decomposition of CO2 on W(111) has been constructed using the nudged elastic band (NEB) method. It was shown that the overall reaction CO2(g) + W(111) f LM1 f LM2 f P1 is exothermic by 61.0 kcal/mol and does not require thermal activation energy because all the calculated transition states (TS1 and TS2, the first and second C-O bond activation transition states) are energetically lower than reactants CO2(g) + W(111). Therefore, we expect the CO2 molecule to be in its dissociative adsorption state (into atomic C and O) on the W(111) surface. Our results for the reaction of CO with W(111) are in good agreement with the reported HREELS data.14 We also have

3348 J. Phys. Chem. C, Vol. 112, No. 9, 2008 predicted the rate constants for CO2 and CO dissociation on the W(111) surface. Acknowledgment. We gratefully acknowledge (1) financial support from the Office of Naval Research under a MURI grant (Prime Award # N00014-04-1-0683 and Subaward # 2794-EUONR-0683), (2) the Emerson Center for Scientific Computation for the use of its resources, and (3) the use of CPU’s from National Center for High-Performance Computing, Taiwan. M.C.L. also wants to thank the National Science Council of Taiwan and Taiwan Semiconductor Manufacturing Company for Distinguished Professorships at the National Chiao Tung University in Hsinchu, Taiwan. Supporting Information Available: Cartesian coordinates (in Å) of all structures reported in this paper and calculated at the rPBE level. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Song, G. M.; Wang, Y. J.; Zhou, Y. Int. J. Refract. Met. Hard. Mater. 2003, 21, 1. (2) Chen, H.-T.; Musaev, D. G.; Lin, M. C. J. Phys. Chem. C 2007, 111, 17333. (3) (a) Liu, Z. M.; Zhou, Y.; Solymosi, F.; White, J. M. J. Phys. Chem. 1989, 93, 4383. (b) Liu, Z. M.; Zhou, Y.; Solymosi, F.; White, J. M. Surf. Sci. 1991, 245, 289. (4) (a) Berko´, A.; Solymosi, F. Surf. Sci. 1986, 171, L498. (b) Egawa, C.; Doi, I.; Naito, S.; Tamaru, K. Surf. Sci. 1986, 176, 491. (5) Rodriguez, J. A.; Clendening, W. D.; Campbell, C. T. J. Phys. Chem. 1989, 93, 5238. (6) (a) Sakurai, M.; Okano, T.; Tuzi, Y. J. Vac. Sci. Technol. A 1987, 5, 431. (b) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Chem. Phys. Lett. 1982, 89, 48. (c) Backx, C.; De Groot, C. P. M.; Biloen, P.; Sachtler, W. M. H. Surf. Sci. 1983, 128, 81. (7) (a) Behner, H.; Spiess, W.; Wedler, G.; Borgmann, D. Surf. Sci. 1986, 175, 276. (b) Bauer, R.; Behner, H.; Borgmann, D.; Pirner, M.; Spiess, W.; Wedler, G. J. Vac. Sci. Technol. A 1987, 5, 1110. (c) Behner, H.; Spieess, W.; Wedler, G.; Borgmann, D.; Freund, H.-J. Surf. Sci. 1987, 184, 335. (d) Freund, H.-J.; Behner, H.; Bartos, B.; Wedler, G.; Kuhlenbeck, H.; Neumann, M.: Surf. Sci. 1987, 180, 550. (e) Pirner, M.; Bauer, R.; Borgmann, D.; Wedler, G. Surf. Sci. 1987, 189/190, 147. (f) Hess, G.; Froitzheim, H.; Baumgartner, C. Surf. Sci. 1995, 138, 331-333. (8) (a) Benziger, J. B.; Madix, R. J. Surf. Sci. 1979, 79, 394. (b) Bartos, B.; Freund, H.-J.; Kuhlenbeck, H.; Neumann, M.; Lindner, H.; Muller, K. Surf. Sci. 1987, 179, 59. (9) (a) Dubois, L. H.; Somorjai, G. A. Surf. Sci. 1980, 91, 514. (b) Solymosi, F.; Kiss, J. Surf. Sci. 1985, 149, 17. (c) Solymosi, F.; Klivenyi,

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