Adsorption and Dissociation of H2O on a W (111) Surface: A

Oct 27, 2007 - Adsorption and reaction of CO and H 2 O on WC(0001) surface: A first-principles investigation. Yu-Jhe Tong , Shiuan-Yau Wu , Hsin-Tsung...
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J. Phys. Chem. C 2007, 111, 17333-17339

17333

Adsorption and Dissociation of H2O on a 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, Atlanta, Georgia 30322 ReceiVed: June 9, 2007; In Final Form: September 6, 2007

The adsorption and dissociation of water on a W(111) surface have been studied at the density functional theory (DFT) level in conjunction with the projected augmented wave approach. The potential energy surface of the water decomposition on the W(111) surface was constructed. It was shown that the barriers for the stepwise H2O dehydrogenation reaction, H2O f 2H(ads) + O(ads), are 1.8 (for HO-H bond activation) and 15.9 (for the O-H bond activation) kcal/mol. The entire process, W(111) + H2O f 2H(ads) + O(ads), is 54.4 kcal/mol exothermic. Calculations show that the formation of W(111)-O + H2(gas) from W(111) + H2O is also exothermic by 23.7 kcal/mol. These results are in good agreement with the temperature-programmed desorption and high-resolution electron energy loss spectroscopy data. On the basis of the calculated PES, we predicted kinetic rate constants for the dissociative adsorption of H2O on the W(111) surface. The structure, vibrational frequency, and binding energy of the W(111)-H2O, W(111)-OH, W(111)-O, and W(111)-H systems were also predicted. It was shown that the most favorable structure of W(111)-H2O corresponds to the coordination of water through its oxygen lone pairs with the W(111) surface (at its top position). The preferable binding sites for the OH, O, and H fragments are top, top, and bridge sites, respectively. The stepwise dissociative adsorption mechanism of H2O on the W(111) surface has also been confirmed by DFT molecular dynamics simulations.

1. Introduction Water is the most abundant compound in the biosphere and covers most real solid surfaces. The interaction of water with solid surfaces1,2 plays a pivotal role in a variety of phenomena in nature such as catalysis, weathering, electrochemistry, corrosion, rock efflorescing, and more. A detailed understanding of the water-surface interaction is essential to design, optimize, and control such processes. Therefore, the elucidation of the water-surface interaction mechanism continues to be the focus of numerous investigations.1,2 For instance, recently, the interaction of water monomers, dimers, and hexamers with Ag(111),3 Cu(111),4 Pd(111),5 Ru(0001),6 and Pt(111)7 surfaces were studied by scanning tunneling microscopy (STM). Density functional theory (DFT) calculations were applied to study water adsorption on Cu(111),8 Cu(110),9 Pt(111), Rh(111), Pd(111), Au(111),10 and Ru(0001)10 surfaces. These and many other11,12 studies have identified two types of interactions involving water molecules on most metal surfaces: (1) the water-surface interaction containing metal-oxygen (M-O) and metalhydrogen (M-HO) bonds and (2) the inter-water interaction via a hydrogen-bonding pattern. Recently, Feibelman has demonstrated that, in fact, the water-surface interaction could be even more complex. They have proposed a new structure pattern for the water-Ru(0001) system, where intact and partially dissociated water molecules alternated on the surface.11 Many experiments on the reaction mechanism of water on W surfaces have also been reported.13-15 It was shown that the adsorption of water on the W surface occurs via a dissociative pathway, H2O f OH + H, while the water desorption proceeded via an OH + OH f H2O + O mechanism. Later, the produced free O atom oxidizes W to WO3. Water adsorption on W(111) * Corresponding author. E-mail: [email protected].

also was investigated for various modified W(111) surfaces using temperature-programmed desorption (TPD) and highresolution electron energy loss spectroscopy (HREELS) methods.16 Bryl and co-workers17 studied the interaction of water with clean and gold-precovered W field emitters and have shown that water decomposes on clean W(111), producing surface oxygen and hydrogen gas. To our best knowledge, there is no theoretical study of water decomposition on W(111). In this paper, we used DFT to study adsorption and dissociation of water on the W(111) surface. 2. Computational Methods All presented calculations were performed by the Vienna ab initio simulation package (VASP).18,19 In these calculations, we used a plane-wave approach20 in conjunction with the PerdewWang (PW91)21 and revised Perdew-Burke-Ernzerhof (rPBE) densitiy functionals.22,23 The Brillouin zone was sampled with the Monkhorst-Pack grid.24 The calculations were carried out the (4 × 4 × 4) and (4 × 4 × 1) Monkhorst-Pack mesh k-points for bulk and surface calculations, respectively. A 400 eV cutoff energy was chosen, which allows convergence to 0.01 eV in the total energy. The p(2 × 2) and p(3 × 3) lateral cells of the W(111) surface were modeled as periodically repeated slabs with six atomic layers (see Figure 1). The bottom three atomic layers were kept frozen and set to the estimated bulk parameters, while the remaining layers were fully relaxed during the calculations. All slabs were separated by a vacuum spacing greater than 15 Å, which guaranteed no interactions between the slabs. In this study, we calculated adsorption energies according to the following equation:

∆Eads ) E[slab + adsorbate] - (E[slab] + E[adsorbate])

10.1021/jp074472c CCC: $37.00 © 2007 American Chemical Society Published on Web 10/27/2007

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Chen et al.

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

TABLE 1: Important Geometry Parameters and Vibrational Frequencies of H2O and OH Molecules Calculated at the rPBE and PW91 Levels of Theory molecule

H2O

OH

symmetry

C2V

C ∞V

calcd r(O-H or H-H) (Å) θ(H-O-H) (deg) Vasym (cm-1) Vsym (cm-1) Vbent (cm-1)

(0.957)b

0.971 104.7 (104.5) 3851 (3856)c 3739 (3741) 1575 (1585)

exptla

calcd

exptla

0.972 0.987 (0.986) 0.970 104.7 3756d 3600 (3642) 3738 3657 1595

a Ref 29(a) for H O and ref 29(b) for OH, respectively. b Data in 2 parentheses are calculated at the PW91 level. c Unscaled vibrational frequencies. d Ref 30.

where E[slab + adsorbate], E[slab], and E[adsorbate] are the calculated electronic energies of adsorbed species on a W(111) surface, a clean surface, and a gas-phase molecule, respectively. Vibrational frequencies of the adsorbed structures were analyzed by diagonalizing the Hessian matrix of selected atoms within the VASP program. The nudged elastic band (NEB) method25,26 was applied to locate the transition states (TS). The NEB method is an efficient method for finding the minimum energy path (MEP) between reactant and product. At least eight images were used for each calculated TS. The rate constants for the dissociation reaction of H2O on the W(111) surface were calculated using the variational RRKM theory27a as implemented in the Variflex code.27b 3. Results and Discussion 3.A. Calculations of Bulk W and Gas-Phase H2O and OH. Computed bulk lattice constants for bulk tungsten by PW91 and rPBE were 3.183 and 3.181 Å, respectively, which are in good agreement with the experimental value of 3.165 Å.28 The calculated W-W bond distance varied within 2.754-2.750 Å, which again is in good agreement with the experimental value of 2.741 Å.28 As summarized in Table 1, PW91 and rPBE calculated geometrical parameters and vibrational frequencies of H2O and OH molecules in a 15 Å cubic box are in line with available experimental data.29,30 In general, the water molecule, as well as its smaller derivatives, OH, O, and H, can be coordinated to the W(111)

surface via several ways. In Figure 1b, we show four different adsorption sites on the W(111) surface, which are top (on one W atom of the first layer) (I), bridge (on the middle of a W-W bond) (II), 3-fold-deep (on top of a third layer W atom) (III), and 3-fold-shallow (on top of a second layer W atom) (IV) sites of the surface. We have calculated all possible structures associated with the coordination of H2O, OH, O, and H to W(111). In Tables 2-4, we present the adsorption energies and vibrational frequencies of the most stable structures. 3.B. Adsorption of H2O. H2O molecules can interact with the W(111) surface via several different ways by utilizing its H-atoms and O lone-pair electrons (see Scheme 1). All located local minima of W(111)-H2O are given in Figure 2. Here, we discuss only the energetically most favorable structures, X-H-up and X-H-parallel, where X represents the absorption sites (I-IV) discussed previously. Next, we discuss only the rPBE calculated data. The PW91 calculated adsorption energies (see Table 2) for water on W(111) are slightly greater (0.05 to 0.25 eV) than those computed at the rPBE level. This finding is in good agreement with that from the previous studies, where the authors have shown that PW91 overestimates chemisorption energies relative to their rPBE calculated and experimental values.31 The two most stable structures correspond to the coordination of the water molecule to the top site (XdI) are I-H-up and I-Hparallel. In both structures, water interacts with a W center through its lone-pair electrons. Calculated adsorption energies for these structures are 9.77 and 12.60 kcal/mol, respectively. For XdII, we have found only one structure, II-H-parallel, which is stable by 8.82 kcal/mol relative to the W(111) + H2O dissociation limit. At the 3-fold-deep (III) and 3-fold-shallow (IV) configurations, only H-parallel structures were found, which are stable by 12.38 and 11.90 kcal/mol relative to the W(111) + H2O dissociation limit, respectively. Thus, these results show that the H-parallel orientation is energetically the most favorable at all active sites. Among these structures, I-Hparallel, III-H-parallel, and IV-H-parallel are energetically very close to each other, and their stability increases in the order I-H-parallel > III-H-parallel > IV-H-parallel. The calculated W-OH2 bond distances in these structures are within 2.3082.316 Å and correlate with the stability trend of those structures. We have performed calculations for H2O adsorption on (1 × 1), (2 × 2), and (3 × 3) ad-layers, corresponding to the coverage of 1 ML, 1/4 ML, and 1/9 ML, respectively. The calculated results are summarized in Table 3, which lists the most favorable H-parallel absorbents. As seen in Table 3, the adsorption energies at high coverage are, in general, more positive than that at low coverage, reflecting the effect of adsorbate interactions. We also studied the adsoption of the water dimer, trimer, and hexamer on the (2 × 2) and (3 × 3) ad-layers. The most stable structures are depicted in Figure 3. Their calculated adsorption energies and important geometry parameters are given in Table S1 (Supporting Information). Water molecules prefer the top site adsorption, whenever possible. They tend to lie down on the surface due to cluster-surface interactions. In the dimer, both water molecules lie very flatly on the surface with hydrogen atoms parallel to the surface. The trimer and hexamer are ring-like structures. In the trimer, each water tilts ∼45° on the surface. The hexamer forms a flat hexagonal ring with six molecules lying at the same distance to the surface. As seen in Figure 3 and Table S1, the trimer is the least stable with adsorption energies of -10.4 and -11.4 kcal/mol for the (2 × 2) and (3 × 3) ad-layers, respectively, and the monomer

Adsorption and Dissociation of H2O on a W(111) Surface

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TABLE 2: Adsorption Energies (kcal/mol) of H2O, OH, O, and H on W(111) Calculated at the rPBE and PW91 (in Parentheses) Levels H2O rPBE (PW91) on-top (I) I-H-up I-H-parallel bridge (II) II-H-up II-H-parallel 3-fold-deep (III) III-H-parallel 3-fold-shallow (IV) IV-H-parallel

-9.77 (-14.94) -12.60 (-18.28) 2.08 (3.19) -8.82 (-17.14) -12.38 (-18.42) -11.90 (-17.37)

OH

O

H

-98.16 (-104.79)

-142.27 (-155.58)

-69.52 (-73.23)

-83.39 (-94.65)

-136.89 (-152.36)

-74.80 (-80.14)

-86.54 (-95.80)

-131.26 (-149.07)

-56.15 (-61.59)

-87.21 (-96.11)

-119.27 (-133.56)

-70.35 (-75.24)

TABLE 3: Adsorption Energies (kcal/mol) of H2O and Most Stable Structures of HO, O, and H on W(111) at Different Coverages Calculated at the rPBE Level H2O

1 ML

1/4 ML

1/9 ML

on-top (I) bridge (II) 3-fold-deep (III) 3-fold-shallow (IV) OH on-top (I) O on-top (I) H bridge (II)

-8.17 -4.78 -8.50 -7.89 -104.10 -153.17 -84.83

-12.60 -8.82 -12.38 -11.90 -98.16 -142.27 -74.80

-13.40 -9.78 -13.33 -12.21 -94.47 -138.75 -78.41

TABLE 4: Vibrational Frequencies of H2O, OH, O, and H Adsorbed on the W(111) Surface Calculated at the rPBE Level frequency (cm-1)

H2O site

Vasym(OH)

Vsym(OH)

Vbent(HOH)

V(W-OH2)

I-H-up I-H-parallel III-H-parallel IV-H-parallel

3800 3557 3454 3542

3639 3461 3365 3447

1504 1537 1528 1538

421 568 621 602

O atom

H atom

site

V(OH)

OH V(W-OH)

V(W-O)

V(W-H)

I II III IV

3766 3712 3648 3652

629 647 708 694

874 607 521 750

1724 1459 1398 1529

SCHEME 1: H2O Molecule Adsorbed at the On-Top Site

3.C. Adsorption of Water Derivatives, OH, O, and H on W(111) Surface. The W(111)-OH, W(111)-O, and W(111)-H structures could be the products of water dissociation on W(111). Therefore, here we have studied the adsorption of the OH molecule and O and H atoms on W(111). All located local minima of W(111)-OH, W(111)-O, and W(111)-H are given in Figure 4. The calculated adsorption energies (at 1/4 ML coverage) of the W(111)-OH, W(111)-O, and W(111)-H structures are summarized in Table 2. In Table 3, we present the adsorption energies of the most stable structures at 1 ML, 1/4 ML, and 1/9 ML coverage. As could be expected, the OH, O, and H radicals interact with the W(111) surface more strongly than the closed-shell water molecule does. Among the many different adsorption sites, the adsorption on the top site, I (on one W atom of the surface), is more favorable for OH and O, while the bridge site, II, is found to be the most favorable binding site for the H atom. As shown in Table 3, unlike the H2O system, the adsorption energies for the OH molecule and O and H atoms increase with increasing coverage. Next, we use the most favorable adsorption sites of OH, O, and H to discuss the dissociation of the water molecule on W(111). 3.D. Dissociation of H2O on W(111) Surface. As mentioned previously, the I-H-parallel conformer is energetically the most stable one among all studied conformers of W(111)-H2O and is chosen to be the starting structure for our study of H2O dissociation on W(111) at 1/4 ML coverage. The dissociation of H2O into hydrogen gas and adsorbated O is expected to include the following elementary processes:

H2O(g) + W(111) f W(111)-H2O(ads) W(111)-H2O(ads) f H(ads)-W(111)-OH (ads) H(ads)-W(111)-OH (ads)) f H(ads)-H(ads)-W(111)-O(ads) H(ads)-H(ads)-W(111)-O(ads) f W(111)-O(ads) + H2(g)

is the most stable with adsorption energies of -12.6 and -12.4 kcal/mol for the (2 × 2) and (3 × 3) ad-layers, respectively, indicating that water molecules prefer to coordinate with W atoms on the top position (I) as individual molecules (rather than as a cluster). In other words, the results show that water does not prefer to form small clusters on the W(111) surface.

As shown in Figure 5 and discussed previously, the adsorption of H2O(g) on W(111) is exothermic by 12.6 kcal/mol (the resulting system will be called LM1). The following dehydrogenation of H2O occurs with a 1.8 kcal/mol barrier calculated from LM1. In TS1, transition state associated with this process, the activated O-H bond is strongly elongated (which is calculated to be 1.018 Å). Meanwhile, the W-O bond length in TS1 is about 2.251 Å, which is 0.057 Å smaller than that in LM1. Analysis shows that the dissociated H atom moves to a W-W bridge position (structure LM2 in Figure 5). The entire adsorption and dehydrogenation process, H2O(g) + W(111) f W(111)-H2O(ads) f H(ads)-W(111)-HO(ads), is calculated to be exothermic by 45.5 kcal/mol. It is noteworthy that we have also investigated the diffusion of water along the [11h 0] direction on the W(111) surface,

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Figure 2. Optimized geometries of adsorbed H2O species at various sites on W(111) calculated at the rPBE and PW91 (in parentheses) levels.

Figure 3. Water (a) monomer, (b) dimer, (c) trimer, and (d) hexamer clusters adsorbed on the (3 × 3)-W(111) surface calculated at the rPBE level.

which was found to proceed via a 10.4 kcal/mol barrier. The value of the diffusion barrier is much larger than the calculated H2O dehydrogenation barrier (1.8 kcal/mol). From the resulting LM2 structure, the decomposition process may proceed via two paths: (1) hydrogen desorption and (2) OH dehydrogenation. The first process, H2 desorption, is calculated to be only 13.8 kcal/mol endothermic. The second process, dehydrogenation of the adsorbed OH group, leads to the adsorption of O(ads) and H(ads) and occurs with a slightly larger barrier, 15.9 kcal/mol, at the transition state TS2. This process is calculated to be exothermic by 8.9 kcal/mol. In the transition state TS2, the H atom of OH moves toward an adjacent W-W bridge site, where the activated O-H bond is elongated to 1.695 Å. The O-W bond length of TS2 is about 1.777 Å, which is 0.034 Å longer than that for product LM3. H2(g) formation from LM3 is calculated to be endothermic by 30.7 kcal/mol. Thus, in the overall process, H2O(g) + W(111) f W(111)-O(ads) + H2(g), the oxidation of W(111) by the water molecule is 23.7 kcal/mol exothermic and expected to be a barrier-less reaction. In addition, we have studied the diffusion of oxygen and hydrogen atoms on the W(111) surface. For this purpose, we

have selected the most favorable bonding sites as the initial stages. An oxygen atom diffusion from an on-top site to a bridge site was calculated to proceed with a 15.8 kcal/mol barrier. The calculated diffusion barrier of the hydrogen atom from a bridge site to a 3-fold-shallow site was 5.9 kcal/mol. The low diffusion barrier of hydrogen provides a promising route for H2 formation at moderate temperatures. The presented data are in qualitative agreement with the experimental TPD result of Chen et al.,16 who proposed that the initial sharp peak is likely due to the removal of hydrogen from the H2O to produce adsorbed OH, while the broader peak is attributed to the subsequent decomposition of OH to hydrogen and surface oxygen. To verify our MEPs using the NEB method in the previous section, MD simulations at 298 to 500 K were carried out. We used the I-H-parallel conformer (LM1) as the fully optimized beginning reactant. ∆t ) 3 fs was applied, and the calculations were iterated until time reached 300 fs. Figure S1a shows the snapshots of configurations calculated according to time evolution, representing the adsorption and dissociation of a H2O molecule, while Figure S1b displays the change of the energy profile. On the basis of MD modeling, the first dehydrogenation of H2O occurs in approximately 12 fs and the second dehydrogenation of OH occurs in approximately 36 fs. The dissociation process reaches a stable 2H(a) + O(a) configuration in approximately 120 fs. This result is consistent with the MEPs, indicating that the adsorption of water on the W surface occurs via two dissociative pathways: H2O f OH + H and OH f O + H. 3.E. Rate Constant Calculations. On the basis of the aforementioned PESs for the dissociation of H2O on the W(111) surface, we have computed the rate constants for the following reactions

H2O(g) + W(111) f W(111)-H2O(ads) (LM1) f OH (ads) + H(ads) (LM2) f O(ads) + H(ads) + H(ads) (LM3) f W(111)-O(ads) + H2(g) For the reaction rate constant calculations, the MEP representing the barrier-less association process H2O(g) + W(111) f H2O(ads) (LM1), which is the rate determining pathway, was calculated along the reaction coordinate M-O, which is stretched from its equilibrium value to 4 Å with a step size of 0.1 Å. At each fixed M-O distance, the bottom three atomic

Adsorption and Dissociation of H2O on a W(111) Surface

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Figure 4. Optimized geometries of adsorbed OH, O, and H species on W(111) calculated at the rPBE and PW91 (in parentheses) levels.

Figure 5. Schematic potential energy profiles for H2O-W(111) calculated at the rPBE level. For the transition state, values of imaginary frequencies, Vi, are also presented.

layers of the W(111) surfaces were fixed, while atoms in the remaining layers and H2O were fully optimized at the rPBE level. The obtained MEP was approximated with a Morse potential, V(r) ) De{1 - exp[-β(R - R0)]}2, where R is the reaction coordinate, R0 is the equilibrium W-O bond distance, and De is the bond energy without zero-point energy corrections. The parameters obtained by fitting the Morse potential to the MEP are R0 ) 2.308, β ) 1.89 Å-2, and De ) 13.2 kcal/mol. Our calculations have been carried out for the temperature range

of 200 to 3000 K. The predicted rate constant (in molecular units, cm3/s) in the broad temperature range can be represented by

k ) 1.76 × 10-11T-0.001 exp[2.52 (kcal mol-1)/RT] The rate constant for the dissociative adsorption process of H2O on W(111) is defined by32

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d[X]surf/dt ) k(θ/As)[X]g which has the unit of a flux, molecule cm-2 s-1. In the equation, θ, As, and [X]g represent the fraction of available surface sites, the surface area, and the gas-phase concentration of H2O in molecules/cm3, respectively. 3.F. Frequency Calculations. Chen et al.16 have measured HREEL spectra of H2O adsorption on the W(111) surface at the temperature range of 90 to 450 K. At 90 K, they observed three main peaks, 1644, 3578, and 3687 cm-1, and a broad band below 1000 cm-1. Peaks at 1644 cm-1, V(HOH), and 3578 cm-1, V(OH), are characteristic of the H2O molecule, while that at 3687 cm-1 is related to the V(OH) of OH adsorbed on the W(111) surface. The peaks below 1000 cm-1 were assigned to rotation and W-OH2 vibration V(W-OH2) (609 cm-1). No spectroscopic changes were observed when the surface was heated to 140 K. However, several changes were observed upon heating to 230 K: (1) the vibrational modes of molecular H2O begin to diminish, (2) the V(OH) intensity of the surface OH groups increases and shifts to 3619 cm-1, and (3) well-defined features at 446, 609, and 764 cm-1 are formed. Upon further heating of the surface, the same three features are identified between 400 and 800 cm-1, and the V(OH) mode disappears. At 450 K, the spectrum is characteristic of an oxygen-modified W(111) surface.33 The HREELS results show that water decomposes on W(111) and produces surface oxygen and gasphase hydrogen. This experimental result is in good agreement with our conclusion from the calculation. Next, we report the calculated (unscaled) frequencies of H2O, OH, O, and H adsorbed on W(111) on different sites (see Table 4). We selected the most stable configurations of H2O adsorption: I-H-up, I-H-parallel, III-H-parallel, and IV-H-parallel. As shown in Table 4, the calculated (unscaled) asymmetric V(OH), symmetric V(OH), bent V(HOH), and V(W-OH2) frequencies of H2O adsorbed on W(111) are within 3454-3800, 33653639, 1504-1538, and 421-621 cm-1, respectively. These values are in good agreement with the experimental values, especially for the I-H-parallel, III-H-parallel, and IV-H-parallel configurations, which are the most stable conformers. Comparison of the calculated asymmetric V(OH) mode for the gasphase and adsorbed H2O (see Table 1) shows the red shifts ranging within 278-374 cm-1. This large red shift is attributable to the weakening of the O-H bond in the W(111)-H2O adsorbate (see Table 2). The calculated V(OH) and V(W-OH) frequencies of the adsorbed OH are 3652-3766 and 629-708 cm-1, respectively, which are consistent with their experimental values of 3619 and 609 cm-1, respectively. The calculated V(W-H) frequencies are 1398-1724 cm-1; however, there are essentially no spectroscopic data for V(W-H) frequencies of adsorbed H. Finally, the calculated V(W-O) frequencies are 521-874 cm-1, which are consistent with experimental values of 400-800 cm-1. 4. Conclusion The mechanism for the adsorption and dissociation of water on a W(111) surface has been elucidated using DFT in conjunction with a projected augmented wave approach. The structures, vibrational frequencies, and adsorption energies of possible adsorbates including H2O and its derivatives, OH and atomic O and H, have been predicted. It was shown that the preferable coordination sites are top, top, top, and bridge for H2O, OH, O, and H, respectively. The calculated adsorption energies are 12.6, 98.2, 142.3, and 64.8 kcal/mol for H2O, OH, O, and H, respectively. A potential energy surface for the

Chen et al. decomposition of water on W(111) has been constructed. According to our calculations, the barriers of H2O dehydrogenation are 1.8 kcal/mol for the H-OH bond activation and 15.9 kcal/mol for the O-H bond activation. All calculated transition states are located below the dissociation limit H2O + W(111). The overall reaction to produce surface oxygen and H2 gas was found to be exothermic by 23.7 kcal/mol. The DFT/MD modeling has been shown to be useful for elucidating the mechanism of H2O on the W(111) surface. Our conclusion is in good agreement with the available data from TPD and HREELS experiments. We also predicted the rate constant for the H2O dissociative adsorption on the W(111) surface. Acknowledgment. We gratefully acknowledge financial support from the Office of Naval Research under a MURI Grant (Prime Award N00014-04-1-0683 and Subaward 2794-EUONR-0683), the Emerson Center for the use of its resources, and the use of CPUs from the National Center for HighPerformance Computing, Taiwan. M.C.L. also thanks the National Science Council of Taiwan and the Taiwan Semiconductor Manufacturing Co. for Distinguished Professorships at the National Chiao Tung University in Hsinchu, Taiwan. Supporting Information Available: (Table S1) Adsorption energies (kcal/mol) and geometry parameters for H2O clusters on (2 × 2)- and (3 × 3)-W(111) surfaces calculated at the rPBE level. (Figure S1a) Snapshots of configurations calculated according to time evolution, representing the adsorption and dissociation of a H2O molecule. (Figure S1b) Change of the energy profile. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1. (2) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (3) Morgenstern, K.; Nieminen, J. Phys. ReV. Lett. 2002, 88, 66102. (4) Morgenstern, K.; Rieder, K.-H. J. Chem. Phys. 2002, 116, 5746. (5) Mitsui, T.; Rose, M. K.; Fomin, E.; Ogletree, D. F.; Salmeron, M. Science (Washington, DC, U.S.) 2002, 297, 1850. (6) Pirug, G.; Ritke, C.; Bonzel, H. P. Surf. Sci. 1991, 241, 289. (7) Morgenstern, K.; Michely, T.; Comsa, G. Phys. ReV. Lett. 1996, 77, 703. (8) Ruuska, H.; Pakkanen, T. A.; Rowley, R. L. J. Phys. Chem. B 2004, 108, 2614. (9) Ren, J.; Meng, S. J. Am. Chem. Soc. 2006, 128, 9282. (10) Meng, S.; Wang, E. G.; Gao, S. Phys. ReV. B 2004, 69, 195404. (11) Feibelman, P. J. Science (Washington, DC, U.S.) 2002, 295, 99. (12) Menzel, D. Science (Washington, DC, U.S.) 2002, 295, 58. (13) Ageev, V. N.; Ionov, N. I.; Ustinov, Y. K. Zh. Tekh. Fiz. 1969, 39, 1337. (14) Goryachkovskii, Y. G.; Kostikov, V. I.; Solodkin, G. A. Zh. Fiz. Khim. 1976, 50, 1959. (15) Ustinov, Y. K.; Ionov, N. I. Zh. Tekh. Fiz. 1967, 37, 2046. (16) Hwu, H. H.; Polizzotti, B. D.; Chen, J. G. J. Phys. Chem. B 2001, 105, 10045. (17) Bryl, R.; Blaszczyszyn, R.; Galewska, E. Vacuum 1997, 48, 332. (18) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (19) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558. (20) Blochl, P. Phys. ReV. B 1994, 17, 953. (21) Perdew, J. P.; Chevary, J. A.; Vosco, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (22) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (23) Zhang, Y.; Yang, W. Phys. ReV. Lett. 1998, 80, 890. (24) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (25) Henkelman, G.; Uberuaga, B. P.; Jo¨nsson, H. J. Chem. Phys. 2000, 113, 9901. (26) Mills, G.; Jo¨nsson, H.; Schenter, G. Surf. Sci. 1995, 324, 305. (27) (a) Gilbert, R. G.; Smith, S. C. Theory of Unimolecular and Recombination Reactions; Blackwell Science: Cambridge, MA, 1990. (b) Klippenstein, S. J.; Wagner, A. F.; Dunbar, R. C.; Wardlaw, D. M.; Robertson, S. H. Variflex 1999.

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