Density Functional Studies of the Adsorption and Dissociation of NOx

Mar 9, 2009 - Department of Chemistry, Emory University, Atlanta, Georgia 30322, Center for Interdisciplinary Molecular Science, Institute of Molecula...
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J. Phys. Chem. C 2009, 113, 5300–5307

Density Functional Studies of the Adsorption and Dissociation of NOx (x ) 1, 2) Molecules on the W(111) Surface Hsin-Tsung Chen,†,§ Hui-Lung Chen,⊥ Shin-Pon Ju,# Djamaladdin G. Musaev,*,† and M. C. Lin*,†,‡ Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322, Center for Interdisciplinary Molecular Science, Institute of Molecular Science, National Chiao Tung UniVersity, Hsinchu, Taiwan 300, National Center for High-Performance Computing, No. 28, Nan-Ke 3rd Rd., Hsin-Shi, Tainan, 741, Taiwan, Department of Chemistry and Institute of Applied Chemistry, Chinese Culture UniVersity, Taipei 111, Taiwan, and Department of Mechanical and Electro-Mechanical Engineering, Center for Nanoscience and Nanotechnology, National Sun-Yat-Sen UniVersity, Kaohsiung, Taiwan 804 ReceiVed: NoVember 24, 2008; ReVised Manuscript ReceiVed: January 20, 2009

Adsorption and dissociation of NOx (x ) 1, 2) molecules on the W(111) surface have been investigated by using the density functional theory (DFT) with the projector-augmented wave (PAW) approach in periodic boundary conditions. The adsorption structures, vibrational frequencies, and binding energies of NO2, NO, N, and O on the W(111) surface were predicted. It was shown that the most favorable adsorption geometry of W(111)/NO2 is the WNO2(IV-µ3-N2, O1, O1) configuration with NO2 at the 3-fold-shallow site of the surface and has an adsorption energy of 79.4 kcal/mol. For W(111)/NO, WNO(II-µ2-N1, O1) with NO at the bridge site is energetically the most favorable one and has an adsorption energy of 74.4 kcal/mol. The N and O atoms are bound preferentially at the bridge and top sites, respectively. The potential energy profiles for the decomposition of NO2 on W(111) were constructed using the nudged elastic band (NEB) method. The barriers for the stepwise NO2-deoxygenation process to N(ads) + 2O(ads) are calculated to be only 0.5-3.7 (for the first step of the reaction, ON-O bond activation) and 0.2-3.0 (for the second step of the reaction, N-O bond activation of the coordinated NO molecule) kcal/mol, with an overall exothermicity of 177.0-185.2 kcal/mol. These findings show that NO2 can easily decompose on the W(111) surface. The rate constants for NO2 and NO dissociation on the surface were also predicted. I. Introduction Elucidation of the mechanisms and factors governing the reaction of small molecules (such as NOx (x ) 1 and 2), N2, COx (x ) 1 and 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. The reaction of combustion gases with surfaces of transition metal clusters, such as tungsten and its alloys, which are important materials and widely used in high-temperature environments,1 has recently attracted significant attention. In this regard, we have previously studied the mechanisms for coordination and dissociation of H2O, COx (x ) 1, 2), HCl, and Cl2 molecules on the W(111) surface.2,3 This paper is our continuing effort on the reactions of W(111), with NOx (x ) 1, 2) molecules which are key pollutants of hydrocarbon combustion in air. In the literature, reactions of NO2 on metal surfaces have been the subject of numerous investigations.4,5 Until now, the adsorption and decomposition of NO2 on Pt(111),6-9 Ru(001),10,11 Ag(110),12,13 Ag(111),14-17 Pd(111),18 Au(111),19-21 and polycrystalline Au22 have been extensively studied using various techniques such as TPD, LEED, EELS, HREELS, XPS, and UPS. These studies have * Corresponding authors. E-mail: [email protected] (M.C.L.); [email protected] (D.G.M.). † Emory University. ‡ National Chiao Tung University. § National Center for High-Performance Computing. ⊥ Chinese Culture University. # National Sun-Yat-Sen University.

Figure 1. Possible isomers of the M(NO2) intermediate.

demonstrated that the NO2 molecule dissociatively adsorbs on Pt(111), Ru(001), Ag(110), Ag(111), and Pd(111) surfaces at low temperatures but molecularly adsorbs on Au(111) and polycrystalline Au. According to these experiments, NO2, unlike CO2, coordinates to metal centers of the surface through either one (monodentate) or both (bidentate in a chelating structure) oxygen atoms or through the central N atom, or by one of its O atoms and the N atom at the same time (see Figure 1). To our best knowledge, the reaction of NO2 on the tungsten surface has not been investigated previously, while the adsorption of NO on tungsten23-26 and modified tungsten27,28 surfaces has been studied experimentally. These studies have shown that NO adsorbs dissociatively on the W(111) surface and likely forms N2 and N2O products following the decomposition and

10.1021/jp8102996 CCC: $40.75  2009 American Chemical Society Published on Web 03/09/2009

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Figure 2. Slabs models for the W(111) surface: (a) side view and (b) top view.

TABLE 1: Adsorption Energies of NO2, NO, N, and O Species on W(111) under Different Coverage WNO2(IV-µ -N2, O1, O1) WNO(II-µ2-N1, O1) WN(II-µ2-N1, N1) WO(I-η1-O1) 3

1 ML

1/4 ML

1/9 ML

72.1 75.9 200.4 153.2

79.4 74.4 198.2 142.3

77.9 73.2 196.9 138.8

recombination reactions. In the present work, we extend our previous studies and investigate the adsorption and dissociation of NO2 and NO on the W(111) surface. II. Computational Methods In the present series of studies, we employ the density functional theory (DFT) plane-wave method, as implemented in the Vienna ab initio simulation package (VASP)29-31 with the projector-augmented wave method (PAW),32,33 to calculate reactants, intermediates, transition states, and products of the reactions W(111) + NOx. Open-shell spin states of these systems were treated by using ISPIN)2 in the calculations. In these studies we used the Perdew-Wang (PW91)34 and the revised Perdew-Burke-Ernzerhof (rPBE) functionals.35,36 The calculations were carried out using (4 × 4 × 4) and (4 × 4 × 1) Monkhorst-Pack mesh k-points37 for bulk and surface calculations, respectively, with a 400 eV cutoff energy. The p(2 × 2) cell of the W(111) surface was modeled as periodically repeated slabs with six atomic layers (see Figure 2a). The bottom three atomic layers were frozen and set to the estimated bulk parameters, while the remaining layers were fully optimized. All slabs were separated by a vacuum spacing greater than 15 Å, which guarantees no interaction between the slabs. It should be mentioned that we have also performed calculations for NO2, NO, N, and O adsorptions for the most stable structures using the (1 × 1), (2 × 2), and (3 × 3) adlayer surface models, corresponding to the coverage of 1 ML, 1/4 ML, and 1/9 ML, respectively. It was shown that the coverage effect is negligible when it is less than 1/4 ML (see Table 1). The adsorption energies are calculated based on the following equation:

∆Eads ) (E[slab] + E[adsorbate]) - E[slab + adsorbate] where E[slab + adsorbate], E[slab], and E[adsorbate] are the calculated electronic energies of adsorbed species on the W(111) surface, clean surface, and gas-phase molecule, respectively. Vibrational frequencies were analyzed by diagonalizing the Hessian matrix of selected atoms within the VASP approach. The nudged elastic band (NEB) method38,39 was applied to locate transition states, and potential energy surfaces (PESs) were

constructed accordingly. The rate constants for the NO2 and NO dissociation on the W(111) surface were calculated using the variational RRKM theory as implemented in the Variflex code.40 III. Results and Discussion In our previous studies,2,3 we have evaluated the computed lattice constants for bulk tungsten and have shown that above presented computational approach provides a reasonable agreement with the available experimental values. Here, we have extended our test to NO2 and NO molecules. As summarized in Table 2, the predicted geometrical parameters and vibrational frequencies of gas-phase NO2 and NO in a 15 Å cubic box are in line with available experimental data. We have also shown that the rPBE functional provides a better agreement on the chemisorption energies of atoms and molecules on the W(111) surface with experiments than PW91 does in our previous studies.2,3 Therefore, the rPBE-calculated results will be used in the following discussion. In general, the NO2 molecule, as well as its derivatives, NO, N, and O, can coordinate to the W(111) surfaces via several different ways, as shown in Figure 2b. In our studies, we considered four different adsorption sites on the W(111) surface: top, I (on one of W atoms of the first layer); bridge, II (on the W-W bond); 3-fold-deep, III (on one of W atoms of the third layer); and 3-fold-shallow, IV (on one of W atoms of the second layer). The notation (Nm, On), used below, indicates that N and O atoms are coordinated to the mth and nth layer W atoms, respectively. Optimizing geometries of the W(111)/NO2, W(111)/NO, W(111)/N, and W(111)/O structures starting from the different coordination modes of substrates on the surface may lead to several different local minima on the potential energy surface, which are associated with the multiple isomeric forms of these species. All of those local minima and the corresponding adsorption energies are shown in Figures 3-5. Cartesian coordinates of all reported structures are given in the Supporting Information. III.1. Adsorption of NO2 Molecules on the W(111) Surface. NO2 has a bent structure 2A1 ground state with the unpaired electron residing on the N atom. As mentioned in the Introduction, NO2 can adsorb on W(111) via several isomeric forms. Resulting W(111)/NO2 structures can be written as N-bonded, WNO2(I-µ1-N1); O-nitrito, WNO2(I-η1-O1); O,O-nitrio, WNO2(Iη2-O1, O1), WNO2(II-µ2-O1, O1); N,O-nitrio, WNO2(I-η2-N1, O1), WNO2(II-µ2-N1, O2); and N,O,O-nitrio, WNO2(III-µ3N3, O1, O1), and WNO2(IV-µ3-N2, O1, O1) as shown in Figure 3.

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Figure 3. Optimized geometries of adsorbed NO2 on W(111) at the rPBE and PW91 (in parentheses) levels of theory. Distances are in Å.

TABLE 2: Geometrical Parameters and Vibrational Frequencies of Gas-Phase NO2 and NO Calculated at the rPBE and PW91 (in Parentheses) Levels of Theory and Some Experimental Data from the Literaturec molecule

NO2

NO

symmetry

C2V

C∞V

r(N-O) (Å) θ(O-N-O-) (deg) Vasym (cm-1) Vsym (cm-1) Vbend (cm-1) a

calcd

expta

calcd

exptb

1.216 (1.214) 133.0 (133.4) 1686 (1657) 1348 (1333) 733 (735)

1.193 134.1 1618 1318 750

1.172 (1.153)

1.151

1940 (1908)

1904

From ref 43. b From ref 44. c Unscaled vibrational frequencies.

As seen from Figure 3, the N,O,O-nitrio isomer WNO2(IV-µ3N2, O1, O1) with NO2 at the 3-fold-shallow site is energetically the most favorable one with an adsorption energy (i.e., W(111)sNO2 interaction energy) of 79.4 kcal/mol among the all reported structures of W(111)/NO2. Structures WNO2(II-µ2-O1, O1), WNO2(IV-µ3-N3, O1, O1), WNO2(II-µ2-N1, O2), WNO2(Iη2-N1, O1), WNO2(I-η1-O1), WNO2(I-η2-O1, O1), and WNO2(Iη1-N1) are 15.5, 22.0, 22.3, 25.7, 27.2, 27.7, and 40.7 kcal/mol higher than WNO2(IV-µ3-N2, O1, O1), respectively. It should be noted that we were not able to locate structures WNO2(II-µ2-N1, O1) and WNO2(II-µ2-N1) which converged to the structures with the dissociated products NO(a) and O(a). Comparison of these data with those for W(111)/COx reported previously3 shows that the energetically most favorable isomers of both W(111)/CO2 and W(111)/NO2 are their corresponding 3-fold-shallow site coordinations. However, as it could be expected, the W(111)sNO2 interaction (79.4 kcal/mol) is much stronger than the W(111)sCO2 interaction (37.6 kcal/mol).

III.2. Adsorption of NO Molecules on the W(111) Surface. Understanding of the nature of M-NO interaction as well as structures and energetics of the W(111)/NO species are important for exploring NO2 dissociation on the W(111) surface. Similarly, NO can coordinate with metal centers of the surface either through its (monodentate) oxygen atom or through the N atom, or by both O and N atoms (bidentate in a bridging structure) also. The W(111)/NO intermediate may have several isomers: WNO(I-η1-N1), WNO(I-η1-O1), WNO(II-µ2-N1, O1), WNO(II-µ2-N2, O1), WNO(II-µ2-N1, O2), WNO(III-η1-N3), WNO(III-η1-O3), WNO(IV-η1-N2), and WNO(IV-η1-O2), presented in Figure 4. As seen from Figure 4, the isomer WNO(II-µ2-N1, O1) is energetically the most stable one among all the calculated W(111)/ NO structures with an adsorption energy of 74.4 kcal/mol. The next stable isomer is found to be WNO(II-µ2-N2, O1), which is only 2.0 kcal/mol higher in energy than the former. The isomers WNO(I-η1-O1), WNO(IV-η1-O2), and WNO(III-η1-O3) with NO coordination to a W atom with its O atom are less stable than those

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Figure 4. Optimized geometries of NO adsorption on W(111) at the rPBE and PW91 (in parentheses) levels of theory. Distances are in Å.

Figure 5. Optimized geometries of adsorbed N and O atoms on W(111) at the rPBE and PW91 (in parentheses) levels of theory. Distances are in Å.

of WNO(I-η1-N1), WNO(IV-η1-N2), and WNO(III-η1-N3) with NO coordination to the W atom with its N atom. These results indicate that the bridge-bonded NO is the most favorable adsorption geometry W(111)/NO, which is similar to that reported for W(111)/ CO.3 One should note that the bridge site structures WNO(II-µ2N2,O3) and WNO(II-µ2-N3,O2) are not stable and converge to the N(ads)/W(111)/O(ads) structure upon optimization. III.3. Adsorption of N and O Atoms on the W(111) Surface. These species may have four different isomers: WX(Iη1-X1), WX(II-µ2-X1,X1), WX(III-µ2-X1,X2), and WX(IV-

η1-X2), where X ) N, O, as shown in Figure 5. It should be noted that the optimization of 3-fold-deep site structures converges to the bridge site WX(II-µ2-X1, X1) species, while optimization of 3-fold-shallow site structures converges to the bridge site WX(III-µ2-X1,X2) structure. As seen from Figure 5, atoms N and O strongly adsorb on the W(111) surface. The bridge site (II) is the most favorable binding site for the N atom, while the top site (I) is the most favorable binding site for the O atom. The calculated adsorption

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TABLE 3: Vibrational Frequencies of NO2, NO, N, and O Adsorbed on the W(111) Surface Calculated at the rPBE Level frequency (cm-1)

site NO2

Vasym(NO)

Vsym(NO)

I-η -N1 I-η1-O1 I-η2-O1, O1 I-η2-N1, O1 II-µ2-O1, O1 II-µ2-N1, O2 IV-µ3-N2, O1, O1 III-µ3-N3, O1, O1

1382 1745 1130 1581 919 1477 924 722

1291 719 1049 914 790 779 829 581

V(N-O)

V(W-NO)

1631 1578 1396 1527 1488 1701 910 818 1129

497 412 289

1

NO I-η1-N1 IV-η1-N2 III-η1-N3 I-η1-O1 IV-η1-O2 III-η1-O3 II-µ2-N1, O1 II-µ2-N1, O2 II-µ2-N2, O1 X) O or N atom I-η1-X1 II-µ2-X1, X1 III-µ2-X1, X2 IV-η1-X2

V(W-O) 874 607 521 750

Vbend(ONO) V(W-NO2) 771 413 844 692 688 500 585 561

296 215 350 351 413 389 420 432 V(W-O)

403 338 448 396 344 382

452 553 509 V(W-N) 909 659 574 818 2

energies of the most favorable structures WN(II-µ -O1, O1) and WO(I-η1-O1) are 198.2 and 142.3 kcal/mol, respectively. Thus, the N atom prefers to occupy the bridge sites between the first-layer W centers, while the O atom prefers to coordinate with one of the W centers. The coordination of N to the bridge position may inhibit the oxygen diffusion into the bulk. This finding is consistent with the observation of Miki et al.26b who have showed that the O atom diffusion is blocked by the N atom. III.4. Frequency Calculations. Vibrational frequencies of W(111)/NO2 have not been reported previously, while the vibrational frequencies of W(111)/NO have been studied experimentally by Zhang et al.28 They have measured HREEL spectra of W(111)/NO at 90-1000 K. At 90 K, a relatively sharp peak (1772 cm-1) and two unresolved shoulders (1691 and 1847 cm-1) were detected. These features are assigned to the V(NO) vibrational mode of NO molecule. When the temperature increases to 300 K, the peak at 1772 cm-1 decreases, while three other peaks at 1258, 1481, and 1610 cm-1 appear. The frequency range of 1258-1610 cm-1 is characteristic of the strongly NO-chemisorbed state on the W(111) surface. Besides, two additional peaks, ∼846 and 994 cm-1, are also found at 300 K. All features at frequencies over 1100 cm-1 of W(111)/NO disappear when the surface is heated up to 600 K. The observed 453-846 and 629-994 cm-1 features are tentatively assigned to the V(W-N) and V(W-O) modes, respectively. The calculated (unscaled) frequencies of NO2, NO, N, and O adsorbed on W(111) are reported in Table 3. As shown in this table, the calculated asymmetric V(NO) and symmetric V(NO) stretches, and bent δ(ONO) and V(W-NO2) frequencies, of the W(111)/NO2 are in the ranges of 722-1745, 581-1291, 500-844, and 296-432 cm-1, respectively. The large red shifts (relative to the V(NO) frequency of the NO2 molecule) are due to the weakening of the N-O bond, indicating that WNO2(II-

µ2-O1, O1), WNO2(IV-µ3-N2, O1, O1), and WNO2(III-µ3-N3, O1, O1) are the most likely precursors for the dissociation reaction. As shown in Table 3, the calculated frequency of WNO(III-η1-O3), 1701 cm-1, is assigned for the N-O stretching vibration of the weakly physisorbed state which corresponds to the experimental observation of 1772 cm-1 detected at 90 K. The calculated V(NO) frequencies of W(111)/NO are within the range of 1396-1631 cm-1, which are consistent with the experimental values28 of 1258-1610 cm-1, and are characteristic of the NO-chemisorbed states on the W(111) surface. Frequencies of WNO(II-µ2-N1, O1), WNO(II-µ2-N1, O2), and WNO(IIµ2-N2, O1) configurations located below the 1200 cm-1 are due to the weak N-O bond. Indeed, in these structures, the calculated N-O bond lengths are 1.382, 1.401, and 1.332 Å (see Figure 4), respectively. Finally, the V(W-N) and V(W-O) frequencies of W(111)/N and W(111)/O systems are calculated to be within 574-909 and 521-874 cm-1, respectively, which are in good agreement with their experimental values (629-994 and 453-846 cm-1, respectively).28 III.5. Potential Energy Surface of the Reaction NO2 + W(111). Here, we construct the potential energy surface (PES) of NO2 + W(111) using the NEB method. For this purpose, we have selected energetically the most stable coordination modes of the NO2, NO, O, and N-groups (such as WNO2(IVµ3-N2, O1, O1), WNO2(II-µ2-O1, O1), and WNO2(IV-µ3-N3, O1, O1), WNO(II-µ2-N1, O1), WNO(II-µ2-N2, O1), WN(IIµ2-N1, N1), and WO(I-η1-O1)). These PESs include the following elementary reactions:

NO2(g) + W(111) f W(111)/NO2(ads) W(111)/NO2(ads) f NO(ads) /W(111)/O(ads) NO(ads) /W(111)/O(ads) f N(ads) /O(ads) /W(111)/O(ads) Important geometry parameters of intermediates, transition states, and products of these reactions are presented in Figure 6. The constructed potential energy surfaces are depicted in Figure 7. Cartesian coordinates of all reported structures are given in the Supporting Information. As seen from Figure 7, the adsorption of NO2(g) on the W(111) surface is exothermic, giving rise to WNO2(IV-µ3-N2, O1, O1), WNO2(II-µ2-O1, O1), and WNO2(IV-µ3-N3, O1, O1) intermediates, which lie 79.4, 63.9, and 57.4 kcal/mol below the reactants, respectively. These processes take place by variational transition states without intrinsic barriers. Let us discuss pathways starting from these three intermediates separately. The first deoxygenation of NO2 from WNO2(IVµ3-N2, O1, O1) leads to O/W(111)/NO, LM1, with the adsorbed NO and O having the depicted WNO(II-µ2-N2, O1) and WO(Iη1-O1) configurations in Figures 4 and 5, respectively. This process is found to be 43.1 kcal/mol exothermic and occurs with a small (2.3 kcal/mol) barrier at transition state TS1. In TS1 (see Figure 6), the breaking N-O bond length is about 1.538 Å, which is 0.160 Å longer than that in the reactant. The second deoxygenation starts from the LM1 intermediate to produce P1a with coadsorbed WN(II-µ2-N1, N2) and two WO(I-η1-O1). This process occurs with a 3.0 kcal/mol barrier at TS2 and is exothermic by 47.7 kcal/mol. As seen in Figure 6, in the transition state TS2, the breaking N-O bond is 1.355 Å. In P1a, the N atom is located at the bridge position between the first and second layer W atoms. The N atom can diffuse to the bridge site between the two first-layer W atoms to form P1b by passing through a barrier of 6.0 kcal/mol. The overall reaction NO2(g) + W(111) f WNO2(IV-µ3-N2, O1, O1) f LM1 f P1a f P1b is calculated to be exothermic by 185.2 kcal/ mol, and the reaction does not require thermal activation energy.

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Figure 6. Geometrical illustration of intermediates, transition states, and products for the NO2-W(111) interactions using the rPBE level of theory.

Figure 7. Schematic potential energy profiles for NO2-W(111) using the rPBE level of theory.

The second path starts from WNO2(II-µ2-O1, O1) passing through a 3.6 kcal/mol energy barrier at the TS3, leading to the formation of LM2 which includes coadsorption of WNO(II-

µ2-N1, O1) and WO(I-η1-O1); the process is exothermic by 53.9 kcal/mol. As seen in Figure 6, the breaking N-O bond length is 2.003 Å at TS3, which has increased by 0.67 Å relative to

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that of WNO2(II-µ2-O1, O1). In the following step of the reaction, the LM2 intermediate overcomes a 1.4 kcal/mol activation barrier at the TS4 to produce P2, with adsorbed WN(II-µ2-N1, N1) and two WO(I-η1-O1). As seen in Figure 6, the breaking N-O bond in TS4 is 1.677 Å. This pathway of the reaction is exothermic by 177.0 kcal/mol and also occurs without thermal activation energy. As seen in Figure 7, the third process begins from WNO2(IVµ3-N3, O1, O1) intermediates, following the path via WNO2(IVµ3-N3, O1, O1) f TS5 f LM3 f TS6 f P3; it is exothermic by 183.3 kcal/mol. The calculated barriers at TS5 and TS6 are very small also. In summary, these calculation have shown that the NO2 molecule can completely dissociate on the W(111) surface without any thermal activation. The overall process is highly exothermic (∼185 kcal/mol). We have also investigated the diffusion of NO2 and NO molecules as well as N and O atoms on the W(111) surface. For this purpose, we choose the most favorable bonding site as an initial stage and the second favorable one as a final stage. Diffusion barrier of the process, WNO2(IV-µ3-N2, O1, O1) f WNO2(II-µ2-O1, O1), is calculated to be 22.1 kcal/mol, which is much higher than the ON-O bond activation barrier (2.3 kcal/mol), initiated from the same WNO2(IV-µ3-N2, O1, O1) reactant. The diffusion barrier of the reaction of WNO(II-µ3-N1, O1) f WNO(I-η1-N1) is found to be 16.6 kcal/mol. An oxygen atom diffusion from the WO(Iη1-O1) structure to the WO(II-µ2-O1, O1) structure has a 15.8 kcal/mol barrier. The calculated diffusion barrier of a nitrogen diffusion from WN(II-µ2-N1, N1) to WN(III-µ2-N1, N2) is 13.5 kcal/mol. The relatively low diffusion barriers of NO, O, and N may provide promising routes for N2 and N2O formation at a moderate temperature condition. These calculations are qualitatively in agreement with available experimental TPD results reported by Zhang et al.,28 in which they proposed that the N2 and N2O formed are likely products of the NO decomposition and recombination reactions. III.6. Rate Constant Calculations. On the basis of the aforementioned PESs for the dissociation of NO2 and NO on the W(111) surface, we have calculated the rate constants for the following two reactions:

NO2(g) + W(111) f

β ) 1.77 Å-1, and De ) 79.41 kcal/mol and R0 ) 2.033, β ) 1.70 Å-1, and De ) 75.21 kcal/mol for NO2(g) + W(111) and NO(g) + W(111) reactions, respectively. The predicted rate constants (in molecular units, cm3/s) in the broad temperature range of 200-3000 K can be represented as

kNO2 ) 1.16 × 10-9T-0.00094 exp(0.53 kcal mol-1 /RT) kNO ) 1.40 × 10-10T-0.00082 exp(0.52 kcal mol-1 /RT) The rate constants for these dissociative adsorption processes are associated with the equation42

d[X]surf /dt ) k(θ/As)[X]g which has the units of a flux, molecule cm-2 s-1. In the above equation, θ, As, and [X]g represent the fraction of available surface sites, the surface area, and the gas-phase concentration of NO2 and NO gases in molecules/cm3, respectively. III.7. Comparison of the Reactions of NOx, COx, and OHx (x ) 1, 2) on the W(111) Surface. In this section, we compare the mechanisms of OHx and COx (x ) 1, 2) on W(111) surface studied previously2,3 with that of NO2 reported above. First, W(111)sNO2 adsorption energy (∆Eads ) 79.4 kcal/mol) is greater than that of W(111)sCO2 and W(111)sOH2 (∆Eads ) 37.5 and 12.6 kcal/mol, respectively), reflecting that the higher ability of the unpaired electron in NO2 interacts more strongly toward W centers. Similarly, the radical species has a larger binding energy (98.2 and 74.4 kcal/mol for OH and NO, respectively) than the close-shell molecule (37.9 kcal/mol for CO). Furthermore, the exothermicities of the overall W(111) + AB reactions decrease in order with AB ) NO2, CO2, and OH2, 185.2, 61.0, and 23.7 kcal/mol, respectively. For all cases, the calculated transition state structures lie below their starting reference points, indicating that no supplement of thermal energy is necessary for the reaction to proceed. In addition, the calculated energy barriers for the first (ON-O, OC-O, and HO-H bonds) and second (N-O, C-O and O-H bonds) X-O bond activation are not very high (within the range of 3.0-24 kcal/mol), indicating that the reactions of COx, NOx, and OHx with W(111) can easily oxidize the surface, which is in good agreement with available experiments.45,46 IV. Conclusions

NO2(ads)(WNO2(IV-µ -N2, O1, O1)) f NO(ads) + O(ads)(LM1) f N(ads) + 2O(ads)(P1b)

(1)

NO(g) + W(111) f NO(ads) f N(ads) + O(ads)

(2)

3

For the reaction rate constant calculations, the stretching potential energy surfaces representing the barrierless association processes, NO2(g) + W(111) f NO2(ads) (WNO2(IV-µ3-N2, O1, O1)) and NO(g) + W(111) f NO(ads), were calculated along the reaction coordinate M-N, which is varied from its equilibrium value to 4.5 Å with the step size of 0.15 Å. At each fixed M-N distance, the bottom three atomic layers of the W(111) surface were fixed, while the remaining layers, NO2 and NO, were fully optimized at the rPBE level. The obtained stretching (M-N 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-N bond distance, and De is the bond energy without zero-point energy corrections. The Morse potential parameters obtained by fitting the stretching potential energy curves are R0 ) 2.083,

From the above presented data one can draw the following conclusions: 1. Adsorption and dissociation of NO2 on the W(111) surface occurs without any thermal barrier and is highly (∼185 kcal/mol) exothermic. It leads to oxidation and nitridation of the W(111) surface. Therefore, we predict that the NO2 molecule will be in its dissociative adsorption state on the W(111) surface. 2. Comparison of the reactions OHx + W(111), COx + W(111), and NO2 + W(111) show that the latter reaction occurs much easier and more exothermic. This trend was explained in terms of the presence of the unpaired electron in the NO2 molecule. Our finding for the reaction of NO with W(111) is in good agreement with the HREELS results reported by Zhang et al.28 3. We have predicted the rate constants for NO2 and NO dissociative adsorption processes 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 the use of its resources,

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