DFT Calculations of Adsorption and Decomposition of N2O on Rh(100

Nov 12, 2010 - Nanosystem Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 ...
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DFT Calculations of Adsorption and Decomposition of N2O on Rh(100) Hideo Orita,*,† Toshitaka Kubo,‡ Tatsuo Matsushima,§ and Anton Kokalj| Nanosystem Research Institute (NRI), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan, Nanosystem Research Institute (NRI), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, Catalysis Research Center, Hokkaido UniVersity, Sapporo 001-0021, Japan, and Jozˇef Stefan Institute, JamoVa 39, 1000 Ljubljana, SloVenia ReceiVed: July 8, 2010; ReVised Manuscript ReceiVed: October 11, 2010

Adsorption and decomposition of N2O on Rh(100) surface has been investigated by density functional theory (DFT) methods with full geometry optimization, transition state search (synchronous transit methods and constrained geometry optimization), and molecular dynamics. More stable adsorption forms (lying-flat form with all three atoms of the N2O molecule interacting with the surface and “horse-like” form with two N atoms attached to the surface) have been found in the present work besides the two forms reported previously (an upright form with the terminal N atom bonding to the surface and a lying form attaching with both terminal N and O atoms to atop sites). Decomposition pathways of N2O with and without coadsorbed oxygen atoms have been identified and compared with the experimental data. The present results suggest the necessity of internal energy partitioning measurements of desorbing species to discuss the transition states of decomposition more definitely. 1. Introduction N2O is a linear triatomic molecule in the gas phase, and has two nitrogen atoms in different chemical environments: the terminal (Nt) and central (Nc) nitrogen atoms. The interaction of N2O with metal surfaces is one of the interesting topics of surface science and catalysis since N2O is not only a greenhouse gas but also an undesirable byproduct in the removal processes of NOx on automobile three-way catalysts. However, the mechanism of its formation and decomposition on the catalysts is not clear at present mainly because of difficulty in obtaining spectroscopic evidence. The decomposition of N2O is very dependent on kinds and surface orientations of metals. For example, N2O does not decompose on fcc (111) surfaces such as Pt,1 Ir,2 Rh,3 and Ni.4 On the other hand, decomposition of N2O is observed on Ni(100),5 Ni(110),6 and Rh(110)3 by using molecular beam technique. The adsorption of N2O on Pd(110) and Rh(110) has been investigated by using the density functional theory (DFT) method.7-9 N2O adsorbs weakly to the surface in two alternative forms, either tilted with attaching the Nt atom to the surface, or lying horizontally in a “roof-like” shape with attaching both the Nt and O atoms to atop sites. The horizontal form of N2O is proposed as the precursor of the inclined desorption of the product N2 observed in the thermal decomposition of N2O.10 Adsorption and decomposition of N2O on Ni(755) has been studied experimentally11,12 as well as computationally.13 From the experiments, it is found that the decomposition of N2O occurs exclusively at the step sites below 200 K, yielding gaseous N2 and leaving atomic oxygen on the surface.11,12 From the calculations,13 several molecular adsorption * To whom correspondence should be addressed. Fax: +81-29-851-5426. Phone: +81-29-861-4835. E-mail: [email protected]. † Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology. ‡ Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology. § Hokkaido University. | Jozˇef Stefan Institute.

structures (bent or linear forms adsorbed upward by attaching with Nt, and the bent one adsorbed horizontally) are found both on the terrace and step. Adsorption energies for the terrace are smaller than those for the step. Much attention has been given to the decomposition of N2O on rhodium surfaces, because rhodium is among the best catalysts for removing nitrogen oxides. Decomposition of N2O on Rh(100) produces two N2 desorption peaks at around 85 K (β2-N2) and 110 K (β1-N2).14 Neither the N2 production nor the N2O desorption can be observed below about 0.1 ML (monolayer). At higher coverage, the sum yield of the N2 production and the N2O desorption increases almost linearly at larger exposures even after the desorption from the N2O multilayer begins. The emission of β2-N2 at low coverage is off-normally concentrated in the plane along either the [001] or [010] direction. Preadsorbed oxygen severely suppresses the N2O decomposition, and especially the β2-N2 peak even at the coverage lower than 1/20 ML. The suppression is complete at the coverage of about 1/4 ML. Although preliminary DFT calculations on adsorption and decomposition of N2O on Rh(100) have been performed by two of the present authors,15 the influence of coadsorbed oxygen atoms has not been examined yet. In the present work, we therefore investigate adsorption and decomposition of N2O with and without coadsorbed atomic oxygen as a function of the adsorbate coverage, because the decomposition of N2O is quite dependent on the composition of adsorbate.14 2. Computational Methods All DFT calculations except molecular dynamics were performed with DMol3 in Materials Studio (version 4.4) of Accelrys Inc. on personal computers. In the DMol3 method,16-18 the physical wave functions are expanded in terms of accurate numerical basis sets. We used the double-numeric quality basis set with polarization functions (DNP). The size of the DNP basis set is comparable to Gaussian 6-31G**, but the DNP is more

10.1021/jp106338t  2010 American Chemical Society Published on Web 11/12/2010

Adsorption and Decomposition of N2O on Rh(100)

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accurate than the Gaussian basis set of the same size.19 Density functional semicore pseudopotential (DSPP)20 was employed only for Rh; DSPP replaces the effect of core electrons inner than the 3d orbitals with a simple potential, and includes some degree of relativistic correction. The gradient-corrected GGA functional, developed by Perdew, Burke, and Ernzerhof (PBE),21 was employed. A Fermi smearing of 0.002 hartree (0.054 eV; 1 hartree ) 27.2114 eV) and a real-space cutoff of 4.6 Å were used to improve computational performance. A (3 × 3) surface supercell with a slab consisting of five (100) layers (45 Rh atoms) was used, with a 30 Å of vacuum region between the adjacent slabs. Adsorbate and the three top layers of metal were allowed to relax in all the geometry optimization calculations without symmetry constraint (i.e., always using P1 symmetry). We used FINE quality integration mesh of the program and 10-7 SCF convergence threshold. The geometry optimization tolerances were 10-5 hartree (2.72 × 10-4 eV), 2 × 10-3 hartree/Å (5.44 × 10-2 eV/Å), and 5 × 10-3 Å for energy, gradient, and displacement convergence, respectively. A 3 × 3 × 1 k-point sampling was used. Adsorption energies (Ead) were computed by subtracting the energies of the gas-phase N2O molecule and surface from the energy of the adsorption system as shown in eq 1.

Ead ) E(N2O/surface) - E(N2O) - E(surface)

(1)

With this definition, a negative Ead corresponds to stable adsorption on the surface. The computationally determined Rh lattice constant of 3.85 Å (cf. 3.80 Å experimentally22) was used for the production of the surfaces. Under the present computational conditions, the N-N and N-O bond lengths for the free N2O molecule were calculated as 1.142 and 1.195 Å, respectively, in good agreement with the experimental values of 1.128 and 1.184 Å.22 A transition state (TS) search was performed with synchronous transit methods.23 These methods rely strongly on reasonable initial and final structures (IS and FS) for a reaction system. Starting from initial and final structures, the synchronous transit methods interpolate a reaction pathway to find a transition state. The linear synchronous transit (LST) method performs a single interpolation to a maximum energy. The quadratic synchronous transit (QST) method alternates searches for an energy maximum with constrained minimizations in order to refine the transition state to a high degree. Complete calculations of LST/ QST begin by performing an LST/Optimization calculation (an LST maximization, followed by an energy minimization in directions conjugate to the reaction pathway). The TS approximation obtained in that way is used to perform a QST maximization. From that point, another conjugate gradient minimization is performed. The cycle is repeated until a stationary point is located. It should be kept in mind that, even with such methods, one is never guaranteed to find other than local pathways. Born-Oppenheimer-type molecular dynamics (MD) simulations were performed with CASTEP24 in Materials Studio, employing the plane-wave basis set with ultrasoft pseudopotentials and the PBE functional. To reduce the high computational costs of MD, the accuracy was slightly compromised by decreasing the vacuum region to 20 Å and k-point sampling to 2 × 2 × 1. A plane-wave cutoff of 300 eV, Gaussian smearing of 0.05 eV, and time step of 1 fs were used (the present computational setup, in particular the size of the supercell, might not be sufficient to observe a hyper-thermally emitted nitrogen molecule before energy dissipation since the interaction of the

Figure 1. Typical adsorption structures of N2O on clean Rh(100) surface for one molecule in the (3 × 3) supercell (1/9 monolayer). Only atoms in the reference supercell are drawn to make the figures simple.

produced nitrogen molecule with the surface is considerably attractive,25 hence the desorption dynamics is not discussed in this paper). All atoms were allowed to move with fixing only the center of mass of the whole system. NVE and NVT (NoseHoover chain thermostat) ensemble simulations were carried out with changing starting and target temperature, respectively, only to survey the possibility of structure transformation due to medium accuracy of the present calculation setups. The results of DMol3 and CASTEP were crosschecked occasionally with PWscf in Quantum ESPRESSO,26 while visualization of results was done with the XCRYSDEN27 program. 3. Results and Discussion 3.1. Adsorption and Decomposition of N2O on Clean Rh(100) Surface. We investigated adsorption structures of N2O at coverages up to 2/9 ML;corresponding to two N2O molecules per (3 × 3) supercell;because the decomposition of N2O is quite dependent on coverage.14 Various adsorption modes were examined, and the resulting optimized structures are either attached with Nt to the surface or lying parallel. The adsorption via the O atom to the surface could not be identified. The typical adsorption structures at the coverage of 1/9 ML are shown in Figure 1. The details of adsorption structures and energies are listed in Table 1. The previous calculations on Rh(100) have already identified two kinds of adsorption forms, i.e., (i) an upright form with Nt bonding to the surface and (ii) a lying form attached with both terminal N and O atoms to atop sites.15 Besides these more stable adsorption forms have been found in the present work: (i) a lying-flat form with all three atoms of the N2O molecule interacting with the surface and (ii) a “horse-like” form with Nt and Nc attached to the

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TABLE 1: Calculated Total Adsorption Energies and Structural Parameters (Bond Distances) of N2O on Clean Rh(100) Surface in the (3 × 3) Supercell Eada/eV

structure

d(Rh-Nt)/Å

d(Rh-Nc)/Å

d(N-N)/Å

d(N-O)/Å

d(Rh-O)/Å

1.29 1.42 1.14 1.16 1.21 1.20

1.34 1.24 1.20 1.20 1.33 1.31

2.06

1.28, 1.29 1.29, 1.29 1.29 1.14 1.29 1.14 1.29 1.20 1.29 1.20 1.41 1.20 1.29 1.20 1.29 1.20

1.34, 1.34 1.33, 1.35 1.35 1.20 1.34 1.19 1.34 1.32 1.34 1.32 1.24 1.33 1.34 1.30 1.35 1.30

2.07, 2.08 2.05, 2.07 2.06

1/9 monolayer lying-flat horse-like atop bridge lying-atop-011 lying-atop-001

-0.78 -0.71 -0.43 -0.28 -0.34 -0.22

2_lying-flat-I 2_lying-flat-II lying-flat + atop-I

-1.31 -1.28 -1.27

lying-flat + atop-II

-1.25

lying-flat + lying-atop-011-I

-1.09

lying-flat + lying-atop-011-II

-1.06

horse-like + lying-atop-011

-0.98

lying-flat + lying-atop-001-I

-0.87

lying-flat + lying-atop-001-II

-0.87

a

1.98, 2.04 1.98, 1.98 2.00 2.18, 2.21 1.99 2.00

2.11 2.12, 2.12

2/9 monolayer 1.98, 2.01, 2.03, 2.08 2.09, 2.12 2.00, 2.00, 2.01, 2.03 2.12, 2.19 flat: 1.98, 2.04 2.12 atop: 2.01 flat: 1.99, 2.05 2.11 atop: 2.02 flat: 1.98, 2.04 2.11 atop-011: 1.99 flat: 1.98, 2.04 2.11 atop-011: 2.00 horse: 1.97, 1.99 2.12, 2.16 atop-011: 1.99 flat: 1.98, 2.03 2.15 atop-001: 2.02 flat: 1.98, 2.03 2.15 atop-001: 2.03

2.07 2.14

2.07 2.06 2.08 2.06 2.08 2.08 2.05 2.14 2.05 2.15

Total adsorption energy of N2O per supercell.

TABLE 2: Calculated Activation Energies on Rh(100) Surface in the (3 × 3) Supercell initial structure lying-flat lying-atop-011 lying-atop-001 atop

final structure 1/9 monolayer N2(a) + O(a) N2(a) + O(a) N2(a) + O(a) lying-atop-011

2/9 2_lying-flat-I 2_lying-flat-II lying-flat + lying-atop-011-II lying-flat atop

monolayer lying-flat + N2(a) + O(a) lying-flat + lying-atop-011-II lying-flat + N2(a) + O(a)

1/9 monolayer with two coadsorbed O atoms N2(a) + O(a) N2(a) + O(a)

E*/eV 0.38 0.06 0.03 0.38 0.40 0.23 0.10 0.36 0.37

surface. The adsorption energies of these two forms are more exothermic than the previously reported ones by 0.28 eV at least, and their decomposition as well as diffusion could not be observed in the present work during several attempts of NVE MD simulation with initial kinetic temperature up to 1000 K (about 0.086 eV kinetic energy). In fact, the activation energy (E*) of the direct decomposition pathway from the lying-flat species is estimated as 0.38 eV by the synchronous transit methods as shown later in the next paragraph for the coverage of 2/9 ML (various calculated activation energies are summarized in Table 2). These species are probably formed preferentially at the initial stage of adsorption that both the N2 production and the N2O desorption could not be observed experimentally. When geometry optimization starts from the lying form with both terminal N and O atoms attached to the adjacent bridge sites, the decomposition to N2(a) + O(a) occurs spontaneously, which suggests that it is important for decomposition to approach the oxygen atom to the bridge site. However, the spontaneous decomposition on Rh(100) was not observed experimentally.14 The adsorption energy of the upright bridge form is 0.28 eV, but the adsorption at the 4-fold hollow

site was not identified. The adsorption energies of the lyingatop-011 and -001 are closer to the adsorption heat (0.28 eV) estimated from the desorption peak temperature (ca. 110 K)14 of β1-N2. The calculated decomposition activation energies for the lying-atop-011 and -001 forms are 0.06 and 0.03 eV, respectively, indicating that the decomposition pathway is slightly activated; note, however, that these small values are not significant and that these species are not populated on the surface because of the large energy difference from the lyingflat form (in the range of 0.44-0.56 eV). The TS for decomposition is quite reactant-like as shown in the next paragraph for the coverage of 2/9 ML. The conversion from the most stable lying-flat to the lying-atop-011 adsorption forms needs the activation energy of at least 0.44 eV as judged from the difference of the two respective adsorption energies. The calculated activation energy of the tilting-down of the atop form to lying-atop-011 is 0.38 eV, which is larger than the difference of the adsorption energy between the atop and the lying-atop011 by 0.29 eV. Therefore, these conversion processes to the lying-atop species and the direct decomposition of the lyingflat form at 1/9 ML need comparable activation energies. For the coverage of 2/9 ML, a large number of adsorption configurations were considered. The typical optimized adsorption structures are shown in Figure 2. The details of adsorption structures and energies are listed also in Table 1. The energy difference between 2_lying-flat and lying-flat + atop configurations is less than 0.06 eV, indicating that these configurations are populated simultaneously. The adsorption energy of the lying-flat form becomes less exothermic by 0.25 eV in passing from 1/9 to 2/9 ML coverage [2 × (-0.78) ) -1.56 eV at 1/9 ML to be compared to -1.31 eV at 2/9 ML]. This can be attributed to sharing the surface Rh atoms by the neighboring molecules at 2/9 ML. After searching several dissociated configurations as the final structure in the synchronous transit methods, we investigated the direct and indirect decomposition pathways of the lying-flat species from the initial state configurations of 2_lying-flat-I and 2_lying-flat-II, respectively. The

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Figure 4. IS, TS, and FS images of decomposition of N2O from the lying-flat + lying-atop-011-II configuration on clean Rh(100) surface in the (3 × 3) supercell.

Figure 2. Typical adsorption structures of N2O on clean Rh(100) surface for two molecules in the (3 × 3) supercell (2/9 monolayer).

Figure 5. Typical adsorption structures of N2O on Rh(100) surface in the presence of one oxygen atom in the (3 × 3) supercell.

lying-flat + lying-atop-011-II f lying-flat + N2(a) + O(a), E2* ) 0.10 eV

Figure 3. IS, TS, and FS images of decomposition of N2O from the 2_lying-flat-I configuration on clean Rh(100) surface in the (3 × 3) supercell.

direct decomposition pathway of the 2_lying-flat-I (initial state) to the most stable dissociated configuration (final state) is shown in Figure 3. The activation energy is determined as 0.40 eV. The TS for this direct decomposition pathway is quite reactantlike, and the structure change mainly occurs along rotation of the second lying-flat species. The Nc-O distance becomes longer from 1.34 to 1.73 Å whereas the N-N distance becomes a little shorter from 1.28 to 1.22 Å. On the other hand, the decomposition starting from the 2_lying-flat-II configuration is a two-step process, i.e.:

2_lying-flat-II(IS1) f lying-flat + lying-atop-011-II(IS2), E1* ) 0.23 eV

The first step corresponds to the transformation of the 2_lyingflat-II configuration to the lying-flat + lying-atop-011-II configuration and the latter then dissociates. The sum of the activation energy is 0.32 eV [i.e., E* ) E(TS2) - E(IS1) ) -0.96 - (-1.28) ) 0.32 eV]. The TS2 for this indirect decomposition is also quite reactant-like as shown in Figure 4, and the structure change mainly involves the elongation of the Nc-O distance of the lying-atop-011 from 1.32 to 1.57 Å and shortening of its Rh-O distance from 2.08 to 1.94 Å (the distances of its N-N and Rh-Nt are shortened slightly from 1.20 and 2.00 Å to 1.18 and 1.97 Å, respectively). By considering repulsive forces from the counterproduct oxygen atom and the transition state structures of the direct and indirect decomposition pathways, the nascent N2 might be rotationally excited parallel and perpendicular to the surface (i.e., so-called helicopter- and cartwheel-like rotations), respectively, but the internal energy partitioning of desorbing molecules has not been studied experimentally at present. These measurements are quite desired to discuss the decomposition process more definitely. 3.2. Adsorption and Decomposition of N2O with Coadsorbed Oxygen Atoms. The typical adsorption structures with coadsorbed oxygen atoms at the coverages of 1/9 and 2/9 ML are shown in Figures 5 and 6, respectively. The details of adsorption energies and structures are listed in Table 3. All the adsorption energies become less exothermic than those without

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Figure 6. Typical adsorption structures of N2O on Rh(100) surface in the presence of two oxygen atoms in the (3 × 3) supercell.

TABLE 3: Calculated Adsorption Energies and Structural Parameters (Bond Distances) of N2O with Coadsorbed Oxygen Atoms on Rh(100) Surface in the (3 × 3) Supercell structure

lying-flat atop lying-atop-011 lying-atop-001

Eada/ d(Rh-Nt)/ d(Rh-Nc)/ d(N-N)/ d(N-O)/ d(Rh-O)/ eV Å Å Å Å Å

-0.69 -0.42 -0.30 -0.13

1/9 monolayer with one coadsorbed O atom 1.98, 2.03 2.17 1.29 2.01 1.14 1.99 1.21 2.01 1.20

1/9 monolayer with two coadsorbed O atoms lying-flat -0.57 1.99, 2.04 2.15 1.28 atop -0.39 2.02 1.14 lying-atop-011 dissociate or convert to lying-flat lying-atop-001 0.02 2.01 1.20

atop lying-flat

1/9 monolayer with three coadsorbed O atoms -0.40 2.03 1.14 0.07 2.02, 2.05 (3.18) 1.25

1.34 1.20 1.33 1.31

2.06 2.08 2.15

1.34 1.20

2.06

1.30

2.15

1.19 1.30

2.17

Figure 7. Transition state search for decomposition from the atop configuration in the presence of two oxygen atoms in the (3 × 3) supercell by stepwise geometry optimization with constraining the z coordinate (Zo) of the oxygen atom in N2O from the surface; energy dependence on Zo and typical images during decomposition. The reference of the z coordinate is the top Rh atom of the unrelaxed surface slab model.

a

Adsorption energy of N2O on Rh(100) preadsorbed with oxygen atoms.

oxygen atom (cf. Table 1). While the adsorption energy of the lying-flat is very sensitive to oxygen coverage and becomes less exothermic from -0.78 to -0.57 eV with increasing the oxygen coverage from 0 to 2/9 ML, that of the atop species changes by only 0.03 eV because the surface Rh atoms are not shared by the atop species and oxygen atoms. Nevertheless, the energy difference between the lying-flat and the atop forms remains large (in range of 0.18-0.27 eV), hence the population of the atop species is small. At the oxygen coverage of 2/9 ML, the lying-atop-011 species cannot stay at a local minimum, whereas the adsorption energy of the lying-atop-001 species becomes almost zero, or rather, slightly endothermic. For the direct decomposition pathway from the lying-flat species in the presence of oxygen atoms at 2/9 ML, the activation energy has been estimated as 0.36 eV by the synchronous transit methods as already shown in Figure 3 in the absence of oxygen atom. On the other hand, for the decomposition pathway from the atop species, the TS search has been carried out by stepwise geometry optimization with constraining the z coordinate of the oxygen atom (Zo) in N2O from the surface not by the synchronous transit methods, because we could not find a reliable intermediate structure for the synchronous transit methods (Figure 7). The adsorption structure with maximum energy (Zo ) 2.49 Å) in Figure 7 is very similar to that of the lying-atop-001, but the adsorption energy is slightly exothermic (-0.02 eV). The oxygen atom in N2O is located at the bridge

site not only just before the large energy fall (Zo ) 1.92 Å) but also after that (Zo ) 1.90 Å). The nascent N2 is swung away from the counterproduct oxygen atom along the Nc-O bond just after this energy fall because it receives repulsive forces from the oxygen atom. The buckling of the surface is quite apparent just after the energy fall, indicating that the produced oxygen atom begins to interact strongly with the surface as the N2 and O go apart from each other. Although not only the energy transfer between surfaces and desorbing molecules but also the internal energy partitioning into the rotational, vibrational, and translational modes are important for desorption dynamics, they have not been studied either experimentally or theoretically. The activation energy for this decomposition pathway from the atop species is determined as 0.37 eV. This value is almost the same as the adsorption energy of the atop species, which indicates its decomposition and desorption can compete with each other. These results are consistent with the suppression of decomposition with coadsorbed oxygen atoms in the experiments.14 In the presence of three oxygen atoms (i.e., 1/3 ML), only the atop species is stable (cf. Table 3), while the adsorption of lyingflat species is endothermic with Ead of 0.07 eV and its Nt atom located high above the surface (3.18 Å), hence the direct decomposition pathway from the lying-flat species becomes impossible. These results at the oxygen coverage of 1/3 ML should correspond to the complete suppression of decomposition at 1/4 ML in the experiments.14

Adsorption and Decomposition of N2O on Rh(100) 4. Conclusion Adsorption and decomposition of N2O on Rh(100) surface has been investigated by density functional theory (DFT) methods with full geometry optimization, transition state search (synchronous transit methods and constrained geometry optimization), and molecular dynamics. Besides the two forms reported previously (an upright form with the terminal N atom bonding to the surface and a lying form attached with both terminal N and O atoms to atop sites), more stable adsorption forms (lying-flat form with all three atoms of the N2O molecule interacting with the surface and “horse-like” form with Nt and Nc attached to the surface) have been found. These more stable forms are probably formed preferentially at the initial stage of adsorption such that neither the N2 production nor the N2O desorption could be observed experimentally. Two decomposition pathways of N2O on clean Rh(100) have been identified (direct and indirect decomposition from lying-flat species). The coadsorbed atomic oxygen reduces the exothermicity of N2O adsorption, and for lying N2O configurations this reduction is substantial. At the oxygen coverage of 2/9 ML, the magnitude of the adsorption energy of the N2O atop species becomes almost degenerate with its activation energy for decomposition thus indicating that the two phenomena can compete with each other. The coadsorption of oxygen therefore leads to the suppression of N2O decomposition. At the oxygen coverage of 1/3 ML, only the adsorption of atop species remains exothermic, while that of the lying-flat species becomes endothermic. The direct decomposition pathway from the lying-flat species becomes impossible. References and Notes (1) Avery, N. R. Surf. Sci. 1983, 131, 501. (2) Cornish, J. C. L.; Avery, N. R. Surf. Sci. 1990, 235, 209. (3) Li, Y.; Bowker, M. Surf. Sci. 1996, 348, 67.

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21449 (4) Va¨terlein, P.; Krause, T.; Ba¨βler, M.; Fink, R.; Umbach, E.; Taborski, J.; Wu¨stenhagen, V.; Wurth, W. Phys. ReV. Lett. 1996, 76, 4749. (5) Hoffman, D. A.; Hudson, J. B. Surf. Sci. 1978, 180, 77. (6) Sau, R.; Hudson, J. B. J. Vac. Sci. Technol. 1981, 18, 607. (7) Kokalj, A.; Kobal, I.; Horino, H.; Ohno, Y.; Matsushima, T. Surf. Sci. 2002, 506, 196. (8) Kokalj, A.; Kobal, I.; Matsushima, T. J. Phys. Chem. B 2003, 107, 2741. (9) Kokalj, A.; Matsushima, T. J. Chem. Phys. 2005, 122, 034708. (10) Matsushima, T. Prog. Surf. Sci. 2007, 82, 435, and references cited therein. (11) Kodama, C.; Orita, H.; Nozoye, H. Appl. Surf. Sci. 1997, 121/122, 579. (12) Orita, H.; Kondoh, H.; Nozoye, H. J. Catal. 1998, 177, 217. (13) Orita, H.; Itoh, N. Surf. Sci. 2004, 550, 166. (14) Matsushima, T. J. Phys. Chem. C 2007, 111, 6422. (15) Matsushima, T.; Kokalj, A. Surf. Sci. 2007, 601, 3996. (16) Delley, B. J. Chem. Phys. 1990, 92, 508. (17) Delley, B. J. Phys. Chem. 1996, 100, 6107. (18) Delley, B. J. Chem. Phys. 2000, 113, 7756. (19) Inada, Y.; Orita, H. J. Comput. Chem. 2008, 29, 225. (20) Delley, B. Phys. ReV. B 2002, 66, 155125. (21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (22) CRC Handbook of Chemistry and Physics, 81st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2000. (23) Govind, N.; Petersen, M.; Fitzgerald, G.; King-Smith, D.; Andzelm, J. Comput. Mater. Sci. 2003, 28, 250. (24) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. Z. Kristallogr. 2005, 220, 567. (25) Kobal, I.; Kokalj, A.; Horino, H.; Ohno, Y.; Matsushima, T. Trends Chem. Phys. 2002, 10, 139. (26) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; Fabris, S.; Fratesi, G.; de Gironcoli, S.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari., P.; Wentzcovitch, R. M. J. Phys.: Condens. Matter 2009, 21, 395502. (Code available from: http://www.quantum-espresso.org/). (27) Kokalj, A. Comput. Mater. Sci. 2003, 28, 155 (Code available from: http://www.xcrysden.org/).

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