Dissociation and Diffusion of Hydrogen on the Mg (0001) Surface

May 22, 2009 - Unlike single substitution, where the impurity atoms always prefer to replace one of the Mg atoms from the second layer, in the case of...
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J. Phys. Chem. C 2009, 113, 10574–10579

Dissociation and Diffusion of Hydrogen on the Mg(0001) Surface: Catalytic Effect of V and Ni Double Substitution S. Banerjee, C. G. S. Pillai, and C. Majumder* Chemistry DiVision, BARC, Trombay, Mumbai-85, India ReceiVed: January 9, 2009; ReVised Manuscript ReceiVed: April 15, 2009

To understand the effect of impurity concentration in hydrogen dissociation and diffusion, an extensive firstprinciples investigation has been performed over doubly substituted Mg surface and the findings have been compared to those of single substitution. The calculations have been carried out using density functional theory within the plane-wave pseudopotential approach employing projector-augmented wave potentials and generalized gradient approximation of Perdew-Burke-Ernzerhof exchange-correlation functional. Unlike single substitution, where the impurity atoms always prefer to replace one of the Mg atoms from the second layer, in the case of double substitution, when Ni and V atoms are doped simultaneously, the most stable configuration shows that Ni and V prefer to substitute Mg atoms from the first and second layers, respectively. The stabilization of Ni on the surface layer results in significant reduction in the dissociation barrier of the hydrogen molecule on the doped Mg surface. I. Introduction In recent years, research in materials science has shown a rapid expansion toward the discovery of advanced materials for sustainable energy. The hydrogen economy is proposed to solve the ill effects of using hydrocarbon fuels in transportation, and other end-use applications where carbon is released into the atmosphere. In this context the storage of hydrogen poses the biggest challenge in a new hydrogen economy because the storage medium must meet the requirements of high gravimetric and volumetric density, fast kinetics, and favorable thermodynamics.1-5 Although molecular hydrogen has very high energy density on a mass basis, as a gas, at ambient conditions, it has very low energy density by volume. If it is to be used as fuel stored onboard the vehicle, pure hydrogen gas must be pressurized or liquefied, which will not only demand other forms of energy but also bring complications in terms of safety aspects. Considering all these issues, solid-state materials offer a practical alternative, and a great deal of effort has been devoted to finding efficient hydrogen storage materials. Magnesium-based systems are considered to be very promising hydrogen storage materials because of their low cost and high hydrogen storage capacity. Ideally, Mg can absorbed 7.66 wt % of hydrogen, which is sufficiently high for onboard hydrogen storage.5 But the problems with magnesium are high thermal stability of the magnesium hydride and its slow kinetics of hydrogen absorption and desorption.6,7 On reaction with hydrogen at 300 °C and 1 bar H2 pressure, Mg forms MgH2 and the heat of formation has been estimated to be -74.5 kJ/ mol H2 (-0.38 eV/H atom).8-10 Because of such high thermodynamical stability of the MgH2, the hydrogen desorption occurs at a very high temperature, which is not suitable for vehicular application. Many experimental and theoretical works have been performed to improve the efficiency of Mg metal to meet the criteria for hydrogen storage material.11-19 It has been found that ball milling of Mg can modify the rate of hydrogen absorption. By ball milling, it is possible to obtain particle size as low as 20 * To whom correspondence should be addressed.

nm so the diffusion path length decreases and the kinetics for hydrogen dissociation and absorption increases. It is also reported that transition-metal elements act as a catalyst for the absorption of hydrogen on the Mg surface, thus improving the kinetics of hydrogen absorption.20-25 It happens so because of the interaction between the 1s orbital of hydrogen and 3d orbital of transition-metal elements, but very little is known about the detailed mechanism of their catalytic action. Arboleda et al. have performed the quantum dynamics calculation to study the dissociative absorption of hydrogen molecule on Mg(0001), Ti(0001), and La(0001) surfaces26 to show that the activation barrier for hydrogen dissociation decreases in the order Mg > Ti > La. Thus, it is envisaged that incorporation of transitionmetal atoms into the Mg host would lead to an improvement of its kinetics toward absorption and desorption of hydrogen. Few theoretical reports are available in this direction, where Ti, Ni, and Pd were used as dopants.27-29 Most of these studies were carried out by substituting one of the Mg atoms from the surface layer. The substitutional doping is justified as by ball milling it is possible to create a vacancy, where the impurity atom can be placed easily. For these cases the dissociative chemisorption of hydrogen molecule occurs almost spontaneously on the catalytic site and the hydrogen atoms occupy the fcc or hcp holes depending upon the nature of the substituent atom. However, they restrict the movement of hydrogen atoms further forming a strong bond with the hydrogen atom and the diffusion barrier becomes quite high. For these cases the diffusion of hydrogen atoms becomes the rate-limiting step. However, the stability of the doped elements was not verified in these papers to know whether they prefer to replace the Mg atoms from the surface layer or from the bulk. In our previous study we find that most of the 3d elements prefer to replace one of the Mg atoms from the second layer rather than from the first layer.30 This has an advantage of reducing the diffusion barrier, but on the other hand, it increases the activation barrier for the obvious reasons. So the most desirable situation could be if we can use two dopants, each substituting one Mg atom from the first layer and the second layer.

10.1021/jp9002092 CCC: $40.75  2009 American Chemical Society Published on Web 05/22/2009

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Here, an attempt to provide such an understanding is made by carrying out first-principles calculations to investigate the structure and stability of the Mg(0001) surface doped with V and Ni atoms and its interaction with hydrogen molecules. For this purpose, first we calculated the stability of the doubly doped Mg surface by placing the dopants at different layers. Once the preferred location was established through stability criteria, we investigated the dissociative chemisorption of hydrogen molecule followed by diffusion of hydrogen atoms on the doubly doped Mg surface and compared these results with those of the singly doped Mg surface. Finally, we analyzed these results in terms of stability, dissociation of H2 molecule on the surface, and diffusion of H atoms on the surface to understand the role of doping toward designing magnesium-based alloys for hydrogen storage materials. II. Computational Details The calculations were performed with density functional theory31 using the plane-wave pseudopotential approach.32 An all-electron description, the projector-augmented wave (PAW) potential,33 were used to describe the electron ion interaction and the generalized gradient approximation of PerdewBurke-Ernzerhof (PBE) was used for exchange correlation energy functional. For all calculations, the plane wave cutoff energy was fixed to 300 eV. With use of the PAW method, the lattice parameters of bulk Mg were calculated to be a ) 3.19 Å and c ) 5.18 Å, which are only 1% in error compared with those of the experimental values.34 The slight difference from the experimental values can be due to room-temperature thermal expansion in the experimental values, which are not being considered in our theoretical calculations. The corrected cohesive energy was estimated to be 1.50 eV/atom, which is in excellent agreement with the experimental value of 1.51 eV/atom.35 For all surface calculations, the clean and doped Mg(0001) surfaces have been modeled with a five-layer slab, each layer of the slab consisting of 12 Mg atoms. The first Brillouin zone was integrated using a (5 × 5 × 1) Monkhorst-Pack grid.36 A lager vacuum space of 15 Å is maintained to ensure sufficient separation between periodic images. The energy was found to be converged within 1 meV and the calculated surface energy was found to be 0.31 eV/atom. These are in good agreement with the experimental results.29,37 To determine activation barriers for dissociation and diffusion of hydrogen on the doped surfaces, the nudged elastic band (NEB) method38 was used. This method involves optimizing a chain of images that connect the reactant and product state obtained by linear interpolation. For each calculation a total of 10 images have been considered including those of the initial and final states. Each image is allowed to move in the direction perpendicular to the hypertangent. Hence, the energy is minimized in all directions except for the direction of the reaction path. III. Results and Discussion In the first part, we briefly describe the results of hydrogenation on singly doped Mg surfaces. On the basis of energetics, it is seen that both Ni and V atoms prefer to substitute one of the Mg atoms from the second layer than from the top surface layer. However, the relative stability in the first and second layer differs for different transition-metal elements. For V, the difference in energy is almost 0.425 eV and in the case of Ni it is 0.13 eV only. The interaction of a hydrogen molecule with doped Mg surfaces suggests that when the impurity atoms are on the top layer the dissociative chemisorption of hydrogen

Figure 1. V and Ni doped at different layers of the Mg(0001) surface, when V and Ni are not occupying the nearby positions. The gray and dark balls indicate V and Ni atoms, respectively. (a), (b), (c), and (d) indicate V1Ni1, V2Ni2, V1Ni2, and V2Ni1, respectively.

molecule occurs spontaneously on the catalytic site and the hydrogen atoms occupy the fcc or hcp holes depending upon the nature of M atom. However, they restrict the movement of hydrogen atoms further, as the diffusion barrier is quite high. Here, the diffusion of hydrogen atoms is the rate-limiting step and Ni is found to be the best catalyst as it shows the lowest activation barrier for diffusion. The situation is reversed when the M atoms substitute one of the Mg atoms from the second layer. In this case the dissociation of hydrogen molecule is controlled by a high activation barrier, but once the hydrogen is absorbed, the mobility of the hydrogen atoms on the surface is easier than that in the previous case. Here the dissociation process is the rate-limiting step. Finally, when all the results are compared, it is inferred that the substitution of V in the Mg lattice will be the best choice considering the requirements of a good hydrogen storage material. So in the present study, by using first-principle calculations, we have determined the stability of Ni- and V-doped Mg(0001) surfaces when they are doped simultaneously by substituting two Mg atoms from the same or different layers. A. Relative Stability of Mg Surface after Substitution of V and Ni. We first describe the initial structural arrangements of the doubly substituted Mg surface that we have considered in our work. The clean Mg(0001) surface under consideration is composed of 60 Mg atoms (a five-layer slab and each layer consists of 12 Mg atoms). Two Mg atoms were substituted by Ni and V atoms, so the resulting surface has been abbreviated as Mg58VNi and the weight percent of doping is estimated to be 7.52 wt %. Now to find the most stable location for Ni and V on the Mg(0001) surface, the relative stabilities of different V- and Ni-substituted Mg surfaces were calculated by placing the doping elements at different locations. We have divided the substitutional arrangement into two parts: (A) when Ni and V are not occupying the nearby positions and (B) when Ni and V substitute two Mg atoms from adjacent positions. For the first case, we have considered four starting geometries, which are shown in Figure 1. (i) Both V and Ni are in the first layer (V1Ni1), (ii) both are in the second layer (V2Ni2), (iii) V is in

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TABLE 1: Substitutional Energies for Different Geometries of V-Ni Composite Doping on Mg(0001) Surface substitutional energies (eV) geometries

no adjacent site

adjacent site

V1Ni1 V2Ni2 V1Ni2 V2Ni1

-7.961 -8.440 -8.092 -8.308

-8.008 -8.489 -7.790 -8.562

the first layer and Ni in the second layer (V1Ni2), and (iv) V is in the second layer and Ni in the first layer (V2Ni1). Similar substitutional patterns with V and Ni at the adjacent sites were also considered in the second case. The stability of the configurations was analyzed by the energy gain as

E ) EMg/VNi(0001) + 2EMg(atom) - EV - ENi - EMg(0001) (1) where EMg/NiV(0001) is the energy of the Ni- and V-substituted Mg(0001) surface, EMg(atom) is the energy of one bulk Mg atom, EV is the energy of the V atom in the free state, ENi is the energy of the Ni atom in the free state, and EMg(0001) is the energy of a clean Mg surface. For the first case where the V and Ni atoms are not occupying the adjacent sites the most stable configuration is obtained by substituting two Mg atoms from the second layer. In this context it should be mentioned that when the Mg surface was substituted by V or Ni separately, both favored the second layer over the top layer.30 The order of stability for all substitutional patterns is listed in Table 1. In the second case where Ni and V occupy adjacent sites (Figure 2) on the Mg surface, then the situation is different. In general, all configurations are more stable than the previous case, except the situation when V remains in the first layer and Ni in the second layer. The most favorable substitutional site is found to be V2Ni1, where V and Ni substitute two Mg atoms from the second and first layer, respectively, and they are adjacent to each other. The Ni-V

Figure 2. V and Ni doped on different layers of the Mg(0001) surface when V and Ni substitute two Mg atoms from the adjacent position. The gray and dark balls indicate V and Ni atoms, respectively. (a), (b), (c), and (d) indicate V1Ni1, V2Ni2, V1Ni2, and V2Ni1, respectively.

Figure 3. Possible absorption sites of the hydrogen atoms after dissociation on the V- and Ni-doped Mg(0001) surface. The small balls indicate hydrogen atoms.

distance for this geometry is 2.32 Å, which is 0.274 Å less than the sum of the atomic radii of Ni and V. Furthermore, the smallest Mg-Mg distance on the top layer adjacent to the Ni atom is found to be 3.02 Å compared to the ideal Mg-Mg distance in the same layer of hcp lattice of 3.21 Å. This results in a local dip at the Ni site and acts as a catalytic center. B. Interaction of Hydrogen with the Most Stable Mg58VNi Surface. After establishing the most stable geometry for the V and Ni doubly substituted Mg surface, we investigated the interaction of hydrogen molecule with it. The interaction of hydrogen molecule with the Mg surface has been carried out in two steps. In the first step, the molecular hydrogen comes close to the Mg surface and dissociates into two H atoms (dissociative chemisorption), and in the second step the hydrogen atoms are diffused along the surface. Both steps require some external energy for the activation barrier to be crossed. The role of a catalyst is to decrease the activation barrier for the dissociation of hydrogen molecule and diffusion of hydrogen atoms. In the present work, V and Ni are used as catalysts to minimize the activation barrier of each process. To start the calculation of molecular hydrogen dissociation on the Mg surface, we find the most favorable location of hydrogen atoms after dissociation at the top of Ni atom. For this purpose we have identified six different hollow sites near the Ni atom where the dissociated hydrogen atoms can be absorbed. The possible locations, as shown in Figure 3, are as follows: (i) Two hydrogen atoms occupy the hcp holes (octahedral site/face) on the surface layer such that one hydrogen

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TABLE 2: Average Binding Energies of Hydrogen Atoms When They Occupy Different Holes at the Surface number

abbreviation

binding energy (eV)

1 2 3 4 5 6

h1 h2 h3 h4 h5 h6

-0.474 -0.374 -0.348 -0.370 -0.419 -0.377

atom is near the V atom (h1), (ii) two hydrogen atoms occupy the hcp holes on the surface layer such that both hydrogen atoms are away from the V atom (h2), (iii) two hydrogen atoms can occupy fcc holes (tetrahedral site/face) on the surface layer such that both hydrogen atoms are nearer to the V atom (h3), (iv) two hydrogen atoms occupy the fcc holes on the surface layer such that one hydrogen atom is nearer to the V atom (h4), (v) one hydrogen atom occupies the fcc hole and the other hcp hole such that the second hydrogen is nearer to the V atom (h5), and (vi) one hydrogen atom occupies the fcc hole and the other hcp hole such that the second hydrogen is away from the V atom (h6). The average binding energy has been calculated from the following equation,

E ) (Etotal_surface+2H - Etotal_surface - EH2) /2

Figure 4. Hydrogen dissociation and diffusion on the most stable Mg58VNi(0001) surface as represented by the initial state (a), after dissociation of hydrogen molecule (b), and after diffusion of hydrogen atoms (c1 and c2).

(2)

where Etotal_surface+2H is the total energy of the system after hydrogen absorption, Etotal_surface is the total energy of the surface, and EH2 is the energy of the hydrogen molecule. The average binding energies for all the above cases have been calculated and are listed in Table 2. It has been found that the most stable state is one where the hydrogen atoms absorb into two nearby hcp positions and one hydrogen atom is nearer to the V atom (h1). The next higher energy location is 0.06 eV higher in energy, where one hydrogen atom occupies the fcc hole and the other hcp hole such that the second hydrogen is nearer to the V atom (h5). Figure 4 shows the top view of the dissociation of hydrogen molecule and diffusion of hydrogen atoms on the Ni- and V-doped Mg(0001) surface when the hydrogen molecule is kept at a distance of 3 Å from the surface. To understand the effect on the electronic structures of these systems, we have analyzed the projected density of state (PDOS) of the most stable configuration. In principle, the transitionmetal elements act as good catalysts because of the availability of d electrons for donation and vacant d states for back-donation. Figure 5 shows the PDOS of the Ni and V before and after interaction with hydrogen. For comparison, we have also depicted the PDOS of only hydrogen before and after interaction. It is seen (Figure 5a) that when hydrogen molecule is at a large distance (3 Å) from the surface, there is no overlap between the Ni(d) orbital and H(s) orbital; a peak of hydrogen appears at -6.99 eV. The d PDOS of Ni (Figure 5b), at the top layer of the Mg surface, shows a broad peak for the d band. With a slight variation to this, the d PDOS of V (Figure 5c), which is in the second layer of the Mg(0001) surface, exhibits a dip in the center of the d band, characteristics of bcc metals. We further note that, unlike Ni, the d PDOS of V shows polarization of the spin channels. After the interaction with hydrogen, two distinct features are observed: (i) the new peak is blue-shifted (more negative energy), corresponding to pure hydrogen (Figure 5d), and (ii) the appearance of a new s band in the Ni and V spectra (Figure 5e,f). Moreover, we find that the d PDOS of V has changed significantly after the interaction with hydrogen,

which suggests there is a strong influence of V atom also for the dissociation of hydrogen molecule. C. NEB Calculations. After optimization of the most favorable adsorption site for dissociated H atoms on the Ni-V doped Mg surface, we calculated the activation barrier for the dissociation of hydrogen molecule along the minimum energy path by NEB method. It has been found that the dissociation of hydrogen molecule on the Ni site followed by migration toward the most stable hollow sites requires an activation barrier of 0.098 eV as shown in Figure 6 by solid circles. This is significantly lower in comparison to that of the undoped Mg surface, where the activation barrier is found to be 0.97 eV. The activation barrier for the diffusion of hydrogen atom has been calculated by taking 10 replicas including the initial and final states. The absorption sites of two hydrogen atoms are not identical so the diffusion barrier was calculated separately for each of the absorbed hydrogen. For the first hydrogen atom, which is absorbed at the hcp site away from the V atom (V-H distance is 4.22 Å), we calculated the barrier for diffusion toward the fcc site as shown in Figure 4(c1). It has been found from the NEB map that it needs an activation barrier of 0.41 eV for the diffusion process as shown by the solid triangle in Figure 6. The result shows that for the diffusion of the first hydrogen atom the presence of V atom does not influence the activation barrier much. For diffusion of the second hydrogen atom, i.e., which is closer to the V atom (V-H distance is 1.884 Å) toward the fcc site (Figure 4(c2)), the activation barrier for diffusion from the hcp site toward the fcc site is 0.66 eV (shown by the squares in Figure 5), higher than that in the previous case. IV. Conclusion In this study we have investigated the hydrogen dissociation and diffusion mechanism on the Mg(0001) surface doped with V and Ni atoms. The efficiency of catalytic behavior of these transition elements has been compared between single and double substitution. All calculations were performed with density functional theory using plane-wave pseudopotential approach. The results revealed that when the Mg surface was doped by

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Figure 5. Projected densities of states for H2 dissociating over Ni- and V-doped Mg surface as a function of the energy. (a), (b), and (c) represent the PDOS of H, Ni, and V, respectively, before interaction (initial state), and (d), (e), and (f) represent the PDOS of H, Ni, and V, respectively, after hydrogen absorption. The s and d orbitals are represented by blue and red lines, respectively.

also facilitates dissociative chemisorptions of the hydrogen molecule on the catalytic site of Mg(0001) surface. We have also seen its effect on the diffusion of hydrogen molecule through the Mg(0001) surface. After dissociation of hydrogen molecule into two hydrogen atoms at the catalytic site in the Mg(0001) surface, two hydrogen atoms show different activation barriers for diffusion depending upon their positions. The hydrogen atom, which is away from the V atom, shows a lower diffusion barrier compared to the other hydrogen atom as a result of the lower influence of the V atom. Finally, we conclude that by increasing the impurity concentration, it is possible to stabilize the dopant on the top layer, which results in significant reduction of the hydrogen molecule dissociation barrier on the Mg surface. References and Notes Figure 6. NEB profiles for the dissociation and diffusion of hydrogen on the Mg55VNi surface when Ni atom is at the first layer and V atom is at the second layer. The relative energies of the initial state, after dissociation of hydrogen molecule on the surface, and after diffusion of hydrogen atoms on the surface are marked by (a), (b), and (c), respectively.

one Ni or V atoms, the most stable configuration showed that the impurity atoms prefer to substitute one of the Mg atoms from the second layer. This results in very negligible effect on the hydrogen dissociation barrier. In contrast, when both V and Ni are doped simultaneously, the most stable configuration shows that V and Ni are placed in the second and first layer, respectively. The stabilization of Ni in the first layer enhances the direct interaction of hydrogen, which results in significant lowering of the dissociation energy barrier, the most desirable property of hydrogen storage. The presence of V atom at the subsurface layer not only stabilizes Ni at the surface layer but

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