Influence of Dopants Ti and Ni on Dehydrogenation Properties of

May 12, 2009 - We report on a study of influence of dopants (Ti and Ni) on the dehydrogenation properties of NaAlH4 using the ab initio density functi...
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J. Phys. Chem. C 2009, 113, 10215–10221

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Influence of Dopants Ti and Ni on Dehydrogenation Properties of NaAlH4: Electronic Structure Mechanisms Y. Song,*,† J. H. Dai,† C. G. Li,† and R. Yang‡ School of Materials Science and Engineering, Harbin Institute of Technology, Weihai Campus, 2 West Wenhua Road, Weihai 264209, China, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China ReceiVed: January 11, 2009; ReVised Manuscript ReceiVed: April 24, 2009

We report on a study of influence of dopants (Ti and Ni) on the dehydrogenation properties of NaAlH4 using the ab initio density functional method. Calculations show that the influence of dopants on the electronic structure is much localized; only the electronic structures of atoms in the vicinity of the dopants were changed. The mechanisms by which Ti improves the dehydrogenation properties of NaAlH4 are sensitive to the occupation of Ti in NaAlH4. If Ti substitutes for an Al atom, it may form Ti-Al intermetallics, and if it occupies an interstitial site, it may “capture” H atoms to form TiH2 phase. In both cases, the [AlH4] groups were dramatically distorted, which is the main reason why Ti dopant improves the dehydrogenation properties of NaAlH4. The influence of Ni on the dehydrogenation properties of NaAlH4 is relatively in weak comparison to the Ti dopant, mainly because the Ni only affects the electronic distribution in its vicinity, and so no significant changes of the [AlH4] groups were observed in Ni doped systems. 1. Introduction In the future new sustainable energy, sources and carriers will be needed. It has emerged in recent years that complex metal hydrides with the general formula MAlH4 (M ) Li or Na) are some of the most promising hydrogen storage materials. Many complex metal hydrides have long been recognized as potential hydrogen storage materials, as they have a high hydrogen weight percentage. However, they are generally characterized by extremely slow hydrogen cycling kinetics and were considered not to be reversible until Bogdanovic and Schwickardi’s 1997 discovery that NaAlH4 could be made reversible in the presence of some catalysts.1 Many studies of the structure and chemical properties of the catalysis alanates have followed. It has been shown that Ti is the additive that most efficiently increases the hydrogenation and dehydrogenation rate.2,3 However, its role is still poorly understood. It has been suggested that one role of the Ti catalyst might be to split the hydrogen4 in order to facilitate migration either of H via interstitials5 or of metal atoms in the form of AlH3 units.6,7 The location of Ti in this compound will help to identify the catalyst effects of Ti on the NaAlH4. A number of experiments have shown that Ti preferably substitutes the Na sites in the bulk lattice8 to give an Al-Ti alloy,9 remains on the surface in an amorphous TiAl3 phase,10 and solves in fcc Al.11 It has been shown theoretically that the substitution of the Ti atom for an Al site can result in a charge defect, a shift the Fermi energy level of the doped system and can induce significant modifications of one or more surrounding [AlH4] groups, improving the kinetics of NaAlH4.12 The total energy calculations using a standard state reference system show that the incorporation of Ti in to NaAlH4 is thermodynamically unstable due to that a positive enthalpy difference between the mixture and the pure * To whom correspondence should be addressed. E-mail: [email protected]. † School of Materials Science and Engineering, Harbin Institute of Technology. ‡ Institute of Metal Research, Chinese Academy of Sciences.

materials, regardless of the substitution behavior of Ti in NaAlH4.13,14 If gas phase atoms are used as the reference state, the Na substitution is favorable.8,15,16 It is well-known that the main factor controlling the dehydrogenation reaction of NaAlH4 is the highly stable [AlH4] group. The stability of the [AlH4] groups in NaAlH4 is reduced by the existence of impurities such as the dopants and the vacancies.12,14,15,17-21 Du et al. have shown that the [AlH4] group adjacent to a Na vacancy becomes strongly distorted, and two Al-H bonds were elongated and weakened due to the presence of a Na vacancy on the (001) surface of NaAlH4.18 They also found that the labile H2 molecule comes initially from a single [AlH4] group and is subsequently followed by other H migrations to stabilize the structure in the presence of a Na vacancy on the (110) surface of NaAlH4.21 In this paper, we use an ab initio simulation to gain an insight as to whether the dopants Ti and Ni influence the dehydrogenation properties of NaAlH4. The method used in this work is briefly described in Section 2. Section 2 gives the results of the calculations and discusses possible mechanisms that Ti uses to improve the dehydrogenation properties of NaAlH4 based on the electronic structure analysis in this section. Conclusions are given in Section 4. 2. Methodology. The electronic structure and the total energy of pure and M (M ) Ti or Ni) doped NaAlH4 were calculated with the VASP code22,23 using the generalized gradient approximation of Perdew and Wang.24 The project augmented wave method was used to span out the valence electron density.25 A cutoff energy of 480 eV and a Gaussian smearing method with an energy broadening of 0.2 eV were used throughout. A k-mesh of 3 × 3 × 3 was used. The criterion for self-consistency in the electronic structure determination was that two consecutive energies differed by less than 0.01 meV. NaAlH4 occurs in a tetragonal structure (space group I41/a) with lattice parameters a ) 0.5021 nm and c ) 1.1346 nm that has two formula units per primitive.26 To check the accuracy of the calculations, a full relaxation was performed and the atomic coordinates and both the size and shape of the cell of

10.1021/jp900254c CCC: $40.75  2009 American Chemical Society Published on Web 05/12/2009

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Song et al.

Figure 3. Charge difference distribution on the (010) plane of NaAlH4.

M substitutes for the Na and Al site and (x ) 0, y ) 0, z ) 1) means M occupies an interstitial site. The initial coordinates of the interstitial site were selected as (0.5, 0.5, 0.75), and a full relaxation of the doped systems was performed. To estimate the reaction enthalpy of NaAlH4 dissociating to Na3AlH6, Al, and H2, the ground state energies of Na3AlH6, the elements, Im3m Na metal, Fm3m Al and Ni metals, P63/mmc Ti metal, and diatomic H2 gas were calculated. Experimental values were used for the bulk lattice constants of these elements, and the H2 gas was simulated using a 1 × 1 × 1 nm3 cell. The calculated value of the reaction enthalpy is -43.2 kJ/molH2, which is close to the experimental measurement of -37 kJ/molH2.1 The stability of the doped systems can be described by the difference in total energy between the doped system and both the undoped NaAlH4 and the pure metal reference systems. We therefore define an occupation energy in eq 1.

Figure 1. Supercell of the NaAlH4 used in this work. The large and dark-red, the medium and light-blue, and the small and light-pink balls denote the Na, the Al, and the H atoms, respectively, while the darkblue ball denotes the interstitial site (Is) where the dopant was initially located.

Eoccu ) E(Na16-xAl16-yMx+y+zH64) - E(Na16Al16H64) {(x + y + z)E(M) - xE(Na) - yE(Al)} (1) Here E(M) denotes the total energy of system M. The calculated total energy, the lattice parameters and the dopant’s coordinates and occupation energy of the considered systems are listed in Table 1. In general, the dopant M extends the lattice of the NaAlH4, especially in the c direction. Table 1 illustrates this; it shows a relatively large change of M’s coordinates in the c direction. The configuration in which Ti substitutes for the Al atom is the most favorable in terms of occupation energy among the systems that were considered.

Figure 2. Density of states of NaAlH4 without dopant.

the pure NaAlH4was measured. The resulting lattice parameters are a ) 0.4973 nm and c ) 1.1098 nm, which are very close to the experimental values26 and other calculations.6 The present work uses a 2 × 2 × 1 supercell containing 16 formula cells of NaAlH4 (Figure 1) to investigate the influence of dopants on the dehydrogenation properties of NaAlH4. The M-doped system (M ) Ti or Ni) is denoted as (Na16-xAl16-yMx+y+z)H64, where (x ) 1, y ) 0, z ) 0) and (x ) 0, y ) 1, z ) 0) refer to the

3. Results and Discussions 3.1. Electronic Structure of NaAlH4. The electronic structure of NaAlH4 was calculated under the above framework. The total density of states of the NaAlH4 (DOS, shown in Figure 2)

TABLE 1: Lattice Parameters (nm), Total Energy E (eV), Positions, and Occupation Energy Eoccu (eV) of Dopants in the Doped NaAlH4 Systems lattice parameter

M coordinates

M

x

y

z

a

b

c

u

V

w

E(Na16-xAl16-yMx+y+zH64)

Eoccu

Ti

1 0 0 1 0 0

0 1 0 0 1 0

0 0 1 0 0 1

0.4950 0.5018 0.5100 0.5033 0.5018 0.5088

0.4960 0.5027 0.4974 0.4924 0.5009 0.5016

1.1368 1.1396 1.1419 1.1356 1.1421 1.1292

0.250 0.500 0.500 0.250 0.531 0.500

0.500 0.500 0.500 0.500 0.429 0.500

0.696 0.452 0.718 0.862 0.588 0.709

-318.575 -318.385 -320.841 -315.918 -314.479 -319.344

2.400 0.204 1.489 2.806 1.858 0.697

Ni

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is similar to that reported by Vajeeston et al. and Auayo and Singh.27,28 There is an approximately 4.5 eV band gap between the conduction bands and the valence bands. The valence bands consist of two crystal-field split manifolds with a width of approximately 3 eV. The Al s, the Na s, p, and the H s electrons contribute to the lower part, and the Al p, the Na p, and the H s electrons dominate the upper part, near the Fermi energy. These peaks form typical sp hydrides. In addition, partial DOSs show that the valence bands are strongly dominated by the H atoms, while the Na atoms control the conduction bands. Both the Al and the H atoms show fewer states in the conduction bands than in the valence bands, implying that the bonding between the Al and the H atoms in [AlH4] group is ionic in nature. Aguayo and Singh also reached this conclusion using LDA calculations.28 However, our GGA calculations show both that a large amount of Na s and p electrons contribute to the bonding and the antibonding states and that the electrons in the valence and conduction bands have an almost equivalent distribution in Na’s PDOSs (Figure 2). It is also noted that Na’s conduction bands are the largest contributor to the conduction bands of the total DOS of pure NaAlH4. This may cause covalent bonding between the Na atom and the [AlH4] group. We plotted the charge difference distribution (CDD) of this system as defined in eq 2 in Figure 3 to gain further insight into the bonding characteristics.

FCDD ) Fsys -

∑ Fiatom

(2)

i

In eq 2, Fsys is the charge distribution of the considered system is the charge distribution of the individual atoms that and Fatom i is calculated using a pseudostructure make up the system. Fatom i in which the ith atoms are kept in their positions and all atoms other than the ith ones are removed. The lattice parameters and the symmetry of this pseudostructure are as same as the original supercell. Figure 3 is the CDD on the (010) plane of pure NaAlH4. Although all the H atoms are equivalent in this compound, the two H atoms in the center [AlH4] group of this plane are labeled

as H1 and H2 to aid comparison with the doped systems. It is clear that the bonding between the Al and the H atoms is ionic, while the interaction between the Na atom and the [AlH4] group is a relatively weak covalent one. Similar bonding characteristics were reported by Aguayo and Singh.28 3.2. Ti Doped NaAlH4. It is well-known that Ti and its compounds are efficient catalysts when used to improve the dehydrogenation properties of NaAlH4. This work considers three different positions for a Ti additive to the system: a substitution for the Na atoms (x ) 1, y ) 0, z ) 0), a substitution for the Al atoms (x ) 0, y ) 1, z ) 0), and the occupation of an interstitial site (x ) 0, y ) 0, z ) 1). The occupation energy shows that the Ti substitution for the Al atoms is the most favorable configuration of the three, a finding consistent with Løuvik and Opalka’s calculations.14 Figure 4a shows the total and partial DOSs of the system in which Ti substitutes for the Na atoms. The PDOSs of the Na atom did not show significant changes when compared to the pure NaAlH4, and so it is not included in Figure 4. The total DOS shows three bonding energy windows, from -9.5 to -6.8 eV, from -6.8 to -3.2 eV, and near the Fermi energy. The lower two parts of the total DOS are similar in shape to the total DOS of pure NaAlH4 but are about 3.2 eV closer to the lower energy range. PDOSs show that the lower two windows are the result of the interaction between the Al and the H atoms, i.e., these bonding peaks in the two energy regions show the interaction within the [AlH4] group. The bonding peak near the Fermi energy is mainly due to the interaction between the Ti d and the Al p electrons, and it enhances the bonding between the Ti atom and the [AlH4] group. Structural analysis shows that the distance between the Ti/Na atom and the [AlH4] group is reduced from 0.373 nm (Na-Al, undoped) to 0.322 nm (Ti-Al, Ti-doped). Another feature in the PDOSs is that equivalent H atoms (H1 and H2 in Figure 4a) become slightly unequivalent by the addition of the Ti atom, possibly due to the attraction of the Ti on nearby H atoms.15 The main bonding peaks of the H1 s electrons in the energy range are from -5.5 to -5.0 eV but appear in the energy range from -4.0 to -3.2 eV for the H2 s electrons. The CDD on the (010) plane of this

Figure 4. Density of states of Ti-doped NaAlH4. (a) Ti substitutes for a Na atom, (b) Ti substitutes for an Al atom, and (c) Ti is located at an interstitial position.

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Figure 5. Charge difference distribution on the (010) plane of Tidoped NaAlH4. (a) Ti substitutes for a Na atom, (b) Ti substitutes for an Al atom, and (c) Ti is located at an interstitial position.

system is plotted in Figure 5a and shows a significant charge difference around the Ti atom. The Ti d electrons tend to distribute in one direction and make covalent bonds with the nearest neighboring [AlH4] groups. The charge distribution within the [AlH4] group is almost constant relative to pure NaAlH4. The substitution of a Ti atom for a Na site in NaAlH4 only has a weak effect on the distortion of the [AlH4] group by dopants, so it is unlikely that the Al-H bonds will be broken by the substitution of the Ti atom for a Na site. Figure 4b shows the total and partial DOSs of a system in which the Ti substitutes for an Al atom in NaAlH4. The note H1′ in this figure denotes the H atom in the [AlH4] group whose Al atom was replaced by a Ti atom. The two bonding areas in the total DOS of this system are mainly due to the strong bonding peaks between the Ti d and H1′ s electrons in the

Song et al. energy ranges from -3.0 to -2.5 eV and from -6.0 to -3.0 eV. The Ti dopant significantly influenced the interactions between the Al and the H atoms within the [AlH4] groups, reducing the group’s stability. Figure 4b shows that the H atoms do not interact equivalently with the Al atom; the stronger bonding between the Al and H2 atoms creates closer bonds than those between the Al and the H1 atoms. This is because the peaks of H2 s electrons (which are in the energy range from -3.0 to -2.5 eV) are much higher than that of the H1 s electrons in the same energy range. These electrons make hybrids with the Al p electrons, forming the Al-H bonds. Figure 4b also shows the peaks in the energy range from -6.0 to -3.0 eV that govern the interactions between the Ti and the Al atoms. These Ti-Al interactions appear again in the CDD on the (010) plane (Figure 5b). Three points are worth noting in Figure 5b: First, when Ti is substituted into a [TiH4] group, it interacts equivalently with all of the H1′ atoms and the H2′ atoms that have a bond length of 0.181 nm; this interaction is ionic in nature. In addition, the Ti “captures” two other H atoms from the nearest neighboring [AlH4] group using approximately 0.187 nm bonds. The Ti atom is now surrounded by six H atoms at distance of 0.181-0.188 nm and four Al atoms at a distance of 0.286-0.298 nm. Similar results were observed when Ti substitutes for an Al atom on the (110) surface of NaAlH4.20 In this case, the distance between the Ti and the H atoms is longer (0.188-0.200 nm) and the Ti atom bonds with three Al atoms at distance of 0.246-0.294 nm without breaking the Al-H bonds.20 In general, the distance between atoms is usually inversely proportional to the strength of the interaction between them. It is therefore unsurprising that the interactions between the Ti and the H atoms when Ti substitutes for an Al atom in bulk are slightly stronger and the distortion of the [AlH4] groups is greater than when the substitution is made on the surface. The other [AlH4] groups, especially the nearest neighbors, were also dramatically distorted by the substitution. The “captured” H atom has been moved away from the [AlH4] group toward to the [TiH4] group, so the Al-H and Ti-H bonds now have lengths of 0.192 and 0.187 nm, respectively. This Al-H bond is about 17% longer than that in the undoped NaAlH4. In terms of the bond length, this H atom is free from the [AlH4] group and is bonded with the Ti atom, i.e., one Al-H bond in the [AlH4] group was broken by the substitution of the Ti for an Al site. Further analysis reveals that the remaining [AlH3] specie has a high symmetry, the Al-H bond lengths are in the range of 0.164-0.166 nm, and the H-Al-H angles are between 100° and 110°. This will aid the formation of the [AlH3] ligand by lowering its formation energy.20 Figure 5 also shows that more electrons are distributed in the area between the Al and the H2 atoms than between the Al and the H1 atoms, implying that an Al-H2 bond is stronger than an Al-H1 bond. This is consistent with the above analysis of the electronic structure. Third, a large amount of electrons are located in the area between the Ti and Al atoms, and both of these atoms show negative CDDs. This means that the Ti and the Al atoms share the electrons between them to form a covalent bond. This interaction causes the Al atoms to move toward to the Ti atom, resulting in a 0.298 nm distance between the Ti and Al atoms (smaller than the Al-Al 0.373 nm distance in undoped NaAlH4). This will help form Ti-Al intermetallics and improve the dehydrogenation properties of NaAlH4. Ti could also occupy the interstitial site. This is the secondmost favorable position in terms of occupation energy in the considered Ti-doped systems. The Ti atom is initially located

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Figure 6. Density of states of Ni-doped NaAlH4. (a) Ni substitutes for a Na atom, (b) Ni substitutes for an Al atom, and (c) Ni is located at an interstitial position.

at the (0.5, 0.5, 0.75) interstitial site and has three [AlH4] groups surrounding it. The distances between the Ti and Al atoms are in the range 0.271-0.289 nm.19 After the relaxation, the Ti atom moves toward the [AlH4] groups, finishing at the position (0.5, 0.5, 0.718). The distance between the Ti and Al atoms has been significantly reduced to 0.245 nm. The H atoms in the [AlH4] groups move from an initial position of (0.383, 0.682, 0.675) toward the Ti atom, finishing at a position of (0.386, 0.646, 0.678). The Ti atom is now surrounded by six H atoms, two per [AlH4] group. The distances between the Ti and the H atoms are now in the range of 0.187-0.192 nm, and of the four Al-H bonds in the [AlH4] groups, two remain unchanged with a bond length of 0.162 nm and two have significantly elongated to a bond length of 0.216 nm. The [AlH4] groups surrounding to the Ti atom have been totally distorted by the interstitial Ti atom. Figures 4c and 5c show the DOSs and the CDD on the (010) plane of this system, respectively. Figure 4c shows that the interaction between the Ti and the H1/H2 atoms is the strongest and that the interaction between the Al and the H1/H2 atoms is reduced as a relatively large amount of Al p electrons move to the antibonding states. There were fewer bonding p electrons in this system than in pure NaAlH4 (Figure 2). This causes the two H atoms (H1 and H2) in the [AlH4] group to move toward to the Ti atom, which significantly reduces the distance between the Ti and H1/H2 atoms from 0.236 nm in the initial state to 0.187 nm after the relaxation. In other words, the Ti “captures” the H atoms that belong to the [AlH4] group, leaving the rest of the group distorted. This may be the one of the reasons that the TiH2 phase was observed experimentally.29,30 Figure 5c is the CDD on the (010) plane of this system. The H1 and H2 atoms have clearly moved away from the Al atom toward to the Ti atom so they can form ionic bonds with it. 3.3. Ni Doped NaAlH4. Nickel is a chemically active element, yet despite its common use as a catalyst, it seems to have little effect on NaAlH4. We reported the first principles calculations on the Ni doped NaAlH4 systems in order to ascertain why Ni does not work with NaAlH4. As with the Tidoped systems, three Ni-doped systems were studied; one with a Ni atom substituting for the Na atom (x ) 1, y ) 0, z ) 0), one with a Ni atom substituting for the Al atom (x ) 0, y ) 1,

z ) 0), and one in which the Ni atom occupies the (0.5, 0.5, 0.75) interstitial site (x ) 0, y ) 0, z ) 1). Parts a-c of Figure 6 show the DOSs of these three systems. In the system where a Ni atom is substituted for a Na atom, the Ni d electrons mainly distribute at the Fermi energy and they are the only electrons at this energy area (Figure 6a). This, coupled with a relatively large occupation energy (2.806 eV in Table 1), indicates that the Ni atom weakly interacts with the matrix. Figure 6a also shows that the sp hybridization between the Al and the H atoms is dominated by the electrons distributed in the energy range from -4.0 to -1.5 eV and that there is a overlap between the Ni d and the Na s, p electrons in the energy region between -5.0 and -2.0 eV. The CDD on the (010) plane (Figure 7a) shows both that the Ni atom moves from the Na’s position to an interstitial position and that the distribution of electrons within the [AlH4] group is almost independent of the addition of the Ni atom. These electronic structural features indicate that the dopant Ni strongly interacts with the Na atoms yet has little influence on the stability of the [AlH4] group. Figures 6b and 7b are the DOSs and the CDD on the (010) plane for the system in which the Ni replaces the Al atom. The Ni atom has strong interactions with the H atoms but only in the group it located, which results in a significant distortion of the [NiH4] group. The small size of the change in the electronic distribution of other [AlH4] groups indicates that the influence of Ni is quite localized. Ni has the lowest occupation energy (0.697 eV) when it occupies the interstitial site among the considered Ni-doped systems. In this system, the Ni could interact with either the Na or the H1/H2 atoms, as the Ni d, the Na p, and the H1/ H2 s electrons all contribute to a bonding peak at the Fermi energy in the total DOS (Figure 6c). The CDD on the (010) plane shows that the bonding is covalent between the Ni and the Na atoms and ionic between the Ni and the H1/H2 atoms (Figure 7c). The other [AlH4] groups do not show significant changes in the electronic structures when compared to the pure NaAlH4 (Figure 3). This means that the Ni has very little influence over dehydrogenation properties of the NaAlH4.

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Song et al. TABLE 2: Dissociation Energy, Edis Defined by eq 3, of the H Atom in the Na16-xAl16-yMx+y+zH64 Systems ∆Edis (eV) M

Ti

Ni

x,y,z

[MH4]

[AlH4]

0, 0, 0

1.69

1.69

1, 0, 0 0, 1, 0 0, 0, 1

0.98

1.01 0.87 0.99

1, 0, 0 0, 1, 0 0, 0, 1

0.52

0.94 1.51 1.25

The Edis of the undoped NaAlH4 (x ) 0, y ) 0, and z ) 0 in eq 3) is 1.69 eV. For the doped systems, if the dopant M occupies an Al site, i.e., one [AlH4] group becomes the [MH4] group and the H atom can dissociate from the [MH4] group or a neighbor [AlH4] group. Consequently, the Edis may be different for the two cases. Table 2 listed the values of the Edis for the considered systems. It can be concluded that, in general, the Edis is reduced by the dopants. In the Ti-doped systems, the lowest Edis appears in the case that the Ti occupies an Al site, a preferred site for Ti to occupy from Table 1. The Edis of the H atom dissociates from the [TiH4] and an [AlH4] group is 0.98 and 0.87 eV, respectively. Comparing with the undoped system, these values are distinctly reduced. In terms of the dissociation energy, the H atoms first dissociate from [AlH4] groups and then from the [TiH4] group. In the Ni-doped systems, the Edis of H atom dissociated from an [AlH4] group is 0.94, 1.51, and 1.25 eV, for the Ni occupies a Na site, an Al site, and an interstitial site, respectively, while Edis ) 0.52 eV if the H dissociates from the [NiH4] group when the Ni occupies an Al site. This implies that the influence of the Ni is quite near, which is consistent with the above electronic structure analysis. From Table 1, we know that the interstitial site is the prefer site for the Ni to occupy, while the electronic structures and Table 2 show that this is not the case that performs the best in improvement of the dehydrogenation properties of NaAlH4. This may be one of the reasons that the Ni has a letter effect on the improvement of the dehydrogenation behaviours of NaAlH4 than that of the Ti dopant. 4. Conclusions Figure 7. Charge difference distribution on the (010) plane of Nidoped NaAlH4. (a) Ni substitutes for a Na atom, (b) Ni substitutes for an Al atom, and (c) Ni is located at an interstitial position.

3.4. Dissociation Energy. To further clarify the effects of the dopants Ti and Ni on the dehydrogenation properties of NaAlH4, we therefore calculated the dissociation energy of hydrogen atom in NaAlH4. The dissociation energy was defined as

1 Edis ) E(Na16-xAl16-yMx+y+zH63) + E(H2) 2 E(Na16-xAl16-yMx+y+zH64)

[

]

(3)

The energy of the H dissociated system E(Na16-xAl16-yMx+y+zH63) is estimated by using the final structure of the Na16-xAl16-yMx+y+zH64 system but kept the geometry of the supercell and the coordinates of atoms unchanged. The value of the energy of a hydrogen molecule, E(H2), is -6.712 eV, calculated using a 1 × 1 × 1 nm supercell.

This paper uses first principles calculations to study Ti/Nidoped NaAlH4 systems. Although some experiments have reported that Ti substitutes for the Na atom, it seems that this is not favorable thermodynamically, as a large positive occupation energy was obtained in the considered systems when compared to the standard systems. The estimated occupation energies indicate that the Al’s position is the Ti’s most favorable position in NaAlH4, whereas Ni performs better in the interstitial position. The dopants Ti and Ni only have a local influence on the electronic structure. The mechanism that Ti uses to improve the dehydrogenation properties of NaAlH4 is sensitive to the position of the Ti in the NaAlH4. If Ti occupies an interstitial site, it is likely that it will interact with H atoms to form a TiH2 phase. If the Ti replaces Al atoms, there are stronger interactions between these atoms and other [AlH4] groups are dramatically distorted. These interactions may be the original forces that drove the formation of the Ti-Al intermetallics that were experimentally observed in Ti-doped NaAlH4. The influence of Ni on the dehydrogenation properties of NaAlH4 is relatively weak when compared to the Ti dopant, largely because Ni only

Influence of Ti and Ni on Dehydrogenation of NaAlH4 affects the electronic distribution in its vicinity not the stability of the [AlH4] groups. Acknowledgment. This work was supported by the National Basic Research Programme of China Grant 2006CB605104, the Natural Science Foundation of Shandong, China (Y2007F61), and the Programme of the Excellent Team of Harbin Institute of Technology. References and Notes (1) B. Bogdanovic, B.; Schwickardi, M. J. Alloys Compd. 1997, 1, 253. (2) Anton, D. L. J. Alloys Compd. 2003, 400, 356. (3) Sun, D. L.; Srimvasan, S. S.; Kabayashi, T.; Kuriyama, N.; Jensen, C. M. J. Phys. Chem. B 2003, 107, 10176. (4) Bellosta Von Colbe, J. M.; Schmidt, W.; Felderhof, M.; Bogdanovic, B.; Schu¨th, F. Angew. Chem., Int. Ed. 2005, 45, 3663. (5) Palumbo, O.; Paolone, A.; Cantelli, R.; Jensen, C. M.; Sulic, M. J. Phys. Chem. B 2006, 110, 9105. (6) Arau´jo, C. M.; Li, S.; Ahuja, R.; Jena, P. Phys. ReV. B 2005, 72, 165101. (7) Fu, O. J.; Ramirez-Cuesta, A. J.; Tsang, S. H. J. Phys. Chem. B 2006, 110, 711. (8) Arau´jo, C. M.; Ahuja, R.; Guille´n, J. M. O. Appl. Phys. Lett. 2005, 86, 251913. (9) Bogdanovic, B.; Brand, R. A.; Marjanovic, A.; Schwickardi, M.; Tolle, J. J. Alloys Compd. 2000, 302, 36. (10) Graetz, J.; Reilly, J. J.; Johnson, J.; Ignatov, A. Y.; Tyson, T. Y. Appl. Phys. Lett. 2004, 85, 500.

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