J. Phys. Chem. B 2006, 110, 25863-25868
25863
A First-Principles Analysis of Hydrogen Interaction in Ti-Doped NaAlH4 Surfaces: Structure and Energetics Jianjun Liu and Qingfeng Ge* Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed: August 25, 2006; In Final Form: October 10, 2006
First principles density functional theory studies have been carried out to investigate the hydrogen interactions in Ti-doped NaAlH4 (001) and (100) surfaces. In both surfaces, Ti was found to energetically favor the interstitial sites formed by three neighboring AlH4- units and interact directly with them. The resulting local structure corresponds to a formula of TiAl3Hx with x ) 12 before hydrogen desorption starts. The hydrogen desorption energies from many positions of TiAl3Hx are reduced considerably as compared with that from the corresponding clean, undoped NaAlH4 surfaces. The almost invariant local environment surrounding Ti during dehydrogenation makes the TiAl3Hx complex a precursor state for the formation of experimentally observed TiAl3. The importance of the complex has been explored by analyzing the structures and energetics accompanying hydrogen desorption from the complex and from the neighboring AlH4- units. The TiAl3Hx has extended effects beyond the locally reducing hydrogen desorption energy. It facilitates low-energy hydrogen desorption by either transferring hydrogen to the TiAl3Hx complex or reducing hydrogen desorption energy in the neighboring AlH4- by linking these AlH4- units with the complex structure. The possible mechanisms for forming octahedral AlH63- were also identified in the vicinity of TiAl3Hx. Desorbing hydrogen atoms between Ti and Al atoms causes a symmetrical expansion of Ti-Al bonds and leads to the formation of octahedral AlH63-.
1. Introduction communication,1
In a recent we identified an invariant local environment surrounding Ti in Ti-doped NaAlH4 (001) surface which we referred to as TiAl3Hx. We also demonstrated that the hydrogen desorption energies due to formation of this complex structure were reduced considerably as compared with that on an undoped clean surface. We concluded that this TiAl3Hx complex structure is a precursor state for the formation of TiAl3 which have been reported in a number of recent experimental studies of Ti-doped NaAlH4.2,3 We further predicted that this complex structure plays important roles in the reversible hydrogenation/dehydrogenation and speculated that the mechanism may involve mobility of hydrogen from the surrounding AlH4- units to the TiAl3Hx complex. Herein, we extend our study of Ti doping to another low index surface, (100), of NaAlH4. We also explored further the dehydrogenation energetics as well as associated structural changes on both surfaces. For hydrogen to be used as fuel on-board a vehicle, an efficient hydrogen storage media with acceptable volume, weight, cost, and safety risk has to be developed.4-6 The seminal work of Bogdanovic and Schwickardi,7 which reported the decomposition of NaAlH4, can be made reversible at reasonable pressures and temperatures by doping the sample with a few mole percent of Ti-containing compounds, stimulating a great deal of interest in exploring Alanate-based materials as reversible hydrogen storage media for on-board applications. Reversible dehydrogenation and rehydrogenation with a hydrogen cycling capacity of ∼4 wt % in Ti-doped NaAlH4 can be routinely achieved with reasonably good kinetics at 120-160 °C.6,8-10 * Address correspondence to this author. E-mail:
[email protected]. Fax: 618 453 6408.
Although Ti-doped NaAlH4 itself will not be able to meet the hydrogen capacity target set by the U.S. Department of Energy,11 it has served as a prototype to understand the mechanism of Ti-promoted heterogeneous dehydrogenation and rehydrogenation processes in materials with similar structures and compositions.12-16 Understanding the structural transformation in Ti-doped NaAlH4 accompanying the reversible hydrogen desorption and adsorption processes will likely provide an important physical basis in the design of novel materials for hydrogen storage with much improved performance. Hydrogen release and uptake from Ti-doped NaAlH4 involves a myriad of complex heterogeneous reaction and mass transfer steps. These steps are accompanied by solid-state phase transitions. The measured kinetics is a balanced result of all steps involved.17-19 A key question that has been constantly asked is the following: What is the role of the added Ti in the process? Many experimental and theoretical studies have been attempted to provide an answer to this question.20-24 On the basis of the observed lattice parameter changes upon doping through mechanical milling,25 Ti cations with varying valances were believed to be incorporated as a bulk dopant. However, the reported lattice parameter change was not supported by other analyses.26 On the other hand, recent studies indicate that a surface-localized species consisting of a nascent binary phase Ti-Al alloy was formed during cyclic dehydrogenation and rehydrogenation processes and the alloy was shown in amorphous form in an X-ray diffraction.2 Combined TEM-EDX study and XAFS measurement determined that Ti was atomically dispersed in the Al phase and forms an Al-Ti alloy.3 On the basis of the fact that Ti-Al compounds have favorable formation energies,2 it was concluded that the alloy is present in the form of TiAl3, consistent with the Ti-Al binary phase
10.1021/jp065527y CCC: $33.50 © 2006 American Chemical Society Published on Web 11/22/2006
25864 J. Phys. Chem. B, Vol. 110, No. 51, 2006 diagram. There is also evidence showing that Ti exists in an invariant environment during dehydrogenation and rehydrogenation processes.27-29 Combined XPS and EXAFS analysis of TiCl3-doped NaAlH4 showed that the local structure around Ti during the first cycle is close to that of the metal Ti but in a more distort state.30 The local structure around the Ti atom consists of both Al and Ti with a Ti-Al distance of 2.79 Å and a Ti-Ti distance of 3.88 Å. Ti hydrides, formed during doping or hydrogenation processes, were also suggested to contribute enhanced kinetics in the reversible dehydrogenation and rehydrogenation processes12 and were supported by similar results obtained by doping the sample with either metallic Ti or TiH2.22 TiH2 phase reflections were also observed in a recent synchrotron X-ray diffraction study of a Al1-yTiy phase in the TiCl3-doped NaAlH4 system.29 Although the formation of Ti hydride is favorable due to the stronger affinity of Ti to hydrogen than Al, the role of Ti hydrides as a catalyst in reversible de-/ hydrogenation processes is not clearly identified. Ti can be introduced to NaAlH4 with different precursors: Ti butoxides,9 Ti halides,9,31,32 TiH2,22,32 and Ti colloids.33,34 There is a general consensus that added Ti is reduced to the zerovalence state, at least after cycling.2,3,35 Therefore, using Ti atom as a dopant to examine the effect of Ti doping is a reasonable choice. Several doping models have been examined in Ti-doped NaAlH4 by using first principles based density functional theory (DFT) calculations. Iniguez and co-workers claimed that doping Ti in the bulk lattice and substituting Na is most favorable.24,36 In contrast, Løvvik and Opalka concluded that doping Ti in the bulk lattice was not thermodynamically stable and the most favorable metastable mode of doping was Ti substituting for Al.37 The seemingly contradicting conclusions were reconciled by a later report of Araujo et al.,38 who showed that the apparent discrepancy was due to the reference states used in the previous studies. Many previous theoretical studies have focused on the models of doping Ti in bulk NaAlH4 lattice.36-38 Even with surface models of NaAlH4, Iniguez and Yildirim examined only various substituional doping modes.24 In contrast, we adopted surface models of NaAlH4 in our study and studied adsorption/insertion modes of doping. The selection of the surface-based models is consistent with the fact that mechanical milling used in many experimental dehydrogenation and rehydrogenation studies produces small particles exposing their surface constantly. We also examined the substituional doping of Na but kept the substituted Na atom in the system by moving it to the surface as adatom or pushing it down into the bulk. We showed that the interstitial doping in the vicinity of the surface with minimal distortion to the lattice is favorable. The impact of doping Ti in NaAlH4 on hydrogen desorption has been examined based on the most favorable interstitial structure and is the focus of the present study. 2. Computational Details Periodic DFT calculations have been carried out with use of the VASP code.39,40 The electron-ion interactions were described by projector augmented wave (PAW)41 and the valence electrons of Al 3s23p1, Na 3s1, H 1s1, and Ti 3d34s1 were treated explicitly with a plane-wave cutoff energy of 400 eV. Test calculations using a cutoff energy up to 500 eV showed that the basis set is well converged at 400 eV. The exchangecorrelation energy was calculated with the PBE form of the generalized gradient-corrected functional.42 At this cutoff energy, the calculated bond length and atomization energy of H2 including zero point energy are 0.751 Å and 4.267 eV,
Liu and Ge respectively, which agree well with the experimental values of 0.741 Å and 4.478 eV at 0 K.43 For Ti2 dimer, the calculated bond length of 1.90 Å is also consistent very well with the experimental measurement of 1.91 Å. Similar parameter set has been used in our previous study of LiBH4.44 The surface Brillouin zone was sampled with the K-points generated by the Monkhorst-Pack scheme with a space less than 0.05 Å-1.45 The bulk structure of NaAlH4 is tetragonal with lattice constants of 4.98 and 11.15 Å measured at 8 K.46 The relaxed lattice parameters from our calculations are 5.01 and 11.12 Å, in good agreement with the above experimental results and theoretical values reported by others.38,47 Slabs with six-layer metal atoms (Al or Na) consisting of 24 NaAlH4 units were constructed on the basis of the relaxed bulk lattice constants to simulate the surfaces. The dimensions of the super-cell to simulate the (001) and (100) surface are 9.96 × 9.96 × 27.61 and 11.05 × 9.96 × 26.80 Å3, respectively. The vacuum space in both cases is larger than 15 Å along z directions. The geometries of the clean slab were optimized by the quasiNewton or the conjugate-graduate method as implemented in VASP. A Gaussian smearing with a width of 0.1 eV was employed to improve the convergence of electronic selfconsistent cycles. A calculation is considered as converged when the changes in energy and force are 1.0 × 10-6 eV and 0.05 eV/Å, respectively. The Al and Na atoms in the bottom three layers of the slab were fixed at their corresponding bulk positions during the relaxation. On the other hand, the Na and Al atoms in the top three layers as well as all the hydrogen atoms were allowed to relax according to the calculated Hellmann-Feynman forces. Neutral Ti atom was then introduced to the super cell and atoms in the slab were further relaxed to simulate the effect of adding Ti on hydrogen interaction in the slab. The energies of the relaxed clean slab were used as references to calculate the binding energy of Ti in different positions of the slab. The reference energy of an isolated Ti atom was calculated by placing the Ti atom in a big box with spin-polarization. Calculations including spin-polarization were performed for selected slab geometries and the results were found to agree with that from calculations without including spin-polarization. 3. Results and Discussion 3.1. Ti Doping in NaAlH4 (001) and (100). As we briefly mentioned in our earlier communication,1 Ti prefers to occupy the interstitial sites in Ti-doped NaAlH4 (001) and form a TiAl3H12 local structure. The binding energy of Ti in the interstitial site on the (001) surface is 3.818 eV. We note that the Ti binding energies reported here were calculated with respect to the undoped surface and a free Ti atom. If the energy of bulk Ti was used as a reference, this state becomes endothermic but still favorable compared with doping Ti in bulk NaAlH4.37 To simulate Ti doping in NaAlH4 (100), we placed Ti atoms in different initial positions in the slab and then allowed positions of the atoms (Na and Al in the top 3 layers as well as all H atoms) to re-optimize together with the Ti atoms. The surface energies of NaAlH4 (100) and (001) were calculated to be 0.327 and 0.146 J/m2, respectively.48 Four stable structures were obtained from 12 initial structures for Ti in NaAlH4 (100). Similar to Ti in NaAlH4 (001), we divided these four structures into two categories according to the position of Ti in the slab. The first category of sites corresponds to Ti occupying the original position of lattice Na+ and pushing Na+ up, as shown in Figure 1a,b. We refer to these two structures as Sub1 and Sub2 since they both are substitutional in nature. The main
Hydrogen Interaction in Ti-Doped NaAlH4 Surfaces
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Figure 2. Top view of the detailed local structure of the TiAl3Hx-100 (x ) 12) complex shown in the box of Figure 1c.
Figure 1. The DFT-GGA relaxed structures of Ti doped in NaAlH4 (100) surface: (a) side view of Ti substituting Na in Sub1 and moving Na to the surface; (b) side view of Ti substituting Na in Sub2 and pushing Na to the surface; (c) top view of Ti occupying the interstitial position in the first layer; and (d) side view of Ti occupying the interstitial position in the second layer.
difference between the two structures shown in Figure 1a,b is the relative position of Ti to the surface in the z-direction: Ti in Sub2 is 0.768 Å lower than that in Sub1 and interacts predominantly with the AlH4- units in the second and third layers, thus making the first layer AlH4- units available for displaced Na+, as circled in the illustrations. As such, Ti in Sub2 is more stable with a binding energy of 3.873 eV. In the second category of sites, Ti atoms are located in the interstitial positions and each connected with three AlH4- hydride units. Parts c and d of Figure 1 show two relaxed structures in which Ti occupies the first-layer (Int1) and second-layer (Int2) interstitial sites, respectively. Although the local structures formed from Ti in the first layer and second layer interstitial sites are very similar, Int1, with a Ti binding energy of 4.165 eV, is 0.307 eV more stable than Int2. Obviously, Int1 is directly accessible from the surface and could be the dominant site for Ti during mechanical milling. In fact, we attempted two initial structures with Ti on the surface and both were relaxed to Int1. Int1 is also the most stable among all relaxed structures for Ti on NaAlH4 (100) in which Ti is directly bound to three AlH4- units. The stronger affinity of Ti toward H than Al makes Ti strip some hydrogen atoms from Al. We note that the insertion of Ti in the interstitial sites has a localized effect: it only affects the immediate neighboring AlH4- units without inducing significant changes further away from this center. A closer look at the local structure of the most stable species formed by adding Ti in NaAlH4 (100) shows that it is very similar to the TiAl3H12 structure isolated in Ti-doped NaAlH4 (001). The local structure in the slab was marked in Figure 1c and the detailed structural information was shown in Figure 2 in a cluster representation. Some of the bonds were drawn to show connections between atoms and do not necessarily mean a single bond is formed between these atoms. For example, the bonds connecting H8 with Ti and A13 in Figure 2 imply that the distances between H8 and Ti or Al are in the range of a chemical bond and H8 is shared by the Ti and Al atoms in the complex. Similar representations were used throughout the
Figure 3. (a) Side view of DFT-GGA relaxed structure of Ti-doped NaAlH4 (001) surface with Ti occupied in interstitial space; (b) detailed local structure of TiAl3Hx-001 (x ) 12) complex shown in part a). (Reprinted with permission from ref 1.)
paper. For comparison, the TiAl3H12 structure isolated from Tidoped NaAlH4 (001) and its cluster representation1 are reproduced here in Figure 3a,b. The two structures are referred to as TiAl3Hx_100 and TiAl3Hx_001, respectively, in later discussions. The similarity between the two structures is self-evident although there are some differences in the details. For example, TiAl3Hx_001 has higher symmetry (C2 in cluster form), whereas the symmetry is broken in TiAl3Hx_100. These results showed that doping Ti in either (100) or (001) of NaAlH4 leads to similar stable structures. In these structures, Ti prefers to occupy the interstitial sites close to the surface. The easy access of these sites from the surface and stability of the resulting structure indicate that the TiAl3H12 complex structure could be formed in the initial doping process of the sample, such as mechanical milling. Four types of bonds are formed in the complex structures: Ti-Al, Al-H, Ti-H, and Ti-H-Al bonds. The formation of Ti-Al bonds was also observed in previous molecular dynamics simulations.24 As such, the complex structure (TiAl3Hx) is a combination of TiAl3 and TiHx as well as Al-H hydrides. The dissociation from this structure may lead to the formation of TiAl3 or Ti-H hydrides. In fact, both TiAl3 alloy phase and TiHx hydride have been detected experimentally although the roles of these species in reversible hydrogen desorption and adsorption processes were not conclusive. For example, it has been reported that doping TiAl3 directly in NaAlH4 resulted in poor kinetics.49 Nevertheless, TiAl3Hx provides a seed for forming TiAl3 and we conclude that the TiAl3Hx complex structure is the precursor state. 3.2. Hydrogen Desorption from the Complex Structures. We explored the effect of doping Ti in NaAlH4 (100) and (001) on hydrogen interaction in the complex hydrides. Our study was based on the most stable interstitial structures on both surfaces. We have demonstrated that doping Ti in NaAlH4 (001) lowers the desorption energy of hydrogen from many positions of the complex structure.1 Herein, we will present results for hydrogen
25866 J. Phys. Chem. B, Vol. 110, No. 51, 2006 TABLE 1: The Hydrogen Desorption Energies Calculated by Periodic DFT-GGA for the Complex TiAl3Hx-100 in Ti-Doped NaAlH4 (100) Surfacea
a
combination of H atoms
desorption energy (eV/H2 molecule)
H1, H2 H3, H4 H5, H6 H11, H12 H8, H11 H7, H12 H3, H4, H5, H6
0.864 0.852 0.877 0.900 1.205 1.317 0.935
See Figure 2 for numbering of atoms.
TABLE 2: The Hydrogen Desorption Energies Calculated by Periodic DFT-GGA for the Complex TiAl3Hx-001 in Ti-Doped NaAlH4 (001) Surfacea
a
combination of H atoms
desorption energy (eV/H2 molecule)
H1, H2 H11, H12 H9, H10 H7, H8 H4, H11 (or H6, H12) H3, H4 (or H5, H6) H3, H4, H5, H6 H1, H2, H3, H4 H1, H2, H4, H11 H1, H2, H11, H12 H4, H11, H5, H6 H3, H4, H11, H12 H4, H11, H6, H12
0.515 0.934 1.265 2.067 0.605 0.668 0.921 0.882 0.892 0.952 1.012 1.103 1.207
The H atoms are numbered in Figure 3b.
desorption from Ti-doped NaAlH4 (100) as well as new results for Ti-doped NaAlH4 (001). The desorption energy of hydrogen is defined as ∆Edes ) (1/n)(Estoi - nEH2 - EHdes) , where Estoi and EHdes are total energies of stoichiometric slab and the slab with H desorbed, EH2 is the total energy of a H2 molecule, and n shows the number of H2 molecules formed. This expression gives the desorption energy of forming one hydrogen molecule. Hydrogen desorption from Ti-doped NaAlH4 (100) was measured on the basis of structure Int1, as shown in Figure 1c and detailed in Figure 2. The desorption energies of hydrogen atoms numbered in Figure 2 were listed in Table 1. The calculated desorption energy from the surface of undoped NaAlH4 (100) is 1.223 eV. As shown in Table 1, all the desorption energies apart from the combination of H7 and H8 are lower than that from the clean surface. Clearly, the addition of Ti to the system weakens Al-H interaction and reduces the hydrogen desorption energy significantly. A reduced energy cost to remove hydrogen from Ti-substituted bulk NaAlH4 was also reported.20 In the local structure, three original AlH4- units connecting Ti lie in the same layer as Ti. Consequently, Ti interacts with three AlH4- units almost equivalently. In contrast, the relative high desorption energies are hydrogen atoms that connect to the Ti atom directly and bridge Ti and Al atoms in the complex. We also included more extensive results for hydrogen desorption from Ti-doped NaAlH4 (001). The new results are now combined with those reported in the previous communication1 and listed in Table 2. The results reported in our previous paper are shown in italics. In addition to H1 and H2 (0.515 eV), other combinations of hydrogen atoms with low desorption energies were found. These combinations include H3, H4 (or
Liu and Ge H5, H6) and H4, H11 (or H6, H12) which have low desorption energies of 0.668 and 0.605 eV, respectively. In contrast, the energy cost to desorb a pair of hydrogen atoms from the same AlH4- unit on the (001) surface of undoped NaAlH4 is 1.383 eV per H2 molecules. These results indicate that addition of Ti in the NaAlH4 system makes hydrogen desorption energetically much more favorable over the direct hydrogen desorption without Ti. The origin of the reduced hydrogen desorption energy in Tidoped NaAlH4 (100) has been attributed to the bonding characteristics associated with the complex, as we demonstrated in our communication.1 As the local structures of the complex are very similar on both surfaces, the electronic structures are also expected to be similar. Indeed, 3d orbitals of Ti interact with p orbitals of Al and s orbitlas of H (bound to Ti) to form σ-bonding orbitals in HOMO and HOMO-1. These bonding interactions strengthen the Ti-Al and Ti-H bonds. On the contrary, s orbitals of H (bound to Al) interact with σ-bonding orbitals of Ti-Al or d orbitals of Ti to produce antibonding orbitals. These bonding and antibonding orbitals which weaken Al-H interaction but favor H-H interaction produced a synergetic effect in the Ti-doped NaAlH4: reduced hydrogen desorption energy. Next, let us take a closer look at the hydrogen desorption energies for hydrogen atoms at different positions. As shown in Table 2, the averaged desorption energies of any four hydrogen atoms are in the range of 0.8-1.2 eV per H2 molecule. On the other hand, if four hydrogen atoms desorb in a stepwise manner, i.e., two at a time, the second step generally has a much higher desorption energy than the first step. For example, the combination of H3, H4, H5, and H6 has an averaged desorption energy of 0.921 eV. We know the combinations of H3 and H4 or H5 and H6 have the same desorption energy of 0.668 eV. The energy cost to desorb H5 and H6 would be 1.174 eV after first desorbing H3 and H4. Similar analysis can be applied to other combinations of four or more hydrogen atoms. We would point out that these analyses were based only on the stability of the dehydrogenated species. The actual reaction sequences will be determined by the landscape of the potential energy surface and the activation barriers of each reaction channel. We work on mapping out some area of the potential energy surface and determining activation barriers of selected reaction pathways by using the nudged elastic band method and its improvements.50,51 We also started to use ab initio molecular dynamic calculations to explore the potential energy surface of the system. We note that although there are some local relaxations, the integrity of the local Ti-Al framework is maintained as hydrogen is desorbed from the complex structure. This Ti-Al framework produces an almost invariant environment during hydrogen desorption. We would also note that hydrogen atoms that bridge the local structure with neighboring AlH4- units were formed accompanying hydrogen desorption. In this case, two relatively weak Al‚‚‚H bonds replaced one stable Al-H bond. The hydrogen bridges so-formed may enable hydrogen transfer from neighboring AlH4- units and help to maintain the active hydrogen desorption center based on the complex structure. 3.3. TiAl3Hx Mediated Hydrogen Desorption. It has been well-established that a small amount (up to 5 mol %) of Ti can enhance dramatically the dehydrogenation/hydrogenation of NaAlH4.9 It has also been shown that this kind of enhancement occurs with many different Ti compounds.9,31,32 Herein, we explored the role of the TiAl3Hx complex in hydrogen desorption on the basis of the structure established on the (001) surface. The reversible dehydrogenation/hydrogenation processes of
Hydrogen Interaction in Ti-Doped NaAlH4 Surfaces
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Figure 5. A possible pathway of forming octahedral structure AlH63in the presence of TiAl3Hx. After desorbing the hydrogen atoms connected with Ti, highlighted in green in part a, an octahedral AlH63structure, highlighted in yellow in part b, starts to form.
TABLE 3: The Hydrogen Desorption Energies Calculated by Periodic DFT-GGA for the Extended TiAl3Hx-001 Complex Structure Including the Neighboring AlH4- Units in the Ti-Doped NaAlH4 (001) Surfacea Figure 4. Hydrogen desorption and transfer from Ti-doped NaAlH4 (001) surface through the TiAl3Hx complex: (a) starting TiAl3H12 structure with the two hydrogen atoms highlighted in green desorbing with low desorption energy; (b) relaxed structure after desorbing two green hydrogen atoms in part a, showing direct connection between TiAl3Hx-001 and surrounding AlH4- units; and (c) details of the boxed local structure in part b.
NaAlH4 in the presence of Ti additives are thought to proceed through the following steps: Ti
Ti
3NaAlH4 798 Na3AlH6 + 2Al + 3H2 798 3NaH + 3Al + 9/2H2 The first step dehydrogenation involves a phase transformation that converts tetrahedral Al in AlH4- into octahedral Al in AlH63-. The subsequent dehydrogenation step is accompanied by more phase transitions. The effect of the complex on the hydrogen interaction in the surrounding AlH4- as well as a possible route for hydrogen transfer from the surrounding AlH4to the complex has been examined. We also identified the possibility of forming Na3AlH6 in the vicinity of the TiAl3Hx complex. We will present the results in turn in the following sections. 3.3.1. Hydrogen Transfer and Dehydrogenation of Surrounding AlH4-. As shown in Table 2, two side hydrogen atoms (H3, H4 or H5, H6) in TiAl3H12 have a low desorption energy. After the pair of hydrogen atoms is desorbed as a H2 molecule, the remaining TiAl3H10 was relaxed to form direct connections between the complex structure and some of the surrounding AlH4-. As illustrated in Figure 4, desorbing H5 and H6 (highlighted in green in Figure 4a) would cost 0.668 eV/H2. The subsequent relaxation links TiAl3H10 with two neighboring AlH4- units as shown in the box of Figure 4b. The relaxed local structure including TiAl3H10 and two neighboring AlH4- units after H2 desorption is detailed in Figure 4c. The distances between the Al atoms in the TiAl3H10 complex and those in neighboring AlH4- units are 2.705 and 3.195 Å, respectively, forming effectively an extended TiAl3H10-2AlH4- complex structure. In this extended complex structure (Figure 4c), hydrogen atoms of the neighboring AlH4- units (H3 and H7) are shared by the Al atoms from TiAl3H10 and the surrounding AlH4- units. The delocalized effect induced by hydrogen desorption from the complex structure may contribute to the catalytic hydrogen desorption through different mechanisms: (1) The observed sharing of hydrogen will enable hydrogen transfer from neighboring AlH4- units to the complex structure from which hydrogen will desorb with lower desorption energy due
a
combination of H atoms
desorption energy (eV/H2 molecule)
H1, H2 H3 (after H1 and H2) H1, H3 H2, H4 H7 H5, H7 H6, H8
0.617 -0.034 0.623 0.782 0.866 0.958 0.962
The numbering of H atoms is shown in Figure 4c.
to weakened Al-H interactions. (2) The sharing/transferring of hydrogen between TiAl3H10 and the neighboring AlH4- units reduces hydrogen desorption energy from these surrounding AlH4-. The second contribution was demonstrated by the results summarized in Table 3. The combinative desorption energy of H1 and H2 is 0.617 eV. After H1 and H2 are desorbed, the interaction of H3 with Al becomes even weaker and the calculated desorption energy is -0.034 eV. Another bridging hydrogen atom (H7) also has a low desorption energy (0.866 eV). In fact, we explored a number of combinative hydrogen desorptions from the neighboring AlH4- on the basis of the extended TiAl3Hx-2AlH4- structure and the results were shown in Table 3. Clearly, many combinations of hydrogen in the extended complex structure have desorption energies that are less than the desorption energy of H2 from the undoped surface (1.383 eV). These results suggest that the formation of TiAl3Hx complex structure weakens the Al-H interactions in both the complex itself and the surrounding AlH4-. The reduced Al-H interactions lower the hydrogen desorption energies from the complex and the neighboring AlH4- units. We would point out that there are other structures that enable the link between TiAl3Hx and the neighboring AlH4- units. In general, when a link between the Al atom of TiAl3Hx and the Al atom of neighboring AlH4- is established, hydrogen bridges will be formed between them. The formation of such an extended structure reduces hydrogen desorption energy and facilitates hydrogen transfer. 3.3.2. Formation of AlH63-. A detailed analysis of the results revealed that desorption of hydrogen from several positions led to the formation of an octahedrally bonded Al. Figure 5 shows one possible mechanism. Starting from the structure shown in Figure 5a, the structure was relaxed to the structure shown in Figure 5b after desorbing 4 H atoms (highlighted in green). In the local structures (highlighted in yellow) in Figure 5b, the Al atom is coordinated with six H atoms, four of which are shared between this Al atom and the Al atoms of the TiAl3Hx complex. Shifting H atoms to the Al atom of the highlighted structure in Figure 5b and breaking the Al-Al bonds will lead to the formation of an AlH63- ion. Similar octahedral structures were
25868 J. Phys. Chem. B, Vol. 110, No. 51, 2006 observed by desorbing all six hydrogen atoms between the Al atom and the Ti atom in the surface plane of the same starting structure. Desorbing hydrogen atoms between Ti and Al atoms causes a symmetrical expansion of Ti-Al bonds from 2.535 Å of the fully hydrogenated complex to 2.782 Å of the TiAl3H8 shown in Figure 5b. The expansion of the Ti-Al bonds in the complex pushes the Al atoms in the complex closer to the Al atoms in the neighboring AlH4-. The shorter Al-Al distances enable H transfer from the complex to the AlH4- and transform the tetrahedral AlH4- to octahedral AlH63-, as shown in Figure 5b. These results indicate that TiAl3Hx may also provide a channel to form the experimentally observed Na3AlH6 in the process of hydrogen desorption. Finally, we note that hydrogen desorption/readsorption in Tidoped NaAlH4 is a complex heterogeneous process involving both bond breaking and bond making steps as well as mass transfer.21,52 The energetics and structures reported herein were based on relaxation after desorbing hydrogen and may only provide a facet of the complex process. Furthermore, the dynamic nature of the actual process was not accounted for with the model system chosen and the methodology used in the present study. The phase transitions accompanying hydrogen desorption from Ti-doped NaAlH4 are a challenging issue. To address this issue, a multiscale approach has to be developed, and the mechanistic understanding from the present study should provide a physical basis and useful data for the large scale modeling. 4. Conclusion In summary, first principles density functional theory calculations were used to analyze hydrogen interactions in Ti-doped NaAlH4. Possible structures after doping Ti in the (001) and (100) surfaces have been determined. On both surfaces, Ti was found to energetically favor the interstitial site and form a complex structure with the three neighboring AlH4-. The complex of TiAl3Hx (x ) 12) that was identified in Ti-doped NaAlH4 (001) was also found in Ti-doped NaAlH4 (100). The formation of such a complex TiAl3Hx structure reduces the hydrogen desorption energies significantly as compared with that from the clean, undoped surface. The reduction in desorption energy due to the formation of the complex structure was similar on both surfaces. Furthermore, this complex maintains an almost invariant local environment surrounding Ti during dehydrogenation. The complete dehydrogenation from the TiAl3Hx complex would lead to the formation of experimentally observed TiAl3. Therefore, we conclude that the TiAl3Hx complex is a precursor state for the formation of TiAl3. Dehydrogenation from the TiAl3Hx complex induces delocalized changes in both structure and energetics: (1) The link between the complex and neighboring AlH4- units through bridging H and/or direct Al-Al bond facilitates hydrogen transfer from/to the surrounding AlH4-. (2) The link also causes the hydrogen desorption energy from the neighboring AlH4to decrease. Therefore, TiAl3Hx not only reduces the hydrogen desorption energy from the local complex but also has an extended effect on the surrounding AlH4- units. Formation of AlH63- in the vicinity of the TiAl3Hx complex has been observed in the relaxed structures after symmetrically desorbing hydrogen between Ti and Al in the complex. Acknowledgment. This work was supported by the U.S. Department of Energy, contract No. DE-FG02-05ER46231. References and Notes (1) Liu, J.; Ge, Q. Chem. Commun. 2006, 1822. (2) Graetz, J.; Reilly, J. J.; Johnson, J.; Ignatov, A. Y.; Tyson, T. A. Appl. Phys. Lett. 2004, 85, 500.
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