Al: A Density

ARTICLE pubs.acs.org/JPCC

Hydride-Assisted Hydrogenation of Ti-Doped NaH/Al: A Density Functional Theory Study Jianjun Liu, Jiamei Yu, and Qingfeng Ge* Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901, United States ABSTRACT: Density-functional theory calculations were performed to study the hydrogenation mechanism of Tidoped NaH/Al, with emphasis on the role of NaH. Results from TiAl3Hx (x = 0-16) clusters showed that hydrogenation of TiAl3 experienced a structural evolution in which TiAl3 organizes into a distorted tetrahedral skeleton in TiAl3Hx (x = 0∼7), but forms a planar T-shape structure in TiAl3Hx (x = 10-16), with TiAl3H8 and TiAl3H9 being transition structures between the tetrahedral and planar structures. Moreover, H2 adsorbs molecularly at the Ti site of TiAl3Hx, and dissociates spontaneously into hydrides when an extra electron was added to the cluster. The electron-assisted dissociation of H2 is manifested as hydride-assisted hydrogenation of Ti-doped NaH/Al, simulated as TiAl3Hx supported on the NaH (001) surface. Geometry optimizations and molecular dynamics trajectories showed that TiAl3Hx clusters are active for H2 dissociation after acquiring hydride from the NaH surface. The present results suggest a possible mechanism of forming NaAlH4 in the recycling of Ti-doped NaAlH4 as a hydrogen storage medium.

1. INTRODUCTION Aluminum complex hydrides represented by MAlH4 (M = K, Na, Li) are considered as attractive candidates for rechargeable hydrogen storage because of their high hydrogen capacities, reversibility, and low cost.1-6 In contrast, although aluminum hydride (AlH3) has some advantages such as a lower dehydrogenation temperature and higher volumetric hydrogen density,7 its regeneration from solid-state Al and gaseous H2 is not considered practical.7-9 In fact, it has been generally agreed that AlH3 can only be considered as a nonreversible hydrogen storage medium.6 The reversible hydrogen release/uptake in MAlH4 can be described as 3MAlH4 TM3 AlH6 þ 2Al þ 3H2 T3MH þ 3Al þ 9=2H2 ð1Þ Obviously, MH is an essential reactant for the regeneration of MAlH4. However, the role of MH in the reversibility of MAlH4 has not been explicitly examined. The studies on complex metal hydrides were stimulated by the pioneer work of Bogdanovi
c et al. who reported that a small amount of Ti-compounds (4 mol %) improves the kinetics of hydrogen release/uptake from NaAlH4-based hydrogen media.10 Since then, NaAlH4 has served as a model system to understand and design novel complex metal hydride-based systems as potential hydrogen storage materials.11-13 In particular, the role of doped Ti in NaAlH4 has been extensively studied from both views of dehydrogenation and rehydrogenation.14-36 Recently, it has been shown that reducing the particle size of NaAlH4 to the low nanometer regime can improve kinetics without adding any catalyst, although the regeneration of NaAlH4 is incomplete.37,38 r 2011 American Chemical Society

This observation has been attributed to the unique properties of nanoparticles, which have a decreased hydrogen diffusion path, more exposed surface area, as well as decreased stability with respect to their bulk counterpart. Regeneration of NaAlH4 from NaH and Al can be kinetically enhanced in the presence of Ti. Direct synthesis of Ti-doped NaAlH4 has been realized from the mixture of NaH and Al together with Ti compounds either in the presence of a liquid organic solvent39-41 or by ball milling.42-45 Fang et al. proposed that the reduced Ti atoms would combine with Al atoms to form Ti-Al clusters.45 On the surface of a Ti-Al cluster, the hydrogen molecule dissociates into hydrides, leading to Ti-Al-H species. On the other hand, this mechanism completely ignores NaH as an essential reactant in the formation of NaAlH4. Without the participation of NaH, the hydrogenation of Ti-doped Al can only lead to AlH3, not NaAlH4. In Ti-doped NaAlH4, both experimental and theoretical studies demonstrated that Ti interacts with Al/AlH4 forming TiAl3 phases in both dehydrided and hydrided states.14,15,17,19,21,33,46 The TiAl3 phase is not in crystalline form, but exists in a highly dispersed amorphous state on the surface of Al in the dehydrided state. A H2/D2 scrambling study performed by Sch€uth et al. suggested that the Ti catalyzes the dissociation of the hydrogen molecule.36 However, the detailed structural evolution of the active species was not clear. Chaudhuri and Muckerman studied the hydrogenation mechanism based on the Al(001) surface and Received: September 10, 2010 Revised: December 1, 2010 Published: January 6, 2011 2522

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Table 1. Calculated and Experimental Equilibrium Distances (Re) and Dissociation Energies (DE) for the Lowest-Energy States of TiH, AlH, and Al2 Re (Å) species

PBE

B3LYP MP2

DE (eV) Exp.

PBE

B3LYP MP2

Exp.

TiH ( Π) 1.755

1.754

1.814 1.744 2.295

2.298

AlH(1Σ)

1.685

1.666

1.645 1.648 2.878

3.032

2.764 2.953

Al2(3Πu)

2.493

2.508

2.469 2.466 1.596

1.156

1.109 1.378

2

0.899 2.120

substituted two next-nearest-neighbor Al atoms with the Ti atoms.22 These authors showed that the hydrogen molecule dissociates into hydrides by the cooperation of two Ti atoms. On the other hand, this model does not account for the role of NaH. Gunaydin et al. analyzed dehydrogenation processes from Ti-doped NaAlH4 and suggested that the diffusion of AlH3 vacancies limits dehydrogenation.47 However, Ti was believed to promote the formation of AlH3 or NaH vacancies but was not included explicitly in the model. Furthermore, NaH was not treated explicitly, as the study focused on dehydrogenation.47 Therefore, a system directly involving NaH is necessary to account for its role in the cyclic process of using NaAlH4 as a hydrogen storage medium. In the present paper, we start with the TiAl3 cluster and examine its structural evolution by adding hydrogen atoms. We anticipate that the structural and energetic evolution of neutral TiAl3Hn (n = 0-16) provides an insight into hydrogenation mechanisms in Ti-doped NaH/Al. On the basis of the lowestenergy clusters, we explore the breaking H-H bond to form hydrides. We then use TiAl3 supported on the NaH surface as a model and analyze the hydrogenation process. The present study provides an understanding of the important role that NaH and TiAl3 play in the formation of NaAlH4.

2. COMPUTATIONAL METHODS Diatomic molecules were calculated using various single determinant methods, including PBE, B3LYP, and MP2, with the 6-311þþG(d, p) basis set, as implemented in the Gaussian 03 package.48 The calculated properties of diatomic molecules were compared with the available experimental values. All bond distances (Re) and dissociation energies (DE) from calculations and experiments49 are listed in Table 1. As shown in Table 1, both B3LYP and PBE gave more reasonable dissociation energies and equilibrium distances for TiH, AlH, and Al2 than the MP2 method. Because the B3LYP hybrid functional has been shown to successfully predict a wide range of molecular properties,50 we chose B3LYP/6-311þþG(d, p) in our calculation of TiAl3Hx clusters. The structural and other properties of neutral TiAl3Hx (x = 0-14) were calculated by using the DFT-B3LYP method. Vibrational frequency analysis was performed on all optimized structures to confirm that the structure was at a true minimum. The vibrational frequency calculations also provide zero-point energies (ZPEs) for the species. In addition, the properties related to electronic structure, including highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) gaps and bond order, were also calculated at the same level of theory. Periodic density functional theory (DFT) calculations including spin-polarization were carried out using the VASP code.51,52 The electron-ion interactions were described by projector augmented wave (PAW) potential, and the valence electrons

Figure 1. The B3LYP/6-311þþG(d, p) optimized lowest-energy structures of the TiAl3Hx where x = 0-16, The pink, gray, and white balls represent Al, Ti, and H atoms, respectively. The torsion angle (TA) of Ti-Al1-Al2-Al3 is shown. The bond distances are marked in angstroms (Å).

were treated explicitly with a plane-wave basis set at a cutoff energy of 400 eV. The nonlocal exchange-correlation energy was calculated with the PBE functional.53 The surface Brillouin zone was sampled with the K-points generated by the Monkhorst-Pack scheme with a space less than 0.05 Å-1.54 The dimension of the supercell to simulate the NaH (001) surface is 9.60  9.60  23.60 Å3, leaving a vacuum space of ∼15 Å in the z direction. The geometries of the slab were optimized by quasi-Newton or conjugate-gradient method as implemented in VASP. A Gaussian smearing with a width of 0.1 eV was employed to improve the convergence of electronic self-consistent cycles. A calculation was considered as converged when the change in energy and maximum force were less than 1.0  10-6 eV and 0.05 eV/Å, respectively.

3. RESULTS AND DISCUSSION 3.1. Structural and Energetic Evolution of TiAl3Hx (x = 0-16) Clusters. During the formation of NaAlH4 catalyzed by

Ti, the H2 molecule dissociates at the Ti site. The resulting hydrides then migrate over to the surrounding Al sites. Recently, we identified that the TiAl3H6, along with other selected transition metal clusters, is of superatomic characteristics and can be used to assemble supramolecular structures.55 Herein, we focus on the structural and energetic evolution of TiAl3Hx to identify the possible hydrogenation reaction pathways. We optimized possible structures of TiAl3Hx (x = 0-2) and determined the most stable structures by comparing the relative energies at each x value. For TiAl3Hx (x g 3), the initial structures of TiAl3Hx were built according to the optimized TiAl3Hx-1. Hydrogen atoms were added to either on-top, bridge, or hollow positions of the TiAl3 tetrahedral structure. The lowest-energy structures from TiAl3 to TiAl3H16 should indicate a possible hydrogenation reaction pathway. 2523

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Figure 2. Hydrogen desorption energy (HDE), HOMO-LUMO gap (HL), and electron affinity (EA) of the optimized TiAl3Hx clusters at the B3LYP/6-311þþG(d, p) level. HDE was calculated based on the equation HDE = -(ETiAl3Hx - ETiAl3Hx-1 - 1/2EH2).

All calculated lowest-energy structures of TiAl3Hx (x = 0-16) are shown in Figure 1. Obviously, the tetrahedral skeleton of TiAl3 is relatively stable up to the addition of seven hydrogen atoms, i.e., TiAl3Hx with x e 7 has a tetrahedral TiAl3 skeleton as its core. Further increasing hydrogen beyond TiAl3H7 leads to the Al-Al bond breaking. Electronic analysis showed that there is a redistribution of electrons from the Al-Al and Ti-Al bonds to the Ti-H and Al-H bonds due to a much larger electron affinity of H than that of either Ti or Al. Consequently, Al-Al bonds are weakened as more hydrogen atoms are added to the cluster. For TiAl3Hx with x g 10, Ti and three Al atoms form a T-shaped planar skeleton by sharing hydrogen atoms between the Al and Ti atoms, with additional hydrogen atoms bound to the Al atoms terminally. We note that dihydrogen complex forms in TiAl3H14 and TiAl3H16 even though the optimizations started with the geometries of separated H atoms. TiAl3H8 and TiAl3H9 are transition structures. We use the Ti-Al1-Al2-Al3 torsion angle to characterize the transition from tetrahedral to planar TiAl3 skeleton. The Ti-Al1-Al2-Al3 torsion angles of TiAl3H8 and TiAl3H9 are 35.1° and 7.4°, respectively, while that of TiAl3H7 is 67.5°. In order to understand the difference of hydrogen interaction in the tetrahedral, transition, and planar structures, we plotted hydrogen desorption energies (HDEs), HOMO-LUMO gap, and electron affinity as a function of hydrogen number. Figure 2 shows these calculated properties at the B3LYP/6-311þþG(d, p) level. Note that the HDE as an indicator of the stability of TiAl3Hx is calculated according to the equation HDE = -(ETiAl3Hx - ETiAl3Hx-1 - 1/2EH2). The electron affinity of the cluster is an important indicator of its ability to gain electrons and correlates with its stability. At the low hydrogen content, the HDEs oscillate between odd or even values of x. For even values of x, HDEs in tetrahedral structures (x < 8) are higher than those in planar structures (x > 8). This indicates that hydrogen desorption from the planar structures is thermodynamically more favorable than that from the tetrahedral structures. On the other hand, the curve of the HOMO-LUMO gap in Figure 2 shows that the planar structures have a larger electronic stability than the tetrahedral structures except for TiAl3H7. Such stability can be attributed to the formation of the closed shell structures in AlH4-, which we demonstrated previously.

Figure 3. Electron-assisted hydrogen dissociation in TiAl3H6, TiAl3H8, and TiAl3H12. The distances of H-H and Ti-H are given in Å. The white, orange, and gray balls represent H, Al, and Ti atoms, respectively.

We would point out that the TiAl3H7 structure with C3v symmetry is very stable, as indicated by the large HOMOLUMO gap (3.55 eV) and vibrational frequencies. The ionic TiAl3H6þ structure is also stable and has a C3v symmetry and a large HOMO-LUMO gap (3.50 eV). A large HOMO-LUMO gap indicates that TiAl3H7 and TiAl3H6þ prefer to neither donate nor receive electrons. The exceptional stability of TiAl3H7 is also an indication of a possible trap, i.e., additional energy will be needed to convert tetrahedral to planar structures. In fact, the formation of TiAl3H7 would impede the hydrogenation/dehydrogenation and should be avoided. Molecular dynamics simulations of TiAl3H7 at different temperatures were performed to examine its isomerization or dissociation using the Atom-Centered Density Matrix Propagation (ADMP) method at the level of B3LYP/6-31(d, p). The result shows that the terminal hydrogen bound to Ti starts dissociating at 900 K. More importantly, the tetrahedral structure of TiAl3 remains intact. Further increasing the temperature to 1000 K, TiAl3H7 starts to dissociate to planar TiAl2H5 and AlH2. Planar TiAl3H12 is another stable species, with the largest HOMO-LUMO gap (3.71 eV) among all structures and relatively large HDE. The relaxed TiAl3H12 cluster structure is very similar to the similar structure isolated in Ti-doped NaAlH4 surfaces.23,24 As more hydrogen atoms are added to the TiAl3H12 cluster, dihydrogen complex starts to form, as shown in TiAl3H14 and TiAl3H16. 3.2. Electron-Assisted H-H Bond Breaking on TiAl3Hx Clusters. One role of doped Ti in NaAlH4 is to dissociate hydrogen molecules to form hydrides.36 As shown above, dihydrogen with the H-H bond distances of 0.76 Å and 0.77 Å are coordinated with Ti in the TiAl3H14 and TiAl3H16 clusters, respectively. By adding an electron to either TiAl3H14 or TiAl3H16, the dihydrogen dissociates spontaneously into hydrides, and the resulting hydrides becoming bound to Ti or Al atoms after optimization. We performed similar calculations for 2524

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Table 2. NBO Charge Analysis of the Neutral and Negatively Charged Dihydrogen Complex As Well As the Hydride Products TiAl3H6 þ H2 f TiAl3H8-

TiAl3H8 þ H2 f TiAl3H10-

TiAl3H12 þ H2 f TiAl3H14-

TiAl3H6 3 3 3 H2

TiAl3H6 3 3 3 H2-

TiAl3H8-

TiAl3H6 3 3 3 H2

TiAl3H6 3 3 H2-

TiAl3H8-

TiAl3H12 3 3 3 H2

TiAl3H12 3 3 3 H2-

TiAl3H14-

-0.34

-0.62

-0.30

-0.52

-0.95

-0.64

0.43

-0.15

-0.11

Al1

0.62

0.36

0.48

0.67

0.57

0.46

1.03

1.00

0.96

Al2

0.68

0.54

0.44

0.87

0.74

0.72

1.00

0.98

0.98

Al3

0.68

0.54

0.44

0.87

0.74

0.72

1.00

0.98

0.96

H

0.04

0.01

-0.21

0.07

0.04

-0.34

0.07

0.04

-0.10

H

0.04

0.01

-0.15

0.05

0.06

-0.19

0.07

0.04

-0.10

Ti

Figure 5. The TiAl3_1 cluster supported by the NaH (001) surface dissociated hydrogen molecule. The distances are given in Å. Color codes are the same as those in Figure 4.

Figure 4. The DFT-PBE relaxed structures of TiAl3 on the NaH (001) surface. The binding energy (BE, unit: eV) of each structure is given. The purple, white, orange, and gray balls represent Na, H, Al, and Ti atoms, respectively.

H2 on the tetrahedral (TiAl3H6), transition (TiAl3H8), and planar (TiAl3H12) structure. Figure 3 shows the structures formed from breaking the H-H bond of adsorbed H2 after adding an extra electron. Similarly to TiAl3H14 and TiAl3H16, the hydrogen molecule adsorbs physically at the Ti site of TiAl3Hx, forming the dihydrogen complex. Geometry optimization of the neutral dihydrogen complex does not result in dissociative adsorption. After adding an electron, H2 dissociates spontaneously to form the hydrides. This led us to postulate that negatively charged TiAl3Hx may be responsible for the formation of NaAlH4 catalyzed by Ti. To understand the mechanism of electron-assisted hydrogen dissociation, we analyzed the natural bond orbital (NBO) charges of TiAl3Hx 3 3 3 H2, TiAl3Hx 3 3 3 H2-, TiAl3Hxþ2- (x = 6, 8, and 12)

species and listed results in Table 2. The TiAl3Hx 3 3 3 H2 and TiAl3Hxþ2 structures were obtained by geometry optimization, and the TiAl3Hx 3 3 3 H2- structure was obtained by adding one electron to the neutral TiAl3Hx 3 3 3 H2 geometry. These calculations allowed us to follow the details of the electron transfer during H-H bond breaking. When TiAl3Hx gains an electron, the electron is largely located on the titanium atom. The electron can be readily donated from the d-orbital of Ti to the σ* antibonding orbital of hydrogen molecule when the cluster is approached by H2. Such electron transfer facilitates H-H bond breaking and Ti-H and Al-H bond formation.25 The present results indicate that the negatively charged TiAl3Hx promotes hydrogen dissociation during the hydrogenation process. In contrast, the neutral TiAl3Hx only physically adsorbs hydrogen molecules, forming a dihydrogen complex. Our results established that the extra electron assists in breaking the H-H bond of the dihydrogen coordinated to TiAl3Hx. A natural question is then where the electron would come from in Ti-doped NaH/Al during regeneration of NaAlH4. We suggest that NaH is the primary resource of electrons. The hydride of NaH may be transferred to TiAl3Hx, forming TiAl3Hxþ1-. The extra electron is then redistributed to the empty d orbitals of Ti, activating the cluster for H2 dissociation at the Ti site. This is followed by hydrogen migration from TiAl3Hx to the surrounding Al atoms. 3.3. Hydrogenation of TiAl3 Cluster Supported by the NaH (001) Surface. In this section, we examine the H-H bond breaking promoted by TiAl3 cluster supported on the NaH(001) surface. This structure allowed us to probe the role of NaH in the regeneration of NaAlH4 from NaH and Al catalyzed by Ti. The interaction between the tetrahedral TiAl3 cluster and the NaH(001) surface was simulated by supporting TiAl3 on the surface. We examined a number of possible positions of TiAl3 on the NaH(001) surface for TiAl3, as shown in Figure 4. Except for TiAl3_5, the tetrahedral TiAl3 skeleton was maintained during optimizations. The binding energies calculated by the formula 2525

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Figure 6. Snapshots from AIMD trajectories for H2 dissociation on TiAl3 and TiAl3H4 clusters supported on NaH (001) surface. Color codes are the same as those in Figure 4, except for green balls for the dissociating H2.

EBE = ENaH(001)-TiAl3 - ENaH(001) - ETiAl3 were used to determine the relative stability. The results show that TiAl3_1 in Figure 4 is the energetically most favorable structure. In this structure, the cluster interacts with the NaH surface through the plane formed by Ti and two of the three Al atoms. Two H-atoms were pulled up by 0.9 and 1.6 Å to bind Ti and Al, respectively. Bader charge analysis shows that TiAl3 gained a total of 0.50 electron with a distribution of Ti: -0.13; Al1: -0.34; Al2: -0.06; Al3: 0.03. The electron transfer from sodium hydrides to TiAl3 can be attributed to the larger electron affinity (EA) of TiAl3 (1.86 eV) than that of H (0.75 eV). As we discussed above, the extra electron on the TiAl3 cluster is expected to assist the H-H bond breaking to form hydrides. We observed transition from tetrahedral structure to planar structure in molecular dynamics trajectories of TiAl3H7 þ H2 on NaH (001) at the temperatures of 25 and 150 °C. The planar structure TiAl3H10 was formed after a hydride was transferred from the NaH surface to TiAl3H7, followed by H2 dissociation. The structural integrity of TiAl3H10 was maintained for a relatively long time. We also observed the reduction of TiAl3 torsion angle along the trajectory, indicating that the planar structures (TiAl3Hx, x = 10-12) are stable on the NaH surface. The structural stability may be attributed to the formation of the stable Ti-Al-H bridge bonds. The electrostatic attraction between the positively charged surface after losing its H- and the negatively charged TiAl3H10 keeps it on the surface. We then studied the hydrogenation process, i.e., dissociating hydrogen molecule and forming hydrides, based on the most stable structures TiAl3_1. Note that the Ti in TiAl3_1 is in direct contact with the surface. Figure 5 shows the dissociation of hydrogen molecule based on the supported cluster structure. Initially, a hydrogen molecule was placed close to the Ti atom at a distance that is similar to the dihydrogen complex. After optimization, the H-H distance in TiAl3_1 was stretched to 2.39 Å. The Ti-H and Al-H bonds form in compensating for the broken H-H bond. The dissociation of hydrogen molecule can be attributed to electron backdonation from the d-orbital of Ti to the σ* antibonding orbital of hydrogen molecule, referred to as homolytic bond cleavage.56,57 Clearly, the extra electron that Ti acquired from hydrides enables the supported TiAl3 to break the H-H bond and form the Ti-H and Al-H bonds. The above results demonstrated that hydride transfer from NaH to TiAl3Hx plays a key role in dissociating H2 and, therefore,

resynthesizing NaAlH4 from NaH and Al. The hydride transfer is expected to occur at the Al-NaH interface in the presence of Ti, leading to the formation of the negatively charged TiAl3Hx species. We showed that these negatively charged species catalyzed H-H bond breaking and hypothesize that the activities will maintain through the hydrogenation process. We tested our hypothesis preliminarily by using the ab initio molecular dynamics (AIMD) implemented in the VASP program. Constant temperature MD runs at 298 K for up to 2 ps were performed to follow the H2 dissociation on the NaH (001)-supported TiAl3Hx clusters. Snapshots of two trajectories starting from dihydrogen complex structures H2 3 3 3 TiAl3 and H2 3 3 3 TiAl3H4 with a time step of 0.5 fs were shown in Figure 6. The results clearly indicate that TiAl3Hx clusters are activated by acquiring H- from the NaH surface. The activated TiAl3Hx can break the H-H bond rapidly to form Ti-H hydrides. In the trajectory starting from H2 3 3 3 TiAl3, we observed that the newly formed hydride migrated to the Al site. We also note that the interaction mode of TiAl3Hx with the NaH surface plays an important role in H2 dissociation. When the Ti-Al-Al side is in contact with the surface, H2 dissociates readily. Within the limited number of MD runs (∼10), we did not observe Ti-Al bond breaking, although at least one of the Al-Al bonds did break. On the other hand, when the Al-Al-Al side was in contact with NaH surface, H2 dissociation was not observed for the duration of the MD simulation (2 ps). Experimentally, the addition of nanocrystalline TiAl3 of >5 nm to NaAlH4 or NaH/Al was found to be inferior to Ti(III)- or Ti(IV)-based dopants.58,59 The lower catalytic effect of nanocrystalline TiAl3 may be attributed to the size effect. Generally, the metal particles larger than 5 nm have surface structure resembling the structure of bulk-like particles.60 The cohesive energy of the most stable L12-phase of TiAl3 with respect to a TiAl3 cluster is computed to be 9.9 eV. This result suggests that the formation of a TiAl3 cluster from bulk TiAl3 is very difficult. Herein, we focused on hydride transfer from NaH to TiAl3Hx clusters as well as its effect on hydrogen dissociation. A detailed study on size-dependent catalytic activity of TiAl3 as well as the chemistry at the TiAl3Hx-Al interface is part of our ongoing research. The present model demonstrated that the TiAl3Hx clusters supported on the NaH surface are active to dissociate hydrogen 2526

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Basic Energy Science Grant DE-FG02-05ER46231. Figure 7. The proposed hydrogenation mechanism based on hydrogenation of the TiAl3 cluster supported by a NaH (001) surface. Color codes are the same as those in Figure 6.

molecule and may be the active species responsible for recycling NaH and Al back to NaAlH4. We note that this model is highly simplified. In Figure 7, we propose a hydrogenation mechanism based on TiAl3 as the active species. In this model, the TiAl3 cluster locates on the surface of an Al phase and in contact with NaH. The H2 molecule from the gas phase dissociates at the Ti site to form hydrides. As the degree of hydriding increases, the products will undergo a phase transformation, forming NaAlH4. The in situ reformation of NaAlH4 from dehydried materials is expected to be a complex process involving chemical reactions, mass transport, and phase transformations. The proposed model captures a key part of the mechanism, i.e., breaking the H-H bond and forming Al-H bond with the assistance of Ti and NaH. During hydriding phase, we anticipate that TiAl3 molecular clusters remain at the interface between the NaH and Al phases. Stumpf et al. reported that dispersed Ti atoms prefer to diffuse to the subsurface on the Al(100) surface.61 Those authors also reported that direct and Al-mediated H-Ti interactions stabilize surface Ti. The proposed interfacial model shown in Figure 7 is generally consistent with the combined experimental and computational results.

4. CONCLUSION In the present paper, we studied TiAl3Hx (x = 0∼16) clusters using DFT-B3LYP with the 6-311þþG(d, p) basis set, and the TiAl3 cluster supported on a solid-state NaH surface using periodic DFT-PBE and PAW potential. On the basis of our results and analysis, we conclude the following: (1) The neutral TiAl3Hx has a distorted tetrahedral TiAl3 skeleton for x = 0-7 and a T-shape planar structure for x > 9. TiAl3H8 and TiAl3H9 are transition structures between tetrahedral and planar structures. The very stable TiAl3H6þ and TiAl3H7 species indicate that there is a significant barrier for transition from the tetrahedral to planar structures. (2) Both tetrahedral and planar TiAl3Hx bind hydrogen molecularly, forming a dihydrogen complex. Adding an extra electron facilitates hydrogen dissociation, converting molecularly bound hydrogen to hydrides. (3) The NaH (001)-supported TiAl3 was used to test the hypothesis based on the cluster results. Our results support the idea that TiAl3Hx gains electronic charge from the hydrides of NaH. The hydrided TiAl3Hx cluster on the NaH surface, which dissociates the H2 molecule at the Ti site in contact with the surface, is proposed as the active species for NaAlH4 formation in the Ti-doped NaAlH4 hydrogen storage materials.

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