First Principles Study of Hydrogen Desorption from the NaAlH4

Development of Catalyst-Enhanced Sodium Alanate as an Advanced Hydrogen-Storage Material for Mobile Applications. Yongfeng Liu , Zhuanghe Ren , Xin ...
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First Principles Study of Hydrogen Desorption from the NaAlH4 Surface Doped by Ti Clusters Giacomo Miceli,†,‡ Matteo Guzzo,†,¶ Clotilde Cucinotta,‡,§ and Marco Bernasconi*,† †

Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via R. Cozzi 53, I-20125, Milano, Italy Department of Chemistry and Applied Biosciences, ETH Zurich, USI Campus, Via Giuseppe Buffi 13, 6900 Lugano, Switzerland

‡

ABSTRACT: On the basis of density functional calculations, we show that a Ti4 cluster adsorbed at the (101) surface of NaAlH4 is able to catalyze both the release of H2 and the formation of AlH52− groups which have been previously proposed to be the mobile species leading to the formation of the Na3AlH6 product during the dehydrogenation of Na alanate.

amorphous phases invisible in X-rays.10−12 Moreover, the lack of any changes in the lattice parameters upon Ti addition up to 5 mol % seems to exclude the incorporation of Ti into bulk NaAlH4.13 Concerning the catalytic activity of Ti, hydrogen− deuterium scrambling experiments14 suggest that the active catalyst resides at the surface to promote the dissociation and formation of H2 molecule. The rate limiting step of the dehydrogenation reaction in the doped system is then suggested to consist of mass transport in the bulk.14,15 By means of first principles simulations, the most mobile species in bulk NaAlH4 that might lead to the growth of Na3AlH6 has been identified with an AlH3 vacancy actually corresponding to the complex formed by an AlH4− vacancy and an AlH52− species. In the dehydrogenation process, Ti should thus catalyze both the release of H2 and the formation of the mobile species.16 On the theoretical side several studies on the catalytic activity of Ti have been carried out on the basis of density functional theory (DFT). Vegge17 showed that the activation barrier for H2 release at the NaAlH4(001) surface can be substantially reduced in the presence of a complex formed by Na vacancies and a TiNa substitutional defect. The same complex is, however, inactive on the more open (110) surface. Du et al.18 ascribed the easier release of H2 to the less anionic character of AlH4 groups in the presence of Na vacancies or Ti/Na-vacancies complexes. The catalytic activity of substitutional Ti at the surface of NaAlH4 was confirmed also by calculations with small (NaAlH4)n clusters.19 Liu et al.20−22 studied the doping of the (001) and (100) surfaces of NaAlH4 by a single interstitial Ti which strongly reacts with neighboring AlH4− tetrahedra.

1. INTRODUCTION Solid state storage is the most promising route to meet the requirements of high gravimetric and volumetric density for onboard hydrogen storage.1−4 In this respect, complex hydrides of light elements made of mixtures of amides, borohydrides, and/ or alanates (e.g., LiNH2/LiBH4 or LiNH2/NaAlH4) are particularly attractive. Several systems in this class have been studied in the search for materials suitable to release hydrogen reversibly at temperatures, speed, and partial pressures required for automotive applications.1−4 While the thermodynamics for H release can be optimized by looking at the formation energy of possible products,2 the kinetics of the transformation is much more difficult to be envisaged. In fact, most of materials under consideration for on-board H-storage suffer from the poor kinetics of the hydrogenation/dehydrogenation processes. In this respect, doping with metals is extensively studied after the pioneering work of Bogdanovic and Schwikard5 who found that doping with Ti and other metals dramatically improves the speed and reversibility of the H release in sodium alanate (NaAlH4) decomposing according to the reaction 1 2 NaAlH 4 ⇌ Na3AlH6 + Al + H2 (1) 3 3 In spite of extensive experimental and theoretical investigations, the microscopic mechanism by which Ti catalyzes the solid state reaction in the prototypical NaAlH4 compound is still a matter of debate. On the other hand, a deeper understanding of the catalytic cycle would greatly aid the design of better performing hydrides. Experimental spectroscopic data6−9 suggest that Ti is in a metallic state (zero oxidation charge) independently of the type of precursor used to dope the alanate. Evidence has been provided on the formation of solid solution of Ti with Al possibly in the form of small clusters or © 2012 American Chemical Society

Received: July 15, 2011 Revised: December 13, 2011 Published: February 6, 2012 4311

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Brillouin zone (BZ) integration was restricted to the Γ point only. The minimum energy path for different reactions was identified by using the nudged elastic band method (NEB)31,32 which provides geometries and activation energies of the transition states. Climbing image and variable springs were used, with kmax = 0.2 au and kmin = 0.1 au according to the notation of refs 31 and 32. A minimization scheme was applied until the residual total forces acting on each image in the direction perpendicular to the path were smaller than 0.05 eV/ Å. Due to the large size of our simulation cell, we did not perform a harmonic vibrational analysis to assess the order of the saddle point. Hereafter positive reaction energies correspond to endothermic processes. The (101) surface was modeled in a slab geometry with six AlH4 layers and a total number of 144 atoms. We fix the inplane lattice parameters to the ab initio equilibrium value of the bulk obtained from a Murnaghan interpolation of energyvolumes points of fully optimized configurations. A 6 × 6 × 6 mesh in the BZ of the tetragonal I41/a (24-atom) cell was used. The theoretical equilibrium lattice parameters a = 5.047 Å and c = 11.345 Å are in good agreement with experimental values of 5.021 and 11.346 Å33 and similar to previous ab initio results.34 The lattice parameters of the surface supercell were then chosen as a = 12.41 Å and b = 10.09 Å. A vacuum 12 Å wide separates the periodically repeated slabs. A side view of the surface model is shown in Figure 1.

They found that the desorption energy of H2 from partially decomposed AlHx groups decreases drastically with respect to what occurs at the clean surface, but activation barriers for the process have not been computed. Actually the release of H2 from H anions leaves a negative charge which cannot accumulate on Ti or on Al, but it must migrate into the bulk in the form of AlHx anions ultimately leading to the formation of AlH63− octahedra. The fate of this negative charge has not been addressed in refs 17 and 18. Liu et al.20 showed instead that desorption of H2 from the Ti center induced the formation of an AlH63− group in which, however, some H are shared with Al bound to Ti. It is therefore unclear whether this defect might migrate to give rise to the Na3AlH6 product. A different approach to the problem was followed by Peles and Van de Walle23 and Wilson-Short et al.24 who proposed that the formation of mobile charged defects in the bulk is promoted by a shift of the Fermi level induced by TiAl substitutional defects. In the theoretical papers quoted above, Ti in the atomic form in surface and bulk sites is in a charge state inconsistent with EXAFS and XANES7,8 experiments where a charge near zero is observed. Moreover, several experimental data suggest that Ti mostly forms AlTi alloys and is not incorporated in atomic form. Although one cannot exclude that only a minor fraction of Ti atoms are catalytically active with properties different from those of the majority of Ti atoms detected by several probes, it would be interesting to see whether Ti in a metallic state as well might be catalytically active. As a possible form of metallic Ti, we choose here a small Ti4 cluster adsorbed at the surface of the alanate. In a previous ab initio work, Vegge17 has shown that a single Ti atom in a substitutional site is able to promote hydrogen release from AlH4 tetrahedra at the more compact (001) surface but is not effective at the open (110) face. Here, we want to investigate whether a Ti cluster is suitable to catalyze hydrogen release also at more open surfaces, as in a nanostructured material also the less stable faces are expected to be exposed. To this aim, we considered the (101) surface which is also present in the equilibrium shape of nanocrystals predicted by the ab initio surface energies and the Wulff construction.17 The (101) surface was modeled in a slab geometry. On the basis of first principles calculations of the activation barriers, we here show that the cluster Ti4 adsorbed at the (101) surface of NaAlH4 induces a partial decomposition of the alanate with a H transfer to the cluster. The adsorbed cluster is able to catalyze both the release of H2 and the formation of the mobile AlH52− species, although the transfer of H to the cluster again turns Ti into a positively charged ion which is inconsistent with the near zero charged state detected experimentally for the majority of Ti atoms.

Figure 1. (a) Side view of the slab model of the clean NaAlH4 (101) surface. (b) Tetrahedral Ti4 cluster 4 Å from the surface. (c) Ti4 cluster adsorbed at the surface after soft landing simulated by ab initio molecular dynamics. Several AlH4 groups in contact with Ti break down by transferring H to the metallic cluster.

The catalytic center is modeled by a cluster of four Ti atoms adsorbed at the surface. The cluster was initially optimized in a tetrahedral geometry (Ti−Ti distance of 2.53 Å) and positioned 4 Å from the surface (Figure 1b). Then a soft landing of the cluster was simulated by Car−Parrinello molecular dynamics with a time step of 0.12 fs and an electronic effective mass of 600 au as implemented in the CPMD code.35 After thermalization at 300 K for 5 ps the geometry of surface−cluster complex was optimized with the PWscf code.29

2. COMPUTATIONAL DETAILS The calculation scheme adopted throughout this work is based on spin unrestricted DFT using the energy functional with generalized-gradient corrections proposed by Perdew−Becke− Ernzerhof (PBE)25 which, as opposed to Becke−Lee−Yang− Parr26,27 functional used in our previous work on MgAlH4,28 correctly reproduces the formation energies of complex hydrides (see ref 2 for a review). We used the PWscf code of the Quantum-Espresso package29 for most geometry optimizations. Ultrasoft pseudopotentials30 from the Quantum-Espresso library were used with 3, 9, and 12 valence electrons for Al, Na, and Ti, respectively. Kohn−Sham orbitals were expanded on a plane-wave basis set up to a kinetic-energy cutoff of 30 Ry.

3. RESULTS The optimized geometry of the catalytic center made of a Ti4 cluster adsorbed at the (101) surface of NaAlH4 is shown in Figure 1c. Several AlH4− groups close to the Ti cluster decompose by transferring H atoms to Ti; Al−Ti bonds are 4312

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atoms attached to the Ti4 cluster. One might also conceive that the barrier for H2 desorption is related to the ease of electron transfer from H ions to Ti. Thus it seems likely that weakly charged H atoms should be easier to be removed from the surface than strongly charged H. As suggested by a previous theoretical work,22 we selected the two H atoms to be desorbed by looking at Bader atomic charge computed within the scheme of ref 36. We identified two candidate H atoms to form H2 with H3 and H4 depicted in Figure 3 that have a Bader charge less

formed too. The tetrahedral symmetry of the cluster is lost with Ti−Ti distances spread in the range 2.26−2.83 Å. Although Ti4 is clearly in a metallic state, in the isolated form, once adsorbed at the surface the transfer of H generates a positive charge to Ti atoms of about 0.6 au according to the Bader analysis within the scheme of ref 36. For the sake of comparison the average Bader charge of Na is 0.86 au. The strong Ti−H bonds promote an easy decomposition of AlH4− tetrahedra. However, once bound to Ti, H atoms very rarely detach as pointed out in previous works.18 Actually, the desorption of molecular H2 from Ti is a strongly endothermic reaction requiring 2.67 eV/ (H2) when two H are adsorbed at the isolated Ti4 as computed in a cubic box with an edge 12 Å long, or at least 2.36 eV/(H2) when two H are removed from the most favorable sites on Ti4 adsorbed at the (101) surface of NaAlH4 (cf. Figure 1c). For sake of comparison, we studied H2 release from the clean surface. Actually, it is possible to form H2 in two possible ways (cf. Figure 2a): from a pair of H atoms coming from the same

Figure 3. Sketch of the region involved in the desorption of H2 close to the Ti cluster. All the Al and H atoms coming from broken AlH4− groups and bound to the Ti cluster are labeled. Atoms H3 and H4 are considered for the release of H2.

negative than the average. They both form a bond with an Al atom (Al1) directly bound to the Ti cluster. The reaction energy to form a desorbed H2 molecule with H3 and H4 is indeed as low as 1.08 eV. The reaction barrier computed by NEB optimization is as low as 1.12 eV which demonstrates that the Ti cluster is able to reduce the activation energy for H desorption by more than 1 eV with respect to the uncatalyzed reaction. The Ti4 cluster is thus an effective catalyst at an open surface such as the (101) face while a single Ti atom is able to catalyze H release at the (001) surface but not at the less compact (110) surface.17 Moreover, to continue its catalytic action, a catalytic center must be able to return to its initial state after the reaction. Actually the release of a neutral H2 molecule from two negatively charged H ions implies an electron transfer to Ti or to Al bound to Ti which will finally charge the catalytic center hindering its further activity. In fact, the negative charge of the AlH4− tetrahedra comes from Na ions which are supposed to keep their oxidation state across the transformation. A mechanism for the discharge of the catalytic center is therefore to be sought. This issue has not actually been addressed in ref 17. A possible route for the discharge of the center is the back-transfer of a H− ion to an AlH4− group leading to the formation of an AlH52− center. Since the H− ion comes from the decomposition of an AlH4− group, the H− backtransfer would lead to a complex formed by an AlH4− vacancy plus an AlH52− species which formally corresponds to the AlH3 vacancy identified in ref 16 as the most mobile species responsible for mass transfer during the alanate decomposition. The mechanism by which Ti promotes the formation of AlH3 vacancies was, however, not addressed in ref 16. Here, we propose that the back-transfer of H− from Ti to AlH4− has the function of both restoring the charge state of the catalytic center and forming the mobile species which ultimately leads to

Figure 2. (a) Desorption of H2 from the clean (101) surface of NaAlH4. Different pairs of H atoms involved in the reaction are highlighted. (b) Final state after desorption of two H atoms belonging to the same AlH4− group. (c) Desorption of two H atoms from two different nearest neighbors AlH4− groups leading to the formation of an Al2H62− dimer.

AlH4− group or from two different nearest neighbors AlH4− groups. In the first case, the formation and desorption of a H2 molecule are endothermic by 1.59 eV. The final structure is shown in Figure 2b with the resulting AlH2− group shifted by 0.75 Å toward the neighboring AlH4− tetrahedron. In the other case, the two tetrahedra involved in the process approach each other forming a dimer, Al2H62−, with a bond length of 2.68 Å (Figure 2c). The reaction is endothermic by only 0.8 eV, and the corresponding activation energy obtained from NEB optimization is 2.24 eV. These values are similar to those obtained by Vegge17 on the (110) surface. Although H2 desorption from the Ti cluster is even more costly than desorption from the clean surface, one can conceive that an easier H release might be possible from partially decomposed AlHx groups with Al bound to Ti. In these complexes the Al−Ti distance is ∼2.6 Å to be compared with the value of 2.74 Å in crystalline TiAl3. To explore this idea we considered the formation of H2 from two H atoms bound to Al 4313

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the formation of the Na3AlH6 product. The activation energies for the formation of H2 and the AlH52− species are both of the order of 1.1 eV which is a value sufficiently low to conclude that in the presence of Ti the rate limiting step might not be the formation of H2 or of the mobile species but the mass transport itself as previously proposed in the literature.14,15 Although we have here addressed reactions at the (101) surface, it can be envisaged that similar processes can be sustained at the other low indexes surfaces of NaAlH4. The partial decomposition of the AlH4− groups in contact with the Ti clusters are driven by the large strength of Ti−H bonds which is presumably little affected by the different crystal fields present at the different surfaces. We remark that although the isolated Ti4 is clearly in a metallic form, once adsorbed at the surface the charge transfer to H induced a positive charge of about 0.6 au on Ti atom according to the Bader analysis of atomic charges. Therefore, also this form of Ti is inconsistent with the near zero charge suggested for the majority of Ti atoms in doped NaAlH4 by several spectroscopic tools. This is not expected to depend on the face of the alanate crystal on which the cluster is adsorbed on but on the very formation of Ti−H bonds. However, we might envisage that also much larger clusters and perhaps even a flat surface of metallic Ti at the interface with the alanate might be able to induce a partial decomposition of the nearby AlH4− groups. In large clusters the majority of Ti atoms would display a near zero charge as only the Ti atoms at the surface of the cluster are bound to H. Could it be the case, large Ti clusters would be a suitable catalytic species with Ti mostly in a near zero charge state as detected experimentally. The clusters would catalyze both the formation of H2 and of the mobile AlH52− species. It remains to be seen whether the same process would be possible in the presence of few Ti atoms incorporated at the surface of the Al nanoparticles produced by the alanate decomposition.

the growth of Na3AlH6 (cf. eq 1) according to the proposal of ref 16. To support this idea we computed the activation energy of the transfer of a H− close to the Ti cluster to an underneath AlH4− group by means of NEB optimization. Among the possible choices, we selected a H ion bound to an AlH3 group forming an Al−Ti bond (Figure 4). This latter H ion points

Figure 4. Formation of an AlH52−/AlH4−vacancy complex. The total energy along the transformation path is shown as obtained from NEB optimization. The insets show the initial state, the intermediate state with H− bound to several Na+ ions, and the final state with a vacant AlH4− site depicted by a sphere and the AlH52− complex highlighted by a semitransparent polyhedron. The transferred H− ion is depicted as a red small sphere.

toward the bulk, and it has a large negative Bader charge. The reaction path identified by NEB optimizations and displayed in Figure 4 shows an intermediate state. First H− moves to an interstitial site coordinated by three Na+, and then it moves to an AlH4− forming an AlH52− group. The overall process is endothermic by 0.6 eV and has an activation barrier of 1.08 eV. The final product of this reaction consists of an AlH52− species and a neighboring AlH 4− vacancy resulting from the decomposition at the Ti cluster which can migrate according to the mechanism proposed in ref 16.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses

Laboratoire des Solides Irradiés, École Polytechnique, CNRS, CEA-DSM, F-91128 Palaiseau, France. § School of Physics and CRANN, College Green D2, Dublin, Ireland. ¶

4. CONCLUSION In summary, on the basis of DFT calculations we propose that Ti in the form of small clusters adsorbed at the surface of NaAlH4 is able to promote the release of molecular hydrogen. Metallic Ti induces the breaking of neighboring AlH 4− tetrahedra with transfer of H to the metal. Due to the high strength of the Ti−H bond, H2 release form the cluster is very energetically demanding. However, a facile desorption of H2 with an activation barrier 1 eV lower than that calculated on the clean surface is possible from partially decomposed AlHx species with Al bound to Ti. The formation of H2 from anionic H induces the transfer of a negative charge to Ti−Al which has to be removed to allow the H2 evolution to continue. We have shown that the removal of the negative charge accumulated on the catalytic center is possible by transferring a H anion from another AlHx species bound to Ti toward an AlH4− deeper in the bulk. This process results in the formation of a complex formed by AlH52− and a neighboring AlH4− vacancy which formally corresponds to an AlH3 vacancy. This species has been proposed in ref 16 to be the mobile defect sustaining

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ACKNOWLEDGMENTS G.M. gratefully acknowledges a Ph.D. student fellowship from the Corimav Consortium. We gratefully acknowledge computer resources from CSCS (Manno, CH) and from CINECA (Italy) and the contribution from E. Spanò in the early stage of the project.

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REFERENCES

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