Catalyzed Rehydrogenation of NaAlH4: Ti and Friends Are Active on

Article pubs.acs.org/JPCC

Catalyzed Rehydrogenation of NaAlH4: Ti and Friends Are Active on NaH Surfaces; Pt and Friends Are Not Terry J. Frankcombe* Research School of Chemistry, Australian National University, ACT 0200, Australia ABSTRACT: Density functional theory calculations have been performed on slabs of NaH doped with transition metals. These calculations suggest barrierless dissociative chemisorption of H2 onto Sc, Ti, and V dopant sites, while indicating that Rh and Pt do not support dissociative chemisorption. Furthermore, sufficient energy is liberated in the H2 adsorption to allow at least one of the adsorbed hydrogen atoms to diffuse away from the adsorption site without additional energy input. Detaching the second adsorbed hydrogen atom requires a small energy input. These results support an active hydrogen pump mechanism being involved in the rehydrogenation of NaAlH4 decomposition products.

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INTRODUCTION The past two decades have seen intense interest in developing lightweight and compact hydrogen storage systems. Since the pioneering work of Bogdanović and Schwickardi in the mid 1990s,1 complex metal hydrides have been considered to be among the most promising materials around which to base a solid phase storage system. Sodium alanate, NaAlH4, remains a favorite, releasing hydrogen gas according to the two reactions NaAlH4 →

1 2 Na3AlH6 + Al + H 2 3 3

there is no consensus emerging on what the catalysis mechanism is. One of the enduring puzzles about the catalysis mechanism is that traditional hydrogenation catalysts are ineffective at catalyzing the hydrogen release reactions. Specifically, species containing traditional hydrogenation catalyst elements such as Pt, Pd, Ir, Rh, and Ru have minimal impact on the hydrogen cycling reactions, if any impact at all. On the other hand, the light transition metals Sc, Ti, and V (and to a lesser extent elements such as Fe, Zr, Hf, and some light lanthanoids) work as effective catalysts.3−5 There have been only a few attempts to explain why only a few transition metal elements catalyze hydrogen storage reactions in sodium alanate system. Peles and van de Walle have suggested that bulk electronic effects of the dopant alters the charged vacancy balance in the material.6 Their analysis gives a marked difference between Ti and Zr doping, but was not extended to a wider range of dopants. Marashdeh et al. have performed density functional theory (DFT) calculations that indicate elements like Pt and Pd do not substitute into NaAlH4 surfaces like Ti, Sc, and Zr, which would prevent catalysis in their “Zipper model”.7 It is not established how the lack of such substitution may effect the many other models of catalytic activity.2 One of the earliest suggestions for the mechanism of catalysis was the “hydrogen pump” or “spillover” mechanism. In this mechanism the catalyst particles on the surface provide a low energy or even barrierless pathway for hydrogen atoms to associatively desorb when releasing H2, and also the other way in rehydrogenation. Many authors have suggested such a

(1)

and Na3AlH6 → 3NaH + Al +

3 H2 2

(2)

For pure sodium alanate, these reactions are practically irreversible. Bogdanović and Schwickardi showed that adding Ti-based dopants to the system increased the rates and decreased the onset temperatures of the hydrogen release reactions 1 and 2. Furthermore, and more importantly, the Tidoped system could be rehydrogenated to the starting material simply by exposing the dehydrogenation products to hydrogen pressure, via a gas−solid reaction described as NaH + Al +

3 H 2 → NaAlH4 2

(3)

Partial rehydrogenation to Na3AlH6 is also possible. Thus the suitability of using NaAlH4 as the storage material in hydrogen storage applications depends critically on appropriate catalytic doping. This has sparked a considerable amount of experimental and theoretical work designed to determine the mechanism of the Ti catalysis of NaAlH4. This work has led to numerous proposals for the catalysis mechanism. However, as demonstrated in a recent review,2 © XXXX American Chemical Society

Received: November 21, 2012 Revised: April 3, 2013

A

dx.doi.org/10.1021/jp311478y | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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used. Atomic positions were optimized to the minimum potential position (either with or without geometric restraints) by quenched molecular dynamics. The positions were allowed to relax until 5−10 relaxation steps changed the energy by substantially less than 10 meV.

catalysis mechanism for Ti doping (see ref 2 and references therein). Some computational studies have supported a hydrogen pump-like mechanism. Particular arrangements of Ti atoms in the surface of solid aluminum have received substantial recent interest,8−13 while Iń ̃iguez and Yildirim show that, whereas H2 dissociatively adsorbs on Ti substituted into the surface of NaAlH4 crystals, H2 molecules physisorb onto pure NaAlH4 without dissociating.14 The presence of a hydrogen pump in doped alanate has received contradictory support from experimental studies. On one hand, Brinks et al.15 observed that adding TbNiAlH1.33 to NaAlH4 has a minimal effect on the dehydrogenation kinetics, when it would be expected to work as a good hydrogen pump catalyst. On the other hand, Bellosta von Colbe et al.16 observed H−D exchange in Ti-doped NaAlH4 under a D2 atmosphere, which suggests the Ti dopant does indeed facilitate dissociative adsorption/associative desorption. However, it should be noted that H−D exchange should not be taken as proof of a net spillover mechanism in operation.17,18 The bulk mass separation inherent in the dehydrogenation of NaAlH4 means that the Ti-doped alanate dehydrogenation and rehydrogenation reactions need not occur through the same mechanism. Indeed, explaining how Ti doping facilitates reaction 3 is a major challenge for many of the catalysis mechanisms that have been proposed.2 In this work the focus is solely on the rehydrogenation reaction 3. In particular, a hydrogen pump mechanism is tested explicitly by studying the interaction of hydrogen with Ti-doped NaAlH4 decomposition products. Doping Ti into and onto Al surfaces has been shown to have mixed effects, depending on the details of the surface, so this work focuses on the NaH product. Note that hydrogen and aluminum atoms need to associate (ultimately to form [AlHn]m− complex ions in the rehydrogenated alanates) for the hydrogenation reaction to proceed. Thus the NaH surface may be viewed as a less convenient location for H2 dissociation than an Al surface. However, it is worth noting that the formation and accumulation of AlHn species in the vicinity of any catalytic site on an Al surface would rapidly alter the nature of the catalytic site, potentially reducing its activity. Thus rapid migration is important to any catalytic activity whether H2 dissociation occurs on an Al or NaH surface. To address questions of dopant selectivity, this work investigates five different dopants: three that catalyze reversible hydrogen storage (Ti, Sc, and V), and two that do not despite being known as effective hydrogenation catalysts (Pt and Rh).

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RESULTS For a hydrogen pump mechanism to be viable for reaction 3, two elements are required: (1) dissociative adsorption of H2 at the catalytic site, and (2) energetically feasible migration of H atoms away from the adsorption site. These two aspects are dealt with separately below for NaH with transition metal atoms substituted into a surface Na site. Catalyzed Dissociative Adsorption. The test used for catalyzed dissociative adsorption was very simple. In turn, each transition metal dopant was substituted into the surface layer of the NaH slab in a Na site and the atomic positions relaxed. A H2 molecule was placed adjacent to the dopant atom in the resulting structure, around 2.8 Å from the surface. This structure was then allowed to relax to yield the minimum potential geometry. In all cases considered, relaxing the initial doped structure (without the extra H2 molecule) brought the five hydrogen atoms adjacent to the surface doping site closer to the transition metal atom. The nearest four surface Na atoms were also drawn closer to the dopant atom. The distortion of the surrounding NaH lattice was greatest for Rh and Pt, and least for Sc. Two distinct behaviors were evident on relaxation with added H2. For the systems with Sc, Ti, and V substituted in the surface, the added H2 molecule attached to the dopant atom and dissociated, with the potential energy continuously decreasing along the relaxation path. This is illustrated in Figure 1. Such behavior on relaxation demonstrates that there is no potential barrier for the dissociative adsorption of H2 onto the surface dopant site. In the relaxed state the nearest surface Na atoms were displaced toward the adsorbed hydrogen atoms, suggesting additional stabilization of the dissociated state by Na−H interactions. The adsorption energy for the process NaH(M) + H 2 → NaH(M)−2H

(4)

where NaH(M) denotes the NaH (001) surface with the transition metal atom M substituted for a surface Na atom, can be calculated as the potential energy of the relaxed doped slab with adsorbed H2, relative to that of the doped slab plus gas phase H2. The potential energy of a relaxed H2 molecule was calculated with the same parameters as the slab calculation. The calculated adsorption energies are then those given in Table 1. These calculated adsorption energies indicate that the dissociative adsorption of H2 onto Sc, Ti, and V sites in the surface of the NaH slab releases substantial amounts of potential energy. The adsorption energies of H2 on the Sc-, Ti-, and V-doped sites (Sc < Ti < V) in some sense correlate with the hydrogen storage performance observed for NaAlH4 doped with those elements,3,4 with more negative adsorption energies corresponding to better catalysis. Relaxation of H2 on NaH slabs doped with Rh and Pt behaved quite differently. Rather than relaxing to a dissociated chemisorbed state, the H2 remained intact in the relaxed geometry, moving into a physisorption well. Illustrated in Figure 2, this geometry had the H2 molecular axis pointing toward the dopant atom, at greater than 3 Å separation for both

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METHODS DFT calculations were performed with the Abinit code.19−22 The Projector Augmented Wave (PAW) method23,24 was used, with the PBE exchange-correlation functional.25,26 The nuclear pseudopotentials used were those distributed with the Abinit code. The core electrons frozen in the PAW potentials were [He] for Na (9 valence electrons), [Ne] for Sc, Ti, and V (11 to 13 valence electrons), [Kr] for Rh (9 valence electrons), and [Xe]4f14 for Pt (10 valence electrons). The H core froze no electrons. A plane wave cutoff of 500 eV was used throughout. Slab calculations were performed under periodic boundary conditions. A NaH slab was modeled with a 2 × 2 × 2 repetition of the crystallographic unit cell, which was taken to have a lattice parameter of 4.8352 Å. A 15 Å vacuum gap was inserted on the z axis to make slabs with an exposed (001) NaH surface. A 4 × 4 × 1 Monkhorst-Pack k point grid27 was B



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substantial potential barrier exists for close approach of the H2 molecule to the substituted Pt atom. Hydrogen Atoms Can Diffuse Away. The energetics of detaching the hydrogen atoms from the adsorption site was investigated by performing a number of relaxed scans of the potential energy of the Ti-doped NaH slab with adsorbed hydrogen. These scans were performed by successive constrained optimizations, moving selected hydrogen nuclei. For each scan the selected hydrogen atom was moved in a particular direction, with the forces acting to reduce the Ti−H distance in that direction projected out of the calculated DFT forces. The selected hydrogen atoms were successively moved in the chosen direction with the remaining degrees of freedom allowed to relax. Note that this does not constrain the Ti or selected H nuclei from moving in directions perpendicular to the chosen direction, so the Ti−H distance can still change. Nor does the constraint prevent the Ti and H atoms from moving relative to the NaH lattice. Thus, the energies calculated along these paths cannot be considered to be minimum energy paths. They do, however, form an upper bound on the minimum energy path energies. The seven hydrogen atoms bonded to the Ti dopant after H2 adsorption can be grouped into three sets of approximately symmetry equivalent atoms, and one of each of these inequivalent atoms was selected for scanning. These atoms are indicated in Figure 3 with the corresponding displacement directions. Detaching the second adsorbed H nucleus after the first was removed (in the opposite direction) was also investigated. Figure 1. Geometries along the NaH(Ti)−H2 optimization path. The (001) surface is toward the top of each pane, horizontal and perpendicular to the page. The corresponding relative energies are −0.06 eV (top), −0.77 eV (middle), and −1.61 eV (bottom).

Table 1. Adsorption Energies (ΔE, eV) and Adsorbed Hydrogen−Hydrogen (H−H) and Hydrogen−Dopant (H− D) Distances (Å) for H2 on Doped NaH (001) dopant

ΔE

H−H

Sc Ti V Rh Pt

−1.74 −1.61 −1.36 −0.01 −0.02

2.777 2.220 1.863 0.771 0.772

H−M 1.943, 1.805, 1.708, 3.045, 3.055,

1.950 1.812 1.712 3.813 3.824

final state Dissociated 2H Dissociated 2H Dissociated 2H Physisorbed H2 Physisorbed H2

Figure 3. Atoms moved for the relaxed scans on NaH(Ti)-2H, and the constraint directions (labeled according to the lattice vectors of the underlying NaH lattice). The [11̅0] direction is directly into the page, with the (001) NaH surface horizontal. The three atoms are denoted “subsurface” (red), “surface” (green), and “adsorbed” (blue). Nuclei shown as in Figure 1.

The energy profiles as each of the selected atoms was moved away from the Ti dopant atom are shown in Figure 4. The energies in Figure 4 are shown relative to the asymptotic NaH(Ti) + H2 energy, meaning that the initial state with dissociated H2 adsorbed on the Ti atom lies at −1.61 eV, as indicated in Table 1. For consistency, removing the second adsorbed H is shown with the same initial energy. When the deepest, subsurface hydrogen atom was displaced along the [221̅] direction (but allowed to relax in perpendicular directions), the energy of the system rose monotonically. When one of the surface hydrogens or the adsorbed hydrogens were moved across the surface (parallel to the [120] and [110] directions, respectively), the energy similarly increased initially,

Figure 2. Optimized NaH(Pt)−H2 geometry.

Rh and Pt. The physisorption state was only weakly bound, as indicated in Table 1. Attempts to force the adsorbed H2 molecule to approach the dopant atom by constraining the z coordinate of one of the adsorbed hydrogen atoms and successively moving it closer to the surface indicated that a C



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is evidence of significant Na−H interaction. The migrating H nucleus remained between 0.6 and 1.6 Å above the plane of the uppermost atoms in the NaH crystal. Note the displacement of the hydrogen atom path between the forth and fifth H imagescorresponding to the discontinuous change in the energy profile at 2.1 Å in Figure 4 which is indicative of the applied geometrical constraint being a suboptimal approximation to an appropriate reaction coordinate. Energies calculated along linear interpolations between the six coordinated hydrogen structures (that is, after detachment of one hydrogen atom from the Ti adsorption site) show that the hydrogen atoms can migrate from the surface adsorption site to subsurface vacancies (and vice versa) over energy barriers less than 0.25 eV high.

Figure 4. Energies along Ti−H detachment paths. The zero of energy corresponds to the potential energy of NaH(Ti) and gas phase H2. Dashed lines on the right connect configurations relaxed from an interpolation to a potential minimum geometry (see text).

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DISCUSSION Three of the traces shown in Figure 4 stay at energies well below zero. This indicates that hydrogen molecules adsorbing onto Ti sites in NaH surfaces liberate more than enough potential energy to transport a hydrogen atom away from the adsorption site to an interstitial or surface adsorbed site. To be more specific, in the adsorption process potential energy equal to the adsorption energy is converted to kinetic energy. This energy will likely remain localized in vibrational motion of the freshly adsorbed hydrogen atoms for a substantial period of time, allowing this energy to find its way into vibrational modes promoting detachment of the hydrogen atoms along paths such as those shown in Figures 4 and 5. After the first hydrogen migrates away more than 0.5 eV of the liberated adsorption energy remains in excess. While between 0.11 and 0.27 eV can be added to this figure from the conversion of the zero point energy of gas phase H2 in the adsorbed state,28 this is not immediately enough to enable the transport of the second adsorbed hydrogen from the adsorption site. However, a sodium alanate rehydrogenation also involves pure aluminum reacting with hydrogen. If aluminum particles are in the vicinity of the adsorption site it is likely that hydrogen atoms can diffuse to aluminum sites. Significant amounts of energy can be liberated by the formation of Al−H bonds, meaning that there is likely sufficient energy in the system for the second adsorbed hydrogen atom to similarly migrate away from the dopant adsorption site, returning the dopant to its initial state and allowing the catalytic H2 → 2H cycle to continue. It should be noted that the small energy barrier for migration of the second adsorbed atom away from the adsorption site compares favorably with the lowest energy barriers calculated for adsorption and diffusion of hydrogen on Al(111).13 While the DFT methodology used in this work is generally considered to be reliable, we note that most benchmarks of solid state DFT methodologies rely on bulk material properties. In contrast, the energetics and electronic properties of solid surfaces have been shown to differ when using different DFT methodologies on a related crystal.29 Though surface energies do not form a significant part of the current results, methodological artifacts in the electronic structure of the NaH surface may change the interactions of ions with the surface, and thus the energy profiles of Figure 4. When viewed as an isolated NaH surface dopant, it becomes relatively simple to rationalize the differences observed between the low group (Sc, Ti, and V) and high group (Pt and Rh) transition metals. Charge balance with the ionic NaH environment means that the bonded cluster around the transition metal dopant is best described as MH54−. Within

before the rate of increase slowed. However, full relaxations from the geometries with the Ti−H distances around 4.4 Å returned to the initial adsorbed state, indicating that there was no unconstrained potential barrier crossed. Rather than continuing the incremental scans from these ∼4.4 Å separation points, the hydrogen atoms were moved to stable interstitiallike locations and fully relaxed. Geometries were interpolated between the last scan geometry and the fully relaxed geometry and relaxed with the original geometry constraints imposed. Thus, while the points on the right of the potential profiles shown in Figure 4 are not truly points on the constrained minimum energy path, they do represent a constrained, optimized path from the initial adsorbed state to a stable geometry with isolated hydrogen atoms. All three of these paths exhibit small but nonzero potential barriers toward reattaching the hydrogen atom to the Ti center. Figure 5 illustrates the hydrogen position relative to the NaH lattice along this constrained path for the removal of the first adsorbed hydrogen atom. Along the latter part of this path, surface Na nuclei shift almost 0.8 Å toward the H nucleus. This

Figure 5. Location of the migrating hydrogen atom, corresponding to the “first adsorbed hydrogen” curve of Figure 4. The NaH (001) surface is in the plane of the page. Nuclei shown as in Figure 1. The view of Figure 3 is from the upper left corner. D



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this cluster, nine metal orbitals (s, p, and d) and five hydrogen s orbitals are available for bonding. A ligand field theory analysis of these orbitals in a C4v symmetry square pyramidal geometry shows that they combine to form five bonding and five antibonding orbitals, with one metal p orbital and three metal d orbitals unchanged. These orbitals are occupied by between 12 4− 4− and 14 valence electrons for ScH4− 5 , TiH5 , or VH5 , and 18 or 4− 4− 19 electrons for RhH5 or PtH5 . (Or, indeed, 17 to 19 electrons for other ineffective dopants such as Ru and Pd.) Ten electrons occupy M−H bonding orbitals. For Sc, Ti, and V as dopants the remaining two to four electrons only partially fill the remaining metal d orbitals, leaving them available for forming additional M−H bonds. For Rh and Pt, however, the four or more additional electrons fill these three metal d orbitals and the metal p orbital, leaving only high energy, antibonding orbitals available for any bonding between the metal center and an incoming H2 molecule. This model is consistent with the additional hydrogen atoms binding diagonally across the surface site (see, e.g., Figure 5). The metal d orbitals available for forming M−H bonds are the degenerate combination of the dxz and dyz orbitals, which is axially symmetric, and the d orbital lying in the NaH surface plane with lobes pointing between the coordinating hydrogen atoms. Principally, it is the in-plane d orbital that forces the alignment of nascent M−H bonds. Secondary interactions with surface Na ions, evident in the shifting of these atoms toward the adsorbed hydrogens, further stabilize this orientation. In this work it is presumed that once a hydrogen atom detaches from the dopant site and reaches an interstitial or stable surface adsorbed site, then it is free to diffuse to adjacent sites independent of the doping. Singh and Eijt have calculated that, in ideal NaH crystals, bulk diffusion of hydrogen is slow due to a high energy barrier.30 However, vacancies and surface effects substantially lower the migration barriers, allowing thermal diffusion of hydrogen. Some form of long-range diffusion is essential if the hydrogen adsorption occurs away from aluminum-containing phases, as the intermediate rehydrogenation product, Na3AlH6, can only form by physically bringing together aluminum and hydrogen nuclei. It is worth noting that positron annihilation techniques have been used to show that the NaH and Al mixtures resulting from NaAlH4 dehydrogenation contain a high concentration of vacancies.31

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Not only do these results support the hydrogen pump mechanism to be involved in rehydrogenation of dehydrogenated doped NaAlH4 (as distinct from the hydrogen release part of the cycle), but it also clearly demonstrates the distinction between early transition metals like Ti and high group transition metals like Pt. Light lanthanoids, also observed to be active catalysts, similarly possess the available d orbitals used to rationalize the observed trends. It should be recognized that the catalytic splitting of H2 is not the only process occurring in NaAlH4 rehydrogenation. Alcontaining and Na-containing species must undergo long-range transport to form Na3AlH6 or NaAlH4. This work does not address the role, if any, of catalytic dopants in metal transport processes. This work is one of only a handful of theoretical studies that have directly calculated energetics for rehydrogenation processes in NaAlH4 decomposition products. To the author’s knowledge, it is the first of these to focus on the NaH product.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 2 6125 8716. Fax: +61 2 6125 0750. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NCI National Facility at the ANU. The Abinit code is a common project of the Université Catholique de Louvain, Corning Incorporated, and other contributors (http://www.abinit.org).

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REFERENCES

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CONCLUSION

The results presented in this paper clearly show that low group transition metals (Sc, Ti, and V), when substituted into the NaH(001) surface, allow barrierless dissociative adsorption of gaseous hydrogen. High group transition metals (Rh and Pt) do not. Furthermore, at least in the case of Ti, the adsorption liberates sufficient latent heat for at least one of the adsorbed hydrogen atoms to detach from the adsorption site and migrate to the surface or subsurface of the NaH crystal. While sufficient energy remains for scrambling among the hydrogen atoms remaining bonded to the Ti, detachment of the second adsorbed hydrogen atom requires a small energy input. Thus, while the hydrogen pump mechanism is not completely catalytic at zero temperature, at elevated temperatures thermal and exothermic side reaction input is likely to be sufficient to drive catalytic hydrogen splitting at dopant sites. It is reasonable to assume that similar behavior should be expected from hydrogen adsorbed onto Sc and V atoms substituted into the NaH surface. E



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Catalyzed Rehydrogenation of NaAlH4: Ti and Friends Are Active on

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