Article
pubs.acs.org/JPCC
Effect of Transition Metal Dopants on Initial Mass Transport in the
Dehydrogenation of NaAlH4: Density Functional Theory Study
Ali Marashdeh,†,‡ Jan-Willem I. Versluis,† Á lvaro Valdés,†,§ Roar A. Olsen,†,∥ Ole Martin Løvvik,∥,⊥
and Geert-Jan Kroes*,†
†
Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
Department of Chemistry, Faculty of Science, Al-Balqa’ Applied University, Salt 19117, Jordan
§
Instituto de Fisica Fundamental (IFF-CSIC), CSIC, Serrano 123, 28006 Madrid, Spain
∥
SINTEF Materials and Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway
⊥
Department of Physics, University of Oslo, Gaustadalléen 21, N-0318 Oslo, Norway
‡
S Supporting Information
*
ABSTRACT: Sodium alanate (NaAlH4) is a prototype system for storage of
hydrogen in chemical form. However, a key experimental finding, that early transition
metals (TMs) like Ti, Zr, and Sc are good catalysts for hydrogen release (and
reuptake) whereas traditional hydrogenation catalysts like Pd and Pt are poor catalysts
for NaAlH4, has so far received little attention. We performed density functional
theory (DFT) calculations at the PW91 generalized gradient approximation level on
Ti, Zr, Sc, Pd, and Pt interacting with the (001) surface of nanocrystalline NaAlH4,
employing a cluster model of the complex metal hydride to study the initial mass
transport in the dehydrogenation process. A key difference between Ti, Zr, and Sc on one hand and Pd and Pt on the other is
that exchange of the early TM atoms with a surface Na ion, whereby Na is pushed on to the surface, is energetically preferred
over surface absorption in an interstitial site, as found for Pd and Pt. These theoretical findings are consistent with a crucial
feature of the TM catalyst being that it can be transported with the reaction boundary as it moves into the bulk, enabling the
starting material to react away while the catalyst eats its way into the bulk and affecting a phase separation between a Na-rich and
an Al-rich phase. Additional periodic DFT/PW91 calculations in which NaAlH4 is modeled as a slab to model dehydrogenation
of larger NaAlH4 particles and which only consider adsorption and absorption of Ti suggest that Ti prefers to absorb interstitially
but with only a small energy preference over a geometry in which Ti has exchanged with Na. Additional nudged elastic band
calculations based on periodic DFT show only a small barrier (0.02 eV) for exchange of Ti with a surface Na atom. The
mechanism inferred from the cluster calculations is therefore consistent with the slab calculations and may well be important.
to thermodynamics, the first step, in which Na3AlH6, Al, and H2
are produced, proceeds spontaneously at close to 30 °C. The
second step, in which Na3AlH6 reacts to NaH, Al, and H2,
proceeds at close to 110 °C. There has been speculation on the
role of intermediate NanAlmHl phases,24 and structural
similarities between NaAlH4 phases and Na3AlH6 have been
invoked to explain the good de(re)hydrogenation properties of
the material.25,26 A key point is that the release and reuptake of
H2 can be made reversible by adding a catalyst like Ti, as
demonstrated in 1997 by Bogdanovic and Schwickardi.14 In
much of the subsequent work aimed at improving the kinetics
of the release and reuptake of H2, Ti was used (in the form of
TiCl327 or of colloid nanoparticles28,29). However, other
transition metals have also been tried. An intriguing observation
is that traditional hydrogenation catalysts like Pd and Pt are
poor catalysts for hydrogen release from NaAlH4,30 while early
transition metals like Ti, Zr,30,31 and Sc32 and actinides (Ce,32
1. INTRODUCTION
The realization of a hydrogen economy requires development
of an efficient system for on-board hydrogen storage.1 At this
stage, promising methods store hydrogen as a pressurized
gas,2−4 a cryogenic liquid,3−5 an adsorbent to carbon
nanotubes,6,7 to water in clathrate hydrates,8−10 and to metal
organic frameworks (MOFs),11−13 and in chemical form.14−20
Thus far, all of these systems have their specific problems.
Pressurising or liquefying hydrogen requires a significant
fraction of the energy present in H2.13,4 Carbon nanotubes
seemed very promising at first,6 but their room-temperature
storage capacity would appear to be too low at close to 1 wt %.7
At (close to) ambient conditions, the storage capacity presently
achieved for clathrates9,10 and MOFs11,13 is likewise too low.
For recent overviews of storage methods see, for instance, refs
21−23.
Thus far, the chemical storage system that comes closest to
meeting practical requirements is the NaAlH4 system.15 Its first
decomposition step gives about 3.7 wt % hydrogen and the
second 1.9 wt % with total hydrogen release of 5.6 wt % for the
overall reaction. Hydrogen is released in two steps. According
© 2012 American Chemical Society
Received: February 6, 2012
Revised: November 22, 2012
Published: November 27, 2012
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Pr,32 and mischmetal (Mm, 42 atom % Ce, 31 atom % La, 18
atom % Nd, and 9 atom % Pr33) are good catalysts (Sc, Ce,
Pr,32 and Mm33 all being even better than Ti). Another
interesting observation is that adding different transition metals
together may produce synergistic effects, as demonstrated for,
for instance, Ti/Zr31 and Ti/Fe.27
Although much progress has been made at improving
catalyzed hydrogen release from and uptake in NaAlH4, the
kinetics of these processes is still too slow.15 As a result, much
recent work aimed at clarifying the role of the much used Ti
catalyst has focused on determining the form in which it is
present. Thus far, Ti has been found to be present in at least
three different forms. First, Ti has been observed to be present
in Al as a Ti−Al alloy of varying compositions,34−40 and it is
now clear that most of the Ti present should be in this form
during cycling.22 Second, Ti was observed to be present as
TiH2 upon doping a mixture of NaH and Al with pure Ti and
ball milling,41,42 following a conjecture that TiH243 should be
the active catalyst. Finally, there are experiments that suggest Ti
to be present in NaAlH4 itself.40,44,45
Calculations employing periodic density functional theory
(DFT) suggest substitution of Ti into the bulk lattice of
NaAlH4 to be energetically unfavorable if standard states of
NaAlH4, Na, Al, and Ti are used as reference states46,47 or if
reactant and product states appropriate for describing doping
reactions are used as reference states.48 This is supported by
recent elaborate calculations49 showing that bulk substitution
with Ti is unlikely to explain its catalytic effect for
dehydrogenation, even if substitutions varying the number of
hydrogen ions bound to Ti and effects of charged defects
discussed in earlier work of van de Walle and co-worker50,51 are
considered. However, periodic DFT calculations find substitution of Ti into the NaAlH4 lattice to be more stable at the
surface than in the bulk.48,52 Calculations employing a cluster
model of NaAlH4 have shown that Ti atoms present on
NaAlH4(001) in monoatomically dispersed form prefer to
exchange with a surface Na ion53,54 and that the resulting
situation (Ti in surface Na sites, the exchanged Na ion
adsorbed on top of it) is energetically preferred over the case of
two separate bulk phases of NaAlH4 and Ti.54 Similar results
were obtained for the TiH2/NaAlH4 system.55 Furthermore,
periodic DFT calculations likewise suggest that the initial
reaction of TiCl3 with NaAlH4 can result in Ti substituting a Na
surface ion.56 Recent plane wave DFT calculations suggested
that it is energetically even more favorable for Ti to occupy a
surface interstitial site than to exchange with a surface Na ion
and give further support to the idea that Ti can be incorporated
into the surface.57,58 Even more recent DFT calculations
suggest that Ti diffusion from surface adsorption sites to the
energetically most favorable surface interstitial site should
process essentially without barrier.59 Recent experimental
observations also give support to the idea that Ti can be
present in the surface of NaAlH4, immediately after doping with
Ti.40
The exact role of the much used Ti catalyst still remains
elusive,15 although several ideas have been put forward. Isotope
scrambling experiments provided evidence that exchange of
gaseous D2 with NaAlH4 only occurred in the presence of Ti
used as dopant.60 This effect was attributed to the presence of a
Ti−Al alloy,60 with support coming from DFT calculations that
show that dissociation of H2 on a surface of Al(001) with Ti
alloyed into it can occur without barriers61,62 or with much
reduced barriers63,64 whereas high barriers are encountered on
low index surfaces of pure Al. However, the experimentalists
pointed out that the hydrogen exchange observed to take place
under steady state conditions occurs much faster than the full
decomposition reaction, suggesting that the key role of Ti
should be to enhance mass transfer of the solid as the ratelimiting step.60 Experiments employing inelastic spectroscopy
have suggested that Ti enhances bulk diffusion of H2 through
the alanate,65−67 but this point is controversial.68
Addressing the role of Ti, calculations have shown that
surface Ti facilitates defect formation in the surface of NaAlH4
thanks to the ease with which Ti can change its oxidation
state.69 Another idea that has been suggested is that Ti enables
formation of mobile AlH3 which would then enable the fast
mass transfer required in the solid state reactions releasing
hydrogen,25,70 and volatile molecular aluminum hydride
molecules have been identified during hydrogen release from
Ti/Sn-doped NaAlH4 using inelastic neutron scattering spectroscopy.71 In a recent study by Felderhoff and Zibrowius the
volatile AlH3 was, for the first time, trapped and identified
through solid state NMR.72 A complete reaction scheme was
proposed for both dehydrogenation and rehydrogenation
where the role of the Ti catalyst is to facilitate dissociation of
surface−near AlH3 to Al and H2 or their recombination. Other
studies have suggested that an additional role of Ti73 or the
associated anion74,75 used in doping may be to improve the
thermodynamics of the system. Perhaps most crucially, several
experiments have shown that the release and uptake of H2 are
associated with massive mass transfer over long (micrometers)
distances.27,36,76,77 The idea that the crucial role of Ti is to
enable mass transfer over large distances is further supported by
experiments showing that partial decomposition of undoped
NaAlH4 particles is possible if they are very small (nanosized),
decomposition starting at a temperature as low as 40 °C.78
Both the nanosizing and the close contact with the porous
carbon matrix are thought to enhance H2 desorption,79 which
may proceed in one instead of two steps if the NaAlH4 particles
are confined to small enough particles.80 Below a certain
particle size, recent DFT and cluster expansion method
calculations showed that due to the effect of particle size on
decomposition thermodynamics NaAlH4 nanoparticles decompose in one step.75 Also, recent isotope exchange experiments
suggest that diffusion of a hydrogen-containing species heavier
than H (AlH3, AlxHy, or NaH) constitutes the rate-limiting
step,81 and recent DFT calculations suggest this species to be
AlH3.82 Recent NMR experiments also support the presence of
a mobile Al-bearing species that is present under ambient
reaction conditions.83 Recent DFT calculations suggest a role
for surface-adsorbed Ti as a catalytic species for splitting
hydrogen from AlH4/AlH3 groups as well as an initiator for Al
nucleation sites.84
In ref 25 three basic mechanisms were proposed in which Ti
would effect the long-range mass transport of Al or Na required
for de- and rehydrogenation. In the first mechanism, long-range
diffusion of metal species through the alanates to the catalyst
would occur as a first step. Gross et al.25 already proposed that
this could involve the AlH3 species. In the second mechanism,
the driving force would be hydrogen desorption at the catalyst
site, followed by long-range transport of the metal species, the
catalyst acting as a hydrogen dissociation and recombination
site and possibly also as a nucleation site. A computational
example of how the second mechanism might come into play
together with a possible reaction center (i.e., “the dissociation
and recombination site”) was recently published.85 In the third
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at the generalized gradient approximation (PW91) level.88 We
also performed periodic DFT calculations on adsorption and
absorption of Ti in which a slab is used to model the (001)
surface of larger NaAlH4 particles. In section 3 we discuss the
surface adsorption and absorption of the TM atoms on/in the
(001) surface of NaAlH4. This section is concluded with a
discussion of the mechanism suggested by DFT cluster
calculations, which could be relevant for dehydrogenation of
nanosized NaAlH4 particles catalyzed by transition metals like
Ti, Sc, and Zr. Our conclusions are summarized in section 4.
mechanism proposed, the catalyst itself would migrate through
the bulk. In this mechanism, “the starting phase is consumed
and product phases are formed at the catalyst while it ‘eats’ its
way through the material”.25 A more extensive overview of the
different proposed mechanisms for the catalytic activity of Ti in
NaAlH4 has been given in a recent review paper by
Frankcombe.86
The goal of the present paper is to determine the crucial
aspect of the role good catalysts like Ti play in the initial mass
transport in the dehydrogenation of NaAlH4. The main idea
underlying our work is that the explanation of the role of the
catalyst should be based on a key experimental observation, that
traditional hydrogenation catalysts like Pd and Pt30 are poor
catalysts for hydrogen release from NaAlH4, while early
transition metals like Ti,14,30 Zr,30 and Sc32 are good catalysts
for hydrogen release (and reuptake). Our starting point is a
model in which the TM is adsorbed on the face of NaAlH4
which has the lowest surface energy (the (001) face), the TM
being present in monoatomically dispersed form. Such a
situation can arise from ballmilling, a technique that employs
mechanical energy to achieve a fine dispersion of the catalyst,
which has been in use in Ti doping of NaAlH4 since 1999,31 or
from the initial reaction of TiCl3 with NaAlH4.56 Our DFT
calculations using a cluster model and discussed below show
that the bad catalysts, Pd and Pt, prefer to absorb interstitially
in the surface of NaAlH4, while the good catalysts, Ti, Zr, and
Sc, all push Na ions out of the surface and thereby effect a
separation between a Na-rich and an Al-rich phase by
exchanging with surface Na ions. This result would suggest
that the crucial role of catalysts like Ti is indeed to promote
long-range mass transport of one of the metal ions, according
to the third mechanism outlined above, in which the catalyst
moves into the starting phase, eating its way through the
material while moving Na ions outward. Our study may be
compared to a recent theoretical study on the absorption of the
3d transition metal elements Sc−Ni in the (001) surface of
NaAlH4.87 This study87 addressed a greater number (8) of TM
elements than our work (5) but less initial adsorption/
absorption geometries. Also, this study87 focused on the
efficiency of the TMs to reduce hydrogen desorption energies
and break H−H and Al−H bonds, whereas our study focuses
on the ability of the TM to initiate mass transport, i.e.,
separation between Na and Al atoms.
As a further test of the suggested mechanism we also
performed periodic DFT calculations on adsorption and
absorption of one of the TM atoms, Ti, to a NaAlH4 slab to
model Ti-catalyzed dehydrogenation of larger NaAlH4 particles.
Interestingly, these calculations predict that Ti would prefer to
absorb interstitially but with only a small energy preference
over a Na-exchanged geometry. Additional nudged elastic band
(NEB) calculations based on periodic DFT show only a
negligible barrier between surface-adsorbed Ti and Ti
exchanged with Na. The mechanism suggested by our cluster
calculations for catalysis by Sc, Ti, and Zr is therefore consistent
with the periodic DFT calculations.
The outline of this paper is as follows. Section 2 explains the
DFT methodology we used to study the interaction of isolated
TM atoms with the (001) face of NaAlH4. As explained there,
in most of the calculations we employ a cluster methodology
using a cluster with an exposed (001) face which is
energetically, electronically, and structurally close to surface
and bulk NaAlH4 and should form a good model system for
nanosized NaAlH4 particles.53 DFT calculations are performed
2. METHOD
DFT Cluster Calculations. As already mentioned in the
Introduction we use a monoatomically dispersed TM atom on a
cluster consisting of 23 functional NaAlH4 units with an
exposed (001) face as a model system for studying the initial
mass transport in the dehydrogenation process. Justifications
for such a choice of model system have been extensively
discussed in earlier contributions.4950
The binding energies of all systems incorporating a NaAlH4
cluster have been calculated using DFT89,90 as implemented in
the ADF code.91 The exchange-correlation energy is approximated at the generalized gradient approximation (GGA) level
using the PW91 functional.88 The basis set used is of a triple-ζ
plus one polarization function (TZP) type. A frozen core was
chosen of 1s on Al as well as Na and of [He]2s2p on Ti as well
as Sc. On Pd and Zr, the frozen core was [Ar]3d and [Kr]4d on
Pt. The general accuracy parameter of ADF91 was set to 4.0
based on a series of convergence tests. This gives an accuracy in
representative test integrals of four significant digits, leading to
the energy differences studied here being accurate to about 10
meV with respect to this parameter. In many of the calculations
we applied a nonzero electronic temperature to overcome
problems with the SCF convergence. However, we ensured that
we eventually ended up in the electronic ground state by
gradually cooling the electrons. The standard ADF fit sets (for
the TZP basis sets) used to represent the deformation density
were replaced by the fit sets corresponding to the quadruple-ζ
plus four polarization functions type basis sets to achieve results
of high enough accuracy.
In our previous studies we found that it was important to
consider both spin-unpolarized and spin-polarized calculations.53,54 All calculations in the present study have therefore
been done at both the spin-polarized and the spin-unpolarized
level. The spin-polarized calculations were performed allowing
one, two, three, and four electrons to be unpaired. Below, only
the value for the energetically most stable state is reported in
the tables. Calculations involving Zr, Pd, and Pt incorporated
scalar relativistic corrections, employing the ZORA method.92
Other computational details of the cluster calculations can be
found in refs 53 and 54.
The bulk crystal structure of a semispherical NaAlH4 cluster
containing 23 formula units (Z = 23) was used as the initial
geometry to perform a geometry optimization of a bare cluster,
with a large exposed (001) surface.53 The (001) face was
selected because it has been shown to be the most stable crystal
face of NaAlH4.93 The geometry-optimized Z = 23 cluster is
energetically, electronically, and structurally close to surface and
bulk NaAlH4 and should form a good model system for
nanosized NaAlH4 particles.53 In all geometry optimizations of
both the bare cluster and the cluster interacting with a TM
atom, we use much stricter convergence criteria concerning the
force, step length, and energy to locate the minimum than the
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Figure 1. Starting geometries are shown for all geometry optimizations of the TM atom interacting with the (001) surface of the Z = 23 NaAlH4
cluster: (a) TM atom adsorbed above a first-layer Na atom between four AlH4− units [TM@top_4AlH4], (b) between two AlH4− units [TM@
top_2AlH4], and (c) above one AlH4− unit [TM@top_AlH4]. Absorption geometries where (d) the TM atom replaces Na in the surface layer and
the exchanged Na atom adsorbs between two AlH4− units [TM@Na_2AlH4], (e) the TM atom replaces Al in the surface layer and the exchanged Al
atom adsorbs between two AlH4− units [TM@Al_2AlH4], (f) the TM atom replaces Na in the surface layer and the exchanged Na atom adsorbs
between four AlH4− units [TM@Na_4AlH4], (g) the TM atom replaces Al in the surface layer and the exchanged Al atom adsorbs between four
AlH4− units [TM@Al_4AlH4]. Absorption geometries where (h) the TM atom is in an interstitial site between the first and the second layer, on the
C2 axis [TM@ inter12_C2], (i) in an interstitial site in the second layer on the C2 axis [TM@ inter2_C2], (j) in an interstitial site in between the
first two layers but not on the C2 axis [TM@ inter12_nosym], (k) in an interstitial site in the surface layer (TM@inter_surface), and (l) a hybrid
absorption case in which the TM atom exchanges with Na and Na is put in the interstitial site in the surface layer (TM@Na_inter_surface).
TM is Sc, Ti, Zr, Pd, or Pt. Specifically, the TM atom was put at
various sites on the (001) surface (adsorption) and in various
interstitial sites in the subsurface region (absorption). We also
tried exchanging the TM atom with either a surface Na atom or
a surface Al atom, placing the exchanged Na or Al atom at
various sites on the surface of the substituted cluster to find the
lowest energy TM + (Z = 23) cluster with Na or Al exchanged
with a surface-adsorbed TM atom. Finally, we also explored a
hybrid absorption case where Ti was put in a surface Na site
and the replaced Na atom was put in a surface interstitial site.
The three initial adsorption geometries, the four initial
absorption geometries used in exploring exchanges with Na
and Al, the four initial interstitial absorption geometries, and
the hybrid absorption geometry are all shown in Figure 1.
standard ADF criteria used in our previous calculations on Ti
interacting with NaAlH4.53 Specifically, in the new calculations
the convergence criterion used for the gradients is 1 mHartree/
Å. We found that the convergence criteria used earlier,53 which
may give results that are accurate enough for smaller systems,
does not yield accurate enough descriptions of the 100+ atom
system studied here. Use of the tighter convergence criterion
leads to some differences between the Ti−NaAlH4 data
presented here and those in our earlier study.49 However, all
conclusions presented in this study still hold. For example, the
main conclusion that “Ti preferably substitutes a lattice Na near
the surface” is seen to be fully supported by the results
presented here.
The bare Z = 23 cluster formed the starting point of
geometry optimizations of the TM + NaAlH4 system, where
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TM atom sits on top of a surface AlH4− unit (TM@top_AlH4,
Figure 1c). In almost all cases, the TM atom barely moves
parallel to the surface during geometry optimization.
Adsorption energies of the relaxed clusters are listed in Table
1 (the choice of the zero of the energy scale has been discussed
The ADF code calculates the total binding energy relative to
spin-restricted spherically symmetric atoms. This is an
unphysical reference state, but all our calculations have been
done relative to this same state of reference. However, since we
have some freedom in how to define our energy reference, we
have chosen reference systems with a definite physical meaning.
When considering one TM atom adsorbed/absorbed on/in the
(Z = 23) cluster our zero of energy has been set to that of the
fully optimized bare Z = 23 cluster plus one TM atom in TM
bulk. In this way the energy reported for a specific TM +
NaAlH4 system represents the energy gained by taking one TM
atom out of TM bulk and putting it on/in the Z = 23 cluster.
Our adsorption/absorption energies are defined in such a way
that a negative number means that it is stable with respect to
the bare (Z = 23) cluster + one TM atom in TM bulk. The
energies of the bulk TM systems have been computed using the
periodic ADF-BAND code94 using the same basis and fit sets as
employed in the cluster calculations.
Periodic DFT Calculations. Periodic DFT calculations
have been performed using the ab initio simulation package
Vienna ab initio simulation program (VASP).95−98 The
approach uses a plane wave basis set, employing projectoraugmented wave (PAW) potentials to treat the core−valence
interaction.99,100 The periodic calculations use the same
functional to treat the exchange correlation effects as the
cluster calculations, i.e., the PW91 functional.88
The NaAlH4 (001) surface was modeled using a surface cell
containing 4 layers of metal atoms (Na and Al) with 16 NaAlH4
units. The dimensions of the supercell are 9.96 × 9.96 × 24.15
Å, incorporating a vacuum distance of 13 Å along the Z
direction. The surface Brillouin zone was sampled using a 2 × 2
× 1 Γ-point centered k-point grid, and a plane wave cutoff
energy of 600 eV was used. These settings result in energy
differences being converged to about 10 meV. The PAW
potentials have been chosen in accordance with treating H 1s,
Na 2p3s, Al 3s3p, and Ti 4s3d as valence electrons.
During the energy minimizations all atoms were allowed to
relax with convergence criteria for energy and force of 1.0 ×
10−7 eV and 0.05 eV/Å, respectively. The nudged elastic band
(NEB) method101,102 is used to find the minimum energy path
(MEP) between the initial and the final states of a chemical
reaction. In this method, a chain of images is used, consisting of
specific geometries of the atoms on their way from the initial to
the final state joined by forces to ensure that the images are
equally spaced. To construct the initial paths, intermediate
images are linearly interpolated from the first and final minimal
geometries. When these interpolated images are nonphysical,
they are manually changed to more physical geometries. By
minimizing the energy of this string of images the MEP is
revealed.
Table 1. Interaction Energies (in eV) Resulting from
Geometry Optimizations of the Transition Metal Atoms Pt,
Pd, Sc, Ti, and Zr Adsorbing on or Absorbing in the (001)
Face of NaAlH4 Are Given for Each Systema
EZ23 + TM@bulk
TM@top_4AlH4
TM@top_2AlH4
TM@top_AlH4
TM@Na_2AlH4
TM@Al_2AlH4
TM@Na_4AlH4
TM@Al_4AlH4
TM@inter12_C2
TM@inter2_C2
TM@inter12_nosym
TM@inter_surface
TM@Na_inter_surface
Pd
Pt
Sc
Ti
Zr
0.00
1.56
0.95
1.13
0.79
1.35
0.98
2.63
0.79
0.77
0.16
0.19
0.72
0.00
0.84
0.14
0.28
0.72
0.82
0.36
0.85
0.97
0.30
−0.99
−0.16
0.27
0.00
1.27
1.78
1.43
−0.67
1.65
0.33
0.68
1.02
1.19
1.10
0.27
1.04
0.00
0.01
2.19
1.74
−1.59
2.66
0.19
2.31
−0.21
0.70
0.39
0.37
−0.84
0.00
2.54
3.54
2.65
−0.50
2.96
0.53
1.55
2.38
2.05
1.84
1.70
0.35
a
Results were obtained modeling NaAlH4 as a cluster. The zero of the
energy scale is for each set of TM data set at the EZ23 + TM@bulk
value, as discussed in the Method section. Notations used in the first
column are explained in Figure 1.
in the Method section). The final optimized geometry is shown
in Figures 2a−6a for the most favorable adsorption geometry
for each TM atom.
As can be seen from Table 1, when restricted to surface
adsorption the Sc, Ti, and Zr atoms all prefer to be above the
surface Na atom, with coordination to hydrogen atoms from
four surrounding AlH4− units (Figures 2a−4a). For all three
TM atoms, the energetically preferred situation is for the TM
atom to be in TM bulk and separate from NaAlH4. The energy
difference between NaAlH4 and TM bulk and the TM atom
surface adsorbed to the NaAlH4(001) face is smaller for Ti and
Sc than for Zr. Interestingly, the stability of the TM atom
adsorbed to the NaAlH4(001) face correlates well with the
quality of the catalyst for dehydrogenation, Sc and Ti being
better catalysts than Zr.30,32
TM atoms Pt and Pd preferentially adsorb at a different site
when compared to Sc, Ti, and Zr: the Pt and Pd atoms prefer
to adsorb on a 2-fold “bridge site” between 2 AlH4 units (TM@
top_2AlH4, Figures 5a and 6a and Table 1). Just like Sc, Ti, and
Zr, Pt and Pd atoms prefer being in their bulk phases over
being atomically adsorbed to the NaAlH4(001) surface.
Absorption of TM Atoms in the (001) Surface of
Nanosized NaAlH4. Next, we tried to absorb the TM atoms in
the surface of NaAlH4(001), either by exchanging the TM atom
with a surface Na or Al atom or by putting the TM atom in an
interstitial position (see Table 1 for the energy of the relaxed
TM + (Z = 23) cluster systems and Figures 2b−6b and 2c−6c).
When making an exchange, the exchanged atom (Na or Al) was
either put on a 2-fold bridge site between two AlH4− units with
no AlH4− unit underneath (TM@Na_2AlH4 or TM@
Al_2AlH4 Figure 1d and 1e) or put in a site between four
AlH4− units (TM@ Na_4AlH4 and TM@Al_4AlH4, Figure 1f
and 1g). In putting the TM atom in an interstitial site, four
3. RESULTS AND DISCUSSION
Adsorption of TM Atoms on the (001) Surface of
Nanosized NaAlH4. The TM atoms (Sc, Ti, Zr, Pd, Pt) were
adsorbed on the (001) surface of the (Z = 23) NaAlH4 cluster
in different positions, after which a geometry optimization of
the whole system was performed. Three different starting
geometries were attempted. In one geometry, the TM atom sits
above a surface Na atom in between 4 AlH4− units (a structure
denoted TM@top_4AlH4, Figure 1a). In the second geometry,
the TM atom sits in between two AlH4− units on a 2-fold Al
bridge site with no AlH4− underneath (denoted TM@
top_2AlH4, Figure 1b). In the third adsorption geometry, the
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Figure 2. Final optimized geometry for the Sc atom adsorbed on/absorbed in different positions relative to the (001) surface of the (Z = 23)
NaAlH4 cluster for the most favorable cases concerning adsorption, absorption with exchange, and absorption in an interstitial site: (a) Sc atom
adsorbed above a first layer Na atom between four AlH4− units [Sc@top_4AlH4], (b) the Sc atom replaces Na in the surface layer and the exchanged
Na atom adsorbs between two AlH4− units [Sc@Na_2AlH4], and (c) the Sc atom absorbs in an interstitial site in the surface layer (Sc@
inter_surface).
Figure 3. Final optimized geometry for the Ti atom adsorbed on/absorbed in different positions relative to the (001) surface of the (Z = 23)
NaAlH4 cluster for the most favorable cases concerning adsorption, absorption with exchange, and absorption in an interstitial site: (a) Ti atom
adsorbed above a first layer Na atom between four AlH4− units [Ti@top_4AlH4], (b) Ti atom replaces Na in the surface layer and the exchanged Na
atom adsorbs between two AlH4− units [Ti@Na_2AlH4], and (c) Ti atom is in an interstitial site between the first and the second layer on the C2
axis [Ti@ inter12_C2].
different possibilities were tried. In the first case, the TM atom
was placed initially in between the first 2 layers on the C2 axis
(TM@inter12_C2) (Figure 1h). In the second case, the TM
atom was placed within the second layer, again on the C2 axis
(TM@inter2_C2) (Figure 1i). In the third case, the TM atom
was placed initially in between the first 2 layers but not on the
C2 axis (TM@inter12_nosym) in a position above a Na ion
(Figure 1j). In the fourth case the atom was placed initially in a
surface interstitial site above an AlH4− unit (TM@inter_surface, Figure 1k), as investigated previously by Liu and Ge.57,58
Finally, we also investigated a hybrid case, exchanging the TM
atom with Na and putting Na in the surface interstitial site
above an AlH4− unit (TM@Na_inter_surface, Figure 1l).
Again, we find a difference between Sc, Ti, and Zr on one
hand and Pt and Pd on the other (see Table 1). The Sc, Ti, and
Zr atoms all prefer to exchange with a surface Na atom, leaving
the Na atom on the surface on a site where it is in between 2
AlH4− units (TM@Na_2AlH4, Figures 2b−4b) and high above
the surface. In all three cases, the TM@Na_2AlH4 absorbed
case is also preferred over the energetically most favorable
adsorption case, TM@top_4AlH4. Furthermore, in all cases the
TM@Na_2AlH4 absorbed case is also preferred over the case
where the TM atom is in its bulk phase but separate from
NaAlH4. On the other hand, the Pd and Pt atoms prefer to be
in an interstitial position, in the TM@inter12_nosym site
(Figures 5c and 6c), and in this case no Na ion is pushed out of
the surface. Considering only the exchanges, for Pd the TM@
Na_2AlH4 geometry is preferred over the other two exchange
geometries, as was the case for Sc, Ti and Zr, but the TM@
Na_4AlH4 geometry is the preferred exchange symmetry for Pt.
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Figure 4. Final optimized geometry for the Zr atom adsorbed on/absorbed in different positions relative to the (001) surface of the (Z = 23)
NaAlH4 cluster for the most favorable cases concerning adsorption, absorption with exchange, and absorption in an interstitial site: (a) Zr atom
adsorbed above a first layer Na atom between four AlH4− units [Zr@top_4AlH4], (b) Zr atom replaces Na in the surface layer and the exchanged Na
atom adsorbs between two AlH4− units [Zr@Na_2AlH4], and (c) Zr atom absorbs in an interstitial site in the surface layer [Zr@ inter_surface].
Figure 5. Final optimized geometry for the Pd atom adsorbed on/absorbed in different positions relative to the (001) surface of the (Z = 23)
NaAlH4 cluster for the most favorable cases concerning adsorption, absorption with exchange, and absorption in an interstitial site: (a) Pd atom
adsorbed above a first layer Na atom between two AlH4− units [Pd@top_2AlH4], (b) Pd atom replaces Na in the surface layer and the exchanged Na
atom adsorbs between two AlH4− units [Pd@Na_2AlH4], and (c) Pd atom absorbs in an interstitial site in between the first two layers but not on
the C2 axis [Pd@ inter12_nosym].
Na is no longer favored over absorption in all surface interstitial
sites for Ti. The idea to explore the TM@Na_2AlH4 geometry
arose when we were doing calculations on Pd and Pt adsorbing
on the (001) face of NaAlH4 and discovered that these atoms
preferred to adsorb between 2 rather than between 4 AlH4−
units, giving rise to the question of whether the same might be
true for Na adsorbing to a substituted surface. A second
comment is that for Sc and Zr the most favorable interstitial site
is TM@inter_surface, while for Ti it is the TM@inter12_C2
site. A final point is that the hybrid absorption mode (TM@
Na_inter_surface) is in no case the preferred absorption mode.
Adsorption and Absorption of Ti to the (001) Face of
Larger NaAlH4 Particles. Cluster calculations suggest a
mechanism for TM catalysis of dehydrogenation of nanosized
NaAlH4 in which exchange of a TM atom with a surface Na ion
leads to an onset of a separation between a Na-rich and an Alrich phase, as required for the first dehydrogenation step. To
In either case the exchanged Na ion is hardly pushed out of the
surface for Pd and Pt (Figures 5b and 6b), in contrast to what is
found for Sc, Ti, and Zr (Figures 2b, 3b, and 4b). The
difference between the preferred absorption sites correlates well
with catalyst activity: Sc, Ti, and Zr, which exchange with Na
and push Na on to the surface, are good catalysts,30,32 while Pt
and Pd, which prefer to go interstitial and leave surface Na ions
in their place, are bad catalysts.30
A few extra comments are in order. The first is that in our
previous work on a single Ti atom adsorbed to the (001) face
of NaAlH453 we did not yet consider the exchange where the
exchanged atom (Na or Al) was put on a 2-fold bridge site, in
between 2 AlH4− units (for instance, the most favorable TM@
Na_2AlH4 structure of Figure 1d). Instead, in the previous
work only the exchanges with the Na atom in between 4 AlH4−
units were considered (for instance, TM@Na_4AlH4). For the
latter geometry, in our present calculations the exchange with
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Figure 6. Final optimized geometry for the Pt atom adsorbed on/absorbed in different positions relative to the (001) surface of the (Z = 23)
NaAlH4 cluster for the most favorable cases concerning adsorption, absorption with exchange, and absorption in an interstitial site: (a) Pt atom
adsorbed above a first layer Na atom between two AlH4− units [Pt@top_2AlH4], (b) Pt atom replaces Na in the surface layer and the exchanged Na
atom adsorbs between four AlH4− units [Pt@Na_4AlH4], and (c) Pt atom absorbs in an interstitial site between layers 1 and 2, away from the C2 axis
(Pt@inter12_nosym).
in the surface interstitial site (Figure 7e) is preferred over the
exchange with the surface Na atom (Ti@Na_2AlH4, Figure 7d)
by 0.12 eV, whereas the exchange is strongly preferred in the
cluster calculations. Our finding that in calculations modeling
NaAlH4 as a slab the interstitial site is the preferred Ti
absorption site is in agreement with earlier periodic DFT
calculations on Ti + NaAlH4(001) by Liu and Ge using the
same (PW91) functional as used by us.57,58
At the present stage it is difficult to fully account for the
differences between the periodic and the cluster calculations.
We recommend undertaking further calculations with the aim
of clarifying whether there are deficiencies in the cluster or the
periodic models or whether the difference is due to a genuinely
physical effect, i.e., significant differences in mechanism
between smaller and larger particles.
Figure 8 shows two MEPS: the first one (NEB1) going from
the initial adsorption state Ti@top_4AlH4 to the surface
interstitial site, Ti@inter_surface, and the second one (NEB2)
going from the initial adsorption state Ti@top_2AlH4 to the
final exchanged geometry (Ti@Na_2AlH4). In NEB1 Ti may
proceed from the energetically preferred Ti@top_2AlH4
adsorption state to the surface interstitial site without
encountering a large barrier (Figure 8), in agreement with
the earlier calculations of Du et al. (see Figure 5 of ref 59). In
NEB2 the negligible very early barrier (0.02 eV) suggests that it
is also possible for Ti to proceed from the most favorable
adsorption state (Ti@top_2AlH4) to the state in which Ti has
exchanged with a Na ion (Ti@Na_2AlH4) without encountering a prohibitively large barrier. Note that it was difficult to fully
converge all geometries in the nudged elastic band, with the
maximum force on an atom in the final nudged elastic band of
0.06 (NEB1) and 0.13 (NEB2) eV/Å, respectively. However,
the barrier geometry itself as well as nearby geometries were all
well converged, the problem with the geometry convergence
being located in the region far away from the barrier where the
potential energy surface is probably rather flat. In addition, the
NEB calculations provide an upper bound to the barrier in
question. We also note that inclusion of zero-point energy
effects104,105 neglected in this study might result in small
further test for this suggested mechanism, we also performed
periodic DFT calculations to model adsorption on and
absorption in the (001) surface of larger NaAlH4 particles for
a subset of the initial geometries considered in the cluster
calculations. The results are presented in Table 2 (the choice of
the zero of the energy scale has been discussed in the Method
section).
Table 2. Interaction Energies (in eV) Resulting from
Geometry Optimizations of Ti Adsorbing on or Absorbing
in the (001) Face of NaAlH4a
system
energy Z23(eV)
energy slab (eV)
Ti@top_4AlH4
Ti@top_2AlH4
Ti@top_AlH4
Ti@Na_4AlH4
Ti@Na_2AlH4
Ti@Al_4AlH4
Ti@Al_2AlH4
Ti@inter_surface
Ti@Na_inter_surface
1.60
3.77
3.32
1.78
0.00
3.90
4.24
1.96
0.75
3.36
1.67
2.60
0.81
0.00
2.15
0.56
−0.12
2.21
a
Results were obtained from cluster (Z23) and periodic DFT
calculations modeling NaAlH4 as a slab. Zero of the energy scale is
in both cases set at the Ti@Na_2AlH4 geometry, as discussed in the
Method section. Notations used in the first column are explained in
Figure 1.
A notable difference with the clusters is that on a NaAlH4
slab the Ti prefers to adsorb on a bridge Al site (Ti@
top_2AlH4, Table 2 and Figure 7b) rather than on the Ti@
top_4AlH4 site (Figure 7a) preferred for the cluster (Table 1).
The preference for the Ti@top_2AlH4 adsorption site in our
periodic calculations is in agreement with the results of earlier
periodic DFT calculations performed by Du et al. (see Figure 1
of ref59) using the PBE functional103 which by construction
yields electronic energies very similar to PW91 energies.103
Turning to absorption, an additional difference with the
cluster calculations is that in the slab calculations the absorption
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Figure 8. Reaction paths obtained from four NEB calculations based
on periodic DFT results. First NEB calculation maps out the reaction
path between the initial Ti@top_4AlH4 adsorption state and the final
interstitial absorption state, proceeding through the energetically
favored Ti@top_2AlH4 adsorption state. Second NEB path connects
the energetically favored initial Ti@top_2AlH4 adsorption state with
the final exchanged Ti@Na_2AlH4 absorption state.
changes to the MEP, but they are believed not to change the
overall picture presented here.
In the slab calculations, the interstitial site is the preferred
absorption geometry. In this geometry, no Na atom is pushed
out of the surface (Figure 7e). However, only a low barrier
exists between the favored adsorption geometry and the
exchanged geometry on a NaAlH4 slab (Figure 8), and the
exchanged geometry is only 0.12 eV less favorable than the
most favorable interstitial geometry in the slab calculations.
Assuming that the slab calculations are representative for Ticatalyzed dehydrogenation from the (001) surface of larger
than nanosized NaAlH4 particles, the onset of dehydrogenation
could therefore also involve an onset of a separation of NaAlH4
in a Na-rich and an Al-rich phase, as effectively occurs in the
cluster calculations through the action of Sc, Ti, and Zr, which
dig themselves into the surface and push a Na atom out to
achieve the energetically favored TM@Na_2AlH4 geometry.
Interpretation with a Zipper Model for Mass Transport. The results of the cluster calculations suggest that in
dehydrogenation of nanosized NaAlH4 an important function
of the catalyst could be to arrange for mass transport by
enabling an initial phase separation between a Na-rich phase
and an Al-rich phase. The “zipper mechanism” outlined below
is essentially a specific example of the third mechanism for mass
transport discussed by Gross et al.,25 in which the catalyst
moves into the starting phase and “eats” its way into the
material, moving one of the two metal atoms (in our case Na)
out of it. A difference in mass transport is exactly what
distinguishes the absorption behavior of the good catalysts (Sc,
Ti, and Zr) from the absorption behavior of the bad catalysts
(Pt and Pd): the good catalysts prefer to push a Na atom onto
the surface by exchanging with it, penetrating the surface and
bringing about a mass separation between a Na-rich and a Napoor phase. In contrast, the bad catalysts (Pt and Pd) are able
to penetrate the surface but do not actively enforce mass
separation by exchanging with either Na or Al and transporting
the exchanged atom away to sit above the surface. Instead, they
simply absorb in an interstitial site.
We previously invoked a “zipper model”54 to explain the
effect one of the good catalysts, Ti, may have, and the same
model can be applied to Sc and Zr. In this model, the
Figure 7. Final optimized geometry for the Ti atom adsorbed on/
absorbed in different positions relative to the (001) surface of the
NaAlH4 slab, for three cases involving adsorption, absorption with
exchange, and absorption in an interstitial site: (a) the Ti atom
adsorbed above a first layer Na atom between four AlH4− units [Ti@
top_4AlH4]; (b) the Ti atom adsorbed in a 3-fold bridge site, in
between two AlH4− units in the surface and above a second layer
AlH4− unit [Ti@top_2AlH4]; (c) the Ti atom adsorbed above a first
layer AlH4− unit [Ti@top_AlH4]; d) the Ti atom replaces Na in the
surface layer and the exchanged Na atom adsorbs between two AlH4−
units [the 2-fold bridge site, Ti@Na_2AlH4]; (e) the Ti atom absorbs
in an interstitial site in the surface layer (Ti@inter_surface).
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interstitial site. Thus, on nanosized NaAlH4 the good catalysts
are seen to initiate mass transport of the heavy metal atoms and
to bring about an initial separation between a Na-rich and an
Al-rich phase. DFT cluster calculations therefore suggest the
catalyst to effect mass transport according to one of three
mechanisms postulated earlier by Gross et al.,25 in which the
catalyst moves into the starting phase and “eats” its way into the
material, moving one of the two metal atoms (in this case Na)
out of it.
A difference found among the good catalysts that correlates
with their activity is that for the best of the three good catalysts
(Sc and Ti), atomic adsorption of the catalyst on the (001) face
of NaAlH4 is energetically less costly than atomic adsorption of
the Zr catalyst, which strongly prefers to be in its own bulk.
This suggests that the ability to disperse the catalyst in atomic
form over the surface of NaAlH4 may also be important to the
functioning of the catalyst and that this ability may be used to
separate the “best catalysts” from the class of “good catalysts”.
We also carried out slab calculations on adsorption and
absorption of Ti on and in the (001) surface of larger NaAlH4
particles. These calculations are consistent with the mechanism
suggested by the cluster calculations. However, there are many
differences between the outcomes of the cluster calculations
and the periodic calculations. Additional calculations are
needed to establish whether the differences are due to
deficiencies of the cluster or the periodic models or to
genuinely physical effects, i.e., whether there are significant
differences in mechanism between smaller and larger particles.
Such calculations would help us to determine which model best
describes Ti-catalyzed dehydrogenation of NaAlH4.
We believe that our calculations have revealed some key
points about what are good catalysts for effecting hydrogen
release from NaAlH4. We hope that experiments can be done to
confirm the interpretations given above of what are features of
good catalysts.
catalytically active TM atom acts as a slider, eating itself into a
NaAlH4 nanoparticle as it reacts away, effectively unzipping the
structure. The Na ion transported away to sit on the surface
could form the nucleus of a Na-rich phase (Na3AlH6 or NaH),
while the TM atom moving into the NaAlH4 could act as a
nucleus for an Al-rich phase (Al with the TM atom alloyed into
it). The precise ordering of the catalytic activity of Sc, Ti, and
Zr can be explained assuming that two things are needed for
the catalyst to be good: (1) it should prefer to exchange with a
surface atom, which would lead to the desired mass transport,
and (2) it should be possible to initially have the catalyst
present on the surface in monoatomically dispersed form.
Assumption 2 would explain why Ti and Sc are better catalysts
than Zr (less energy is needed to keep Ti and Sc separate from
their bulk phase and present in monatomic form on the surface
than for Zr). In the latter two cases, the energy required to have
the TM be present in monatomic form on the surface could, for
instance, be provided in mechanical form, through ballmilling.
Note that an important part of the zipper model is the
directionality introduced by the propensity of the Ti/Sc/Zr
atom to move into the “fresh” NaAlH4 structure while at the
same time “pushing” a Na atom in the opposite direction.
Another possible short-range separation mechanism could be
invoked through diffusion of Na vacancies, which has also been
shown to be a low-activation process.51 However, we are not
aware of any indications that Na vacancies could introduce the
same directionality as we demonstrated for the Ti/Sc/Zr
exchange mechanism with Na. We should also note that we so
far have not treated the interface between the two separating
phases by, e.g., building a Na-rich phase in direct contact with
our NaAlH4 surface. This would be an interesting direction to
pursue in the future.
The slab calculations on adsorption and absorption of Ti on
and in the (001) surface of a NaAlH4 slab are consistent with
the mechanism suggested by the cluster calculations. Whether
or not this mechanism may be important will depend on the
competition with the barrierless absorption in the interstitial
site and the correctness of the picture the slab model presents
of Ti absorption on a NaAlH4 (001) face. Further research is
needed to explore this and to uncover the causes of the
different energetic preferences of adsorption and absorption
geometries in cluster and in slab calculations and which model
(cluster or slab) presents the best description of Ti-catalyzed
dehydrogenation of NaAlH4.
■
ASSOCIATED CONTENT
S Supporting Information
*
Final optimized geometries obtained with the ADF code for the
TM atom adsorbed on/absorbed in different positions relative
to the (001) surface of the (Z = 23) NaAlH4 cluster for the
most favorable cases concerning adsorption, absorption with
exchange, and absorption in an interstitial site (Tables S1−15);
optimized geometries of the Ti atom interacting with the (001)
surface of the NaAlH4 slab obtained with the VASP code
(Tables S16−20). This material is available free of charge via
the Internet at http://pubs.acs.org.
4. CONCLUSIONS
We performed DFT cluster and periodic DFT calculations on
the interaction of a single TM atom (Sc, Ti, Zr, Pd, and Pt)
with the (001) face of NaAlH4 using a nanosized cluster model
to study the initial mass transport in the dehydrogenation
process. In the DFT calculations, the PW91 GGA was used and
both spin-unpolarized and spin-polarized calculations were
performed. Calculations involving Zr, Pd, and Pt incorporated
scalar relativistic corrections, employing the ZORA method.
Calculations were performed in an attempt to explain a key
experimental finding that Sc, Ti, and Zr are good catalysts for
hydrogen release (and reuptake) while the traditional hydrogenation catalysts Pt and Pd are poor catalysts for hydrogen
storage in NaAlH4.
The key difference found in the cluster calculations between
the good catalysts (Sc, Ti, and Zr) and the bad catalysts (Pt and
Pd) is that the good catalysts prefer to exchange with a surface
Na atom while the bad catalysts prefer to absorb in an
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: g.j.kroes@chem.leidenuniv.nl.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The work presented here has been supported by a grant from
the Dutch research council NWO under the ACTS Hydrogen
programme and by a grant of computer time by the Dutch
National Computing facilities Foundation (NCF).
■
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