Ti-doped NaAlH4 Research Articles

Catalytic effect and reaction mechanism of Ti doped in NaAlH4: A review

Catalytic effect and hydrogen reaction mechanism of Ti doped in NaAlH4 were elaborated in this paper, and current viewpoints about Ti active species in hydrogen reaction were discussed. In a further step, the possibility and practicality of the hydrogen reaction mechanism of Ti-doped NaAlH4 were elucidated. They could be summarized as follows: while the current theory about the hydrogen reaction mechanism of Ti-doped NaAlH4 should be further improved and modified, the research on Ti-doped NaAlH4 would be a recommendable pattern for the catalyst research in other metal complex hydrides.

Nature of Hydrogen Interaction and Saturation on Small Titanium Clusters

This work explores the nature of interaction and saturation of hydrogen molecules on small titanium clusters using ab initio calculations. Molecular dynamics simulations and ensuing charge density maps were used to gain insight into the key steps involved in dissociation of the hydrogen molecule on the metal clusters. The mechanistic insights gleaned from these simulations were subsequently utilized to obtain realistic models of the hydrogen saturated titanium clusters. It was found that the most stable hydrogen saturated titanium clusters involve hydrogen multicenter bonds. The observed peaks in the experimental mass and photoelectron spectra of hydrogen saturated titanium clusters are attributed to structures possessing hydrogen multicenter bonds. Hydrogen multicenter bonds are also ascribed to the origin of the broad shoulder in the vibrational spectra of hydrogen cycled Ti-doped NaAlH4 reported in a recent study.

Structure and dynamics of Ti–Al–H compounds in Ti-doped NaAlH4

Structure and stability of pristine and modified NaAlH4 are first investigated using density functional theory (DFT) with plane-wave basis and PW91 functional. Vacancy and Ti-dopant effects in the sodium alanate bulk, (0 0 1) surface, and on-top (0 0 1) surface are then studied considering Na and interstitial lattice sites. Calculated substitution energies of Ti-doped (0 0 1) NaAlH4 surfaces have shown almost equal probability of substitution at both lattice and interstitial sites. Ti–Al–H complexes are formed depending on the accessible AlH4 groups around the Ti dopant. TiAl2H7 and TiAl2H2 complexes are found after geometry optimising doped-NaAlH4 surface models. Their stability and dynamics over time at 423 and 448 K are investigated using periodic density functional molecular dynamics (DFT-MD) simulations. Results have shown increased association of Al and H with the complexes as time evolves. DFT-MD simulations show evolution from TiAl2H7 to TiAl5H7 as time and temperature increase in case of Ti dopant at Na surface site (Ti → SNa), and evolution from TiAl2H2 to TiAl3H6 at 423 K and TiAl3H7 at 448 K in case of Ti dopant on-top of the interstitial surface site (Ti → TI) with time.

Dehydrogenation kinetics of Ti-doped NaAlH 4—Influence of Ti precursors and preparation methods

We investigated at 423 K in vacuo the hydrogen desorption kinetics of Ti-doped sodium aluminum hydride ( NaAlH 4 ) prepared with using different Ti precursors and procedures. Initial distinction in kinetics among specimens vanishes through several cycles of dehydrogenation at 473 K in vacuo and rehydrogenation at 423 K under 10 MPa of H 2 . Considering that the temperature of dehydrogenation cycle is above the melting point of NaAlH 4 , we can assume that the difference in physical nature of the materials, such as particle size, phase distribution and so on, disappears through cycling. The present study tells that the chemical nature of the Ti related species which enhances the reaction kinetics of alanate becomes identical whatever Ti presursor is used and, however, the material is prepared.

The catalytic role of an isolated-Ti atom in the hydrogenation of Ti-doped Al(001) surface: An ab initio density functional theory calculation

Recent work [S. Chaudhuri, J.T. Muckerman, J. Phys. Chem. B 109 (2005) 6952] reported that two Ti-substituted atoms on an Al(0 0 1) surface can form a catalytically active site for the dissociation of H2, but the diffusion barrier of atomic H away from Ti site is as high as 1.57 eV. By using ab initio density functional calculations, we found that two hydrogen molecules can dissociate on isolated-Ti atom doped Al(0 0 1) surface with small activation barriers (0.21 and 0.235 eV for first and second H2, respectively). Additionally, the diffusion barrier of atomic H away from Ti site is also moderate (0.47 eV). These results contribute further towards understanding the improved kinetics observed in recycling of hydrogen with Ti-doped NaAlH4.

Determination of the Heat of Hydride Formation/Decomposition by High-Pressure Differential Scanning Calorimetry (HP-DSC)

Among the thermodynamic properties of novel materials for solid-state hydrogen storage, the heat of formation/decomposition of hydrides is the most important parameter to evaluate the stability of the compound and its temperature and pressure of operation. In this work, the desorption and absorption behaviors of three different classes of hydrides are investigated under different hydrogen pressures using high-pressure differential scanning calorimetry (HP-DSC). The HP-DSC technique is used to estimate the equilibrium pressures as a function of temperature, from which the heat of formation is derived. The relevance of this procedure is demonstrated for (i) magnesium-based compounds (Ni-doped MgH2), (ii) Mg-Co-based ternary hydrides (Mg-CoHx) and (iii) Alanate complex hydrides (Ti-doped NaAlH4). From these results, it can be concluded that HP-DSC is a powerful tool to obtain a good approximation of the thermodynamic properties of hydride compounds by a simple and fast study of desorption and absorption properties under different pressures.

Improving Hydrogen Storage Performance of NaAlH4 by Novel Two-Step Milling Method

A selective two-step doping method was developed to prepare metallic Ti-doped NaAlH4, in which a catalytic amount of metallic Ti was first milled with NaH and then milled again after adding Al powder (NaH:Al=1:1). As determined by the combined X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) examinations, the utilization of this novel doping method has produced a more homogeneous dispersion of the fine Ti hydride particles in the NaH/Al matrix, compared to the sample prepared by the traditional one-step milling. As a result, both the hydrogen capacity and de-/hydriding kinetics of the doped materials have been markedly improved. For the NaH + Al + 0.04Ti mixture mechanically milled for 20 h, the cycling hydrogen capacity of the two-step milled sample reached to over 4.3 wt %, about 40% higher than the sample prepared by the traditional one-step method. Moreover, it was observed that the cycling performance of the doped material was highly stable over several hydriding/dehydriding cycles. On the basis of these results, a discussion regarding the nature of catalytically active species was given.

A first-principles study of Sc-doped NaAlH 4 for reversible hydrogen storage

Density function theory calculations have been carried out to investigate energetics, structure, and NMR chemical shielding of Sc-doped NaAlH 4 (0 0 1) surface. Sc atom was found to occupy the interstitial site and interact directly with three immediate neighboring AlH 4 − groups and indirectly with two additional AlH 4 − groups, forming an extended complex structure of ScAl 3H 12–2AlH 4. In the ScAl 3H 12–2AlH 4 structure, the Al–Al distances between the Al atom in ScAl 3H 12 and the Al in the neighboring AlH 4 − are 3.154 Å. Hydrogen desorption energies from many positions of the complex were reduced significantly from that of the clean surface. This extended structure is more efficient for dehydrogenating NaAlH 4 than the similar TiAl 3H 12 complex that was identified in the Ti-doped NaAlH 4 surfaces. Furthermore, our calculated NMR shielding of Sc-doped NaAlH 4 shows that the Al atoms in direct contact with Sc have very different shielding values from the Al atoms in the center of H tetrahedra. These result may provide an explanation for the 27Al MAS NMR chemical shifts observed experimentally.

The hydrogen storage properties and microstructure of Ti-doped sodium aluminum hydride prepared by ball-milling

The hydrogen storage properties and microstructures of Ti-doped NaAlH 4 complex hydrides prepared by hydrogenation of ball-milled NaH/Al mixture with x mol% Ti powder ( x = 0 , 4 , 6 , 10 ) were investigated. It is found that hydrogen as ball-milling atmosphere is better than argon. The reversible hydrogen storage properties improve with increasing Ti content. As the milling time ( t ) extends from 1 to 24 h, the hydrogen desorption capacity increases first and then decreases, and reaches a maximum capacity of 4.25 wt% at t = 16 . The catalytic mechanism for hydrogen storage behavior of Ti-doped NaAlH 4 is attributed to the presence of active small TiH 1.924 and TiAl particles, which are scattered on the surface of much larger NaAlH 4 (NaH/Al) globelets, acting as the catalytic active sites for the complex compound and playing an important catalytic role in the hydriding-dehydriding process.

A First-Principles Analysis of Hydrogen Interaction in Ti-Doped NaAlH4 Surfaces: Structure and Energetics

First principles density functional theory studies have been carried out to investigate the hydrogen interactions in Ti-doped NaAlH4 (001) and (100) surfaces. In both surfaces, Ti was found to energetically favor the interstitial sites formed by three neighboring AlH4- units and interact directly with them. The resulting local structure corresponds to a formula of TiAl3Hx with x = 12 before hydrogen desorption starts. The hydrogen desorption energies from many positions of TiAl3Hx are reduced considerably as compared with that from the corresponding clean, undoped NaAlH4 surfaces. The almost invariant local environment surrounding Ti during dehydrogenation makes the TiAl3Hx complex a precursor state for the formation of experimentally observed TiAl3. The importance of the complex has been explored by analyzing the structures and energetics accompanying hydrogen desorption from the complex and from the neighboring AlH4- units. The TiAl3Hx has extended effects beyond the locally reducing hydrogen desorption energy. It facilitates low-energy hydrogen desorption by either transferring hydrogen to the TiAl3Hx complex or reducing hydrogen desorption energy in the neighboring AlH4- by linking these AlH4- units with the complex structure. The possible mechanisms for forming octahedral AlH6(3-) were also identified in the vicinity of TiAl3Hx. Desorbing hydrogen atoms between Ti and Al atoms causes a symmetrical expansion of Ti-Al bonds and leads to the formation of octahedral AlH6(3-).

Reversible hydrogen storage properties of NaAlH 4 catalyzed with scandium

A systematic study was carried out to compare the reversible hydrogen storage properties of Sc-doped NaAlH 4 with Ti-doped NaAlH 4. Uncycled and cycled (five times) samples of 1, 2 and 4 mol% Sc-doped NaAlH 4 and 4 mol% Ti-doped NaAlH 4 were examined. Temperature programmed desorption, constant temperature desorption, and constant temperature cycling results revealed the following: Sc-doped NaAlH 4 realized very fast hydrogenation kinetics, reasonably fast dehydrogenation kinetics and minimal hydrogen capacity losses during cycling over a broad range of temperatures without any additives or co-dopants. Sc-doped NaAlH 4 also exhibited comparable kinetics and a higher hydrogen storage capacity compared to Ti-doped NaAlH 4, depending on the temperature, metal dopant level, and cycling. Sc exhibited this superior performance over Ti perhaps because Sc is an earlier transition metal than Ti.

How carbon affects hydrogen desorption in NaAlH 4 and Ti-doped NaAlH 4

The hydrogen storage properties of doped and undoped NaAlH 4 samples are studied after mixing them with different percentages of high surface carbon. Manually mixed samples are compared with ball milled ones; it was found that manual mixing was a simple and effective way to dope NaAlH 4. A morphological and micro-structural analysis has been carried out in order to understand the effect of carbon. Carbon added samples show a marked enhancement of hydrogen desorption rate. The desorption temperature and the total hydrogen content remain almost unchanged for undoped sample. The desorption temperatures of Ti-doped samples increase with carbon content.

Impact of preparation conditions on hydrogen storage performance of metallic Ti-doped NaAlH 4

Contrary to the popular opinion, it has been found that metallic Ti powder can be directly utilized as dopant precursor to prepare catalytically enhanced NaAlH 4 through mechanical milling. As a novel method for preparation of catalytically enhanced NaAlH 4, direct metallic Ti-doping possesses practical advantage over the state-of-the-art doping technology: elimination of the inactive by-products and the gas impurities that are highly detrimental to fuel cell operation. A systematic investigation along this new approach was performed under various preparation conditions, including different starting materials, milling atmosphere and milling time. The materials thus prepared under different conditions were found to share some common features, while at the meantime, differ significantly from each other on hydrogen storage performance. A comprehensive understanding of these results provides valuable insight into a series of fundamental questions in catalytically enhanced Ti-NaAlH 4 system.

Kinetic Behavior of Ti-Doped NaAlH4 When Cocatalyzed with Carbon Nanostructures

The effects of SWNTs, MWNTs, AC, C(60), and G when used as a cocatalyst with Ti on the dehydrogenation and hydrogenation kinetics of NaAlH(4) were investigated for the first time in the important temperature range of 90 to 250 degrees C. All five carbons exhibited significant, sustaining, and synergistic cocatalytic effects on the dehydrogenation and hydrogenation kinetics of Ti-doped NaAlH(4) that persisted through charge and discharge cycling. SWNTs were the best cocatalyst, G was the worst cocatalyst, and all five carbons were inactive as a catalyst unless Ti was present. The carbon most likely was imparting an electronic contribution through the interaction of its facile pi-electrons with Ti through a hydrogen spillover mechanism, which explained why one carbon was better than another one in terms of optimal aromatic character, out-of-plane exposure of pi-electrons, and interaction of pi-bonds with neighboring sheets.

Electron microscopy study of Ti-doped sodium aluminum hydride prepared by mechanical milling NaH∕Al with Ti powder

Quite recently, we have demonstrated that mechanical milling NaH∕Al mixture with metallic Ti powder resulted in the formation of catalytically active Ti hydride. In the present study, the morphology and the distribution state of the catalytically active Ti hydride and the parent phases of the as-prepared and cycled samples were examined by electron microscopy in combination with energy dispersive x-ray analysis. It was observed that the de-∕hydriding cycles resulted in a substantial change on the particle size and grain size of Al. In particular, a specific correlation between the distribution states of Al and Ti hydride was developed after de-∕hydriding cycles. In addition, a variation of milling atmosphere during the doping process was found to lead to a varied distribution state of Ti hydride phase. Understanding of these structural characteristics, especially in combination with the previously observed hydrogen storage performance of the materials, has provided insight into the catalytic mechanism and reaction processes of metallic Ti-doped NaAlH4.

A density functional theory study of Ti-doped NaAlH 4 clusters

Density functional theory calculations have been performed on Ti-doped NaAlH 4 clusters. First the electronic structure and stability of undoped clusters of different size and shape were studied, and then one of these clusters was chosen as a model system for a nano-sized NaAlH 4 particle. A Ti atom added to the surface of this model preferably substituted a lattice Na near the surface, when using the NaAlH 4 cluster with Ti adsorbed as the reference system and keeping the substituted atoms within the model. This may be a first step towards a model explaining the role of Ti during dehydrogenation and hydrogenation.

A precursor state for formation of TiAl3 complex in reversible hydrogen desorption/adsorption from Ti-doped NaAlH4

The structure of a TiAl3Hx complex for the formation of a TiAl3 binary phase that could play important roles in the reversible de-/hydrogenation of Ti-doped NaAlH4 has been identified on the basis of first principles density functional theory studies.

Characterization of titanium dopants in sodium alanate by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectra were obtained for samples of Ti-doped NaAlH4 subjected to different numbers of cycles of dehydrogenation/re-hydrogenation. Ti is observed to evolve from its initial Ti(III) state through a series of Ti(0) species during the first 5 cycles. Although the conversion of Ti(III) to Ti(0) occurs much more readily for TiCl3-doped samples than those prepared with TiF3, in both cases the evolution of Ti follows the same sequence that involves 3 distinguishable Ti(0) species and ends in the predominance of the same single Ti(0) species. The spectrum of a sample of NaAlH4 containing 2 mol% of cubic Al3Ti is distinctly different than any of those observed for the Ti(0) species that arise during the hydrogen cycling of the hydride. The major changes in the nature of the predominant Ti species have little if any effect on the dehydrogenation kinetics, which strongly suggests that the profoundly enhanced hydrogen cycling kinetics of Ti-doped NaAlH4 are due to a Ti species present in only a relatively minor amount.

Formula omitted] and [formula omitted] MAS NMR and [formula omitted] multiple quantum studies of Ti-doped NaAlH 4

Previous X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) studies on Ti-doped NaAlH 4 revealed the reaction products of two heavily doped (33.3 at.%) samples. This investigation revealed that nano-crystalline or amorphous Al 2O 3 forms from the possible coordination of aluminum with the oxygen atom of the furan ring system from added tetrahydrofuran (THF) in solvent-mixed samples, and that TiAl 3 forms in mechanically-milled samples [E.H. Majzoub, J.L. Herberg, R. Stumpf, R.S. Maxwell, J. Alloys Compd. 394 (2005) 265], indicating the importance of understanding the processing conditions of these potentially important hydrogen storage materials. The present paper provides a more sophisticated NMR investigation of these materials and resolves some unanswered questions. On heavily doped (33.3 at.%) solvent-mixed samples, Al 27 Magic Angle Spinning (MAS) NMR Al 27 multiple quantum MAS (MQMAS) indicates the presence of an oxide layer of Al 2O 3 on the surfaces of potentially bulk nanocrystalline Ti, nanocrystalline TiAl 3, and/or metallic aluminum. The H 1 MAS NMR data also indicate the possible coordination of aluminum with oxygen atoms in the THF molecules. In addition, the H 1 MAS NMR and H 1 spin-lattice relaxation (T 1) measurements are consistent with the presence of TiH 2. These results are in agreement with recent XAFS measurements indicating both Al and H within the first few coordination shells of Ti in the doped alanate.

Exploration of the Nature of Active Ti Species in Metallic Ti-Doped NaAlH4

Clarification of the nature of active Ti species has been a key challenge in developing Ti-doped NaAlH(4) as a potential hydrogen storage medium. Previously, it has been greatly hindered by the invisibility of Ti-containing species in conventional analysis techniques. In the present study, for the first time, the catalytically active Ti-containing species have been definitely identified by X-ray diffraction in the hydrides doped with metallic Ti. It was found that mechanical milling of a NaH/Al mixture or NaAlH(4) with metallic Ti powder resulted in the formation of nanocrystalline Ti hydrides. The variation of the preparation conditions during the doping process leads to a slight composition variation of the Ti hydrides. The catalytic enhancement arising upon doping the hydride with commercial TiH(2) was quite similar to that achieved in the hydrides doped with metallic Ti. Moreover, the cycling stability that was previously established in metallic Ti-doped hydrides was also observed in the hydrides doped with TiH(2). These results clearly demonstrate that the in situ formed Ti hydrides act as active species to catalyze the reversible dehydrogenation of NaAlH(4). The mechanism by which Ti hydrides catalyze the reversible de-/hydrogenation reactions of NaAlH(4) was discussed.