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Abstract

Previous X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) studies on Ti-doped NaAlH4 revealed the reaction products of two heavily doped (33.3 at.%) samples. This investigation revealed that nano-crystalline or amorphous Al2O3 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 TiAl3 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, Al27 Magic Angle Spinning (MAS) NMR Al27 multiple quantum MAS (MQMAS) indicates the presence of an oxide layer of Al2O3 on the surfaces of potentially bulk nanocrystalline Ti, nanocrystalline TiAl3, and/or metallic aluminum. The H1 MAS NMR data also indicate the possible coordination of aluminum with oxygen atoms in the THF molecules. In addition, the H1 MAS NMR and H1 spin-lattice relaxation (T1) measurements are consistent with the presence of TiH2. 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.

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... . , [26] (ab-initio), Ti Al TiAl 3 . Ti 3 Ti-Al-H [21] (a) ; (b) ; (c) Ti-Al-H Al, TiAl 3 1) [27,28] . Cantelli 2 TiCl 3 , P-C-I ...
... This is expected since those compounds are prevented from establishing a long-range order due to the spatial confinement at the nanometer scale imposed by the CMK-3 carbon matrix. 32,33 The assessment that indeed oxidic compounds result from the reaction of LiAlH 4 with the THF molecules is additionally supported by the results of the XPS investigations that are discussed as follows. Figure 6 shows the XP spectra of the infiltrated material as well as the final composite LA@CMK-3. ...
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... Fig. 3 shows the 27 Al MAS NMR spectra of the Al-modified CeO 2 oxides and of pure γ-Al 2 O 3 for comparison. The NMR spectrum of γ-Al 2 O 3 (Fig. 3(e)) shows two peaks at 64.9 and 8.2 ppm, which are assigned to tetrahedrally and octahedrally coordinated Al 3+ , respectively [19,20]. In the results for the xAl-(1-x)Ce oxides, new peaks appear at 27, 34, and 40 ppm. ...
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... 46-1212), which presumably formed by the oxidation of aluminum when negligible amount of O 2 is present in the reaction medium. The formation of Al 2 O 3 has been previously observed by Herberg et al. using the Al 27 nuclear magnetic resonance spectroscopy [14]. The XRD spectra of transition metal-doped alanates, however, do not show any peak of transition metal halides; which is not unexpected as the metal halides are present only in very small quantities. ...
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Article
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An experimental exploration of the transport rate of hydrogen through amorphous Al2O3 layers is presented. Significant changes in the transport rate were observed when changing the thickness of the oxide from 1 to 3 nm. By coating the oxide with a catalytically active Pd layer, a fast dissociation of hydrogen is allowed, enabling a separation between the limitation imposed by dissociation and the transport through the oxide.
Chapter
IntroductionMechanical Processing of SolidsPreparation and Modification of Hydrogen Storage MaterialsAbout the Mechanism of Mechanically Induced Transformations in SolidsConclusions Acknowledgements
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The influence of titanium and vanadium on the hydrogen transport rate through thin amorphous alumina films is addressed. Only small changes in the transport rate are observed when the Al 2 O 3 are covered with titanium or vanadium. This is in stark contrast to results with a Pd overlayer, which enhances the transport by an order of magnitude. Similarly, when titanium is embedded into the alumina the transport rate is faster than for the covered case but still slower than the undoped reference. Embedding vanadium in the alumina does not yield an increase in uptake rate compared to the vanadium covered oxide layers. These results add to the understanding of the hydrogen uptake of oxidized metals, especially the alanates, where the addition of titanium has been found to significantly enhance the rate of hydrogen uptake. The current findings eliminate two possible routes for the catalysis of alanates by Ti, namely dissociation and effective diffusion short-cuts formed by Ti. Finally, no photocatalytic enhancement was noticed on the titanium covered samples.
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Periodic density functional theory calculations with plane-wave basis set and projector-augmented wave potentials have been carried out to investigate the stability and hydrogen interaction in the NaAlH4(001) surfaces doped with 3d transition-metal (TM) elements. A complex structure, TMAl 3H12, in which the TM atom occupies the interstitial position formed from three AlH4- groups, is the most stable structure for TM = Sc to Co. The stability of the complex structure, as well as the hydrogen desorption energies from different positions of the complex structure, was found to follow the 18-electron rule in general. The electron-deficient TMAl3Hx. tends to get more electrons by coordinating with the surrounding Al-H bonds and H-H bond, or by losing the "outside" hydrogen atoms. On the other hand, the electron-rich complex loses its excess electrons easily by releasing A1HV., which resulted in the formation of a new catalytic center, or by desorbing H2. By cycling between the electron-deficient and electron-rich states, TMAl 3Hx. acted as an active center in reversible hydrogen release/uptake processes. Electronic structure analysis revealed that the electron transfer between hydrogen and Al groups mediated by the d orbitais of TMs played important roles in hydrogen release/ uptake from alanate-based materials. The exchange of ligands can be described as a a-bond metathesis process catalyzed by transition metals through a dihydrogen complex. Early transition metals are more efficient to reduce hydrogen desorption energy and break H-H and Al-H bonds as a result of balanced electron-accepting/backdonating abilities, making them better candidates as catalysts. The present analyses are consistent with the experimental observations.
Article
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.
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We study halide interactions with NaAlH4 using first-principles calculations based on density functional theory. First, we determine which products can be formed when TiX3 (X=F, Cl) is ball milled with NaAlH4. Then we investigate impurity incorporation, finding that fluorine may substitute for hydrogen, or form a charged interstitial defect. Fluorine substitution for hydrogen has been proposed as a means of destabilizing hydrides, but we find this to be unlikely for either NaAlH4 or Na3AlH6. Fluorine interstitials are most stable as charged defects; however, their concentration will likely be too low to shift the Fermi level. The migration barrier for interstitial fluorine is low enough to easily allow diffusion during cycling through the hydride phases. Chlorine has a large size mismatch with hydrogen and every chlorine-related defect studied is found to have a high formation energy. Chlorine is therefore unlikely to incorporate in NaAlH4.
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Multinuclear and multidimensional solid state NMR techniques including magic-angle-spinning (MAS) and multiple-quantum (MQ) MAS experiments have been used to characterize various AlH_3 samples. At least three distinct polymorphic AlH_3 phases have been prepared by desolvating the alane etherate product from its organometallic synthesis. MAS-NMR spectra for the ^(1)H and ^(27)Al nuclei have been obtained on a variety of AlH_3 samples that include the β- and γ-phases as well as the α-phase. ^(27)Al MAS NMR was found to respond with high sensitivity for showing differences in spatial arrangements of AlH_6 octahedra in the three polymorphs studied. Based on the characteristic NMR signatures determined, phase transition of the γ-AlH_3 to the α-AlH_3 was studied at room and high temperatures. Direct decomposition of the γ-AlH_3 to aluminum metal at room temperature was also unambiguously confirmed by NMR studies.
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We present a systematic study of the DO22-structure trialuminide intermetallic alloys using 27Al NMR spectroscopy. The quadrupole splittings, Knight shifts, and spin-lattice relaxation times on Al3Ti, Al3V, Al3Nb, and Al3Ta have been identified. Knight-shift tensors were isolated by observation of quadrupole satellite lines and fitting to the central-transition powder patterns. The results are associated with the local electronic density of states for each crystallographic site. Universally small isotropic Knight shifts and long T1’s are consistent with low Fermi-surface densities of states indicating the importance of Fermi-surface features for the phase stability of these alloys. Larger anisotropic Knight shifts occurring at aluminum site I indicate strong hybridization at this site, and the electric-field-gradient tensors confirm the strong ab plane bonding configuration. Local-moment magnetism is found in Al3V, yet electrically this material appears very similar to the other DO22 aluminides.
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Ti K -edge x-ray absorption near-edge spectroscopy was used to explore the Ti valence and coordination in Ti-activated sodium alanate. An empirical relationship was established between the Ti valence and the Ti K -edge onset based on a set of standards. This relationship was used to estimate oxidation states of the titanium catalyst in 2 and 4 mol % Ti-doped Na Al H 4 . The results demonstrate that the formal titanium valence is zero in doped sodium alanate and nearly invariant during hydrogen cycling. A qualitative comparison of the edge fine structure suggests that the Ti is present on the surface in the form of amorphous Ti Al 3 .
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The reaction of Al2Cl6 with an excess of Ph2Si(OH)2 in THF in the presence of pyridine yielded new anionic and cyclic aluminosiloxanes: the structure of the anionic complex is composed of separated pyridinium cations and aluminosiloxane anions with a tetrahedral arrangement around the Al atom, which is similar to that in natural aluminosilicates; the core of the cyclic aluminosiloxane is a twelve-membered Al2Si4O6 ring in a chair conformation, which contains a Cl group on each of the two Al atoms.
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We have been investigating several compounds belonging to the Y2O3-Al2O3 phase diagram by means of aluminum-27, oxygen-17, and yttrium-89 high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy. All aluminum, oxygen and yttrium sites of the five crystalline compounds (C-Y2O3, Y4Al2O9, YAlO3, Y3Al5O12, and alpha -Al2O3) have been resolved and the NMR parameters deduced. Al-VI sites exhibit isotropic chemical shift delta (iso) between 0 and 10 ppm and small quadrupolar coupling constant C-Q, whereas Al-IV are located between 70 and 80 ppm and show large C-Q. O-17 resonances are characterized by small Co and delta (iso) ranging from 72 ppm (Al2O3) to 358 ppm (Y2O3) and showing a strong sensitivity to the nature of the second coordination sphere of oxygen as well as its coordination number. Y-89 delta (iso) are found between 184 and 314 ppm and are not strongly correlated to the coordination state of yttrium, the chemical shift anisotropy being found moderate (similar to 100 ppm). On the basis of the results obtained with the crystalline phases, we have characterized the oxygen environment in a glassy sample of composition Y3Al5O12 and described the variation of the local structure during the crystallization process of YAlO3 from the sol-gel raw product. In the vitreous state, oxygen's environments can be described in terms of OYk Al4-k, sites, with respective populations not distributed in a purely random fashion, and two types of 5-fold-like aluminum environments can be evidenced. A quantitative description of the heat treatments of the YAlO3 precursor is given by O-17 NMR experiments in terms of Y4Al2O9, Y3Al5O12, and amorphous phases formed. New oxygen environments are also evidenced and may be attributed to the hexagonal metastable form of YAlO3.
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γ-Al2O3 and γ-Al2O3 impregnated with phosphorus and/or molybdenum, amorphous AlPO4, and Al2(MoO4)3 have been studied by the recently introduced multiple-quantum magic angle spinning (MQMAS) NMR and off-resonance nutation NMR. Average quadrupolar coupling parameters of the resonances in the compounds were determined with 27Al off-resonance nutation spectroscopy. The MQMAS NMR experiment was used to increase the resolution of the spectra in order to gain insight in the distribution of the quadrupole parameters and to resolve overlapping lines from surface species on γ-Al2O3 supports. This combined use of advanced NMR techniques provided information about the bulk and surface structure of γ-Al2O3- and γ-Al2O3-supported catalyst precursors. When phosphorus and molybdenum loadings below “monolayer” coverage are employed, the single pulse spectra did not reveal the formation of new aluminum-containing compounds. At higher phosphorus loading the formation of a new phase was observed that from the MQMAS experiment could be clearly assigned to amorphous AlPO4. In a calcined sample containing both molybdenum and phosphorus, 27Al NMR showed that Al2(MoO4)3 and some AlPO4 had been formed, which could not be detected by XRD and hardly by single-pulse-excitation (SPE) MAS NMR. The fact that the observed distribution of aluminum atoms over octahedral and tetrahedral positions did not change with the loading of the support (except when AlPO4 was formed) led to the conclusion that aluminum atoms with both types of coordination are found not only in the bulk of γ-Al2O3 but also on its surface in about the same ratio.
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The crystalline microporous aluminophosphate AlPO4-14 was studied using 27Al NMR spectroscopy, with both conventional MAS and two-dimensional multiple-quantum MAS (MQMAS) techniques. Data obtained for the four non-equivalent framework aluminum atoms detected in AlPO4-14 are reported. They are found to occur with the ratio 1:1:1:1 according to the proposed crystal structure. The aim of this paper is also to compare the NMR methods used in this work with those found in literature. It is concluded that the new MQMAS method is by now the best approach to distinguish easily the non-equivalent Al atoms in the structure of such materials. It is also very efficient to obtain rapidly the necessary information about distribution of chemical shifts and quadrupolar interactions.
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Sodium aluminum hydrides have gained attention due to their high hydrogen weight percent (5.5% ideal) compared to interstitial hydrides, and as a model for hydrides with even higher hydrogen weight fraction. The purpose of this paper is to investigate the Ti-compounds that are formed under solution-doping techniques, such as wet doping in solvents such as tetrahydrofuran (THF). Compound formation in Ti-doped sodium aluminum hydrides is investigated using X-ray diffraction (XRD) and magic angle spinning (MAS) nuclear magnetic resonance (NMR). We present lattice parameter measurements of crushed single crystals, which were exposed to Ti during growth. Rietveld refinements indicate no lattice parameter change and thus no solubility for Ti in NaAlH4 by this method of exposure. In addition, X-ray diffraction data indicate that no Ti substitutes in NaH, the final decomposition product for the alanate. Reaction products of completely reacted (33.3 at.%-doped) samples that were solvent-mixed or mechanically milled are investigated. Formation of TiAl3 is observed in mechanically milled materials, but not solution mixed samples, where bonding to THF likely stabilizes Ti-based nano-clusters. The Ti in these clusters is activated by mechanical milling.
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Intermetallic aluminide alloys with the L12, DO22, DO23, and Al2Ti crystal structures have been examined by NMR using the 27Al resonance to investigate how changes in crystal structure affect the local environment as indicated by hyperfine interactions at the atomic nucleus. All nonequivalent Al sites have been clearly resolved. Further, the environment of the transition metal component in each intermetallic crystal structure has been investigated using the 45Sc resonance in the L12 structure of Al3Sc, the 49Ti resonance in the DO22 structure of Al3Ti, and in Al2Ti, the 51V resonance in the DO22 structure of Al3V and the 91Zr resonance in the DO23 structure of Al3Zr. Using a combination of static and magic-angle spinning Fourier transform NMR, and static field sweep spectroscopy, the isotropic Knight shifts (Kiso) and the nuclear quadrupole coupling constants (Cq) have been determined for all nuclei and all sites. In some cases the 27Al axial Knight shift (Kax) has been obtained. In the trialuminides each Al site located by NMR has been identified with the corresponding site in the unit cell of the crystal structure.
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The addition of aluminum to titanium is known to reduce the embrittling effect of hydrogen on the metal. NMR measurements of hydrogen have been performed on Ti-Al-H alloys in order to ascertain how aluminum affects the behavior of hydrogen in titanium. NMR absorption curves and the temperature dependence of T1 were obtained for various concentrations of hydrogen in titanium-aluminum alloys representative of the random and ordered phases of the Ti-Al system. The results are compared to those of hydrogen in aluminum free titanium and show that the hydrogen is located in three different crystallographic environments having different diffusion characteristics. Heat treatment seems to shift the hydrogen sites and a tentative crystallographic model is presented to explain our results. The role of aluminum on the inhibition of hydrogen embrittlement of titanium is discussed and our data indicates that heat treatment may be beneficial in alleviating this problem. It is also shown how the techniques applied in this study can be used to find quantitatively the optimum heat treatment and alloy concentration for reducing hydrogen embrittlement.
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With the description of more and more complex one-and two-dimensional NMR experiments comes the need to develop methods to make a comprehensive interpretation of the various different experiments that can be carried out on the same sample or series of related samples. We present some examples of modelling one-and two-dimensional solid-state NMR spectra of I = 1 2 spin and quadrupolar nuclei, using laboratory-developed software that is made available to the NMR community. Copyright  2001 John Wiley & Sons, Ltd.
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The processes occurring in the course of two sequential hydrogen discharging and recharging cycles of Ti-doped sodium alanate were investigated in parallel using XRD analysis and solid-state NMR spectroscopy. Both methods demonstrate that in hydrogen storage cycles (Eq. (1)) the majority phases involved are NaAlH4, Na3AlH6, Al and NaH. Only traces of other, as yet unidentified phases are observed, one of which has been tentatively assigned to an Al–Ti alloy on the basis of XRD analysis. The unsatisfactory hydrogen storage capacities heretofore observed in cycle tests are shown to be due entirely to the reaction of Na3AlH6 with Al and hydrogen to NaAlH4 (Eq. (1), 2nd hydrogenation step) being incomplete. Using XRD and NMR methods it has been shown that a higher level of rehydrogenation can be achieved by adding an excess of Al powder.
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Solid-state nuclear magnetic resonance is used to study the thermal decomposition of lithium tetrahydroaluminate into metallic aluminum, hydrogen and trilithium hexahydroaluminate. Aluminum sites in LiAlH4 and Li3AlH6 were characterized using static, magic angle spinning (MAS) and multiple-quantum MAS NMR. By applying the in situ NMR method, it has been demonstrated that melting is not a prerequisite for the decomposition of LiAlH4. Based on the observed data, a decomposition path has been established that is consistent with the concentrations of observed Al species at various stages of the thermally induced reaction.
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Two-dimensional 27Al multiple-quantum magic angle spinning (MQMAS) NMR spectroscopy is used to extract the isotropic chemical shifts and quadrupolar parameters of five amorphous aluminosilicates, all of approximately mullite composition (3Al2O3·2SiO2) but of widely differing synthetic origin. Three principal types of Al site are apparent in each sample: two of these are conventionally assigned to 4- and 6-coordinate Al, while the nature of the third site, observed at a shift of , remains a subject of debate. In some of the more anhydrous samples, two 6-coordinate Al sites are observed. Significant distributions of isotropic chemical shifts and quadrupolar parameters are evident in each of the Al sites resolved in the two-dimensional spectra and lineshape fitting is used to estimate the means and widths of these. Additional data are obtained from 27Al{1H} CPMAS NMR experiments and suggest that the protons in the samples are most closely associated with particular 6-coordinate Al sites. The NMR results from the five samples are compiled and compared with those reported for other amorphous and crystalline aluminosilicates and the possible nature of the site is discussed.
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Mechanical processing of polycrystalline LiAlH4 in the presence of titanium- and iron-based catalysts induces the transformation of LiAlH4 into Li3AlH6, Al and H2 at room temperature. Several catalysts were tested and it was established that their activity gradually decreases from TiCl4 to Fe in the series TiCl4>Al3Ti≫Al22Fe3Ti8>Al3Fe>Fe. The high catalytic activity of TiCl4 has been attributed to microcrystalline intermetallic Al3Ti, which rapidly forms in situ from TiCl4 and LiAlH4 during mechanical processing and then acts as a heterogeneous dehydrogenation catalyst.
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1H spin diffusion times of toluene (MB) and tetrahydrofuran (THF) adsorbed on a series of porous solids (charcoal, SiO2 and Al2O3) were measured by a selective inversion technique. The experimental results show that they cover a wide range (from less than one millisecond to several hundreds of milliseconds). For all samples, a tri-exponential behavior was observed in the magnetization recovery processes of the negative peaks. This is attributed to the existence of the two different kinds of spin diffusion processes in addition to the T1 relaxation. One is assigned to the intermolecular spin diffusion between the surface acidic protons of the adsorbent and the organic molecules adsorbed on the solid surface, the other to the intramolecular spin diffusion of adsorbed molecules. Due to hydrogen bonding between the surface hydroxyl groups and the adsorbate, the intermolecular spin diffusion of THF adsorbed on various solids is more effective compared to that of adsorbed MB. In addition, the intermolecular 1H spin diffusion between charcoal and adsorbed THF molecules was confirmed by indirect measurement suggested by Tekely et al.
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Two-dimensional magic-angle spinning (triple quantum, single quantum) correlation pulse sequences and phase cycles based on the technique of Frydman and Harwood for the reconstruction of the isotropic spectrum of half-integer spin quadrupolar nuclei broadened to second-order are described. These sequences provide pure absorption mode two-dimensional lineshapes and increased sensitivity. Experimental examples on spin I = 3/2 (87Rb in RbNO3) and I = 5/2 (27Al in NaSi3AlO8) are presented. The isotropic chemical shift and quadrupolar coupling parameters could be obtained from a simple analysis of the triple quantum filtered single quantum magic-angle spinning cross-sections.
Article
5QMAS experiments on spin-5/2 systems display a low sensitivity compared with their 3QMAS counterparts. Nevertheless, the superior resolution of 5QMAS over 3QMAS makes these experiments a favorable choice for many materials. We report an enhancement scheme for the 5QMAS experiment, using an improved five-quantum excitation pulse scheme combined with a FAM-II conversion pulse. The results are verified experimentally on a polycrystalline sample of gamma-(27)Al(2)O(3), showing an enhancement factor of 2.4 over the simple two-pulse (CW) 5QMAS scheme. Numerical computations of the efficiency parameter epsilon support these results.