Energetics and structure of single Ti defects and their influence on the decomposition of NaAlH(4)

The energetics and structure of various types of single extrinsic Ti defects in NaAlH(4) bulk and (001) slab at the hydriding/dehydriding critical point environment were studied systematically. It is found that the most favorable situation is Ti substituting Al at the subsurface (Ti(Al)(2nd)), which has the highest coordination number for extrinsic Ti ions. The most stable Ti defect in the 1st layer is located at the Al rich interstitial site, namely Ti(i)(1st), accompanied with remarkable strength of Ti-H/Al bond and local geometry deformation at the 1st layer around Ti. Deeper insight of the formation mechanism of Ti defects is obtained by dividing the formation enthalpy of Ti defects into three terms, which are contributed from the cost of removing a substituted host atom if necessary, the cost of structure deformation, and the gain of bonding between Ti and its surrounding ions in the formation of the defects. This associates the formation energy directly with the local structure of Ti defects. For the first time, we adopt H(f)(H), H(f)(H-H), H(f)(AlH(3)) and H(f)(Na) to discuss the hydrogen release ability of the Ti doped NaAlH(4). We find that TiAl(4)H(20) and TiAl(3)H(12) complexes are formed around Ti(Al)(2nd) and Ti(i)(1st) respectively, which significantly promotes the dehydriding ability of NaAlH(4). What is more, the catalyst mechanism of Ti on the decomposition of NaAlH(4) is linked to the AlH(3) mechanism according to our calculations.     

TiCl3-enhanced NaAlH4: impact of excess al and development of the Al1-yTiy phase during cycling

NaAlH(4) with TiCl(3) and Al were mixed by ball-milling and cycled three times. The hydrogen storage properties were monitored during cycling, and the products were characterized by synchrotron X-ray diffraction. Because of the previously described formation of Al(1)(-)(y)Ti(y) with y approximately 0.15 during cycling that traps Al beyond the amount associated with the formation of NaCl, some Na(3)AlH(6) has no free Al to react with to form NaAlH(4). This was counteracted in the present work by adding a stoichiometric amount of Al that increases the theoretical storage capacity. Due to limitations in metal diffusion small amounts of Na(3)AlH(6) were still detected. When approximately 7 mol % more Al than the stoichiometric amount was added, the observed storage capacity increased significantly, and the Na(3)AlH(6) content was negligible after prolonged rehydrogenation. Cycled NaAlH(4) + 10 mol % TiCl(3) were desorbed to two different levels, and the diffraction patterns were compared. There is no change in unit-cell dimensions during desorption, and there is no sign of changes in the bulk composition of the Al(1)(-)(y)Ti(y) phase during a cycle. Adding pure Ti to a NaH + Al mixture by ball-milling in argon or hydrogen results in formation of TiH(2) that is stable during at least one cycle.     

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.     

掺杂元素Ti和Ni对NaAlH4放氢性能影响的第一原理研究

利用第一性原理方法研究了掺杂元素Ti, Ni对NaAlH4放氢性能的影响. 计算表明: Ti在NaAlH4中倾向于替代Al原子, 而Ni则倾向于占据间隙位置. 电子结构分析显示Ti替代NaAlH4中的Al位置时与近邻的Al原子产生强烈的相互作用, 破坏[AlH4]基团的结构, 从而改善NaAlH4的放氢性能. Ti替代Na或占据间隙位置时Ti与H原子间存在较强的相互作用, 有可能诱发TiH2相而改善NaAlH4的放氢性能. 与Ti相比Ni对NaAlH4放氢性能的影响较小, 仅当Ni占据间隙位置时才可能对[AlH4]基团产生一定影响. 总体而言, Ti对NaAlH4放氢性能的影响强于Ni的作用, 这与实验观测相吻合.     

Equilibrium structure and Ti-catalyzed H2 desorption in NaAlH4 nanoparticles from density functional theory

Improving the hydrogen ab- and desorption kinetics in complex hydrides is essential if these materials are to be used as reversible hydrogen storage media in the transport sector. Although reductions in particle size and the addition of titanium based compounds have been found to improve the kinetics significantly, the physical understanding remains elusive. Density functional theory is used to calculate the energy of the potential low energy surfaces of NaAlH(4) to establish the equilibrium particle shape, and furthermore to determine the deposition energy of Ti/TiH(2) and the substitutional energy for Ti@Al and Ti@Na-sites on the exposed facets. The substitutional processes are energetically preferred and the Na-vacancy formation energy is found to be strongly reduced in the presence of Ti. The barrier for H(2) desorption is found to depend significantly on surface morphology and in particular on the presence of Ti, where the activation energy for H(2) desorption on NaAlH(4){001} surfaces can drop to 0.98 eV--in good agreement with the experimentally observed activation energy for dehydrogenation.     

Synchrotron X-ray studies of Al(1-y)Ti(y) formation and re-hydriding inhibition in Ti-enhanced NaAlH4

NaAlH4 samples with Ti additives (TiCl3, TiF3, and Ti(OBu)4) have been investigated by synchrotron X-ray diffraction in order to unveil the nature of Ti. No crystalline Ti-containing phases were observed after ball milling of NaAlH4 with the additives, neither as a solid solution in NaAlH4 nor as secondary phases. However, after cycling, a high-angle shoulder of Al is observed in the same position with 10% TiCl3 as that with 2% Ti(OBu)4, but with considerably higher intensity, indicating that the shoulder is caused by Ti. After prolonged reabsorption, there is only a small fraction of free Al phase left to react with Na3AlH6, whereas the shoulder caused by Al(1-y)Ti(y) is dominating. The Ti-containing phase causing the shoulder therefore contains less Ti than Al3Ti, and the aluminum in this phase is too strongly bound to react with Na3AlH6 to form NaAlH4. The composition of the Al(1-y)Ti(y) phase is estimated from quantitative phase analysis of powder X-ray diffraction data to be Al(0.85)Ti(0.15). Formation of this phase may explain the reduction of capacity beyond the theoretical reduction from the dead weight of the additive and the reaction between the additive and NaAlH4.     

Orbital-free density functional theory applied to NaAlH4

We present the application of orbital-free density functional theory (OF-DFT) to NaAlH(4), a potential hydrogen storage material, and related systems. Although the simple Al and NaH structures are reproduced reasonably well by OF-DFT, the approach fails for the more complex NaAlH(4) structure. Calculations on AlH(3) show that the failure to describe the Al-H interaction is related to the kinetic energy functionals used rather than the local pseudopotentials which are required within the OF-DFT approach. Thus, systems such as NaAlH(4) present a challenge which awaits the development of more reliable orbital-free kinetic energy functionals.     

络合氢化物Ti-NaAlH4的制备与储氢特性

采用Ti粉为催化剂前驱体、预处理Al粉和NaH为合成原料, 通过机械球磨-加氢方法合成出络合氢化物Ti-NaAlH4, 系统研究了球磨保护气氛、球磨时间和氢化加氢压力等制备参数对其储氢性能的影响. 结果表明, 制备方法对Ti-NaAlH4储氢特性有很大影响. 与氩气保护气氛相比, 在氢气气氛中球磨制备的复合物具有更高的吸放氢性能. 在氢气保护气氛下, 随着球磨时间从6 h增至24 h, 复合物的吸氢容量和吸氢速率先增后减, 12 h时达到最佳值, 而复合物的放氢容量和放氢速率则逐渐增高; 进一步延长球磨时间会使颗粒发生团聚, 从而导致吸氢性能下降. 随着氢化加氢压力从7.5 MPa升至13.5 MPa, 复合物的吸氢容量(质量分数)由2.83%逐渐增至4.21%. 复合物球磨后出现的Na3AlH6中间氢化物相表明, 在氢气下掺Ti球磨对NaH和Al的氢化反应起到很好的促进作用.     

Evolution of the local structure around Ti atoms in NaAlH4 doped with TiCl3 or Ti13.6THF by ball milling using X-ray absorption and X-ray photoelectron spectroscopy

X-ray absorption and X-ray photoelectron spectroscopy are used to investigate NaAlH4 doped with 5 mol % of Ti on the basis of either TiCl3 or Ti13.6THF by ball milling. X-ray photoelectron spectroscopy (XPS) analysis of TiCl3 or Ti colloid doped samples indicates that Ti species do not remain on the sample surface but are driven into the material with increasing milling time. The surface concentration of Ti continues to decrease during subsequent cycles under hydrogen. After several cycles, it reaches a constant value of 0.5 at. % independently of the nature of the precursor. Moreover, metallic aluminum is already present at the surface after 2 min of ball milling in the case of TiCl3 doped Na-alanate, whereas it is totally absent in the case of Ti colloid doped samples at any milling time. Upon cycling, the atomic concentration of metallic Al at the surface evolves with the reaction under hydrogen, in contrast to the Ti concentration. Analysis of the binding energies of samples doped with TiCl3 or Ti colloid, after eight desorption/absorption cycles, reveals that the Na, O, and Ti environment remains the same, while the Al environment undergoes changes. According to the extended X-ray absorption fine structure (EXAFS) analysis of TiCl3 doped Na-alanate, the local structure around Ti during the first cycle is close to that of metallic Ti but in a more distorted state. In the case of the Ti colloid doped sample, a stripping of the oxygen shell occurs. After eight cycles, a similar intermetallic phase between Ti and Al is present in the hydrogenated state of TiCl3 or Ti colloid doped samples. The local structure around Ti atoms after eight cycles consists of Al and Ti backscatterers with a Ti-Al distance of 2.79 angstroms and a Ti-Ti distance of 3.88 angstroms. This local structure is not exactly the TiAl3 phase because it differs significantly from the alloy phase in its fine structure and lacks long-range order. Volumetric measurements performed on these samples indicate that the formation of this local structure is responsible for the reduction of the reversible hydrogen capacity with the increasing number of cycles. Moreover, the formation of the alloy-like phase is correlated with a decrease of the desorption/absorption reaction rate.     

Understanding the role of Ti in reversible hydrogen storage as sodium alanate: a combined experimental and density functional theoretical approach

We report the results of an experimental and theoretical study of hydrogen storage in sodium alanate (NaAlH(4)). Reversible hydrogen storage in this material is dependent on the presence of 2-4% Ti dopant. Our combined study shows that the role of Ti may be linked entirely to Ti-containing active catalytic sites in the metallic Al phase present in the dehydrogenated NaAlH(4). The EXAFS data presented here show that dehydrogenated samples contain a highly disordered distribution of Ti-Al distances with no long-range order beyond the second coordination sphere. We have used density functional theory techniques to calculate the chemical potential of possible Ti arrangements on an Al(001) surface for Ti coverages ranging from 0.125 to 0.5 monolayer (ML) and have identified those that can chemisorb molecular hydrogen via spontaneous or only moderately activated pathways. The chemisorption process exhibits a characteristic nodal symmetry property for the low-barrier sites: the incipient doped surface-H(2) adduct's highest occupied molecular orbital (HOMO) incorporates the sigma antibonding molecular orbital of hydrogen, allowing the transfer of charge density from the surface to dissociate the molecular hydrogen. This work also proposes a plausible mechanism for the transport of an aluminum hydride species back into the NaH lattice that is supported by Car-Parrinello molecular dynamics (CPMD) simulations of the stability and mobility of aluminum clusters (alanes) on Al(001). As an experimental validation of the proposed role of titanium and the subsequent diffusion of alanes, we demonstrate experimentally that AlH(3) reacts with NaH to form NaAlH(4) without any requirement of a catalyst or hydrogen overpressure.     

Quasiclassical trajectory calculations of hydrogen absorption in the (NaAlH4)2Ti system on a model analytical potential energy surface

We performed a quasiclassical trajectory dynamics study on a model analytical 21-dimensional (7 active atoms) potential energy surface (PES) to examine in detail the mechanism of the hydrogen absorption in a simple (NaAlH(4))(2)Ti model system. The reaction involves a capture of H(2) by the Ti centre and formation of the (η(2)-H(2))Ti(NaAlH(3))(2) coordination complex containing the side-on bonded dihydrogen ligand. The calculated rate constant corresponds to a very fast capture of H(2) by the Ti coordination sphere without a demonstrable barrier. This implies that this step is not the rate-determining step in the complex multi-step process of the NaAlH(4) recovery. The model analytical PES captures the essence of this reaction well and the corresponding energy contours compare favourably to those based on the all-atom hybrid density functional theory calculations.     

Thermodynamics and kinetics of NaAlH4 nanocluster decomposition

Reactive nanoparticles are of great interest for applications ranging from catalysis to energy storage. However, efforts to relate cluster size to thermodynamic stability and chemical reactivity are hampered by broad pore size distributions and poorly characterized chemical environments in many microporous templates. Metal hydrides are an important example of this problem. Theoretical calculations suggest that reducing their critical dimension to the nanoscale can in some cases considerably destabilize these materials and there is clear experimental evidence for accelerated kinetics, making hydrogen storage applications more attractive in some cases. However, quantitative measurements establishing the influence of size on thermodynamics are lacking, primarily because carbon aerogels often used as supports provide inadequate control over size and pore chemistry. Here, we employ the nanoporous metal-organic framework (MOF) Cu-BTC (also known as HKUST-1) as a template to synthesize and confine the complex hydride NaAlH(4). The well-defined crystalline structure and monodisperse pore dimensions of this MOF allow detailed, quantitative probing of the thermodynamics and kinetics of H(2) desorption from 1-nm NaAlH(4) clusters (NaAlH(4)@Cu-BTC) without the ambiguity associated with amorphous templates. Hydrogen evolution rates were measured as a function of time and temperature using the Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry method, in which sample mass changes are correlated with a complete analysis of evolved gases. NaAlH(4)@Cu-BTC undergoes a single-step dehydrogenation reaction in which the Na(3)AlH(6) intermediate formed during decomposition of the bulk hydride is not observed. Comparison of the thermodynamically controlled quasi-equilibrium reaction pathways in the bulk and nanoscale materials shows that the nanoclusters are slightly stabilized by confinement, having an H(2) desorption enthalpy that is 7 kJ (mol H(2))(-1) higher than the bulk material. In addition, the activation energy for desorption is only 53 kJ (mol H(2))(-1), more than 60 kJ (mol H(2))(-1) lower than the bulk. When combined with first-principles calculations of cluster thermodynamics, these data suggest that although interactions with the pore walls play a role in stabilizing these particles, size exerts the greater influence on the thermodynamics and reaction rates.     

A Density Functional Theory Study on the Effect of Ge Alloying on Hydrogen Desorption from SiGe Alloy Surfaces

We have used density functional theory to investigate hydrogen desorption from SiGe alloy surfaces, and the effect of Ge alloying on the kinetics of hydrogen desorption via the prepairing and interdimer mechanisms. We find that the calculated activation barriers of the prepairing mechanism are affected by the surface atom bonded to the desorbing hydrogen atoms. On the other hand, our calculations show that the activation barrier for hydrogen desorption via the 2H interdimer mechanism is affected by all four surface atoms of the two neighboring dimers. For the 4H interdimer mechanism, we have shown that the activation barrier for hydrogen desorption is not significantly higher than the endothermicity of hydrogen desorption. We also find that the calculated activation barriers of the interdimer mechanisms are generally lower than those of the prepairing mechanism. In addition, our calculations show that surface Ge atoms on neighboring dimers on SiGe alloy surfaces have a minor effect on the calculated activation barriers of both the prepairing and interdimer mechanisms, which indicates that the effect of Ge alloying on hydrogen desorption is local in nature. We also discuss the effects of cluster size and constraints on the calculated reaction energies and activation barriers of hydrogen desorption via the two mechanisms.     

First-principles study of Ti-catalyzed hydrogen chemisorption on an Al surface: a critical first step for reversible hydrogen storage in NaAlH4

Complex metal hydrides are perhaps the most promising hydrogen storage materials for a gradual transformation to a hydrogen-based economy. We have used a computational approach to aid the ongoing experimental effort to understand the reversible hydrogen storage in Ti-doped NaAlH(4) and propose a plausible first step in the rehydrogenation mechanism. The study provides insight into the catalytic role played by the Ti atoms on an Al surface in the chemisorption of molecular hydrogen and identifies the local arrangement of the Ti atoms responsible for the process. Our results can potentially lead to ways of making other similar metal hydrides reversible.     

Fast H-vacancy dynamics during alanate decomposition by anelastic spectroscopy. proposition of a model for Ti-enhanced hydrogen transport

A systematic study of the dehydrogenation process of undoped and of catalyzed NaAlH4 by means of anelastic spectroscopy is presented. Evidence is reported of the formation of a highly mobile species during decomposition, which has been identified in off-stoichiometric AlH6-x units, giving rise to fast H vacancy local dynamics. The formation of such stoichiometry defects starts at temperatures much lower in Ti doped than in undoped samples, and concomitantly with the decomposition reaction. The catalyst atoms decrease the energy barrier to be overcome by H to break the bond, thus enhancing the kinetics of the chemical reactions and decreasing the temperature at which the dehydrogenation processes take place. The experimental data show that not all the hydrogen released by the formula units during the evolution of decomposition evolves out of the sample, but part of it remains in the lattice and migrates on a long-range scale within the sample. We identify, in this H mobilized population, the species which induces the fast tetragonal to monoclinic phase transformation accompanying decomposition. A partial spontaneous thermally activated regression of decomposition has also been observed by aging experiments. A model is proposed which accounts for the action of the Ti catalyst and for the atomistic mechanism of decomposition.     

Hydrogen storage in LiAlH4: predictions of the crystal structures and reaction mechanisms of intermediate phases from quantum mechanics

We use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH(4). First, we explore the decomposition mechanism of monoclinic LiAlH(4) into monoclinic Li(3)AlH(6) plus face-centered cubic (fcc) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH(4) and Li(3)AlH(6) agree well with experimental data. On the other hand, the alternative decomposition of LiAlH(4) into LiAlH(2) plus H(2) is predicted to be unstable with respect to that through Li(3)AlH(6). Next, we investigate thermal decomposition of Li(3)AlH(6) into fcc LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH(6) to form the stable Li-H-Li-H complex. Then, two sequential H(2) desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li(3)AlH(6)(monoclinic)-->3 LiH(fcc)+Al(fcc)+3/2 H(2) is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s)+Al(s)-->LiAl(s)+12H(2)(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl.     

Raman and inelastic neutron scattering study on a melt-infiltrated composite of NaAlH4 and nanoporous carbon

Inelastic neutron scattering and Raman scattering spectra of a melt-infiltrated composite of NaAlH(4) and active carbon fibers have been measured at low temperature for two sample conditions: as prepared and subjected to hydrogen desorption-absorption cycling. After a careful data analysis, the present experimental results have been compared to the corresponding spectroscopic data taken from bulk NaAlH(4) and Na(3)AlH(6). Evident signatures induced by infiltration process onto the NaAlH(4) phonon bands have been detected, showing up as a strong peak broadening and smoothing together with, in some cases, an energy shift. Traces of Na(3)AlH(6), appearing as an extra intensity between 130 and 200 meV, seem also confirmed. A substantial agreement between neutron and Raman measurements has been found for the as-prepared melt-infiltrated sample, while for the cycled sample the two techniques produced rather dissimilar results. However, this apparent discrepancy can be explained by considering the different penetration depths of the two spectroscopic probes. Further work, both experimental and based on ab initio simulations, is surely needed in order to rationalize the finding of the present measurements.     

The structure of magnesium alanate

Mg(AlH(4))(2) was produced as a nanocrystalline powder by metathesis of NaAlH(4) and MgCl(2). Starting with a structure estimation which was developed from an evaluation of FTIR data and comparison of structural properties of two solvent adducts, quantum chemical calculations were performed on the density functional theory (DFT) level. The calculated atomic positions were used to simulate an X-ray powder diffraction pattern, based on a trigonal unit cell. The simulated pattern was congruent to experimental data. Thus, magnesium alanate exhibits a CdI(2) layer structure, the layers being formed by Mg atoms occupying the Cd sites and AlH(4) tedrahedra occupying the sites of the iodine atoms in CdI(2).     

A novel catalyst precursor K2TiF6 with remarkable synergetic effects of K, Ti and F together on reversible hydrogen storage of NaAlH4

A synergetic effect of K, Ti and F together on improving the reversible hydrogen storage properties of NaAlH(4) is found by intruding K(2)TiF(6) as catalyst precursor. Around 4.4 wt% of hydrogen can be released from the NaAlH(4)-0.025 K(2)TiF(6) sample within 40 min at 140 °C.