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.     

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.     

Quantitative evaluation of catalytic effect of metal chlorides on the decomposition reaction of NaAlH4

The hydrogen desorption (or decomposition) reaction of NaAlH₄ is expressed as , and its desorption rate is accelerated by mixing metal chloride catalysts (e.g., TiCl₃). This catalytic effect of metal chlorides, MCl_n, is theoretically estimated in a quantitative way using atomization energy concept. The atomization energies, ΔE_M for metal ion and ΔE_Cl for chloride ion in various metal chlorides are evaluated using the energy density analysis of the total energy. It is shown that the hydrogen desorption reaction rate increases with increasing n × ΔE_M values of metal chlorides. This indicates that the metal ion in MCl_n interacts mainly with hydrogen or [AlH₄]^? complex anion in NaAlH₄. To confirm this calculated result, experiments are performed using NaAlH₄ mixed with Ti‐based catalysts. The hydrogen desorption rate is enhanced in the order, TiCl₃ > TiO₂ > Ti metal nanopowder, indicating that the Ti ions in TiCl₃ or TiO₂ work to promote the catalytic reaction more effectively than the neutral Ti atoms in Ti metal nanopowder. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

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.     

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.     

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.     

The influence of chemical structure of sulfonamides on the course of their thermal decomposition

The thermal decomposition of several sulfonamides and potassium salts of sulfonamides was investigated. The analyses were performed using a derivatograph in an air atmosphere, sample sizes were from 50 to 200 mg and heating rate from 2.5 to 20 K min^-1. It has been established, that the thermal destruction of studied compounds occurs via three stages with formation of potassium carbonate as a final product of the complete combustion of potassium salts of sulfonamides. The temperature ranges, in which the analyzed compounds undergo thermal transformations were established. For evaluation of the results the principal component analysis (PCA) was applied. By this method the influence of the specific functional groups on the thermal decomposition of sulfonamides and potassium salts of sulfonamides was determined. It has also been recognized, that better discrimination among the analyzed compounds is obtained for the data set of the DTA. This revised version was published online in July 2006 with corrections to the Cover Date.     

Energetics and mechanism of the decomposition of trifluoromethanol

The thermal instability of alpha-fluoroalcohols is generally attributed to a unimolecular 1,2-elimination of HF, but the barrier to intramolecular HF elimination from CF3OH is predicted to be 45.1 +/- 2 kcal/mol. The thermochemical parameters of trifluoromethanol were calculated using coupled-cluster theory (CCSD(T)) extrapolated to the complete basis set limit. High barriers of 42.9, 43.1, and 45.0 kcal/mol were predicted for the unimolecular decompositions of CH2FOH, CHF2OH, and CF3OH, respectively. These barriers are lowered substantially if cyclic H-bonded dimers of CF3OH with complexation energies of approximately 5 kcal/mol are involved. A six-membered ring dimer has an energy barrier of 28.7 kcal/mol and an eight-membered dimer has an energy barrier of 32.9 kcal/mol. Complexes of CF3OH with HF lead to strong H-bonded dimers, trimers and tetramers with complexation energies of approximately 6, 11, and 16 kcal/mol, respectively. The dimer, CH3OH:HF, and the trimers, CF3OH:2HF and (CH3OH)2:HF, have decomposition energy barriers of 26.7, 20.3, and 22.8 kcal/mol, respectively. The tetramer (CH3OH:HF)2 gives rise to elimination of two HF molecules with a barrier of 32.5 kcal/mol. Either CF3OH or HF can act as catalysts for HF-elimination via an H-transfer relay. Because HF is one of the decomposition products, the decomposition reactions become autocatalytic. If the energies due to complexation for the CF3OH-HF adducts are not dissipated, the effective barriers to HF elimination are lowered from approximately 20 to approximately 9 kcal/mol, which reconciles the computational results with the experimentally observed stabilities.     

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.     

Synergetic effects of NaAlH4-TiF3 co-additive on dehydriding reaction of Mg(AlH4)2

The effects of Na Al H4, Ti F3 and Na Al H4-Ti F3co-additive on dehydriding reaction of Mg(Al H4)2are systematically investigated. The onset dehydrogenation temperature of the co-doped Mg(Al H4)2composites decreased to 74°C, which is about 59°C lower than that of pure Mg(Al H4)2. The dehydrogenation kinetics of Na Al H4-Ti F3co-doped Mg(Al H4)2sample was also improved, which released about 94%hydrogen within 48 min, but no visible hydrogen was released from pure Mg(Al H4)2under the same conditions. The activation energy of co-doped Mg(Al H4)2was 85.6 k J mol-1, which was significantly lower than that of additive-free Mg(Al H4)2sample. The synergetic effects of Na Al H4 and Ti F3 on the dehydrogenation performance of Mg(Al H4)2were confirmed. In addition, a possible catalytic mechanism is discussed, regarding the different roles of Na Al H4 and Ti F3 on Mg(Al H4)2.     

The influence of defects on the morphology of Si (111) etched in NH4F

We have implemented a kinetic Monte Carlo (KMC) simulation to study the effects of wafer miscut and wafer defects on the morphologies of Si (111) surfaces etched in NH4F. Although a conventional KMC simulation reproduced previously published results, it failed to produce the morphologies observed in our experiments. By introducing both dopant sites and lattice defect sites into the model, we are able to simulate samples having different dopant elements and densities as well as different defect concentrations. Using the modified KMC simulation, the simulated surface morphologies agree well with the morphologies observed in our experiments. The enhanced model also gives insights to the formation mechanism for multiple level stacking pits, a notable morphology on the etched surfaces of samples with very small miscut angles.     

The influence of solvent structure on the solvolysis oftrans-dichlorotetra (4-ethylpyridine) cobalt(III) perchlorate

Summary The solvolysis oftrans-[Co(4-Etpy)₄Cl₂]ClO₄, was followed spectrophotometrically in water/isopropanol at different temperatures. The activation energy varied nonlinearly with the mole fraction of the co-solvent, ₂. The plot of logk versus D _s ^–1 was also non-linear. These features were attributed to the differential solvation of the initial and transition states. On plotting H versus S, the points fall very close to straight line. The isokinetic temperature was found to be 334K, indicating that the solvolysis reaction is controlled by S and not H. The change in H and S with the mole fraction of the cosolvent shows extrema at the composition range where changes in solvent structure occur. The influence of the solvent structure on the complex ion in the transition state dominates over that in the initial state, where –G _t ⁰ [Co(4-Etpy)₄Cl]²⁺>–G _t ⁰ [Co(4-Etpy)₄Cl₂]⁺.     

Hyperfine structure investigation on the (3d 4)3 G 4,5 levels of47Ti

Using two-step laser spectroscopy with the sideband technique the hyperfine structure (hfs) splittings of the levels (3d ⁴)³ G _4,5 in⁴⁷Ti have been measured. These are the first hfs measurements of the levels belonging to the 3d ^N+2 configuration of the system (3d+4s) ^N+2. The experimental results indicate a strong increase of the core polarization effect due to excitation of both 4s-electrons to the open 3d-shell.     

Ti(AlBr4)2

Ti(AlBr₄)₂ The solid Ti(AlBr₄)₂ is prepared and its crystal structure is determined. The discussion is concerned with the change of the structure of the gaseous compound during its conversion into the solid state.     

Electrospray ionization in the study of the polycondensation of Ti(O-i-C3H7)4 and Ti(O-n-C4H9)4

The polycondensation of Ti(O-i-C3H7)4 (1) and Ti(O-n-C4H9)4 (2), precursors widely employed in sol-gel processes, has been investigated by electrospray ionization mass spectrometry. By analysis of 10(-6) M methanol solutions of compounds 1 and 2, the same ionic species are detected, proving that the first step in the polycondensation reaction is the i-propyl (or n-butyl) alcohol-methanol complete exchange. This reaction leads to the Ti(OCH3)4 (3) species, representing the synthon of the polycondensation. Various oligomers of 3 have been detected and characterized by MS/MS experiments, and the related mechanisms have been discussed. A minor oligomeric series due to hydroxyl-containing polycondensation products has also been characterized.     

The crystal structure and surface energy of NaAlH4: a comparison of DFT methodologies

This paper presents a comparison of the bulk structure, cleavage energies, and local densities of states of solid NaAlH4 calculated using several different density functional theory methodologies. Good agreement is obtained for the bulk crystal structure. Larger differences become apparent for the calculated surface energies and local densities of states. The (001) NaAlH4 surface is clearly identified as the most stable surface, followed by the (112) and (101) surfaces, with the (100) surface being the least stable. We present an analysis of the local density of states of atoms in the exposed NaAlH4 surface.     

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-).     

The influence of the structure and composition of complexes of the type CuA2X2 on the courses of their thermal decomposition

The influence of different degrees of distortion of the coordination polyhedra in some Cu(II) complexes on the courses of their thermal destruction was studied. It was found that differences in the thermal stability and stoichiometry of thermal decomposition may be satisfactorily explained by the different degrees of distortion of the coordination polyhedra in the Cu(II) complexes under discussion. This fact appears to be significant with respect to the chemical reactivities of these complexes, though the influence of other factors cannot be excluded.

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Zusammenfassung Der Einfluß der verschiedenen Grade der Koordinations-Polyeder-Verzerrung in einigen Cu(II) Komplexen auf den Verlauf ihrer thermischen Zersetzung wurde untersucht. Es wurde festgestellt, daß Unterschiede in der thermischen Stabilität und der Stöchiometrie der thermischen Zersetzung durch die verschiedenen Verzerrungsgrade der Koordinations-Polyeder in den erörterten Cu(II) Komplexen befriedigend erklärt werden können. Diese Tatsache scheint hinsichtlich der chemischen Reaktivität dieser Komplexe von grosser Bedeutung zu sein, obwohl der Einfluß anderer Faktoren nicht ausgeschlossen werden kann.

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Résumé On a étudié l'influence des différents degrés de distortion des polyhèdres de coordination sur l'allure de leur décomposition thermique, dans le cas de quelques complexes du Cu(II). On a trouvé que les différences de stabilité thermique et de stoechiométrie pouvaient expliquer de manière satisfaisante les différents degrés de distortion des polyhèdres de coordination dans les complexes du Cu(II) étudiés. Ce fait semble être plutôt significatif de la réactivité chimique de ces complexes, bien que l'influence d'autres facteurs ne doive pas être exclue.

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Cu(II) . , -, Cu(II). , -, , .

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Presented at the 7th Czechoslovak National Conference on Thermal Analysis (TERMANAL); The High Tatras, 1976.     

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.