Synchrotron X-ray studies of Ti-doped NaAlH4

Pure and doped NaAlH(4) with 5 mol % Ti on the basis of Ti(13).6THF have been investigated by means of X-ray synchrotron radiation. The Rietveld method has been used to study the possible substitution of Ti inside the NaAlH(4) structure and/or the presence of vacancies. This study indicates that there is no significant variation of the lattice parameter once the Na Alanate is doped with the Ti cluster. From the refinement of the site occupation factors, the substitution of Ti on the Na site can be excluded. A slight improvement was found when Ti was substituted on the Al site, but it is not significant enough to say that Ti definitely substitutes for Al in the Alanate phase. Additionally, there is no evidence for vacancy formation in Ti-colloid-doped sodium Alanate.     

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.     

Towards a structure-performance relationship for hydrogen storage in Ti-doped NaAlH4 nanoparticles

Hydrogen storage properties of Ti-doped nanosized (~20 nm) NaAlH(4) supported on carbon nanofibers were affected by the stage at which Ti was introduced. When Ti was deposited first followed by NaAlH(4), sorption properties were superior to the case where NaAlH(4) was deposited first followed by NaAlH(4). This was the result of both a smaller NaAlH(4) particle size and the more extensive catalytic action of Ti in the former material.     

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-).     

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.     

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.     

On the nature of active species in cationic ring-opening polymerization of cyclodimethylsiloxanes

Contrarily to cationic ring-opening polymerization of cyclic ethers and of some other cyclic monomers, for which direct identification of the various types of active centres has been made in a few cases, the nature of the species active in the polymerization of cyclo-dimethylsiloxanes is not yet known. However, some provisional conclusions about the possible mechanisms may be deduced from the wide variation in the types of products and in the kinetics observed according to either the size of the cyclic monomer (D₃, D₄, D₅, D₆) and to the type of initiation (chemically, or radiation induced). For polymerizations with either protonic or non-protonic initiators, made in CH₂Cl₂ near room temperature, the smaller cycle D₃ behaves quite differently from D₄, D₅ and D₆. D₃ is more reactive in both homo- and copolymerizations. It gives small cycles of other types and the effect of water on the reaction may be quite different. A discussion of the data leads to the conclusion that polymer growth for most cyclosiloxanes involves activated esters, while it may occur for D₃ on different sites such as oxonium or silanol groups. Polymerization of D₃, D₄ and D₅ initiated in bulk at 90°C by high energy radiation, in high purity conditions, has also been shown to be cationic but the active centres concentration is much lower, and the propagation rate constants much higher, than in chemically initiated polymerizations. The global rates, the monomer reactivities in copolymerization and the types of cycles are similar for D₃, D₄ and D₅, which is attributed to propagation occurring on very reactive silicenium ions, either free or in the same solvation state.     

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.     

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.     

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.     

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.     

The metallic orbital and the nature of metals

In 1938 it was noticed (L. Pauling, Phys. Rev.54, 899, 1938) that about 0.72 of the nine outer spd orbitals per atom of a transition metal remain unoccupied by bonding electrons, unpaired ferromagnetic electrons, or unshared electron pairs. In 1948 this 0.72 orbital per atom was identified (L. Pauling, Nature (London)161, 1019, 1948; Proc. Roy. Soc. A196, 343, 1949) as required for the unsynchronized resonance that confers metallic properties on a substance, and it was named the metallic orbital. A statistical theory of unsynchronized resonance of covalent bonds in a metal with atoms restricted by the electroneutrality principle to forming bonds only in number v ? 1, v, and v + 1, with v the metallic valence, has now been developed. This theory leads directly to the value 0.70 ± 0.02 for the number of metallic orbitals per atom, in reasonable agreement with the empirical value, and to the conclusion that M⁺, M⁰, and M^? occur in the ratios near 28:44:28. It leads also to the conclusions that stability of a metal or alloy increases with increase in the ligancy and for a given value of the ligancy is a maximum for valence equal to half the ligancy. These results with consideration of the repulsion of unshared electron pairs on adjacent atoms go far toward explaining the selection of different structures by different elemental metals and intermetallic compounds.     

The nature of the active species in bis(imino)pyridyl cobalt ethylene polymerisation catalysts

Studies on cobalt ethylene polymerisation catalysts bearing bis(imino)pyridine ligands strongly indicate that the activated species is not the anticipated cobalt(II) alkyl cation.     

The nature of paramagnetic species in nitrogen doped TiO2 active in visible light photocatalysis

Nitrogen doped TiO2, a novel photocatalyst active in the decomposition of organic pollutants using visible light, contains two different types of paramagnetic centres (neutral NO radicals and NO2(2-) type radical ions respectively) which are likely related to specific properties of the solid.