One-step direct synthesis of a Ti-doped sodium alanate hydrogen storage material

Titanium-doped NaAlH4 which has among the best reported hydrogen adsorption-desorption rates, has been synthesized in a one-step direct hydrogenation process under ball-milling in a special mill, allowing process parameters such as temperature and pressure to be recorded in situ.     

Sodium alanate nanoparticles--linking size to hydrogen storage properties

Important limitations in the application of light metal hydrides for hydrogen storage are slow kinetics and poor reversibility. To alleviate these problems doping and ball-milling are commonly applied, for NaAlH 4 leading to particle sizes down to 150 nm. By wet-chemical synthesis we have prepared carbon nanofiber-supported NaAlH 4 with discrete particle size ranges of 1-10 microm, 19-30 nm, and 2-10 nm. The hydrogen desorption temperatures and activation energies decreased from 186 degrees C and 116 kJ.mol (-1) for the largest particles to 70 degrees C and 58 kJ.mol (-1) for the smallest particles. In addition, decreasing particle sizes lowered the pressures needed for reloading. This reported size-performance correlation for NaAlH 4 may guide hydrogen storage research for a wide range of nanostructured light (metal) hydrides.     

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.     

Thermodynamically tuning LiBH4 by fluorine anion doping for hydrogen storage: A density functional study

The decomposition reaction of LiBH₄ with and without F anion doping was investigated by first-principles calculations. According to the calculation results, doping LiBH₄ with F anion may generate F lattice substitution in both the hydrogenated (LiBH₄) and dehydrogenated (LiH) states of the hydride, and accordingly result in a favorable thermodynamics modification, even to a level suitable for onboard H-storage application. Experimentally evidencing these theoretical predictions may pave a new way to pursue improved hydrogen storage properties of LiBH₄ and other related complex hydrides.     

The Faraday effect for sodium anion in solution

The Faraday effect for sodium dissolved in EDA indicates a magentic moment of about one Bohr magneton for the excited state involved in the 650 nm absorption. Comparison with results for iodide ion suggest a structure for the 650 nm band. A simple connection between ellipticity and derivative of absorption is pointed out and used in analyzing the data.     

Hydrogen-deuterium exchange experiments to probe the decomposition reaction of sodium alanate

NaAlH(4) is the archetypical complex hydride for hydrogen storage. The extraordinary effect of dopants on the sorption kinetics triggered the investigation of this empirical finding. In this paper, a short review of the state of the art is given. To gain further understanding of the mechanisms involved we label the interacting species during the sorption process. This was experimentally realized by hydrogen-deuterium exchange measurements during the decomposition of NaAlH(4) followed by thermogravimetry, Raman spectroscopy and mass spectrometry. By these experiments we are able to obtain specific information on the diffusing species and formation of intermediates. The activation energy of tracer diffusion in NaAlH(4) is found to be 0.28 eV. The results are evidence for a vacancy-mediated desorption process of NaAlH(4).     

Anodic oxidation of olefins in presence of fluorine anion

Russian Chemical Bulletin -     

The replacement of vinylic hydrogen by fluorine

In 3$\text{H}$, 4$\text{H}$-tetrafluoro-3-thiolen and 1$\text{H}$,2$\text{H}$-octafluorocyclohexene, the conversion of -CHCH- into -CHCF- by fluorination over KCoF₄ takes place by saturation to -CHFCHF- followed by dehydrofluorination, and not by direct replacement of a vinylic hydrogen.     

Differentiating between hydrogen and fluorine on a diamond surface

We have computed the interaction energies for pyridine (C₅H₅N) and (CH₃)₃PO with H and F atoms on the surface of diamond(111) as a function of the type of neighboring surface atom. The pyridine-H and -F interaction energies differ by about 5 kcal/mol, which is only about one-third of that found for a one-dimensional model for the surface. The difference in the interaction energies for (CH₃)₃PO is larger. However, the (CH₃)₃PO-neighbor interaction energy is larger than for pyridine, so that the (CH₃)₃PO-H interaction becomes repulsive for six neighboring F atoms. Substituting CN for F dramatically increases the repulsion between the surface atoms and molecules. The repulsion is sufficiently large that H/CN does not appear to be better than H/F as a possible way to store data on a surface. While pyridine shows some potential as a possible probe to differentiate between H and F on a diamond surface, it is not ideal. Received: 7 April 1997 / Accepted: 20 May 1997     

基于绿色氢科学理念构筑从低碳制氢到高效储氢的氢能体系

本文从低碳制氢和高效储氢的角度思考及探讨氢能体系绿色化发展过程中的关键科学问题.提出＂绿色氢科学＂理念与＂低碳制氢,高效储氢＂技术发展路线图,以期通过相关科学问题的认识,来构建具有高度原子经济性及可持续性的绿色氢能体系.     

Clustering of Ti on a C60 surface and its effect on hydrogen storage

Recent efforts in finding materials suitable for storing hydrogen with large gravimetric density have focused attention on carbon-based nanostructures. Unfortunately, pure carbon nanotubes and fullerenes are unsuitable as hydrogen storage materials because of the weak bonding of the hydrogen molecules to the carbon frame. It has been shown very recently that coating of carbon nanostructures with isolated transition metal atoms such as Sc and Ti can increase the binding energy of hydrogen and lead to high storage capacity (up to 8 wt % hydrogen, which is 1.6 times the U.S. Department of Energy target set for 2005). This prediction has led to a great deal of excitement in the fuel cell community [see The Fuel Cell Review, http://fcr.iop.org/articles/features/2/7/4]. However, this prediction depends on the assumption that the metal atoms coated on the fullerene surface will remain isolated. Using first-principles calculations based on density functional theory, we show that Ti atoms would prefer to cluster on the C60 surface, which can significantly alter the nature of hydrogen bonding, thus affecting not only the amount of stored hydrogen but also their thermodynamics and kinetics.     

The mesomeric effect of fluorine

A three-parameter model is used to explain the shifts in π orbital ionization potential of a range of fluorinated molecules. Using results from photoelectron spectroscopy, it appears that the shifts are not completely explicable in terms of inductive effects alone. A mesomeric term, derived from second-order perturbation theory, is found to be comparable in magnitude with inductive effects, contrary to the present theories based on ultraviolet absorption data. Values for some unperturbed π energy levels, at present the subject of discussion, are deduced from the results.     

Macrocyclic anion receptors based on directed hydrogen bonding interactions

Natural anion receptors use charge-neutral dipoles to bind small anions with high affinities and selectivities. A convergent and rigid display of hydrogen bond donors such as amide, thiourea and urea functional groups in macrocyclic scaffolds would be one of the most efficient ways to create synthetic anion receptors that mimic natural ones. In this article, we present examples of natural anion receptors and discuss the synthesis of neutral macrocyclic receptors and their anion binding properties.     

Determination of carbon and hydrogen in organic fluorine compounds

A rapid determination of carbon and hydrogen in organic fluorine compounds is described. Magnesium oxide is used both as an absorbent for fluorine and as an aid to complete combustion. The time required for one determination in series is under 30 min. The method is very widely applicable.     

Supramolecular assistance to regioselectivity in the reactions of chlorocyclophosphazenes with sodium oxyanions: macrocyclic effect and anion dependence

Comparative studies of the naphtholysis of crown-bearing tetrachlorocyclotriphosphazene 2 and its acyclic analogue, nongem-diethoxytetrachlorocyclotriphosphazene 5 (which was synthesized and isolated in the pure cis form for the purpose of this study), reveal a significant macrocyclic effect of the substituent on both the kinetics and regiochemistry of chlorine substitution in the N(3)P(3) ring. Whereas substitution of 5 with sodium naphthoxide provided moderate yields of the respective sterically and electronically favored nongem-naphthoxydiethoxy derivative as the major product, isolated as the mixture of cis and trans isomers 8 + 9, the corresponding reaction of crown substrate 2 resulted in highly regioselective formation of the gem-to-macrocycle substituted mononaphthoxy PNP-crown derivative 11a. The importance of the PNP-crown related effects for the regio- and stereocontrol of the substitution of chlorine atoms in the N(3)P(3) ring is discussed in terms of supramolecular sodium cation-assisted interactions between the ring oxy substituent(s) and incoming oxyanions. To approach the problem of the anion-dependence of the regioselectivity, a comparison has been drawn between the reactions of the PNP-crown substrate 2 with sodium arylates derived from beta-naphthol (10a) and phenol (10b), carried out with equimolar or 100% excess sodium oxyanion with respect to 2, and the corresponding reactions of 2 with sodium monoenolates derived from the beta-dicarbonyl compounds: acetylacetone (10c) and ethyl acetylacetate (10d). The effectiveness of supramolecularly assisted transition state stabilization is found to be dependent not only on the dimensional complementarity between the cation and the macrocycle but also on the properties of the four interacting ionic centers according to the HSAB principle and on the nature of the electronic interaction of the PNP-crown substituent with the N(3)P(3) ring, the latter determining the extent of regiocontrol to either one or two macrocyclic chloride functions. The increase of the extent of regioselectivity of gem-substitution at the PNP-macrocycle when passing from arylate to beta-dicarbonyl enolate oxy anions is related to the observed trends in the (31)P NMR chemical shifts and refers to the electronic structure of the oxy substituent.     

Chemical-clathrate hybrid hydrogen storage: storage in both guest and host

Hydrogen storage from two independent sources of the same material represents a novel approach to the hydrogen storage problem, yielding storage capacities greater than either of the individual constituents. Here we report a novel hydrogen storage scheme in which recoverable hydrogen is stored molecularly within clathrate cavities as well as chemically in the clathrate host material. X-ray diffraction and Raman spectroscopic measurements confirm the formation of beta-hydroquinone (beta-HQ) clathrate with molecular hydrogen. Hydrogen within the beta-HQ clathrate vibrates at considerably lower frequency than hydrogen in the free gaseous phase and rotates nondegenerately with splitting comparable to the rotational constant. Compared with water-based clathrate hydrate phases, the beta-HQ+H2 clathrate shows remarkable stability over a range of p-T conditions. Subsequent to clathrate decomposition, the host HQ was used to directly power a PEM fuel cell. With one H2 molecule per cavity, 0.61 wt % hydrogen may be stored in the beta-HQ clathrate cavities. When this amount is combined with complete dehydrogenation of the host hydroxyl hydrogens, the maximum hydrogen storage capacity increases nearly 300% to 2.43 wt %.     

Lanthanide luminescent anion sensing: evidence of multiple anion recognition through hydrogen bonding and metal ion coordination

The delayed lanthanide luminescence of the terbium [Tb(III)] diaryl-urea complex 1xTb is significantly enhanced upon sensing of dihydrogenphosphate (H2PO4(-)) in CH3CN, which occurs through multiple anion binding through hydrogen bonding interactions and potential metal ion coordination to Tb(III).     

Analyzing excited-state processes and optical signatures of a ratiomeric fluorine anion sensor: a quantum look

Recently, the spectroscopic signatures of a benzoselenadiazole derivative have been investigated in the framework of designing a new ratiometric fluoride sensor(Saravanan et al., Org Lett, 2014, 16: 354–357). It was suggested that this sensor is undergoing excited-state intramolecular proton transfer. In this work, we provide a new look at these experimental data, using a state-of-the-art time-dependent density functional theory approach to mimic the spectroscopic signatures. New insights about the nature of the excited-state processes are obtained.