Compositional flexibility in Li-N-H materials: implications for ammonia catalysis and hydrogen storage.

Li-N-H materials, particularly lithium amide and lithium imide, have been explored for use in a variety of energy storage applications in recent years. Compositional variation within the parent lithium imide, anti-fluorite crystal structure has been related to both its facile storage of hydrogen and impressive catalytic performance for the decomposition of ammonia. Here, we explore the controlled solid-state synthesis of Li-N-H solid-solution anti-fluorite structures ranging from amide-dominated (Li4/3(NH2)2/3(NH)1/3 or Li1.

Bulk phase behavior of lithium imide-metal nitride ammonia decomposition catalysts.

Lithium imide is a promising new catalyst for the production of hydrogen from ammonia. Its catalytic activity has been reported to be significantly enhanced through its use as a composite with various transition metal nitrides. In this work, two of these composite catalysts (with manganese nitride and iron nitride) were examined using in situ neutron and X-ray powder diffraction experiments in order to explore the bulk phases present during ammonia decomposition.

Neutron diffraction and gravimetric study of the manganese nitriding reaction under ammonia decomposition conditions.

Manganese and its nitrides have recently been shown to co-catalyse the ammonia decomposition reaction. The nitriding reaction of manganese under ammonia decomposition conditions is studied in situ simultaneously by thermogravimetric analysis and neutron diffraction. Combining these complementary measurements has yielded information on the rate of manganese nitriding as well as the elucidation of a gamut of different manganese nitride phases.

Neutron diffraction and gravimetric study of the iron nitriding reaction under ammonia decomposition conditions.

Ammonia decomposition over iron catalysts is known to be affected by whether the iron exists in elemental form or as a nitride. In situ neutron diffraction studies with simultaneous gravimetric analysis were performed on the nitriding and denitriding reactions of iron under ammonia decomposition conditions. The gravimetric analysis agrees well with the Rietveld analysis of the neutron diffraction data, both of which confirm that the form of the iron catalyst is strongly dependent on ammonia decomposition conditions.

Isotopic studies of the ammonia decomposition reaction using lithium imide catalyst.

Ammonia decomposition using N labelled ammonia was performed over a lithium imide catalyst with mass spectrometry. The results show that all the nitrogen is released from the bulk of the lithium imide catalyst during the ammonia decomposition reaction, but that the decomposition itself occurs at the catalyst surface; they also indicate that lithium imide decomposes ammonia and does not merely act as a promoter to transition metal catalysts.

Observation of Binding and Rotation of Methane and Hydrogen within a Functional Metal-Organic Framework.

The key requirement for a portable store of natural gas is to maximize the amount of gas within the smallest possible space. The packing of methane (CH4) in a given storage medium at the highest possible density is, therefore, a highly desirable but challenging target. We report a microporous hydroxyl-decorated material, MFM-300(In) (MFM = Manchester Framework Material, replacing the NOTT designation), which displays a high volumetric uptake of 202 v/v at 298 K and 35 bar for CH4 and 488 v/v at 77 K and 20 bar for H2.

Ammonia decomposition catalysis using lithium-calcium imide.

Lithium-calcium imide is explored as a catalyst for the decomposition of ammonia. It shows the highest ammonia decomposition activity yet reported for a pure light metal amide or imide, comparable to lithium imide-amide at high temperature, with superior conversion observed at lower temperatures. Importantly, the post-reaction mass recovery of lithium-calcium imide is almost complete, indicating that it may be easier to contain than the other amide-imide catalysts reported to date.

Dimer-mediated cation diffusion in the stoichiometric ionic conductor Li3N.

Non-equilibrium molecular dynamics has been used to model cation diffusion in stoichiometric Li3N over the temperature range 50 < T/K < 800. The resulting diffusion coefficients are in excellent agreement with the available experimental data. We present a detailed atomistic account of the diffusion process.

Isotopic studies of the ammonia decomposition reaction mediated by sodium amide.

We demonstrate that the ammonia decomposition reaction catalysed by sodium amide proceeds under a different mechanism to ammonia decomposition over transition metal catalysts. Isotopic variants of ammonia and sodium amide reveal a significant kinetic isotope effect in contrast to the nickel-catalysed reaction where there is no such effect. The bulk composition of the catalyst is also shown to affect the kinetics of the ammonia decomposition reaction.

Diffusion in Liâ‚‚O studied by non-equilibrium molecular dynamics for 873 < T/K < 1603.

The use of non-equilibrium molecular dynamics facilitates the calculation of the cation diffusion constant of Li2O at temperatures too low to be accessible by other methods. Excellent agreement with experimental diffusion coefficients has been obtained over the temperature range 873 < T/K < 1603. Diffusion below 1200 K was shown to be dominated by a concerted nearest-neighbour hopping process, whereas in the high-temperature superionic region an additional mechanism involving a six-coordinate interstitial cation site in the anti-fluorite structure becomes increasingly dominant.

Ammonia decomposition catalysis using non-stoichiometric lithium imide.

We demonstrate that non-stoichiometric lithium imide is a highly active catalyst for the production of high-purity hydrogen from ammonia, with superior ammonia decomposition activity to a number of other catalyst materials. Neutron powder diffraction measurements reveal that the catalyst deviates from pure imide stoichiometry under ammonia flow, with active catalytic behaviour observed across a range of stoichiometry values near the imide. These measurements also show that hydrogen from the ammonia is exchanged with, and incorporated into, the bulk catalyst material, in a significant departure from existing ammonia decomposition catalysts.

Understanding composition-property relationships in Ti-Cr-V-Mo alloys for optimisation of hydrogen storage in pressurised tanks.

The location of hydrogen within Ti-Cr-V-Mo alloys has been investigated during hydrogen absorption and desorption using in situ neutron powder diffraction and inelastic neutron scattering. Neutron powder diffraction identifies a low hydrogen equilibration pressure body-centred tetragonal phase that undergoes a martensitic phase transition to a face-centred cubic phase at high hydrogen equilibration pressures. The average location of the hydrogen in each phase has been identified from the neutron powder diffraction data although inelastic neutron scattering combined with density functional theory calculations show that the local structure is more complex than it appears from the average structure.

Hydrogen production from ammonia using sodium amide.

This paper presents a new type of process for the cracking of ammonia (NH3) that is an alternative to the use of rare or transition metal catalysts. Effecting the decomposition of NH3 using the concurrent stoichiometric decomposition and regeneration of sodium amide (NaNH2) via sodium metal (Na), this represents a significant departure in reaction mechanism compared with traditional surface catalysts. In variable-temperature NH3 decomposition experiments, using a simple flow reactor, the Na/NaNH2 system shows superior performance to supported nickel and ruthenium catalysts, reaching 99.

In situ X-ray powder diffraction studies of hydrogen storage and release in the Li-N-H system.

We report the experimental investigation of hydrogen storage and release in the lithium amide-lithium hydride composite (Li-N-H) system. Investigation of hydrogenation and dehydrogenation reactions of the system through in situ synchrotron X-ray powder diffraction experiments allowed for the observation of the formation and evolution of non-stoichiometric intermediate species of the form Li1+xNH2-x. This result is consistent with the proposed Frenkel-defect mechanism for these reactions.

Molecular dynamics investigation of the disordered crystal structure of hexagonal LiBH4.

The crystal structure of the hexagonal phase of solid lithium borohydride (LiBH4) is studied by ab initio molecular dynamics simulations of both the low and high-temperature phases. A temperature range of 200-535 K is simulated with the aim of characterising the disorder in the high-temperature structure in detail. The mechanism and kinetics of the reorientational motion of the borohydride units (BH4(-)) are determined and are consistent with published neutron scattering experiments; it is found that rotational diffusivity increases by an order of magnitude at the phase transition temperature.

Selectivity and direct visualization of carbon dioxide and sulfur dioxide in a decorated porous host.

Understanding the mechanism by which porous solids trap harmful gases such as CO(2) and SO(2) is essential for the design of new materials for their selective removal. Materials functionalized with amine groups dominate this field, largely because of their potential to form carbamates through H(2)N(δ(-))···C(δ(+))O(2) interactions, thereby trapping CO(2) covalently. However, the use of these materials is energy-intensive, with significant environmental impact.

Ab initio nonequilibrium molecular dynamics in the solid superionic conductor LiBH4.

The color-diffusion algorithm is applied to ab initio molecular dynamics simulation of hexagonal LiBH(4) to determine the lithium diffusion coefficient and diffusion mechanisms. Even in the best solid lithium ion conductors, the time scale of ion diffusion is too long to be readily accessible by ab initio molecular dynamics at a reasonable computational cost. In our nonequilibrium method, rare events are accelerated by the application of an artificial external field acting on the mobile species; the system response to this perturbation is accurately described in the framework of linear response theory and is directly related to the diffusion coefficient, thus resulting in a controllable approximation.

Nature of the band gap and origin of the conductivity of PbO2 revealed by theory and experiment.

Lead dioxide has been used for over a century in the lead-acid battery. Many fundamental questions concerning PbO2 remain unanswered, principally: (i) is the bulk material a metal or a semiconductor, and (ii) what is the source of the high levels of conductivity? We calculate the electronic structure and defect physics of PbO2, using a hybrid density functional, and show that it is an n-type semiconductor with a small indirect band gap of ∼0.2  eV.

Two new cluster ions, Ga[GaH3]4(5-) with a neopentane structure in Rb8Ga5H15 and [GaH2]n(n-) with a polyethylene structure in Rb(n)(GaH2)n, represent a new class of compounds with direct Ga-Ga bonds mimicking common hydrocarbons.

The first examples of a new class of gallium hydride clusters with direct Ga-Ga bonds and common hydrocarbon structures are reported. Neutron powder diffraction was used to find a Ga[GaH(3)](4)(5-) cluster ion with a neopentane structure in a novel cubic structure type of Rb(8)Ga(5)H(15). Another cluster ion with a polyethylene structure, [GaH(2)](n)(n-), was found in a second novel (RbGaH(2))(n) hydride.

A combined experimental inelastic neutron scattering, Raman and ab initio lattice dynamics study of α-lithium amidoborane.

A combination of inelastic neutron scattering (INS) spectroscopy and Raman spectroscopy with periodic density functional theory calculations is used to provide a complete assignment of the vibrational spectra of α-lithium amidoborane (α-LiNH(2)BH(3)). The Born charge density and the atomic motion up to the decomposition temperature have been modelled. These models not only explain the nature of bonding in α-LiNH(2)BH(3) but also provide an insight into the atomic mechanisms of its decomposition.

Effective hydrogen storage: a strategic chemistry challenge.

This paper gives an overview of the current status and future potential of hydrogen storage from a chemistry perspective and is based on the concluding presentation of the Faraday Discussion 151--Hydrogen Storage Materials. The safe, effective and economical storage of hydrogen is one of the main scientific and technological challenges in the move towards a low-carbon economy. One key sector is transportation where future vehicles will most likely be developed around a balance of battery-electric and hydrogen fuel-cell electric technologies.

Pore with gate: modulating hydrogen storage in metal-organic framework materials via cation exchange.

A range of anionic metal-organic framework (MOF) materials has been prepared by combination of In(III) with tetracarboxylate isophthalate-based ligands. These materials incorporate organic cations, either H2ppz2+ (ppz = piperazine) or Me2NH2+, that are hydrogen bonded to the pore wall. These cations act as a gate controlling entry of N2 and H2 gas into and out of the porous host.

Diffusion-equation method for crystallographic figure of merits.

Global optimization methods play a significant role in crystallography, particularly in structure solution from powder diffraction data. This paper presents the mathematical foundations for a diffusion-equation-based optimization method. The diffusion equation is best known for describing how heat propagates in matter.

Potassium(I) amidotrihydroborate: structure and hydrogen release.

Potassium(I) amidotrihydroborate (KNH(2)BH(3)) is a newly developed potential hydrogen storage material representing a completely different structural motif within the alkali metal amidotrihydroborate group. Evolution of 6.5 wt % hydrogen starting at temperatures as low as 80 degrees C is observed and shows a significant change in the hydrogen release profile, as compared to the corresponding lithium and sodium compounds.

Stepwise phase transition in the formation of lithium amidoborane.

A stepwise phase transition in the formation of lithium amidoborane via the solid-state reaction of lithium hydride and ammonia borane has been identified and investigated. Structural analyses reveal that a lithium amidoborane-ammonia borane complex (LiNH(2)BH(3).NH(3)BH(3)) and two allotropes of lithium amidoborane (denoted as alpha- and beta-LiNH(2)BH(3), both of which adopt orthorhombic symmetry) were formed in the process of synthesis.