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. The neutron diffraction background was shown to be related to the extent of the ammonia decomposition and therefore the gas composition. From this and the sample mass, implications about the rate-limiting steps for nitriding by ammonia and nitriding by nitrogen are discussed

Rapid synthesis of BiOBrxI1-x photocatalysts : insights to the visible-light photocatalytic activity and strong deviation from Vegard’s Law

This work was supported by the Royal Society for international collaboration grants (IE160277 and IE/CNSFC170670) and Sir John Houghton Fellowship in Jesus College at University of Oxford. ZJ appreciated the institutional GCRF fund from EPSRC and JG appreciates the EUSTICE scholarship from University of Southampton.A series of visible-light-responsive BiOBrxI1-x solid solutions were prepared by a rapid and efficient ultrasonication synthesis and applied in photodegradation of Rhodamine B in aqueous solution. The detailed characterisations showed that the lattice parameters and their band structures of the BiOBrxI1-x solid solutions significantly deviated from the well-established Vegard’s law for solid solution materials. The Mulliken electronegativity and valence band XPS analyses revealed that the substitution of Br by less electronegative iodine can simultaneously modulate the edges of conductance and valence band of the BiOBr, leading to nonlinear dependence of bandgap (Eg) on the halogen anion concentrations. Although the solid solution displayed superior RhB photodegration activity to BiOI, only Br-rich BiOBrxI1-x solid solutions (x>0.5) were more active than BiOBr and BiOI, with the optimal one is BiOBr0.75I0.25. The Br-dependence of bandstructure and photocatalytic activity for the BiOBrxI1-x solid solutions as well as their rate-limiting radical species were also clarified based on experimental and theoretical analyses.PostprintPeer reviewe

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. The basis of this improved recovery is that the catalyst is, at least partially, solid across the temperature range studied under ammonia flow. However, lithium–calcium imide itself is only stable at low and high temperatures under ammonia, with in situ powder diffraction showing the decomposition of the catalyst to lithium amide–imide and calcium imide at intermediate temperatures of 200–460 °C.</p

Spin-ice physics in cadmium cyanide

Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1–5 K. Here, we show that non-magnetic cadmium cyanide, Cd(CN)2, exhibits analogous behaviour to magnetic spin-ices, but does so on a temperature scale that is nearly two orders of magnitude greater. The electric dipole moments of cyanide ions in Cd(CN)2 assume the role of magnetic pseudospins, with the difference in energy scale reflecting the increased strength of electric vs magnetic dipolar interactions. As a result, spin-ice physics influences the structural behaviour of Cd(CN)2 even at room temperature.ISSN:2041-172

2023 Roadmap on ammonia as a carbon-free fuel

The 15 short chapters that form this 2023 ammonia-for-energy roadmap provide a comprehensive assessment of the current worldwide ammonia landscape and the future opportunities and associated challenges facing the use of ammonia, not only in the part that it can play in terms of the future displacement of fossil-fuel reserves towards massive, long-term, carbon-free energy storage and heat and power provision, but also in its broader holistic impacts that touch all three components of the future global food-water-energy nexus

Materials for hydrogen-based energy storage - past, recent progress and future outlook

Globally, the accelerating use of renewable energy sources, enabled by increased efficiencies and reduced costs, and driven by the need to mitigate the effects of climate change, has significantly increased research in the areas of renewable energy production, storage, distribution and end-use. Central to this discussion is the use of hydrogen, as a clean, efficient energy vector for energy storage. This review, by experts of Task 32, “Hydrogen-based Energy Storage” of the International Energy Agency, Hydrogen TCP, reports on the development over the last 6 years of hydrogen storage materials, methods and techniques, including electrochemical and thermal storage systems. An overview is given on the background to the various methods, the current state of development and the future prospects. The following areas are covered; porous materials, liquid hydrogen carriers, complex hydrides, intermetallic hydrides, electrochemical storage of energy, thermal energy storage, hydrogen energy systems and an outlook is presented for future prospects and research on hydrogen-based energy storage

Light metal amides for hydrogen storage and ammonia decomposition

Hydrogen has long been touted as an alternative fuel which could form the basis of a sustainable energy system: the hydrogen economy. This thesis advances the application of light metal amide materials in the realisation of this transformative potential. One of the most vexing technical challenges to the widespread adoption of hydrogen in transportation applications is its low volumetric energy density, which makes the storage of a sufficient amount of hydrogen in a vehicle very difficult. In their conventional application, light metal amides (M(NH2)x),where M is a Group I or II metal) have been promoted as a means of storing large quantities hydrogen in the solid state, significantly increasing this energy density. This thesis highlights the impressive characteristics of amide-based materials, primarily the facile nature of the reversibility of the hydrogen storage reaction, as a model for the development and optimisation of solid-state hydrogen stores. The study of the relationship between the crystal structures of the relevant materials and their hydrogen storage properties through in situ X-ray and neutron powder diffraction measurements is reported for the lithium amide - lithium hydride (Li-N-H) hydrogen store. These investigations provide strong evidence for ionic mobility as the basis of reversible hydrogen storage in the Li-N-H system. The hydrogen storage and release reactions are seen to progress through a continuum of non-stoichiometric states, a transformation which is facilitated by its topotactic nature. The structural and energetic properties of these non-stoichiometric phases are reported, showing that they are intrinsically disordered and thermodynamically unstable relative to their parent structures. The study of the behaviour of the Li-N-H system is extended to many tens of hydrogenation-dehydrogenation cycles to examine practical performance, confirming the mechanism of capacity loss through the formation of parasitic lithium hydride, and showing that the addition of nitrogen improves the cycling lifetime of the system. An unexplored aspect of light metal amide chemistry is also presented, where the hydrogen storage and release reactions of sodium amide are performed simultaneously. Together, these reactions effect the chemical decomposition of ammonia. Ammonia is a high energy density liquid hydrogen carrier which has been largely overlooked, partly due to the difficulty extracting its stored hydrogen. This work demonstrates a new method of ammonia decomposition which gives comparable performance to the expensive rare-metal catalysts which are currently used for the productions of high-purity hydrogen. A survey of the ammonia decomposition efficiency of a number of light metal amides and imides is presented, showing that it is not only amides which decompose into their constituent elements (such as sodium amide) which are active in ammonia decomposition, but also imide-forming amides. Indeed, imides and imide-forming amides are shown to be advantageous from the perspective of containing the catalyst material. Neutron diffraction and isotope exchange measurements provide some initial insights into the mechanism of reaction, identifying clear avenues for development of these systems, and inviting further discussion of the potential of ammonia as a sustainable energy vector.</p

Light metal amides for hydrogen storage and ammonia decomposition

Hydrogen has long been touted as an alternative fuel which could form the basis of a sustainable energy system: the hydrogen economy. This thesis advances the application of light metal amide materials in the realisation of this transformative potential. One of the most vexing technical challenges to the widespread adoption of hydrogen in transportation applications is its low volumetric energy density, which makes the storage of a sufficient amount of hydrogen in a vehicle very difficult. In their conventional application, light metal amides (M(NH2)x),where M is a Group I or II metal) have been promoted as a means of storing large quantities hydrogen in the solid state, significantly increasing this energy density. This thesis highlights the impressive characteristics of amide-based materials, primarily the facile nature of the reversibility of the hydrogen storage reaction, as a model for the development and optimisation of solid-state hydrogen stores. The study of the relationship between the crystal structures of the relevant materials and their hydrogen storage properties through in situ X-ray and neutron powder diffraction measurements is reported for the lithium amide - lithium hydride (Li-N-H) hydrogen store. These investigations provide strong evidence for ionic mobility as the basis of reversible hydrogen storage in the Li-N-H system. The hydrogen storage and release reactions are seen to progress through a continuum of non-stoichiometric states, a transformation which is facilitated by its topotactic nature. The structural and energetic properties of these non-stoichiometric phases are reported, showing that they are intrinsically disordered and thermodynamically unstable relative to their parent structures. The study of the behaviour of the Li-N-H system is extended to many tens of hydrogenation-dehydrogenation cycles to examine practical performance, confirming the mechanism of capacity loss through the formation of parasitic lithium hydride, and showing that the addition of nitrogen improves the cycling lifetime of the system. An unexplored aspect of light metal amide chemistry is also presented, where the hydrogen storage and release reactions of sodium amide are performed simultaneously. Together, these reactions effect the chemical decomposition of ammonia. Ammonia is a high energy density liquid hydrogen carrier which has been largely overlooked, partly due to the difficulty extracting its stored hydrogen. This work demonstrates a new method of ammonia decomposition which gives comparable performance to the expensive rare-metal catalysts which are currently used for the productions of high-purity hydrogen. A survey of the ammonia decomposition efficiency of a number of light metal amides and imides is presented, showing that it is not only amides which decompose into their constituent elements (such as sodium amide) which are active in ammonia decomposition, but also imide-forming amides. Indeed, imides and imide-forming amides are shown to be advantageous from the perspective of containing the catalyst material. Neutron diffraction and isotope exchange measurements provide some initial insights into the mechanism of reaction, identifying clear avenues for development of these systems, and inviting further discussion of the potential of ammonia as a sustainable energy vector.</p

Facilitating green ammonia manufacture under milder conditions:what do heterogeneous catalyst formulations have to offer?

Ammonia production is one of the largest industrial processes, and is currently responsible for over 1.5% of global greenhouse gas emissions. Decarbonising this process, yielding ‘green ammonia’, is critical not only for sustainable fertilizer production, but also to unlocking ammonia's potential as a zero-carbon fuel and hydrogen store. In this perspective, we critically assess the role of cutting-edge heterogeneous catalysts to facilitate milder ammonia synthesis conditions that will help unlock cheaper, smaller-scale, renewables-coupled ammonia production. The highly-optimised performance of catalysts under the high temperatures and pressures of the Haber–Bosch process stands in contrast to the largely mediocre activity levels reported at lower temperatures and pressures. We identify the recent advances in catalyst design that help overcome the sluggish kinetics of nitrogen activation under these conditions and undertake a categorized analysis of improved activity achieved in a range of heterogeneous catalysts. Building on these observations, we develop a ‘catalyst efficiency’ analysis which helps uncover the success of a holistic approach — one that addresses the issues of nitrogen activation, hydrogenation of adsorbed nitrogen species, and engineering of materials to maximize the utilization of active sites — for achieving the elusive combination of high-activity, low-temperature formulations. Furthermore, we present a discussion on the industrial considerations to catalyst development, emphasising the importance of catalyst lifetime in addition to catalyst activity. This assessment is critical to ensuring that high productivities can translate into real advances in commercial ammonia synthesis