Mechanism of Hydrogenation Reaction in the Li−Mg−N−H System

Leng, Haiyan; Ichikawa, Takayuki; Hino, Satoshi; Nakagawa, Toshifumi; Fujii, H.

doi:10.1021/jp0504571

Cited by 79 publications

(80 citation statements)

References 23 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…As mentioned above, there are two proposals for the dehydrogenation mechanism. Regardless of which mechanism plays the main role, there will be dissociation of the chemical bonds (e.g., NÀH, MgÀN, or LiÀH) of the reactants and the evolution of intermediates (e.g., gas intermediate NH 3 [17][18][19] or complex solid intermediates [15,20,36] ). Therefore, by activating each component of this composite deliberately and looking at the changes in the reaction kinetics, we may be able to clarify the origin of the kinetic barrier.…”

Section: Resultsmentioning

confidence: 99%

Solid–Solid Heterogeneous Catalysis: The Role of Potassium in Promoting the Dehydrogenation of the Mg(NH₂)₂/2 LiH Composite

et al. 2013

View full text Add to dashboard Cite

Considerable efforts have been devoted to the catalytic modification of hydrogen storage materials. The K‐modified Mg(NH2)2/2 LiH composite is a typical model for such studies. In this work, we analyze the origin of the kinetic barrier in the first step of the dehydrogenation and investigate how K catalyzes this heterogeneous solid‐state reaction. Our results indicate that the interface reaction of Mg(NH2)2 and LiH is the main source of the kinetic barrier at the early stage of the dehydrogenation for the intensively ball‐milled Mg(NH2)2/2 LiH sample. K can effectively activate Mg(NH2)2 as well as promote LiH to participate in the dehydrogenation. Three K species of KH, K2Mg(NH2)4, and Li3K(NH2)4 likely transform circularly in the dehydrogenation (KH↔K2Mg(NH2)4↔KLi3(NH2)4), which creates a more energy‐favorable pathway and thus leads to the overall kinetic enhancement. This catalytic role of K in the amide/hydride system is different from the conventional catalysis of transition metals in the alanate system.

show abstract

Section: Resultsmentioning

confidence: 99%

Solid–Solid Heterogeneous Catalysis: The Role of Potassium in Promoting the Dehydrogenation of the Mg(NH₂)₂/2 LiH Composite

et al. 2013

View full text Add to dashboard Cite

show abstract

“…An XRD analysis has confirmed that the above reaction is reversible at 200 °C under 3 MPa of hydrogen [8,17]. However, during dehydrogenation at 250 °C, Aoki et al observed that 5.1 mass% hydrogen had been desorbed [18].…”

Section: Introductionmentioning

confidence: 94%

Affects of Mechanical Milling and Metal Oxide Additives on Sorption Kinetics of 1:1 LiNH2/MgH2 Mixture

2011

View full text Add to dashboard Cite

Abstract:The destabilized complex hydride system composed of LiNH 2 :MgH 2 (1:1 molar ratio) is one of the leading candidates of hydrogen storage with a reversible hydrogen storage capacity of 8.1 wt%. A low sorption enthalpy of ~32 kJ/mole H 2 was first predicted by Alapati et al. utilizing first principle density function theory (DFT) calculations and has been subsequently confirmed empirically by Lu et al. through differential thermal analysis (DTA). This enthalpy suggests that favorable sorption kinetics should be obtainable at temperatures in the range of 160 °C to 200 °C. Preliminary experiments reported in the literature indicate that sorption kinetics are substantially lower than expected in this temperature range despite favorable thermodynamics. Systematic isothermal and isobaric sorption experiments were performed using a Sievert's apparatus to form a baseline data set by which to compare kinetic results over the pressure and temperature range anticipated for use of this material as a hydrogen storage media. Various material preparation methods and compositional modifications were performed in attempts to increase the kinetics while lowering the sorption temperatures. This paper outlines the results of these systematic tests and describes a number of beneficial additions which influence kinetics as well as NH 3 formation.

show abstract

“…The finding has stimulated much experimental research to enhance the H-storage efficiency. 3 To design high-performance materials, an understanding of basic properties is of great importance. However, the atomic structure of Li 2 NH appears to be controversial.…”

mentioning

confidence: 99%

“…The lattice constant was taken to be the experimental value. The PES on the 32 3 grid was interpolated into a finer grid 88 3 , to obtain converged eigenstates. Each point on the PES was the result of a wave function optimization using DFT.…”

mentioning

confidence: 99%

“…In the DFT calculations, exchange and correlation were treated within the Perdew-Burke-Ernzerhof generalized gradient approximation, and the ionic cores were described by ultrasoft pseudopotentials. 7 The cutoff energy for the plane-wave expansion was set to be 330 eV, and the electronic Brillouin zone was sampled using a 12 3 MonkhorstPack k-point grid, which are sufficient to give well-converged total energies. The protonic wave function calculations were made at the Γ point, and the cutoff for the protonic wave function grids was set to be 1074 meV, which are also sufficient to give converged results.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Quantum Delocalization of Hydrogen in the Li₂NH Crystal

2006

View full text Add to dashboard Cite

By mapping out potential energy surfaces from density-functional theory (DFT) and solving a protonic Schrödinger equation, we find that the H atom in a unit cell of the Li 2 NH crystal shows remarkably strong quantum behavior, leading to the delocalization of H over six octahedral sites around each N. This can be rationalized in terms of rapid coherent tunneling among these equivalent octahedral sites. Structural and dynamical consequences of our finding are discussed. Since the Li-N-H compounds are considered promising candidates for H-storage, understanding of these fundamental properties will be useful toward improving the performance of the material.Owing to its light mass, hydrogen present in materials can exhibit significant quantum effects, such as tunneling. 1 There has been increasing interest in designing high-performance materials with hydrogen-storage capability; such effects can be of relevance in determining the basic properties of these systems, including, for example, the energetics and location of H absorption and also the diffusivity and transport. To a good approximation, it often suffices to include the minimum amount of quantum mechanics for the H by adding in the zero-point energy (ZPE) computed via a harmonic approximation. On the other hand, if the localizing potential is not sufficiently deep, the H atoms can tunnel between equivalent sites in the material, broadening localized vibrational states into bands and rendering a harmonic approximation invalid. In such cases, the H atom in its ground state is delocalized over several sites. Crystalline environments containing ordered absorption sites can ideally supply such a situation. However, such phenomena are quite rare. In body-centered cubic (bcc) metals, for example, quantum tunneling between equivalent interstitial sites is known. 1 In this letter, we report first-principles calculations on the Li 2 NH crystal, in which the proton has been treated quantum mechanically by solving the Schrödinger equation for a proton in the potential energy surface (PES) derived from density-functional theory (DFT). What emerges from this study is that the proton in this material exhibits very strong quantum behavior, leading to the delocalization of the H atom in certain sites around the N atom.Li 2 NH is a newly emerged H-storage material. 2 Along with LiNH 2 , these materials exhibit promising features for Hstorage: a H-storage capacity of 11.5 wt % was reported, which is well above the desired value for vehicular applications. The finding has stimulated much experimental research to enhance the H-storage efficiency. 3 To design high-performance materials, an understanding of basic properties is of great importance. However, the atomic structure of Li 2 NH appears to be controversial. Three models have been proposed in experiments, suggesting different H adsorption sites. In Figure 1a, the H atoms are at the 4b sites in a face-centered cubic (fcc) structure with Fm3 hm symmetry, 4 and in parts b and c of Figure 1, the H atoms are at the 48h (Fm3 hm) ...

show abstract

Mechanism of Hydrogenation Reaction in the Li−Mg−N−H System

Cited by 79 publications

References 23 publications

Solid–Solid Heterogeneous Catalysis: The Role of Potassium in Promoting the Dehydrogenation of the Mg(NH₂)₂/2 LiH Composite

Solid–Solid Heterogeneous Catalysis: The Role of Potassium in Promoting the Dehydrogenation of the Mg(NH₂)₂/2 LiH Composite

Affects of Mechanical Milling and Metal Oxide Additives on Sorption Kinetics of 1:1 LiNH2/MgH2 Mixture

Quantum Delocalization of Hydrogen in the Li₂NH Crystal

Contact Info

Product

Resources

About

Mechanism of Hydrogenation Reaction in the Li−Mg−N−H System

Cited by 79 publications

References 23 publications

Solid–Solid Heterogeneous Catalysis: The Role of Potassium in Promoting the Dehydrogenation of the Mg(NH2)2/2 LiH Composite

Solid–Solid Heterogeneous Catalysis: The Role of Potassium in Promoting the Dehydrogenation of the Mg(NH2)2/2 LiH Composite

Affects of Mechanical Milling and Metal Oxide Additives on Sorption Kinetics of 1:1 LiNH2/MgH2 Mixture

Quantum Delocalization of Hydrogen in the Li2NH Crystal

Contact Info

Product

Resources

About

Solid–Solid Heterogeneous Catalysis: The Role of Potassium in Promoting the Dehydrogenation of the Mg(NH₂)₂/2 LiH Composite

Solid–Solid Heterogeneous Catalysis: The Role of Potassium in Promoting the Dehydrogenation of the Mg(NH₂)₂/2 LiH Composite

Quantum Delocalization of Hydrogen in the Li₂NH Crystal