Hydrogen storage properties of Li-Mg-N-H systems with different ratios of LiH/Mg(NH2)2

In this work, the hydrogen desorption and structural properties of the Li-Mg-N-H systems with different LiH/Mg(NH2)2 ratios are systemically investigated. The results indicate that the system with the LiH/Mg(NH2)2 ratio of 6/3 transforms into Li2NH and MgNH, and then, the mixture forms an unknown phase by a solid-solid reaction, which presumably is the ternary imide Li2Mg(NH)2; the system with the LiH/Mg(NH2)2 ratio of 8/3 transforms into 4Li2NH and Mg3N2 after releasing H2 at T < 400 degrees C; the system with the LiH/Mg(NH2)2 ratio of 12/3 transforms into 4Li3N and Mg3N2 after releasing H2 at T > 400 degrees C, where the LiMgN phase is formed by the reaction between Li3N and Mg3N2. The characteristics of the phase transformations and the thermal gas desorption behaviors in these Li-Mg-N-H systems could be reasonably explained by the ammonia mediated reaction model, irrespective of the difference in the LiH/Mg(NH2)2 ratios.     

Polymorphism of heptalithium nitridovanadate(V) Li7[VN4

The system Li-V-N was studied by means of X-ray and neutron powder diffraction, thermal and chemical analyses, and XAS spectroscopy at the vanadium K-edge. Three polymorphs of Li(7)[VN(4)] have been established from X-ray and neutron powder diffraction (gamma-Li(7)[VN(4)], space group Pfourmacr;3n, No. 218, a = 960.90(4) pm, V = 887.23(6) x 10(6) pm(3), Z = 8; beta-Li(7)[VN(4)], space group Pathremacr;, No. 205, a = 959.48(3) pm, V = 883.31(5) x 10(6) pm(3), Z = 8; alpha-Li(7)[VN(4)], P4(2)/nmc, No. 137, a = 675.90(2) pm, c = 488.34(2) pm, V = 223.09(1) x 10(6) pm(3), Z = 2). Crystallographic and phase relations are discussed. All three modifications are diamagnetic, indicating vanadium in the oxidation state +5. The V-K XAS spectra support the oxidation state assignment, the non-centrosymmetric coordination (tetrahedral), and the nearly identical second coordination sphere of vanadium, made up from Li in all three phases. The 3d-related features of the spectra display strongly localized properties. The phase transitions appear to be reconstructive; no direct group-subgroup symmetry relations of the crystal structures exist. The formation of solid solutions between Li(2)O and beta-Li(7)[VN(4)] with the general formula Li(1.75)((V(0.25(1)(-)(x))Li(0.25)(x))(N(1)(-)(x)O(x)())) with 0 相似文献   

Mechanism of hydrogenation reaction in the Li-Mg-N-H system

The Li-Mg-N-H system composed of 3 Mg(NH2)2 and 8 LiH reversibly desorbs/absorbs approximately 7 wt % of H2 at 120-200 degrees C and transforms into 4 Li2NH and Mg3N2 after dehydrogenation. In this work, the mechanism of the hydrogenation reaction from 4 Li2NH and Mg3N2 to 8 LiH and 3 Mg(NH2)2 was investigated in detail. Experimental results indicate that 4 Li2NH is first hydrogenated into 4 LiH and 4 LiNH2. At the next step, 4 LiNH2 decomposes into 2 Li2NH and 2 NH3, and the emitted 2 NH3 reacts with (1/2) Mg3N2 and produces the (3/2) Mg(NH2)2 phase, while the produced 2 Li2NH is hydrogenated into 2 LiH and 2 LiNH2 again. Such successive steps continue until all 4 Li2NH and Mg3N2 completely transform into 8 LiH and 3 Mg(NH2)2 by hydrogenation.     

Li24[MnN3]3N2 and Li5[(Li1-xMnx)N]3, two intermediates in the decomposition path of Li7[MnN4] to Li2[(Li1-xMnx)N]: an experimental and theoretical study

The crystal structure of Li7[Mn(V)N4] was re-determined. Isolated tetrahedral [Mn(V)N4](7-) ions are arranged with lithium cations to form a superstructure of the CaF2 anti-type (P4bar3n, No. 218, a = 956.0(1) pm, Z = 8). According to measurements of the magnetic susceptibility, the manganese (tetrahedral coordination) is in a d(2) S = 1 state. Thermal treatment of Li7[Mn(V)N4] under argon in the presence of elemental lithium at various temperatures leads to Li24[Mn(III)N3]3N2, Li5[(Li1-xMnx)N]3, and Li2[(Li1-xMn(I)x)N], respectively. Li24[Mn(III)N3]3N2 (P3bar1c, No. 163, a = 582.58(6) pm, c = 1784.1(3) pm, Z = 4/3) crystallizes in a trigonal unit cell, containing slightly, but significantly nonplanar trigonal [MnN3](6-) units with C3v symmetry. Measurements of the magnetic susceptibility reveal a d(4) S = 1 spin-state for the manganese (trigonal coordination). Nonrelativistic spin-polarized DFT calculations with different molecular models lead to the conclusion that restrictions in the Li-N substructure are responsible for the distortion from planarity of the [Mn(III)N3](6-). Li5[(Li1-xMnx)N]3 (x = 0.59(1), P6bar2m, No. 189, a = 635.9(3) pm, c = 381.7(2) pm, Z = 1) is an isotype of Li5[(Li1-xNix)N]3 with manganese in an average oxidation state of about +1.6. The crystal structure is a defect variant of the alpha-Li3N structure type with the transition metal in linear coordination by nitrogen. Li2[(Li1-xMn(I)x)N] (x = 0.67(1), P6/mmm, No. 191, a = 371.25(4) pm, c = 382.12(6) pm, Z = 1) crystallizes in the alpha-Li3N = Li2[LiN] structure with partial substitution of the linearly nitrogen-coordinated Li-species by manganese(I). Measurements of the magnetic susceptibility are consistent with manganese (linear coordination) in a low-spin d(6) S = 1 state.     

Reindarstellung und Kristallstruktur von Lithiumnitridhydrid,Li4NH,Li4ND

Preparation and Crystal Structure of Lithium Nitride Hydride, Li₄NH, Li₄ND Single phase Li₄NH was prepared by the reaction of Li₃N and LiH at 490°C. Its structure has been solved from x-ray and time-of-flight neutron powder diffraction data. Li₄NH crystallizes in an ordered variant of the Li₂O structure. N and H occupy the sites of two interpenetrating “extended” diamond lattices. Li occupies all N₂H₂ tetrahedral voids and is found to be shifted into a N₂H tetrahedral face. As a result H is in compressed tetrahedral coordination by Li, while N is in bisdisphenoidal coordination by Li. Alternatively, the Li₄NH structure may be regarded as a [Li₄N]⁺threedimensional net, its voids being filled up with H^?. Li₄NH is a reactive solid, which decomposes to imide when in contact with N₂ or H₂ at some 400°C.     

Hydrogen desorption mechanism in a Li-N-H system by means of the isotopic exchange technique

The hydrogen desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H2 was examined by thermal desorption mass spectrometry, thermogravimetric analysis, and Fourier transform IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results obtained indicate that the hydrogen desorption reaction proceeds through the following two-step elementary reactions mediated by ammonia: 2LiNH2 --> Li2NH + NH3 and LiH + NH3 --> LiNH2 + H2, where hydrogen molecules are randomly formed from four equivalent hydrogen atoms in a hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability.     

Ab initio study on the hydrogen desorption from MH-NH3 (M = Li, Na, K) hydrogen storage systems

The hydrogen storage system LiH + NH(3) ? LiNH(2) + H(2) is one of the most promising hydrogen storage systems, where the reaction yield can be increased by replacing Li in LiH with other alkali metals (Na or K) in order of Li < Na < K. In this paper, we have studied the alkali metal M (M = Li, Na, K) dependence of the reactivity of MH with NH(3) by calculating the potential barrier of the H(2) desorption process from the reaction of an M(2)H(2) cluster with an NH(3) molecule based on the ab initio structure optimization method. We have shown that the height of the potential barrier becomes lower in order of Li, Na, and K, where the difference of the potential barrier in Li and Na is relatively smaller than that in Na and K, and this tendency is consistent with the recent experimental results. We have also shown that the H-H distance of the H(2) dimer at the transition state takes larger distance and the change of the potential energy around the transition state becomes softer in order of Li, Na, and K. There are almost no M dependence in the charge of the H atom in NH(3) before the reaction, while that of the H atom in M(2)H(2) takes larger negative value in order of Li, Na, and K. We have also performed molecular dynamics simulations on the M(2)H(2)-NH(3) system and succeeded to reproduce the H(2) desorption from the reaction of Na(2)H(2) with NH(3).     

Binding energies of small lithium clusters (Li(n)) and hydrogenated lithium clusters (Li(n)H)

Large coupled cluster computations utilizing the Dunning weighted correlation-consistent polarized core-valence (cc-pwCVXZ) hierarchy of basis sets have been conducted, resulting in a panoply of internally consistent geometries and atomization energies for small Li(n) and Li(n)H (n=1-4) clusters. In contrast to previous ab initio results, we predict a monotonic increase in atomization energies per atom with increasing cluster size for lithium clusters, in accordance with the historical Knudsen-effusion measurements of Wu. For hydrogenated lithium clusters, our results support previous theoretical work concerning the relatively low atomization energy per atom for Li(2)H compared to LiH and Li(3)H. The CCSD(T)/cc-pwCVQZ atomization energies for LiH, Li(2)H, Li(3)H, and the most stable isomer of Li(4)H, including zero-point energy corrections, are 55.7, 79.6, 113.0, and 130.6 kcal/mol, respectively. The latter results are not consistent with the most recent experiments of Wu.     

Local defects enhanced dehydrogenation kinetics of the NaBH(4)-added Li-Mg-N-H system

A mechanistic understanding on the enhanced kinetics of hydrogen storage in the NaBH(4)-added Mg(NH(2))(2)-2LiH system is provided by carrying out experimental investigations associated with first-principles calculations. It is found that the operating temperatures for hydrogen desorption of the Mg(NH(2))(2)-2LiH system are reduced by introducing NaBH(4), and the NaBH(4) species seems almost unchanged during dehydrogenation/hydrogenation process. First-principles calculations reveal that the presence of NaBH(4) in the Mg(NH(2))(2)-2LiH system facilitates the formation of Mg vacancies in Mg(NH(2))(2). The appearance of Mg vacancies not only weakens the N-H bonds but also promotes the diffusion of atoms and/or ions, consequently resulting in the improvement of the reaction kinetics of hydrogen desorption/absorption of the NaBH(4)-added Mg(NH(2))(2)-2LiH system. This finding provides us with a deep insight into the role played by NaBH(4) in the Li-Mg-N-H system, as well as ideas for designing high-performance catalysts for metal-N-H-based hydrogen storage media.     

Reaction paths between LiNH2 and LiH with effects of nitrides

The solid-state reaction between LiNH2 and LiH potentially offers an effective route for hydrogen storage if it can be tailored to meet all the requirements for practical applications. To date, there still exists large uncertainty on the mechanism of the reaction--whether it is mediated by a transient NH3 or directly between LiNH2 and LiH. In an effort to clarify this issue and improve the reactivity, the effects of selected nitrides were investigated here by temperature-programmed desorption, X-ray diffraction, in-situ infrared analysis, and hydrogen titration. The results show that the reaction of LiNH2 with LiH below 300 degrees C is a heterogeneous solid-state reaction controlled by Li+ diffusion from LiH to LiNH2 across the interface. At the LiNH2/LiH interface, an ammonium ion Li2NH2+ and a penta-coordinated nitrogen Li2NH3 could be the intermediate states leading to the production of hydrogen and the formation of lithium imide. In addition, it is identified that BN is an efficient "catalyst" that improves Li+ diffusion and hence the kinetics of the reaction between LiNH2 and LiH. Hydrogen is fully released within 7 h at 200 degrees C with BN addition, rather than several days without the modification.     

LiAlH4与Li3AlH6的成键特性及热力学稳定性

采用基于密度泛函理论(DFT)的平面波赝势(PW-PP)方法, 计算了LiAlH4分解反应中各个产物的晶胞参数、电子结构、生成焓和分解反应的反应焓. 反应中各固态、气态物质的晶胞的结构优化后的晶格参数与相应的实验值均符合得较好. 对LiAlH4与Li3AlH6的电子结构分析均表明, 其中的Al—H键为共价键、Li—H键为离子键. 对各分解反应的反应焓计算结果表明, (1) LiAlH4→1/3Li3AlH6+2/3Al+H2,(2) 1/3Li3AlH6→LiH+1/3Al+1/2H2及(3) LiH+Al→LiAl+1/2H2均为吸热反应, 298 K时计算的反应焓分别为14.3、14.9 与50.9 kJ·mol-1, 与相应的实验值符合得较好.     

Identification of destabilized metal hydrides for hydrogen storage using first principles calculations

Hydrides of period 2 and 3 elements are promising candidates for hydrogen storage but typically have heats of reaction that are too high to be of use for fuel cell vehicles. Recent experimental work has focused on destabilizing metal hydrides through alloying with other elements. A very large number of possible destabilized metal hydride reaction schemes exist. The thermodynamic data required to assess the enthalpies of these reactions, however, are not available in many cases. We have used first principles density functional theory calculations to predict the reaction enthalpies for more than 100 destabilization reactions that have not previously been reported. Many of these reactions are predicted not be useful for reversible hydrogen storage, having calculated reaction enthalpies that are either too high or too low. More importantly, our calculations identify five promising reaction schemes that merit experimental study: 3LiNH(2) + 2LiH + Si --> Li(5)N(3)Si + 4H(2), 4LiBH(4) + MgH(2) --> 4LiH + MgB(4) + 7H(2), 7LiBH(4) + MgH(2) --> 7LiH + MgB(7) + 11.5H(2), CaH(2) + 6LiBH(4) --> CaB(6) + 6LiH + 10H(2), and LiNH(2) + MgH(2) --> LiMgN + 2H(2).     

Stability and structure of Li(n)H molecules (n=3-6): experimental and density functional study

The molecules Li(3)H and Li(4)H have been identified in mass-spectrometric measurements over solutions of hydrogen in liquid Li, and the gaseous equilibria of the reactions: Li(3)H+Li=Li(2)H+Li(2), Li(3)H+Li(2)=Li(2)H+Li(3), Li(3)H+Li=LiH+Li(3), Li(3)H+LiH=2Li(2)H, and Li(4)H+Li(2)=Li(3)H+Li(3) have been measured. Density functional calculations of Li(n)H molecules (n=3-6) provide structures, vibrational frequencies, ionization energies, and free energy functions of these molecules, and these are used to estimate the enthalpies of these reactions and the atomization energies of Li(3)H (119.4 kcal/mol) and Li(4)H (151.8 kcal/mol).     

Hydrogen storage in LiAlH4: predictions of the crystal structures and reaction mechanisms of intermediate phases from quantum mechanics

We use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH(4). First, we explore the decomposition mechanism of monoclinic LiAlH(4) into monoclinic Li(3)AlH(6) plus face-centered cubic (fcc) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH(4) and Li(3)AlH(6) agree well with experimental data. On the other hand, the alternative decomposition of LiAlH(4) into LiAlH(2) plus H(2) is predicted to be unstable with respect to that through Li(3)AlH(6). Next, we investigate thermal decomposition of Li(3)AlH(6) into fcc LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH(6) to form the stable Li-H-Li-H complex. Then, two sequential H(2) desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li(3)AlH(6)(monoclinic)-->3 LiH(fcc)+Al(fcc)+3/2 H(2) is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s)+Al(s)-->LiAl(s)+12H(2)(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl.     

Polylithiated tetraaminosilanes: synthesis and characterization of (Et2O.Li)4[Si(Nnaph)4] and X-ray structure of (THF.Li3[Si(NiPr)3(NHiPr)])2

The treatment of SiCl4 with 4 equiv of Li2(Nnaph) (naph = 1-naphthyl) in diethyl ether gives (Et2O.Li)4[Si(Nnaph)4] (4), which, upon reaction with excess tBuNH3Cl or MeO3SCF3, generates Si[N(H)naph]4 (5) or Si[N(Me)naph]4 (6), respectively. The centrosymmetric dimer (THF.Li3[Si(NiPr)3(NHiPr)])2 (7), formed via trilithiation of Si[N(H)iPr]4 with n-butyllithium, consists of a bis-THF-solvated Li6(NiPr)6 cyclic ladder bicapped by two SiN(H)iPr units. Crystal data for 7: C32H74Li6N8O2Si2, monoclinic, P2(1)/n, a = 10.661(7) A, b = 16.964(5) A, c = 12.405(4) A, beta = 93.22(4) degrees, V = 2239.9(15) A3, and Z = 2.     

Correlation between composition and hydrogen storage behaviors of the Li2NH-MgNH combination system

Hydrogen storage performances of a Li(2)NH-xMgNH combination system (x = 0, 0.5, 1 and 2) are investigated for the first time. It is found that the hydrogenated samples with MgNH exhibit a significant reduction in the dehydrogenation temperatures. Mechanistic investigations reveal that there is a strong dependence of the hydrogen storage reaction process on the molar ratio between MgNH and Li(2)NH. As a consequence, tuning of thermodynamics is achieved for hydrogen storage in the Li(2)NH-xMgNH system by changing the reaction routes, which is ascertained to be the primary reason for the reduction in the operating temperature for hydrogen desorption. Specifically, it is found that under 105 atm hydrogen (140-280 °C) 5.6 wt% hydrogen is reversibly stored in the Li(2)NH-0.5MgNH combination system, which is greater than in the well-investigated Mg(NH(2))(2)-2LiH system.     

Main group multiple C-H/N-H bond activation of a diamine and isolation of a molecular dilithium zincate hydride: experimental and DFT evidence for alkali metal-zinc synergistic effects

The surprising transformation of the saturated diamine (iPr)NHCH(2)CH(2)NH(iPr) to the unsaturated diazaethene [(iPr)NCH═CHN(iPr)](2-) via the synergic mixture nBuM, (tBu)(2)Zn and TMEDA (where M = Li, Na; TMEDA = N,N,N',N'-tetramethylethylenediamine) has been investigated by multinuclear NMR spectroscopic studies and DFT calculations. Several pertinent intermediary and related compounds (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2) (3), (TMEDA)Li[(iPr)NCH(2)CH(2)CH(2)N(iPr)]Zn(tBu) (5), {(THF)Li[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)}(2) (6), and {(TMEDA)Na[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)}(2) (11), characterized by single-crystal X-ray diffraction, are discussed in relation to their role in the formation of (TMEDA)M[(iPr)NCH═CHN(iPr)]Zn(tBu) (M = Li, 1; Na, 10). In addition, the dilithio zincate molecular hydride [(TMEDA)Li](2)[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)H 7 has been synthesized from the reaction of (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2)3 with nBuLi(TMEDA) and also characterized by both X-ray crystallographic and NMR spectroscopic studies. The retention of the Li-H bond of 7 in solution was confirmed by (7)Li-(1)H HSQC experiments. Also, the (7)Li NMR spectrum of 7 in C(6)D(6) solution allowed for the rare observation of a scalar (1)J(Li-H) coupling constant of 13.3 Hz. Possible mechanisms for the transformation from diamine to diazaethene, a process involving the formal breakage of four bonds, have been determined computationally using density functional theory. The dominant mechanism, starting from (TMEDA)Li[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu) (4), involves the formation of a hydride intermediate and leads directly to the observed diazaethene product. In addition the existence of 7 in equilibrium with 4 through the dynamic association and dissociation of a (TMEDA)LiH ligand, also provides a secondary mechanism for the formation of the diazaethene. The two reaction pathways (i.e., starting from 4 or 7) are quite distinct and provide excellent examples in which the two distinct metals in the system are able to interact synergically to catalyze this otherwise challenging transformation.     

Infrared spectra and theoretical calculations of lithium hydride clusters in solid hydrogen, neon, and argon

A matrix isolation IR study of laser-ablated lithium atom reactions with H2 has been performed in solid para-hydrogen, normal hydrogen, neon, and argon. The LiH molecule and (LiH)(2,3,4) clusters were identified by IR spectra with isotopic substitution (HD, D(2), and H(2) + D(2)) and comparison to frequencies calculated by density functional theory and the MP2 method. The LiH diatomic molecule is highly polarized and associates additional H(2) to form primary (H(2))(2)LiH chemical complexes surrounded by a physical cage of solid hydrogen where the ortho and para spin states form three different primary complexes and play a role in the identification of the bis-dihydrogen complex and in characterization of the matrix cage. The highly ionic rhombic (LiH)(2) dimer, which is trapped in solid matrices, is calculated to be 22 kcal/mol more stable than the inverse hydrogen bonded linear LiH-LiH dimer, which is not observed here. The cyclic lithium hydride trimer and tetramer clusters were also observed. Although the spontaneous reaction of 2 Li and H(2) to form (LiH)(2) occurs on annealing in solid H(2), the formation of higher clusters requires visible irradiation. We observed the simplest possible chemical reduction of dihydrogen using two lithium valence electrons to form the rhombic (LiH)(2) dimer.     

On the composition and crystal structure of the new quaternary hydride phase Li4BN3H10

X-ray data on single crystals of the quaternary metal hydride near the composition LiB(0.33)N(0.67)H(2.67), previously identified as "Li3BN2H8", reveal that its true composition is Li4BN3H10. The structure has body-centered-cubic symmetry [space group I2(1)3, cell parameter a = 10.679(1)-10.672(1) Angstroms] and contains an ordered arrangement of BH4- and NH2- anions in the molar ratio 1:3. The borohydride anion has an almost ideal tetrahedral geometry (angleH-B-H approximately 108-114 degrees), while the amide anion has a nearly tetrahedral bond angle (angleH-N-H approximately 106 degrees). Three symmetry-independent Li atom sites are surrounded by BH4- and NH2- anions in various distorted tetrahedral configurations, one by two B and two N atoms, another by four N atoms, and the third by one B and three N atoms. The Li configuration around B is nearly tetrahedral, while that around N resembles a distorted saddlelike configuration, similar to those in LiBH4 and LiNH2, respectively.