Superior hydrogen storage properties of LiBH4 catalyzed by Mg(AlH4)2

Mg(AlH(4))(2) is found to provide a synergistic effect on improving the de-/rehydrogenation properties of LiBH(4). The Mg(AlH(4))(2)-catalyzed LiBH(4) exhibits lower dehydrogenation temperature and faster de-/rehydrogenation kinetics than the individually MgH(2)- or Al-catalyzed LiBH(4).     

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).     

First crystal structure studies of CaAlH5

A new member of the aluminum hydride family, CaAlH5, is formed during the decomposition of Ca(AlH4)2. The crystal structure of this new compound was calculated by density functional theory band-structure calculations and confirmed by X-ray powder diffraction analysis. The structure crystallizes in space group P2(1)/n (No. 14), with a = 8.3797(9) angstroms, b = 6.9293(8) angstroms, c = 9.8138(11) angstroms, beta = 93.78(1) degrees, and Z = 8.     

Dehydrogenation reaction for Na-O-H system: a first-principles study.

The crystal structures, electronic, dielectric, and vibrational properties of NaH, Na(2)O and NaOH are systematically investigated by first-principles calculations and the quasiharmonic approximation. The phonon dispersion relations and the phonon density of states of the phases and their thermodynamic functions including the heat capacity, the vibrational enthalpy, and the vibrational entropy are calculated using a direct force-constant method. Based on these results, the dehydrogenation reaction, NaH+NaOH-->H(2)+Na(2)O, is predicted to take place at 528 K, which is in agreement with the experimental observed value.     

Reaction dynamics of Ca(4s3d 1D2)+CH4-->CaH(X 2Sigma+)+CH3: reaction pathway and energy disposal for the CaH product

The reaction pathway for Ca(4s3d 1D2)+CH4-->CaH(X 2Sigma+)+CH3 has been investigated by using a pump-probe technique in combination with potential-energy surface (PES) calculations. The nascent product distributions of CaH have been characterized with Boltzmann rotational temperatures of 1013+/-102 and 834+/-70 K for the v=0 and 1 levels, respectively, and a Boltzmann vibrational temperature of 1313+/-173 K. The rotational and vibrational energy partitions in CaH have been estimated to be 461+/-45 and 252+/-15 cm(-1), respectively. According to the PES calculations, the pathway favors an insertion mechanism. Ca(3 1D2) approaches CH4 along an attractive potential surface in a C2v (or Cs) symmetry and then the collision complex undergoes nonadiabatic transition to the reactive ground-state surface. An Arrhenius plot shows a potential-energy requirement of 2695+/-149 cm(-1), which accounts for the endothermicity of 2930 cm(-1) for the reaction scheme. The Ca-C bond distance in the transition state structure is short enough to allow for tight orbital overlap between CaH and CH3. The strong coupling between the moieties renders the energy transfer sufficient from CaH into the CH3 radical. As compared to the Ca(4 1P1) reaction, the dissociation lifetime of the intermediate complex with less excess energy is prolonged so as to cause much less vibrational energy disposal into CaH.     

Functions of MgH(2) in hydrogen storage reactions of the 6LiBH(4)-CaH(2) reactive hydride composite

A significant improvement of hydrogen storage properties was achieved by introducing MgH(2) into the 6LiBH(4)-CaH(2) system. It was found that ～8.0 wt% of hydrogen could be reversibly stored in a 6LiBH(4)-CaH(2)-3MgH(2) composite below 400 °C and 100 bar of hydrogen pressure with a stepwise reaction, which is superior to the pristine 6LiBH(4)-CaH(2) and LiBH(4) samples. Upon dehydriding, MgH(2) first decomposed to convert to Mg and liberate hydrogen with an on-set temperature of ～290 °C. Subsequently, LiBH(4) reacted with CaH(2) to form CaB(6) and LiH in addition to further hydrogen release. Hydrogen desorption from the 6LiBH(4)-CaH(2)-3MgH(2) composite finished at ～430 °C in non-isothermal model, a 160 °C reduction relative to the 6LiBH(4)-CaH(2) sample. JMA analyses revealed that hydrogen desorption was a diffusion-controlled reaction rather than an interface reaction-controlled process. The newly produced Mg of the first-step dehydrogenation possibly acts as the heterogeneous nucleation center of the resultant products of the second-step dehydrogenation, which diminishes the energy barrier and facilitates nucleation and growth, consequently reducing the operating temperature and improving the kinetics of hydrogen storage.     

A new ammine dual-cation (Li, Mg) borohydride: synthesis, structure, and dehydrogenation enhancement

A new ammine dual-cation borohydride, LiMg(BH(4))(3)(NH(3))(2), has been successfully synthesized simply by ball-milling of Mg(BH(4))(2) and LiBH(4)·NH(3). Structure analysis of the synthesized LiMg(BH(4))(3)(NH(3))(2) revealed that it crystallized in the space group P6(3) (no. 173) with lattice parameters of a=b=8.0002(1) ?, c=8.4276(1) ?, α=β=90°, and γ=120° at 50 °C. A three-dimensional architecture is built up through corner-connecting BH(4) units. Strong N-H···H-B dihydrogen bonds exist between the NH(3) and BH(4) units, enabling LiMg(BH(4))(3)(NH(3))(2) to undergo dehydrogenation at a much lower temperature. Dehydrogenation studies have revealed that the LiMg(BH(4))(3)(NH(3))(2)/LiBH(4) composite is able to release over 8 wt% hydrogen below 200 °C, which is comparable to that released by Mg(BH(4))(3)(NH(3))(2). More importantly, it was found that release of the byproduct NH(3) in this system can be completely suppressed by adjusting the ratio of Mg(BH(4))(2) and LiBH(4)·NH(3). This chemical control route highlights a potential method for modifying the dehydrogenation properties of other ammine borohydride systems.     

Vibrational properties of CaAlH5 and α-AlH3 with different AlH6 networks studied by inelastic neutron scattering

We performed a combined study using inelastic neutron scattering (INS) and first-principles calculations of the vibrational properties of CaAlH(5) and α-AlH(3) with different AlH(6) networks, a zigzag one-dimensional AlH(6) network for CaAlH(5), and a three-dimensional AlH(6) network for α-AlH(3). Both materials showed qualitatively similar INS spectra, in which CaAlH(5)/α-AlH(3) was mainly divided into three regions: (i) the translational modes (318/316 cm(-1)), (ii) the librational modes of the octahedral AlH(6) units (external molecular motion) in the lower frequency range and H-Al-H bond-bending modes (intra molecular motion) at a higher frequency (420-1157/513-1038 cm(-1)), and (iii) the Al-H bond-stretching modes (1238-1750/1486-1942 cm(-1)). In region ii, the appearance of both librational and bond-bending modes was determined by the networked nature of the octahedral AlH(6) units. In addition, the librational modes of AlH(6) on α-AlH(3) exhibit higher frequencies than CaAlH(5) due to the tighter bonding between the octahedral AlH(6) units. With regard to average frequencies for the Al-H, ω(S), bond-stretching modes, and average Al-H bond distances on the aluminum-based hydrides including CaAlH(5) and α-AlH(3), ω(S) showed lower frequencies that correlate with lengthening of the Al-H bond distances.     

Hetero‐π‐systems from 2 + 2 cycloreversion,part 2.1Ab initio thermochemical study of heterocyclobutanes 2 + 2 cycloreversion to form heteroethenes H2C=X (X=NH,O, SiH2, PH,S)

Ab initio and DFT thermochemical study of diradical mechanism of 2 + 2 cycloreversion of parent heterocyclobutanes and 1,3‐diheterocyclobutanes, cyclo‐(CH₂CH₂CH₂X), and cyclo‐(CH₂XCH₂X), where X = NH, O, SiH₂, PH, S, was undertaken by calculating closed‐shell singlet molecules at three levels of theory: MP4/6‐311G(d)//MP2/6‐31G(d)+ZPE, MP4/6‐311G(d,p)//MP2/6‐31G (d,p)+ZPE, and B3LYP/6‐311+G(d,p)+ZPE. The enthalpies of 2 + 2 cycloreversion decrease on going from group 14 to group 16 elements, being substantially higher for the second row elements. Normally endothermic 2 + 2 cycloreversion is predicted to be exothermic for 1,3‐diazetidine and 1,3‐dioxtane. Strain energies of the four‐membered rings were calculated via the appropriate homodesmic reactions. The enthalpies of ring opening via the every possible one‐bond homolysis that results in the formation of the corresponding 1,4‐diradical were found by subtracting the strain energies from the central bond dissociation energies of the heterobutanes CH₃CH₂—CH₂XH, CH₃CH₂—XCH₃, and HXCH₂—XCH₃. The latter energies were determined via the enthalpies of the appropriate dehydrocondensation reactions, using C—H and X—H bond energies in CH₃XH calculated at G2 level of theory. Except 1,3‐disiletane, in which ring‐opening enthalpy attains 69.7 kcal/mol, the enthalpies of the most economical ring openings do not exceed 60.7 kcal/mol. The 1,4‐diradical decomposition enthalpies found as differences between 2 + 2 cycloreversion and ring‐opening enthalpies were negative, the least exothermicity was calculated for ⋅ CH₂SiH₂CH₂CH₂. The only exception was 1,3‐disiletane, which being diradical, CH₂SiH₂CH₂SiH₂, decomposed endothermically. Since decomposition of the diradical containing two silicon atoms required extra energy, raising the enthalpy of the overall reaction to 78.9 kcal/mol, 1,3‐disiletane was predicted to be highly resisting to 2 + 2 cycloreversion. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:704–720, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20377     

Infrared spectra of aluminum hydrides in solid hydrogen: Al2H4 and Al2H6

The reaction of laser-ablated Al atoms and normal-H(2) during co-deposition at 3.5 K produces AlH, AlH(2), and AlH(3) based on infrared spectra and the results of isotopic substitution (D(2), H(2) + D(2) mixtures, HD). Four new bands are assigned to Al(2)H(4) from annealing, photochemistry, and agreement with frequencies calculated using density functional theory. Ultraviolet photolysis markedly increases the yield of AlH(3) and seven new absorptions for Al(2)H(6) in the infrared spectrum of the solid hydrogen sample. These frequencies include terminal Al-H(2) and bridge Al-H-Al stretching and AlH(2) bending modes, which are accurately predicted by quantum chemical calculations for dibridged Al(2)H(6), a molecule isostructural with diborane. Annealing these samples to remove the H(2) matrix decreases the sharp AlH(3) and Al(2)H(6) absorptions and forms broad 1720 +/- 20 and 720 +/- 20 cm(-1) bands, which are due to solid (AlH(3))(n). Complementary experiments with thermal Al atoms and para-H(2) at 2.4 K give similar spectra and most product frequencies within 2 cm(-1). Although many volatile binary boron hydride compounds are known, binary aluminum hydride chemistry is limited to the polymeric (AlH(3))( solid. Our experimental characterization of the dibridged Al(2)H(6) molecule provides an important link between the chemistries of boron and aluminum.     

Synthesis, crystal structures, and hydrogen-storage properties of Eu(AlH4)2 and Sr(AlH4)2 and of their decomposition intermediates, EuAlH5 and SrAlH5

Complex Eu(AlH(4))(2) and Sr(AlH(4))(2) hydrides have been prepared by a mechanochemical metathesis reaction from NaAlH(4) and europium or strontium chlorides. The crystal structures were solved from powder X-ray diffraction data in combination with solid-state (27)Al NMR spectroscopy. The thermolysis pathway was analyzed in detail, allowing identification of new intermediate EuAlH(5)/SrAlH(5) compounds. Rehydrogenation experiments indicate that the second decomposition step is reversible.     

化学计量比和放氢背压对LiBH4+xMg2NiH4体系放氢性能的影响

研究了不同化学计量比(x=0.25, 0.5, 0.75, 1.0, 1.25)和放氢背压(1×10-4和0.4 MPa)对LiBH4+xMg2NiH4复合体系吸放氢性能的影响. 结果表明, 随着化学计量比(x)的增加, 复合体系的放氢温度逐渐降低, 放氢动力学性能得到提高, 但放氢容量逐渐降低; 其中, 在1×10-4和0.4 MPa初始放氢背压下, LiBH4+0.75Mg2NiH4体系具有最佳放氢动力学性能和较高的储氢容量. 结果表明, 放氢背压和化学计量比均会对高温下液相LiBH4 与固态Mg2NiH4 的润湿性产生影响, 进而影响复合体系的放氢路径和放氢动力学性能.     

Reaction pathway for the nonadiabatic reaction of Ca(4s3d 1D)+H2-->CaH(X 2Sigma+)+H

The reaction pathway and the nascent CaH product distribution in the reaction Ca(4s3d (1)D)+H(2)-->CaH(X (2)Sigma(+))+H are obtained using a pump-probe technique. The Ca atom is first prepared in the 3 (1)D state by a two-photon absorption, and then in brief time delay the laser-induced fluorescence of the reaction product CaH is monitored. The CaH(v=0,1) distributions appear to be single peaked, as characterized by Boltzmann rotational temperature of 807+/-38 K (v=0) and 684+/-77 K (v=1). The vibrational population ratio of CaH(v=0)/CaH(v=1) is determined to be 3.3+/-0.1, while the v=2 population is not detectable. The fractions of the available energy partitioning into rotation, vibration, and translation are estimated to be 0.36+/-0.05, 0.28+/-0.04, and 0.36+/-0.05, respectively. With the aid of the potential energy surfaces calculations, the current reaction should favor a near C(2v) collision configuration. The temperature dependence measurement yields a positive slope, indicative of the reaction occurrence without any potential barrier. The colliding species are anticipated to follow an attractive 1B(2) (or 2A') surface and then transit nonadiabatically to the reactive ground state surface.     

Reaction pathway and potential barrier for the CaH product in the reaction of Ca(4s4p1P1) + H2 --> CaH(X2Sigma+) + H

The nascent CaH product in the reaction Ca(4s4p1P1) + H2 --> CaH(X2Sigma+) + H is obtained using a pump-probe technique. The CaH(v = 0,1) distributions, with a population ratio of CaH(v = 0)/CaH(v = 1) = 2.7+/-0.2, may be characterized by low Boltzmann rotational temperature. According to Arrhenius theory, the temperature dependence measurement yields a potential barrier of 3820+/-480 cm(-1) for the current reaction. As a result of the potential energy surfaces (PES) calculations, the reaction pathway favors a Ca insertion into the H2 bond along a (near) C2v geometric approach. As the H2 bond is elongated, the configurational mixing between the orbital components of the 4p and nearby low-lying 3d state with the same symmetry makes significant the nonadiabatic transition between the 5A' and 2A' surface in the repulsive limbs. Therefore, the collision species are anticipated to track along the 5A' surface, then undergo nonadiabatic transition to the inner limb of the 2A' surface, and finally cross to the reactive 1A' surface. The observed energy barrier probably accounts for the energy requirement to surmount the repulsive hill in the entrance. The findings of the nascent CaH product distributions may be reasonably interpreted from the nature of the intermediate structure and lifetime after the 2A'-1A' surface transition. The distinct product distributions between the Ca(4 1P1) and Mg(3 1P1) reactions with H2 may also be realized with the aid of the PES calculations.     

In situ X-ray Raman spectroscopy of LiBH4

X-Ray Raman Spectroscopy (XRS) is used to study the electronic properties of bulk lithium borohydride (LiBH(4)) and LiBH(4) in porous carbon nano-composites (LiBH(4)/C) during dehydrogenation. The lithium (Li), boron (B) and carbon (C) K-edges are studied and compared with calculations of the starting material and intermediate compounds. Comparison of the B and C K-edge XRS spectra of the as-prepared samples with rehydrogenated samples shows that the B and C electronic structure is largely regained after rehydrogenation. Both Li and C K-edge spectra show that during dehydrogenation, part of the Li intercalates into the porous carbon. This study shows that XRS in combination with calculations is a promising tool to study the electronic properties of nano-crystalline light-weight materials for energy storage.     

Crystal structures, lattice potential energies, and thermochemical properties of crystalline compounds (1-C(n)H(2n+1)NH3)2ZnCl4(s) (n = 8, 10, 12, and 13)

As part of our ongoing project involving the study of (1-C(n)H(2n+1)NH(3))(2)MCl(4)(s) (where M is a divalent metal ion and n = 8-18), we have synthesized the compounds (1-C(n)H(2n+1)NH(3))(2)ZnCl(4)(s) (n = 8, 10, 12, and 13), and the details of the structures are reported herein. All of the compounds were crystallized in the monoclinic form with the space group P2(1)/n for (1-C(8)H(17)NH(3))(2)ZnCl(4)(s), P21/c for (1-C(10)H(21)NH(3))(2)ZnCl(4)(s), P2(1)/c for (1-C(12)H(25)NH(3))(2)ZnCl(4)(s), and P2(1)/m for (1-C(13)H(27)NH(3))(2)ZnCl(4)(s). The lattice potential energies and ionic volumes of the cations and the common anion of the title compounds were obtained from crystallographic data. Molar enthalpies of dissolution of the four compounds at various molalities were measured at 298.15 K in the double-distilled water. According to Pitzer's theory, molar enthalpies of dissolution of the title compounds at infinite dilution were obtained. Finally, using the values of molar enthalpies of dissolution at infinite dilution (Δ(s)H(m)(∞)) and other auxiliary thermodynamic data, the enthalpy change of the dissociation of [ZnCl(4)](2-)(g) for the reaction [ZnCl(4)](2-)(g)→ Zn(2+)(g) + 4Cl(-)(g) was obtained, and then the hydration enthalpies of cations were calculated by designing a thermochemical cycle.     

Synthesis and properties of calcium alanate and two solvent adducts

Several ways to synthesize solvated and desolvated calcium tetrahydroaluminate by wet-chemical and mechanochemical methods are presented. The products were characterized by elemental analysis, X-ray diffraction (XRD), and infrared spectroscopy (FTIR). The crystal structure of Ca(AlH(4))(2).4THF was determined. After desolvation, an ultrafine powder was obtained. IR data and the mass balance suggest a compound with the composition Ca(AlH(4))(2), containing tetrahedral [AlH(4)] groups.     

3LiBH4/CeF3体系的放氢性能及机制

采用球磨法制备了3LiBH₄/CeF₃反应体系, 通过压力-组成-温度(PCT)测试仪、 X射线衍射仪(XRD)和傅里叶变换红外光谱仪(FTIR)研究了体系的放氢性能、 反应机制及性能改善机理. 结果表明, 3LiBH₄/CeF₃体系在295 ℃左右快速放氢, 总放氢量为4.1%(质量分数). 放氢过程中CeF₃与LiBH₄直接发生反应: 3LiBH₄+CeF₃1/2CeB₆+1/2CeH₂+3LiF+11/2H₂. 与纯LiBH₄相比, 放氢热力学稳定性和表观活化能的降低是3LiBH₄/CeF₃体系放氢温度下降的主要原因.     

Stability and reversibility of LiBH4

LiBH4 is a complex hydride and exhibits a high gravimetric hydrogen density of 18.5 wt %. Therefore it is a promising hydrogen storage material for mobile applications. The stability of LiBH4 was investigated by pcT (pressure, concentration, and temperature) measurements under constant hydrogen flows and extrapolated to equilibrium. According to the van 't Hoff equation the following thermodynamic parameters are determined for the desorption: enthalpy of reaction DeltarH = 74 kJ mol-1 H2 and entropy of reaction DeltarS = 115 J K-1 mol-1 H2. LiBH4 decomposes to LiH + B + 3/2H2 and can theoretically release 13.9 wt % hydrogen for this reaction. It is shown that the reaction can be reversed at a temperature of 600 degrees C and at a pressure of 155 bar. The formation of LiBH4 was confirmed by XRD (X-ray diffraction). In the rehydrided material 8.3 wt % hydrogen was desorbed in a TPD (temperature-programmed desorption) measurement compared to 10.9 wt % desorbed in the first dehydrogenation.     

Thermodynamic properties of molecular borane phosphines, alane amines, and phosphine alanes and the [BH(4)(-)][PH(4)(+)], [AlH(4)(-)][NH(4)(+)], and [AlH(4)(-)][PH(4)(+)] salts for chemical hydrogen storage systems from ab initio electronic structure theory

The heats of formation for the molecules BH(3)PH(3), BH(2)PH(2), HBPH, AlH(3)NH(3), AlH(2)NH(2), HAlNH, AlH(3)PH(3), AlH(2)PH(2), HAlPH, AlH(4)(-), PH(3), PH(4), and PH(4)(+), as well as the diatomics BP, AlN, and AlP, have been calculated by using ab initio molecular orbital theory. The coupled cluster with single and double excitations and perturbative triples method (CCSD(T)) was employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple-zeta, to extrapolate to the complete basis set limit. Additional d core functions were used for Al and P. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. Geometries were calculated at the CCSD(T) level up through at least aug-cc-pVTZ and frequencies were calculated at the CCSD(T)/aug-cc-pVDZ level. The heats of formation of the salts [BH(4)(-)][PH(4)(+)](s), [AlH(4)(-)][NH(4)(+)](s), and [AlH(4)(-)][PH(4)(+)](s) have been estimated by using an empirical expression for the lattice energy and the calculated heats of formation of the two component ions. The calculations show that both AlH(3)NH(3)(g) and [AlH(4)(-)][NH(4)(+)](s) can serve as good hydrogen storage systems that release H(2) in a slightly exothermic process. In addition, AlH(3)PH(3) and the salts [AlH(4)(-)][PH(4)(+)] and [BH(4)(-)][PH(4)(+)] have the potential to serve as H(2) storage systems. The hydride affinity of AlH(3) is calculated to be -70.4 kcal/mol at 298 K. The proton affinity of PH(3) is calculated to be 187.8 kcal/mol at 298 K in excellent agreement with the experimental value of 188 kcal/mol. PH(4) is calculated to be barely stable with respect to loss of a hydrogen to form PH(3).