Enhanced hydrogen storage properties of NaBH4–Mg(BH4)2 composites by NdF3 addition

As two important members of complex hydrides, Mg(BH₄)₂ and NaBH₄ have a high gravimetric capacity (14.9 and 10.8 ​wt%, respectively). In this study, the Mg(BH₄)₂ was synthesized by the ion exchange method. Afterwards, the Mg(BH₄)₂ and NaBH₄ composites with different amounts (30, 40 and 50 ​wt%) of NdF₃ were prepared by mechanical milling. Effects of the NdF₃ on the microstructural evolution and hydrogen storage properties were investigated. The results show that NdF₃ catalyst can significantly improve the dehydrogenation kinetics of the eutectic composites of NaBH₄–Mg(BH₄)₂. The onset hydrogen desorption temperature of the composites is about 88.6 ​°C, which is about 110 ​°C lower than that of Mg(BH₄)₂ and NaBH₄ composites. Mg(BH₄)₂–NaBH₄-0.5NdF₃ composites can released 5.2 ​wt% H₂ at 250 ​°C within 30 ​min, and the dehydrogenation capacity is significantly higher than that of Mg(BH₄)₂–NaBH₄ composites. The analysis of the dehydrogenation mechanism reveals that NdF₃ takes participate in the reaction to generate NaMgF₃ to promote the dehydrogenation reaction process of the composites.     

Improvement on hydrogen generation properties of Zr(BH_4)_4·8NH_3

Hydrolysis of Zr(BH_4)_4·8NH_3 in deionized water can generate high purity hydrogen at room temperature.However, the sluggish hydrolysis kinetics of Zr(BH_4)_4·8NH_3 hinders its practical use. To improve its hydrogen generation properties, the effects of magnetic stirring, changing hydrolysis solution and tuning the ammonia coordination number on the hydrolysis properties of Zr(BH_4)_4·8NH_3 were investigated. Results show that both changing hydrolysis solution and tuning the ammonia coordination number can enhance the hydrolysis kinetics.The hydrolysis kinetics properties of Zr(BH_4)_4·8NH_3 were significantly improved in MgCl_2 and CoCl_2 solutions.The Zr(BH_4)_4·8NH_3(x ≤ 8) samples were synthesized by a ball-milling method with different ammonization time(10, 60 and 180 min). Both the hydrolysis kinetics and hydrogen yield of Zr(BH_4)_4·8NH_3(x ≤ 8) were enhanced as the ammonia coordination number(x) decreased. Thus, tuning ammonia coordination number is an effective way to control the hydrolysis properties of Zr(BH_4)_4·8NH_3(x ≤ 8).     

Current Research Progress in Magnesium Borohydride for Hydrogen Storage (A review)

Hydrogen storage in solid-state materials is believed to be a most promising hydrogen-storage technology for high efficiency, low risk and low cost. Mg(BH₄)₂ is regarded as one of most potential materials in hydrogen storage areas in view of its high hydrogen capacities (14.9 ​wt% and 145–147 ​kg ​cm⁻³). However, the drawbacks of Mg(BH₄)₂ including high desorption temperatures (about 250 ​°C–580 ​°C), sluggish kinetics, and poor reversibility make it difficult to be used for onboard hydrogen storage of fuel cell vehicles. A lot of researches on improving the dehydrogenation reaction thermodynamics and kinetics have been done, mainly including: additives or catalysts doping, nanoconfining Mg(BH₄)₂ in nanoporous hosts, forming reactive hydrides systems, multi-cation/anion composites or other derivatives of Mg(BH₄)₂. Some favorable results have been obtained. This review provides an overview of current research progress in magnesium borohydride, including: synthesis methods, crystal structures, decomposition behaviors, as well as emphasized performance improvements for hydrogen storage.     

Encumbering the intramolecular π donation by using a bridge: A strategy for designing metal-free compounds to hydrogen activation

On the basis of the FLP (frustrated Lewis pair) principle, a new strategy has been proposed to construct the “frustration” in designing metal-free hydrogen activation compounds, by using FMO (frontier molecular orbital) analyses and quantum mechanics calculations. Unlike the known FLPs which use bulky substituents to prevent them from forming stable Lewis acid/base complexes, the new approach encumbers the intramolecular π donation from the electron donor to the acceptor (e.g. in BH₂NH₂) by using a CH₂ bridge (giving BH₂CH₂NH₂). The strategy is simple and effective. Its effectiveness is demonstrated by the small hydrogen activation energy (12.0 kcal/mol) of the model molecule (BH₂CH₂NH₂), which is significantly less than the 42.7 kcal/mol of BH₂NH₂ and also less than the 18.5 kcal/mol of BH₂PH₂ whose derivative, R₂PB(C₆F₅)₂, has been experimentally shown to be able to activate hydrogen. We also exemplified how to use the strategy to design experimentally more realizable molecules. The example shows promises as a hydrogen activation agent. The strategy can be used to design metal-free catalysts for direct hydrogenation.     

Thermodynamics and kinetics of alumina extraction from fly ash using an ammonium hydrogen sulfate roasting method

A novel method was developed for extracting alumina (Al₂O₃) from fly ash using an ammonium hydrogen sulfate (NH₄HSO₄) roasting process, and the thermodynamics and kinetics of this method were investigated. The thermodynamic results were verified experimentally. Thermodynamic calculations show that mullite present in the fly ash can react with NH₄HSO₄ in the 298–723 K range. Process optimization reveals that the extraction rate can reach up to 90.95% when the fly ash reacts with NH₄HSO₄ at a 1:8 mole ratio of Al₂O₃/NH₄HSO₄ at 673 K for 60 min. Kinetic analysis indicates that the NH₄HSO₄ roasting process follows the shrinking unreacted core model, and inner diffusion through the product layer is the rate-controlling step. The activation energy is calculated to be 16.627 kJ/mol; and the kinetic equation can be expressed as 1 ? (2/3)α ? (1 ? α)2/3 = 0.0374t exp[?16627/(RT)], where α is the extraction rate and t is the roasting temperature.     

Theoretical study of N(C)―H…H―B multi-dihydrogen bonds

The optimized geometries, frequencies and interaction energy corrected with basis set superposition error (BSSE) of the multi-dihydrogen bond complexes C₄H₄NH…BH₄^–. and CH≡CH…BH₄^–. have been calculated at both the B3LYP/6-311++G** and the MP2/6-311++G** levels. The calculations were per-formed to study the nature of the N―H…H3―B and C―H…H2―B red shift multi dihydrogen bond in complex C4H4NH…BH^–₄ and CH≡CH…BH4–. The BSSE-corrected multi-dihydrogen bond interaction en-ergy of complex I (C₄H₄NH…BH₄^–.) and complexⅡ(CH≡CH…BH₄^–.) is -76.62 and -33.79 kJ/mol (MP2/6- 311++G**), respectively. From the natural bond orbital（NBO）analysis, we detailedly discussed the orbital interactions, electron density transfers, rehybridizations and the essential of the correlative bond length changes in the two complexes. In addition, solvent effect on the geometric structures, vibration frequencies and interaction energy of the monomer and complexes was studied in detail. It is relevant to the relatively dielectric constants (ε).     

Low temperature hydrothermal synthesis and electrochemical performances of LiFePO4 microspheres as a cathode material for lithium-ion batteries

Mesoporous LiFePO4 microspheres were simply synthesized by a low temperature(130℃),template-free hydrothermal route using low cost LiOH,Fe(NO3)3 and NH4H2PO4 as starting raw materials.These microspheres are composed of densely aggregated LiFePO4 nanoparticles and filled with interconnected mesochannels,which demonstrates not only a high tap density(≥1.4 g cm-3),a high capacity of 150 mAh g-1(～90% of its theoretical capacity) at 0.5 C rate,but also a ≥ 80% utilization of its theoretical capacity at a high rate of 1 C.In addition,the hydrothermal synthesis developed in this work is simple and cost-effective,it may provide a new route for production of the LiFePO4 material in battery applications.     

Catalytic effect of a novel MgC_(0.5)Co_3 compound on the dehydrogenation of MgH_2

The application of magnesium hydride(MgH_2) is limited due to the high reaction temperature and slow kinetics during dehydrogenation. In order to ameliorate the dehydrogenation property of MgH_2, MgC_(0.5)Co_3 compound with induction and catalytic effects was introduced into the Mg/MgH_2 system via ball-milling and hydriding combustion methods in present study. Compared to the pure MgH_2,the initial hydrogen desorption temperature of MgH_2–MgC_(0.5)Co_3 composite lowered to 237°C, decreasing by 141°C. At 325°C the MgH_2–MgC_(0.5)Co_3 composite could release 4.38 wt% H_2 within 60 min, which is 4.5 times the capacity of hydrogen released by as milled-MgH_2. Besides, the hydrogen desorption activation energy of the MgH_2–MgC_(0.5)Co_3 composite was dramatically reduced to 126.7 ± 1.4 k J/mol. It was observed that MgC_(0.5)Co_3 was chemically stable and no chemical transformation occurred after cycling, which not only inhibited the nucleation and growth of composite particles, but also had a positive effect on the hydrogen desorption reaction of MgH_2 due to its catalytic effect.This study may provide references for designing and synthesizing Mg–C–Co alloy compound for the Mg-based hydrogen storage area.     

The preparation and performance of high activity Ni-Cr binary catalysts for direct borohydride fuel cells

Ni-Cr binary anode catalysts used in direct borohydride fuel cells (DBFCs) were prepared using a constant potential electrodepo- sition method. Compared with pure metal Ni catalysts, the electro-oxidation currents of borohydride ions (BH-4) more than doubled when using Ni-Cr binary catalysts under the same conditions. The enhanced activity of the Ni-Cr binary catalysts could be attributed to the change in distribution of BH-4ions on the surface of the Ni electrode. This is due to Cr electrodeposits, which allows a greater number of hydrogen atoms to catalyze from each tetrahedron BH-4 ion. The performance of Ni-Cr binary catalysts could also be improved by optimizing the electrodeposition conditions. The results show that Ni-Cr binary catalysts are optimally prepared using an electrodeposition method of 1s with a Cr3+ concentration of 0.2 mol L-1 and a potential of -0.100 V.     

Investigation on the coordinate structures of the rare earth metal complexes with edta and cydta ligands

0　IntroductionThegreatattentionhasbeenpaidtorareearthorradioactiverareearthmetalcomplexesfortheirvariousbiologicactivi ties[1 3] .Forexamples,theradioactiverareearthmetal1 53SmⅢ com plexeswerewidelyusedfortumourtherapyofbrain ,liver,lung ,heartandbonetissues[4 5] ;forrareearthmetalEuⅢandTbⅢemitinguniquefluorescence ,theircompoundswereappliedfordiagnosesofvariousdiseases[6 ,7] ;forrareearthmetalGdⅢcontainingthemosthigh spinsingleelectrons,itscomplexeswereusuallyusedforcontrastagentsofmag…     

Effects of the hierarchical pyrolysis polyaniline on reversible hydrogen storage of LiBH4

In this work,LiBH_4–20 wt%PP composite was prepared by ball-milling with as-synthesized hierarchical pyrolysis polyaniline(PP)and LiBH_4,and the hydrogen sorption performance as well as catalytic mechanism of the composite was studied.It is found that the onset desorption temperature of the composite decreases to 75°C,almost 235°C lower than that of the milled LiBH_4.Moreover,the composite could release 4.1 wt%H_2and rehydrogenate a total of 4.4 wt%H_2when the temperature raiseds up to 400°C,showing an outstanding reversibility,which even 3.9 wt%H_2can be kept after five cycles.Through scanning electron microscopy(SEM)observation and X-ray diffraction(XRD)analysis,we found that the PP surface forms some nanoholes after hydrogenation-dehydrogenation cycles,which leads to the confinement of some LiBH_4in the PP nanoporous structure,therefore,the hydrogen sorption kinetics and reversibility are significantly enhanced.In addition,we also found the oxygenic groups of the PP can react with LiBH_4forming LiBO_2and Li_3BO_3,where the containing Li–B–O bonds loaded in the porous structure of the PP catalyze the hydrogenation reaction of LiBH_4.     

EPR and UV—vis studies of biomimetic complexes of molybdoenzyme and tungstoenzyme

The biomimetic synthesis and the study of the ca-talysis mechanism of molybdenum and tungsten oxotransfer enzyme have been attracted considerable at-tention for a long time, because the oxotransfer enzymes play an important part in the nitrogen, sulfur and carbon cycles in the biological system[1—4]. With the development of the X-ray crystallography, it is possible to determine the structure of molybdenum and tungsten enzymes. However, due to the complexity and dedicated nature of the protein…     

Ammonium record over the last 96 years from the muztagata glacier in Central Asia

As a major alkaline gas in the atmosphere, ammonia （NH3） plays an important role in atmospheric chemistry. However, there is little knowledge about NH3 variations in the Central Asia. Here we analyzed the ammonium （NH4^＋） history recorded in an ice core from the East Pamir in Central Asia, which was drilled on the Mt. Muztagata at the elevation of 7010 m a.s.I, in 2003. The core was carefully dated and NH; concentration history during 1907-2002 was reconstructed. The result shows that NH; concentration remained approximately constant until the 1930s after a sudden decrease at the very beginning of the 20th century, followed by a minimum in 1940 before increasing steadily to the peak at the end of 1990s. It is found that the annual mean NH4^＋ concentration was strongly associated with the Northern Hemisphere temperature, suggesting the impact of temperature on NH3 emissions in the Central Asia. Moreover, an increase of NH4^＋ concentration after 1940 also reflects the enhancement of NH3 emissions from anthropogenic sources such as fertilizer applications and livestock wastes in the 20th century.     

Reaction condition optimization and kinetic investigation of roasting zinc oxide ore using (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript>

An orthogonal test was used to optimize the reaction conditions of roasting zinc oxide ore using (NH₄)₂SO₄. The optimized reaction conditions are defined as an (NH₄)₂SO₄/zinc molar ratio of 1.4:1, a roasting temperature of 440°C, and a thermostatic time of 60 min. The molar ratio of (NH₄)₂SO₄/zinc is the most predominant factor and the roasting temperature is the second significant factor that governs the zinc extraction. Thermogravimetric–differential thermal analysis was used for (NH₄)₂SO₄ and zinc mixed in a molar ratio of 1.4:1 at the heating rates of 5, 10, 15, and 20 K·min-1. Two strong endothermic peaks indicate that the complex chemical reactions occur at approximately 290°C and 400°C. XRD analysis was employed to examine the transformations of mineral phases during roasting process. Kinetic parameters, including reaction apparent activation energy, reaction order, and frequency factor, were calculated by the Doyle–Ozawa and Kissinger methods. Corresponding to the two endothermic peaks, the kinetic equations were obtained.     

Excited-state solvation dynamics of meso-tetrakis(4-sulfonatophenyl) porphyrin in solid imidazolium-sulfonate-based ionic liquids

The steady-state absorption and fluorescence emission positions of diprotonated meso-tetrakis(4-sulfonatophenyl) porphyrin (H₄TPPS^2?) are dependent on the polarity of the selected two solid ionic liquids (ILs) and are red-shifted with the increase of cation size. The solvation dynamics process of H₄TPPS^2? in these ILs occurs on two well-separated time scales. The short components with 121.2–128.6 ps arise from the local motion of the ion-pairs in close proximity to the porphine core, and the long components with 1056.6–1261.8 ps are due to the collective translation motions of the ion-pairs. The dynamic Stokes shifts and the relaxation times increase with the increasing cation size of the ILs.     

Study on the hydrogen storage property of (TiZr0.1)xCr1.7-yFeyMn0.3 (1.05

Jigang Li  Li Xu  Xiaojing Jiang  Xingguo Li 《自然科学进展(英文版)》2018,28(4):470-477

Improved hydrogen desorption properties of Co-doped Li<Subscript>2</Subscript>BNH<Subscript>6</Subscript>

The hydrogen desorption properties of Li 2 BNH 6 were improved by doping with cobalt. With the addition of CoCl 2 (7 wt%), more than 8 wt% of hydrogen was released from Li 2 BNH 6 at temperatures below 210°C, which is approximately 90°C lower than that of pristine Li 2 BNH 6 . X-ray diffraction, Fourier transform-infrared and Raman characterizations revealed that the dehydrogenation was a stepwise process with the formation of intermediates Li 4 BN 3 H 10 and LiBH 4 and final products of Li 3 BN 2 and LiH. The introduction of Co greatly accelerated the dehydrogenation of Li 4 BN 3 H 10 . X-ray absorption near-edge structure measurements revealed that Co and CoB species formed during ball milling of CoCl 2 with LiBH 4 and LiNH 2 , which may function as catalyst in the subsequent dehydrogenation.     

Kinetics of C2(a3Πu) radical reactions with NO, N2O, O2, H2 and NH3

Pulsed laser photolysis/laser-induced fluorescence (LP-LIF) is utilized to measure rate constants for C₂(a³Π_u) reactions with NO, N₂O, O₂, H₂ and NH₃. Multiphoton dissociation of C₂Cl₄ at 266 nm is employed for the generation of C₂(a³Π_u) radicals. The C₂(a³Π_u) concentration is monitored by the fluorescence of the (0, 0) band of the (d³Π_g&#8596;a³Π_u) transition at 516.5 nm. C₂(a³Π_u) removal rate constants for the reactions are determined as k_NO = (5.46 ± 0.10) × 10^-11 cm³ molecule^-1 s^-1 , k_N2O = (1.63 ± 0.20) × 10^-13 cm³ molecule^-1 s^-1 , k_N2O = (1.58 ± 0.16) × 10^-11 cm³ molecule^-1 s^-1, k_O2 = (5.92 ± 1.00) × 10^-14 cm³ molecule^-1 s^-1, k_H2< 1.0× 10^-14 cm³ molecule^-1 s^-1. Based on the data analysis and theoretical calculation, we suggest that the C₂(a³Π_u) reactions with H₂ and NH₃ proceed via the hydrogen abstraction mechanism, barriers exist at the entrance channel of the reactions of C₂(a³Π_u) with H₂ and NH₃.     

CNa42+团簇的结构及其储氢性能研究

氢是一种理想的二次能源,它将成为化石燃料最有希望的替代能源之一,也是亟待开发的重要能源。而氢能的储存成本高,危险性大是急需解决的问题。理论上预测CNa_4~(2+)的储氢性能,通过理论分析,发现了每个CNa_4~(2+)团簇最多可与16个H2分子有效结合,获得23.5%的质量储氢密度。在B3LYP理论水平上,H2分子与CNa_4~(2+)团簇的平均相互作用能在2.107～4.948Kcal/mol之间。由于CNa_4~(2+)的质量储氢密度在7.1～23.7wt%之间,符合美国能源部的要求目标(5.5 wt%)。研究结果表明,CNa_4~(2+)在一定环境条件下可逆吸放氢性能良好,可作为潜在的理想高容量储氢材料。     

Structural and electrical properties of HCl-polyaniline-Ag composites synthesized by polymerization using Ag-coated (NH4)2S2O8 powder

Ag nanoparticles were sputter-deposited on ammonium persulfate ((NH₄)₂S₂O₈) powder to obtain (NH₄)₂S₂O₈-Ag powder, which was used to synthesize the HCl-doped polyaniline-Ag (HCl-PANI-Ag) composite via a polymerization procedure. The Ag nanoparticles were dispersed in the HCl-PANI matrix, and their sizes mainly ranged from 3 to 6 nm. The Ag nanoparticles did not affect the structure of emeraldine salt in the composite, and they increased the ordered crystalline regions in the HCl-PANI matrix. The HCl-PANI-Ag composite had a conductivity of (6.8 ±0.1) S/cm, which is about four times larger than that of the HCl-PANI. The charge transport mechanism in the composite is explained by the three-dimensional Mott variable-range hopping (3D-Mott-VRH).