Hydrogen storage in magnesium clusters: quantum chemical study

Magnesium hydride is cheap and contains 7.7 wt % hydrogen, making it one of the most attractive hydrogen storage materials. However, thermodynamics dictate that hydrogen desorption from bulk magnesium hydride only takes place at or above 300 degrees C, which is a major impediment for practical application. A few results in the literature, related to disordered materials and very thin layers, indicate that lower desorption temperatures are possible. We systematically investigated the effect of crystal grain size on the thermodynamic stability of magnesium and magnesium hydride, using ab initio Hartree-Fock and density functional theory calculations. Also, the stepwise desorption of hydrogen was followed in detail. As expected, both magnesium and magnesium hydride become less stable with decreasing cluster size, notably for clusters smaller than 20 magnesium atoms. However, magnesium hydride destabilizes more strongly than magnesium. As a result, the hydrogen desorption energy decreases significantly when the crystal grain size becomes smaller than approximately 1.3 nm. For instance, an MgH2 crystallite size of 0.9 nm corresponds to a desorption temperature of only 200 degrees C. This predicted decrease of the hydrogen desorption temperature is an important step toward the application of Mg as a hydrogen storage material.     

Modeling the sorption dynamics of NaH using a reactive force field

We have parametrized a reactive force field for NaH, ReaxFF(NaH), against a training set of ab initio derived data. To ascertain that ReaxFF(NaH) is properly parametrized, a comparison between ab initio heats of formation of small representative NaH clusters with ReaxFF(NaH) was done. The results and trend of ReaxFF(NaH) are found to be consistent with ab initio values. Further validation includes comparing the equations of state of condensed phases of Na and NaH as calculated from ab initio and ReaxFF(NaH). There is a good match between the two results, showing that ReaxFF(NaH) is correctly parametrized by the ab initio training set. ReaxFF(NaH) has been used to study the dynamics of hydrogen desorption in NaH particles. We find that ReaxFF(NaH) properly describes the surface molecular hydrogen charge transfer during the abstraction process. Results on heat of desorption versus cluster size shows that there is a strong dependence on the heat of desorption on the particle size, which implies that nanostructuring enhances desorption process. To gain more insight into the structural transformations of NaH during thermal decomposition, we performed a heating run in a molecular dynamics simulation. These runs exhibit a series of drops in potential energy, associated with cluster fragmentation and desorption of molecular hydrogen. This is consistent with experimental evidence that NaH dissociates at its melting point into smaller fragments.     

Synthesis, structural and hydrogenation properties of Mg-rich MgH2-TiH2 nanocomposites prepared by reactive ball milling under hydrogen gas

MgH(2)-TiH(2) nanocomposites have been obtained by reactive ball milling of elemental powders under 8 MPa of hydrogen pressure. The composites consist of a mixture of β-rutile MgH(2), γ-orthorhombic high pressure MgH(2) and ε-tetragonal TiH(2) phases with nanosized crystallites ranging from 4 to 12 nm. In situ hydrogen absorption curves on milling reveal that nanocomposite formation occurs in less than 50 min through the consecutive synthesis of the TiH(2) and MgH(2) phases. The abrasive and catalytic properties of TiH(2) speed up the formation of the MgH(2) phase. Thermodynamic, kinetic and cycling hydrogenation properties have been determined for the 0.7MgH(2)-0.3TiH(2) composite and compared to nanometric MgH(2). Only the MgH(2) phase desorbs hydrogen reversibly at moderate temperature (523 to 598 K) and pressure (10(-3) to 1 MPa). The presence of TiH(2) does not modify the thermodynamic properties of the Mg/MgH(2) system. However, the MgH(2)-TiH(2) nanocomposite exhibits outstanding kinetic properties and cycling stability. At 573 K, H-sorption takes place in less than 100 s. This is 20 times faster than for a pure nanometric MgH(2) powder. We demonstrate that the TiH(2) phase inhibits grain coarsening of Mg, which allows extended nucleation of the MgH(2) phase in Mg nanoparticles before a continuous and blocking MgH(2) hydride layer is formed. The low crystallinity of the TiH(2) phase and its hydrogenation properties are also compatible with a gateway mechanism for hydrogen transfer from the gas phase to Mg. Mg-rich MgH(2)-TiH(2) nanocomposites are an excellent media for hydrogen storage at moderate temperatures.     

Body centered cubic magnesium niobium hydride with facile room temperature absorption and four weight percent reversible capacity

We have synthesized a new metastable metal hydride with promising hydrogen storage properties. Body centered cubic (bcc) magnesium niobium hydride (Mg(0.75)Nb(0.25))H(2) possesses 4.5 wt% hydrogen gravimetric density, with 4 wt% being reversible. Volumetric hydrogen absorption measurements yield an enthalpy of hydride formation of -53 kJ mol(-1) H(2), which indicates a significant thermodynamic destabilization relative to the baseline -77 kJ mol(-1) H(2) for rutile MgH(2). The hydrogenation cycling kinetics are remarkable. At room temperature and 1 bar hydrogen it takes 30 minutes to absorb a 1.5 μm thick film at sorption cycle 1, and 1 minute at cycle 5. Reversible desorption is achieved in about 60 minutes at 175 °C. Using ab initio calculations we have examined the thermodynamic stability of metallic alloys with hexagonal close packed (hcp) versus bcc crystal structure. Moreover we have analyzed the formation energies of the alloy hydrides that are bcc, rutile or fluorite.     

High-throughput synthesis and screening of hydrogen-storage alloys

Libraries of mixed-metal hydride materials are synthesized on a silicon microfabricated array of "hot-plate" MEMS devices, which allow high-throughput screening using temperature programmed desorption and infrared thermography. The heating plate of the MEMS device is a membrane with low heat capacity, allowing fast and localized temperature control and the extraction of calorimetric data from thermography. The combination of the synthetic method and screening chip allows a fast determination of the desorption temperature and hydrogen content of the materials. Mixed metal hydrides are synthesized directly. The potential of the method is exemplified by presenting results for the sorption properties of Mg xNi 1- x hydride thin-film materials. The results are consistent with the literature, showing the highest hydrogen capacity and desorption temperature for the MgH 2 phase in Mg-rich compositions and the promotion of a lower temperature desorption from the Mg 2NiH 4 phase, with a concomitant reduction in hydrogen capacity.     

Mg2 FeH6 纳米晶的制备及储氢性能

在室温和氩气气氛下, 以MgH2 和纳米Fe为原料, 采用机械合金化(球磨法)制备了Mg2FeH6纳米晶. 考察了球磨参数(时间、 转速)对产物的影响, 对所制备的Mg2FeH6 纳米晶的组成、 结构和形貌进行了表征, 并对其储氢性能进行了测试. 结果表明, 所制备的Mg2FeH6纳米晶为立方结构, 纯度较高(91.4%), 其晶粒尺寸较小, 约为10~30 nm, 但团聚现象较为严重. Mg2FeH6纳米晶具有较低的活化能和较好的吸放氢动力学性能, 其放氢的脱附焓和脱附熵分别为(-42.8±2) kJ/mol和(-72.0±3) J/(mol·K). 在503 K和6 kPa的氢气压力下, Mg2FeH6纳米晶在70 min内放氢量达到2.5%(质量分数); 在2 MPa的氢气压力下, 上述放氢产物具有较快的起始吸氢速率.     

Well‐Defined Molecular Magnesium Hydride Clusters: Relationship between Size and Hydrogen‐Elimination Temperature

A new tetranuclear magnesium hydride cluster, [{ NN ‐(MgH)₂}₂], which was based on a N? N‐coupled bis‐β‐diketiminate ligand ( NN ^2?), was obtained from the reaction of [{ NN ‐(MgnBu)₂}₂] with PhSiH₃. Its crystal structure reveals an almost‐tetrahedral arrangement of Mg atoms and two different sets of hydride ions, which give rise to a coupling in the NMR spectrum (J=8.5 Hz). To shed light on the relationship between the cluster size and H₂ release, the thermal decomposition of [{ NN ‐(MgH)₂}₂] and two closely related systems that were based on similar ligands, that is, an octanuclear magnesium hydride cluster and a dimeric magnesium hydride species, have been investigated in detail. A lowering of the H₂‐desorption temperature with decreasing cluster size is observed, in line with previously reported theoretical predictions on (MgH₂)_n model systems. Deuterium‐labeling studies further demonstrate that the released H₂ solely originates from the oxidative coupling of two hydride ligands and not from other hydrogen sources, such as the β‐diketiminate ligands. Analysis of the DFT‐computed electron density in [{ NN ‐(MgH)₂}₂] reveals a counterintuitive interaction between two formally closed‐shell H^? ligands that are separated by 3.106 Å. This weak interaction could play an important role in H₂ desorption. Although the molecular product after H₂ release could not be characterized experimentally, DFT calculations on the proposed decomposition product, that is, the low‐valence tetranuclear Mg(I) cluster [( NN ‐Mg₂)₂], predict a structure with two almost‐parallel, localized Mg? Mg bonds. As in a previously reported β‐diketiminate Mg^I dimer, the Mg? Mg bond is not characterized by a bond critical point, but instead displays a local maximum of electron density midway between the atoms, that is, a non‐nuclear attractor (NNA). Interestingly, both of the NNAs in [( NN ‐Mg₂)₂] are connected through a bond path that suggests that there is bonding between all four Mg^I atoms.     

Efficient hydrogen storage with the combination of lightweight Mg/MgH2 and nanostructures

Efficient hydrogen storage plays a key role in realizing the incoming hydrogen economy. However, it still remains a great challenge to develop hydrogen storage media with high capacity, favourable thermodynamics, fast kinetics, controllable reversibility, long cycle life, low cost and high safety. To achieve this goal, the combination of lightweight materials and nanostructures should offer great opportunities. In this article, we review recent advances in the field of chemical hydrogen storage that couples lightweight materials and nanostructures, focusing on Mg/MgH(2)-based systems. Selective theoretical and experimental studies on Mg/MgH(2) nanostructures are overviewed, with the emphasis on illustrating the influences of nanostructures on the hydrogenation/dehydrogenation mechanisms and hydrogen storage properties such as capacity, thermodynamics and kinetics. In particular, theoretical studies have shown that the thermodynamics of Mg/MgH(2) clusters below 2 nm change more prominently as particle size decreases.     

Remarkable hydrogen storage properties for nanocrystalline MgH2 synthesised by the hydrogenolysis of Grignard reagents

The possibility of generating MgH(2) nanoparticles from Grignard reagents was investigated. To this aim, five Grignard compounds, i.e. di-n-butylmagnesium, tert-butylmagnesium chloride, allylmagnesium bromide, m-tolylmagnesium chloride, and methylmagnesium bromide were selected for the potential inductive effect of their hydrocarbon group in leading to various magnesium nanostructures at low temperatures. The thermolysis of these Grignard reagents was characterised in order to determine the optimal conditions for the formation of MgH(2). In particular, the use of di-n-butylmagnesium was found to lead to self-assembled and stabilized nanocrystalline MgH(2) structures with an impressive hydrogen storage capacity, i.e. 6.8 mass%, and remarkable hydrogen kinetics far superior to that of milled or nanoconfined magnesium. Hence, it was possible to achieve hydrogen desorption without any catalyst at 250 °C in less than 2 h, while at 300 °C, hydrogen desorption took only 15 min. These superior performances are believed to result from the unique physical properties of the MgH(2) nanocrystalline architecture obtained after hydrogenolysis of di-n-butylmagnesium.     

Hydrogen cycling of niobium and vanadium catalyzed nanostructured magnesium

The reaction of hydrogen gas with magnesium metal, which is important for hydrogen storage purposes, is enhanced significantly by the addition of catalysts such as Nb and V and by using nanostructured powders. In situ neutron diffraction on MgNb(0.05) and MgV(0.05) powders give a detailed insight on the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During the early stage of hydriding (and deuteriding), a MgH(1< x < 2) phase is observed, which does not occur in bulk MgH(2) and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of nanostructured magnesium. It is shown that under relevant experimental conditions, the niobium catalyst is present as NbH(1). Second, a hitherto unknown Mg-Nb perovskite phase could be identified that has to result from mechanical alloying of Nb and the MgO layer of the particles. Vanadium is not visible in the diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2-20 nm. Nanostructuring and catalyzing the Mg enhance the adsorption speed that much that now temperature variations effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH(2) layers.     

Binding of molecular magnesium hydrides to a zirconocene-enyne template

An enyne-zirconium complex stabilizes molecular magnesium hydride (MgH(2) ) and even a molecular hydride, nC(4) H(9) MgH. These systems feature magnesium olefin π?complexation.     

Altered thermodynamic and kinetic properties of MgH(2) infiltrated in microporous scaffold

MgH(2) nanoparticles with a size of <3 nm were formed by direct hydrogenation of Bu(2)Mg inside the pores of a carbon scaffold. The activation energy for the dehydrogenation was lowered by 52 kJ mol(-1) compared to the bulk material, and a significantly reduced reaction enthalpy of 63.8 ± 0.5 kJ mol(-1) and entropy (117.2 ± 0.8 J mol(-1)) was found for the nanoconfined system.     

Theoretical study on the small clusters of LiH, NaH, BeH(2), and MgH(2)

High-level ab initio molecular orbital theory is used to calculate the geometries, vibrational frequencies, atomic charges, and binding energies of the small clusters (LiH)(n), (NaH)(n), (BeH(2))(n), and (MgH(2))(n) (n = 1-4). For (LiH)(n) and (NaH)(n), there are planar cyclic structures when n = 2, 3. We have found the cubic structure T(d) in addition to the planar cyclic D(4)(h) when n = 4. The D(4)(h) is less stable than the T(d) geometry. For (BeH(2))(n) and (MgH(2))(n), when n = 3, there are three kinds of structures: chain C(2)(v), planar cyclic D(3)(h), and hat-like C(2)(v). The C(2)(v) geometry is more stable than the others. When n = 4, there are four kinds of structures: chain D(2)(h), cubic T(d), string-like C(2), and cubic transformation C(1). The most stable compounds in the families of (LiH)(n), (NaH)(n), (BeH(2))(n), and (MgH(2))(n) are cubic T(d), cubic T(d), chain D(2)(h), and string-like C(2) geometries, respectively, when n = 4. Calculated binding energies range from -24 to -37 kcal/mol for (LiH)(n) and --19 to -30 kcal/mol for (NaH)(n), (BeH(2))(n), and (MgH(2))(n). The hydrogen atoms in hydride clusters always have negative charges. The atomic charges of planar cyclic structures are weaker than those of cubic structures, and there is a tendency of reducing along with the increase of the cluster size. The vibrational frequencies of planar cyclic structures have consistent tendency, too. It indicates that the bond distance increases with the ionic character of the bond.     

Determination of the heat of hydride formation/decomposition by high-pressure differential scanning calorimetry (HP-DSC)

Among the thermodynamic properties of novel materials for solid-state hydrogen storage, the heat of formation/decomposition of hydrides is the most important parameter to evaluate the stability of the compound and its temperature and pressure of operation. In this work, the desorption and absorption behaviors of three different classes of hydrides are investigated under different hydrogen pressures using high-pressure differential scanning calorimetry (HP-DSC). The HP-DSC technique is used to estimate the equilibrium pressures as a function of temperature, from which the heat of formation is derived. The relevance of this procedure is demonstrated for (i) magnesium-based compounds (Ni-doped MgH2), (ii) Mg-Co-based ternary hydrides (Mg-CoHx) and (iii) Alanate complex hydrides (Ti-doped NaAlH4). From these results, it can be concluded that HP-DSC is a powerful tool to obtain a good approximation of the thermodynamic properties of hydride compounds by a simple and fast study of desorption and absorption properties under different pressures.     

Intensity and wavelength control of a single molecule reaction: simulation of photodissociation of cold-trapped MgH+

Photodissociation of cold magnesium hydride ions MgH(+) leading to either Mg(+)+H or Mg+H(+) is simulated from first principles. The purpose is to study the possibility of single molecule control of the products in the presence of two laser fields. The system evolves on four electronic potential-energy curves, X(1) Sigma, A(1) Sigma, B(1) Pi, and C(1) Sigma. These potential-energy curves are calculated from first principles using multireference self-consistent field theory. The accuracy of the electronic potential curves has been checked by calculating the energies of the rovibrational eigenstates and comparing them to experimental findings. The photodissociation dynamics has furthermore been simulated by solving the time-dependent Schrodinger equation. It is shown that the branching ratio of the two dissociation channels, Mg(+)+H or Mg+H(+), can be controlled by changing the intensity and wavelength of the two driving laser fields.     

Interface reactions and stability of a hydride composite (NaBH4 + MgH2)

The use of the interaction of two hydrides is a well-known concept used to increase the hydrogen equilibrium pressure of composite mixtures in comparison to that of pure systems. The thermodynamics and reaction kinetics of such hydride composites are reviewed and experimentally verified using the example NaBH(4) + MgH(2). Particular emphasis is placed on the measurement of the kinetics and stability using thermodesorption experiments and measurements of pressure-composition isotherms, respectively. The interface reactions in the composite reaction were analysed by in situ X-ray photoelectron spectroscopy and by simultaneously probing D(2) desorption from NaBD(4) and H(2) desorption from MgH(2). The observed destabilisation is in quantitative agreement with the calculated thermodynamic properties, including enthalpy and entropy. The results are discussed with respect to kinetic limitations of the hydrogen desorption mechanism at interfaces. General aspects of modifying hydrogen sorption properties via hydride composites are given.     

Reversible Dihydrogen Activation and Catalytic H/D Exchange with Group 10 Heterometallic Complexes**

Reaction of a hexagonal planar palladium complex featuring a [PdMg₃H₃] core with H₂ is reversible and leads to the formation of a new [PdMg₂H₄] tetrahydride species alongside an equivalent of a magnesium hydride co-product [MgH]. While the reversibility of this process prevented isolation of [PdMg₂H₄], analogous [PtMg₂H₄] and [PtZn₂H₄] complexes could be isolated and characterised through independent syntheses. Computational analysis (DFT, AIM, NCIPlot) of the bonding in a series of heterometallic tetrahydride compounds (Ni–Pt; Mg and Zn) suggests that these complexes are best described as square planar with marginal metal-metal interactions; the strength of which increases slightly as group 10 is descended and increases from Mg to Zn. DFT calculations support a mechanism for H₂ activation involving a ligand-assisted oxidative addition to Pd. These findings were exploited to develop a catalytic protocol for H/D exchange into magnesium hydride and zinc hydride bonds.     

Effect of formic acid addition on water cluster stability and structure

Computational chemistry simulations were performed to determine the effect that the addition of a single formic acid molecule has on the structure and stability of protonated water clusters. Previous experimental studies showed that addition of formic acid to protonated pure water results in higher intensities of large-sized clusters when compared to pure water and methanol-water mixed clusters. For larger, protonated clusters, molecular dynamics simulations were performed on H(+)(H(2)O)(n), H(+)(H(2)O)(n)CH(3)OH, and H(+)(H(2)O)(n)CHOOH clusters, 19-28 molecules in size, using a reactive force field (ReaxFF). Based on these computations, formic acid-water clusters were found to have significantly higher binding energies per molecule. Addition of formic acid to a water cluster was found to alter the structure of the hydrogen-bonding network, creating selective sites within the cluster, enabling the formation of new hydrogen bonds, and increasing both the stability of the cluster and its rate of growth.     

Self-assembled mesoporous Co and Ni-ferrite spherical clusters consisting of spinel nanocrystals prepared using a template-free approach

Based on a self-assembly strategy, spherical mesoporous cobalt and nickel ferrite nanocrystal clusters with a large surface area and narrow size distribution were successfully synthesized for the first time via a template-free solvothermal process in ethylene glycol and subsequent heat treatment. In this work, the mesopores in the ferrite clusters were derived mainly from interior voids between aggregated primary nanoparticles (with crystallite size of less than 7 nm) and disordered particle packing domains. The concentration of sodium acetate is shown herein to play a crucial role in the formation of mesoporous ferrite spherical clusters. These ferrite clusters were characterized in detail using wide-angle X-ray diffraction, thermogravimetric-differential thermal analysis, (57)Fe M?ssbauer spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, standard and high-resolution transmission electron microscopy, and other techniques. The results confirmed the formation of both pure-phase ferrite clusters with highly crystalline spinel structure, uniform size (about 160 nm) and spherical morphology, and worm-like mesopore structures. The BET specific surface areas and mean pore sizes of the mesoporous Co and Ni-ferrite clusters were as high as 160 m(2) g(-1) and 182 m(2) g(-1), and 7.91 nm and 6.87 nm, respectively. A model for the formation of the spherical clusters in our system is proposed on the basis of the results. The magnetic properties of both samples were investigated at 300 K, and it was found that these materials are superparamagnetic.     

Formation of palladium hydride nanoparticles in Pd/C catalyst as evidenced by <Emphasis Type="Italic">in situ</Emphasis> XAS data

X-ray absorption spectroscopy was used to investigate the formation of palladium hydride upon treatment of metallic Pd nanoparticles 0.9–2 nm in size supported on carbon-carbonaceous material SIB UNIT? with hydrogen. Nanoparticles with the size as small as 0.9 nm can still form hydride PdH. Changes in the chemical composition of gaseous medium cause a reversible transformation of metallic palladium nanoparticles to palladium hydride nanoparticles with tentative stoichiometry PdH_0.6.