Why Hydrogen Dissociation Catalysts do not Work for Hydrogenation of Magnesium

Provision of atomic hydrogen by hydrogen dissociation catalysts only moderately accelerates the hydrogenation rate of magnesium. They shed light on this well-known but technically challenging fact through a combined approach using an unconventional surface science technique together with Density Functional Theory (DFT) calculations. The calculations demonstrate the drastic electronic structure changes during transformation of Mg to MgH2 , which make fractional hydrogen coverage on the surface, as well as substoichiometric hydrogen content in the bulk energetically unfavorable. Reflecting Electron Energy Loss Spectroscopy (REELS) is used to measure the surface and bulk plasmon during hydrogen sorption in magnesium. The measurements show that the hydrogenation proceeds via the growth of magnesium hydride without the presence of chemisorbed hydrogen on the metallic magnesium surface exactly as indicated by the calculations. This is due to the low stability of sub-stoichiometric amounts of chemisorbed H correlating with the unfavorable charge state of Mg. They are merely bound to the unchanged adjacent Mg layers, thereby explaining the failure of classical hydrogenation catalysts, which effectively only hydrogenate Mg in their direct vicinity. The acceleration of hydrogen sorption kinetics in Mg must affect the polarization in the interface between Mg and MgH2 during hydrogenation.

Structural Phase Transitions in closo-Dicarbadodecaboranes C2B10H12

The crystal structures of three thermal polymorphs (I, II, and III) for each isomer of closo-dicarbadodecaboranes C₂B₁₀H₁₂ (ortho, meta, and para) have been determined by combining synchrotron radiation X-ray powder diffraction and density functional theory calculations. The structures are in agreement with previous calorimetric and spectroscopic studies. The difference between rotatory phases (plastic crystals) I and II lies in isotropic rotations in the former and anisotropic rotations of the icosahedral clusters in the latter. Phase I is the cubic close packing (ccp) of rotating closo-molecules C₂B₁₀H₁₂ in the space group Fm3̅. Phase II is the ccp of rotating closo-molecules C₂B₁₀H₁₂ in the cubic space group Pa3̅. The preferred rotational axis in II varies with the isomer. The ordered phases III are orthorhombic (meta) or monoclinic (ortho and para) deformations of the cubic unit cell of the disordered phases I and II. The ordering in the phase III of the ortho-isomer carrying the biggest electrical dipole moment creates a twofold superstructure w.r.t. the cubic unit cell. The thermal polymorphism for C₂B₁₀H₁₂ and related metal salts can be explained by division of the cohesive intercluster interactions into two categories (i) dispersive cohesive interaction with additional Coulombic components in the metal salts and (ii) anisotropic local interaction resulting from nonuniform charge distribution around icosahedral clusters. The local interactions are averaged out by thermally activated cluster dynamics (rotations and rotational jumps) which effectively increase the symmetry of the cluster. The C₂B₁₀H₁₂ molecules resist at least as well as the CB₁₁H₁₂^- anion to the oxidation, and both clusters form easily a mixed compound. This allows designing solid electrolytes such as Na_x(CB₁₁H₁₂)_x(C₂B₁₀H₁₂)_1-x, where the cation content may be varied and the temperature of transition into the disordered conducting phase is decreased.

Study of the Temperature- and Pressure-Dependent Structural Properties of Alkali Hydrido-closo-borate Compounds

In this work, we report on the structural properties of alkali hydrido-closo-(car)borates, a promising class of solid-state electrolyte materials, using high-pressure and temperature-dependent X-ray diffraction experiments combined with density functional theory (DFT) calculations. The mechanical properties are determined via pressure-dependent diffraction studies and DFT calculations; the shear moduli appear to be very low for all studied compounds, revealing their high malleability (that can be beneficial for the manufacturing and stable cycling of all-solid-state batteries). The thermodiffraction experiments also reveal a high coefficient of thermal expansion for these materials. We discover a pressure-induced phase transition for K₂B₁₂H₁₂ from Fm3̅ to Pnnm symmetry around 2 GPa. A temperature-induced phase transition for Li₂B₁₀H₁₀ was also observed for the first time by thermodiffraction, and the crystal structure determined by combining experimental data and DFT calculations. Interestingly, all phases of the studied compounds (including newly discovered high-pressure and high-temperature phases) may be related via a group–subgroup relationship, with the notable exception of the room-temperature phase of Li₂B₁₀H₁₀.

Herein, we study the pressure and temperature dependencies of alkali hydrido-closo-borates in extracting the mechanical properties of this class of compounds that have a promising future as solid electrolytes. In our research, we have discovered and determined two new high-pressure and high-temperature crystal structures.

Surface Properties of the Hydrogen-Titanium System

Titanium is an excellent getter material, catalyzes gas-solid reactions such as hydrogen absorption in lightweight metal hydrides and complex metal hydrides and has recently been shown as a potential ammonia synthesis catalyst. However, knowledge of the surface properties of this metal is limited when it absorbs large quantities of hydrogen at operation conditions. Both the conceptual description of such a surface as well as the experimental determination of surface hydrogen concentration on hydride-forming metals is challenging due to the dynamic bulk properties and the incompatibility of traditional surface science methods with the hydrogen pressure needed to form the metal hydride, respectively. In this paper, the surface pressure-composition isotherms of the titanium-hydrogen system are measured by operando reflecting electron energy loss spectroscopy (REELS). The titanium thin films were deposited on and hydrogenated through a palladium membrane, which provides an atomic hydrogen source under ultrahigh vacuum conditions. The measurements are supported by density functional theory calculations providing a complete picture of the hydrogen-deficient surface of TiH₂ being the basis of its high catalytic activity.

Response to Comment on "High-surface-area corundum by mechanochemically induced phase transformation of boehmite"

Li et al commented that our report claims that methods reported thus far cannot enable the production of high-purity corundum with surface areas greater than 100 m² g^-1, and that our obtained material could be porous aggregates rather than nanoparticles. We disagree with both of these suggestions.

High-surface-area corundum by mechanochemically induced phase transformation of boehmite

In its nanoparticulate form, corundum (α-Al2O3) could lead to several applications. However, its production into nanoparticles (NPs) is greatly hampered by the high activation energy barrier for its formation from cubic close-packed oxides and the sporadic nature of its nucleation. We report a simple synthesis of nanometer-sized α-Al2O3 (particle diameter ~13 nm, surface areas ~140 m2 g−1) by the mechanochemical dehydration of boehmite (γ-AlOOH) at room temperature. This transformation is accompanied by severe microstructural rearrangements and might involve the formation of rare mineral phases, diaspore and tohdite, as intermediates. Thermodynamic calculations indicate that this transformation is driven by the shift in stability from boehmite to α-Al2O3 caused by milling impacts on the surface energy. Structural water in boehmite plays a crucial role in generating and stabilizing α-Al2O3 NPs.

Pressure-induced phase transitions in Na2B12H12, structural investigation on a candidate for solid-state electrolyte

closo-Borates, such as Na2B12H12, are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressure in situ powder X-ray diffraction on Na2B12H12 was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primary-order parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling.

Theoretical description of alkali metal closo-boranes - towards the crystal structure of MgB12H12

Solid state closo-borane salts of alkali metals have very high ionic conductivity. This makes them interesting for practical applications as solid state electrolytes, and has triggered extensive research efforts. Improvement and understanding of their properties require accurate theoretical description of their static and dynamical properties. In this work, we report accuracy assessment of density functional theory in the description of solids with B12H122- anions. We show that these aromatic anions interact via weak dispersive forces. For that reason, non-local exchange-correlation functionals give better description of structural properties and phonons in Li2B12H12 and Na2B12H12. Numerically efficient semi-local methods provide satisfactory results when applied in structure volumes obtained in a non-local method. An extensive structural search for stable crystalline phases of MgB12H12 predicts a new denser lattice with C2/c symmetry that is stabilized by van der Waals interactions. These structures might be discovered as anhydrous MgB12H12 in high pressure experiments, avoiding the amorphous state at ambient pressures.

Dynamics of the Coordination Complexes in a Solid-State Mg Electrolyte

Coordination complexes of magnesium borohydride show promising properties as solid electrolytes for magnesium ion batteries and warrant a thorough microscopic description of factors governing their mobility properties. Here, the dynamics of Mg(BH₄)₂-diglyme_0.5 on the atomic level are investigated by means of quasielastic neutron scattering supported by density functional theory calculations and IR and NMR spectroscopy. Employing deuterium labeling, we can unambiguously separate all the hydrogen-containing electrolyte components, which facilitate Mg²⁺ transport, and provide a detailed analytical description of their motions on the picosecond time scale. The planar diglyme chain coordinating the central Mg atom appears to be flexible, while two dynamically different groups of [BH₄]^- anions undergo reorientations. The latter has important implications for the thermal stability and conductivity of Mg(BH₄)₂-diglyme_0.5 and demonstrates that the presence of excess Mg(BH₄)₂ units in partially chelated Mg complexes may improve the overall performance of related solid-state electrolytes.

Synthesis and crystal structure of solvent-free dodecahydro closo-dodecaborate of nickel, NiB12H12

Ball milling of Na₂B₁₂H₁₂ + NiCl₂ mixtures followed by hydration in air produces closo-borate containing octahedral hexa-aqua complex. The water molecules are easily removed by heating under dynamic vacuum leading first to octahedral tetra-aqua complex, and then to anhydrous pseudo-cubic NiB₁₂H₁₂.

Solid Aluminum Borohydrides for Prospective Hydrogen Storage

Metal borohydrides are intensively researched as high-capacity hydrogen storage materials. Aluminum is a cheap, light, and abundant element and Al³⁺ can serve as a template for reversible dehydrogenation. However, Al(BH₄ )₃ , containing 16.9 wt % of hydrogen, has a low boiling point, is explosive on air and has poor storage stability. A new family of mixed-cation borohydrides M[Al(BH₄ )₄ ], which are all solid under ambient conditions, show diverse thermal decomposition behaviors: Al(BH₄ )₃ is released for M=Li⁺ or Na⁺ , whereas heavier derivatives evolve hydrogen and diborane. NH₄ [Al(BH₄ )₄ ], containing both protic and hydridic hydrogen, has the lowest decomposition temperature of 35 °C and yields Al(BH₄ )₃ ⋅NHBH and hydrogen. The decomposition temperatures, correlated with the cations' ionic potential, show that M[Al(BH₄ )₄ ] species are in the most practical stability window. This family of solids, with convenient and versatile properties, puts aluminum borohydride chemistry in the mainstream of hydrogen storage research, for example, for the development of reactive hydride composites with increased hydrogen content.

Comparative single atom heterogeneous catalysts (SAHCs) on different platforms: a theoretical approach

The nature of SAHC interactions with the matrix is crucial as it controls the electronic structure of the atom, its charge, the coordination pattern and the overall catalytic ensemble. We have checked all these aspects by studying the same single atom in oxides, metals and carbon nitride.

Understanding and Tuning the Intrinsic Hydrophobicity of Rare-Earth Oxides: A DFT+U Study

Rare-earth oxides (REOs) possess a remarkable intrinsic hydrophobicity, making them candidates for a myriad of applications. Although the superhydrophobicity of REOs has been explored experimentally, the atomistic details of the structure at the oxide-water interface are still not well understood. In this work, we report a density functional theory study of the interaction between water and CeO2, Nd2O3, and α-Al2O3 to explain their different wettability. The wetting of the metal oxide surface is controlled by geometric and electronic factors. While the electronic term is related to the acid-base properties of the surface layer, the geometric factor depends on the matching between adsorption sites and oxygen atoms from the hexagonal water network. For all the metal oxides considered here, water dissociation is confined to the first oxide-water layer. Hydroxyl groups on α-Al2O3 are responsible for the strong oxide-water interaction, and thus, both Al- and hydroxyl-terminated wet. On CeO2, the intrinsic hydrophobicity of the clean surface disappears when lattice hydroxyl groups (created by the reaction of water with oxygen vacancies) are present as they dominate the interaction and drive wetting. Therefore, hydroxyls may convert a intrinsic nonwetting surface into a wetting one. Finally, we also report that surface modifications, like cation substitution, do not change the acid-base character of the surface, and thus they show the same nonwetting properties as native CeO2 or Nd2O3.

Challenges in the synthetic routes to Mn(BH4)2: insight into intermediate compounds

We have studied formation of Mn(BH₄)₂ and intermediates [{M(Et₂O)₂}Mn₂(BH₄)₅] in the reaction of MnCl₂ with MBH₄ (M = Li⁺, Na⁺, K⁺) in Et₂O.

Reevaluation of the structure and fundamental physical properties of dawsonites by DFT studies

Dawsonite-type compounds, with the general formula MAlCO(3)(OH)(2), where M = Na(+), K(+), or NH(4)(+), recently have become attractive materials because of their potential interest in geochemical CO(2) sequestration, CO(2) capture in power plants, and heterogeneous catalysis. However, the number of studies assessing the properties of these materials is limited. In the present paper, we report a theoretical reevaluation of the structural and essential physicochemical properties of Na-, K-, and NH(4)-dawsonites as determined by density functional theory (DFT) investigations. The calculated structure of Na- and K-dawsonites is in good agreement with previous data, while for NH(4)AlCO(3)(OH)(2), the calculations suggest orientation disorder of the ammonium ions in the structure. The normal-mode analysis, electronic and bonding properties, and elastic properties are reported for the three analogue dawsonites. The calculated formation enthalpy is -1714, -1699, and -1655 kJ/mol for K-, Na-, and NH(4)-dawsonite, respectively. This study comprises a first step toward a better understanding of the diversity of dawsonite intrinsic properties, which is required to tune their practical applications.

Structural stability of complex hydrides: LiBH4 revisited

A systematic approach to study the phase stability of LiBH4 based on ab initio calculations is presented. Three thermodynamically stable phases are identified and a new phase of Cc symmetry is proposed for the first time for a complex hydride. The x-ray diffraction pattern and vibrational spectra of the Cc structure agree well with recently reported experimental data on LiBH4. Calculations of the free energy at finite temperatures suggest that the experimentally proposed P6(3)mc phase is unstable at T>0 K.

When Langmuir is too simple: H2 dissociation on Pd(111) at high coverage

Recent experiments of H2 adsorption on Pd(111) [Nature (London) 422, 705 (2003)]] have questioned the classical Langmuir picture of second order adsorption kinetics at high surface coverage requiring pairs of empty sites for the dissociative chemisorption. Experiments find that at least three empty sites are needed. Through density functional theory, we find that H2 dissociation is favored on ensembles of sites that involve a Pd atom with no direct interaction with adsorbed hydrogen. Such active sites are formed by aggregation of at least 3 H-free sites revealing the complex structure of the "active sites."

A negative surface energy for alumina

The surface energy of a solid measures the energy cost of increasing the surface area. All normal solids therefore have a positive surface energy-if it had been negative, the solid would disintegrate. For this reason it is also generally believed that when certain ceramics can be found in a highly porous form, this is a metastable state, which will eventually sinter into the bulk solid at high temperatures. We present theoretical evidence suggesting that for theta-alumina, the surface energy is strongly dependent on the size of the crystallites, and that for some facets it is negative for thicknesses larger than approximately 1 nm. This suggests a completely new picture of porous alumina in which the high-surface-area, nanocrystalline form is the thermodynamic ground state. The negative surface energy is found to be related to a particularly strongly adsorbed state of dissociated water on some alumina surfaces. We also present new experimental evidence based on infrared spectroscopy, in conjunction with X-ray diffraction and surface-area measurements, that theta-alumina has indeed very stable surface OH groups at high temperatures, and that this form of alumina does not sinter even at temperatures up to 1,300 K.