Search

Diborane (B2H6) is a hydride gas often employed in high-purity industrial surface processes such as chemical vapor deposition or epitaxial layer growth. The use of diborane at industrial scales is complicated by the formation of higher-order borane contaminants in pure diborane gas via a complex series of gas-phase reactions. An advanced, rationally designed sorbent could stabilize diborane through interfacial interactions, dramatically reducing the decomposition rate without permanently trapping the molecule. However, the design of such a sorbent would require a nuanced understanding of diborane's fundamental surface chemistry, about which little is known. In the work presented in this thesis, a novel ultra-high vacuum (UHV) system was designed and employed to characterize the fundamental interactions of diborane with a variety of surfaces. In situ Fourier-transform infrared (FTIR) spectroscopy and temperature-programmed desorption (TPD) experiments were used in conjunction with density-functional theory (DFT) calculations to elucidate binding geometries and interaction mechanisms. On non-functionalized model surfaces such as CaF2 or amorphous carbon, diborane adsorbed only at cryogenic temperatures. Hydroxylated surfaces such as amorphous silica (SiO2) adsorbed significantly more diborane, which remained at slightly higher temperatures. FTIR spectra indicated the presence of hydrogen bonding between diborane and surface hydroxyl groups. DFT calculations revealed that the interaction takes the form of a novel bifurcated dihydrogen bond. In contrast with previous reports, diborane exhibited only weak interactions with the surface hydroxyl groups of silica. DFT calculations further elucidated that the irreversible reaction of diborane with surface hydroxyls is only possible in the presence of a second nucleophile (such as adventitious water). On the metal-organic framework (MOF) UiO-66 NH2, unique chemistry was observed in which diborane reacted with the –NH2 groups

Diborane (B2H6) is a hydride gas often employed in high-purity industrial surface processes such as chemical vapor deposition or epitaxial layer growth. The use of diborane at industrial scales is complicated by the formation of higher-order borane contaminants in pure diborane gas via a complex series of gas-phase reactions. An advanced, rationally designed sorbent could stabilize diborane through interfacial interactions, dramatically reducing the decomposition rate without permanently trapping the molecule. However, the design of such a sorbent would require a nuanced understanding of diborane's fundamental surface chemistry, about which little is known. In the work presented in this thesis, a novel ultra-high vacuum (UHV) system was designed and employed to characterize the fundamental interactions of diborane with a variety of surfaces. In situ Fourier-transform infrared (FTIR) spectroscopy and temperature-programmed desorption (TPD) experiments were used in conjunction with density-functional theory (DFT) calculations to elucidate binding geometries and interaction mechanisms. On non-functionalized model surfaces such as CaF2 or amorphous carbon, diborane adsorbed only at cryogenic temperatures. Hydroxylated surfaces such as amorphous silica (SiO2) adsorbed significantly more diborane, which remained at slightly higher temperatures. FTIR spectra indicated the presence of hydrogen bonding between diborane and surface hydroxyl groups. DFT calculations revealed that the interaction takes the form of a novel bifurcated dihydrogen bond. In contrast with previous reports, diborane exhibited only weak interactions with the surface hydroxyl groups of silica. DFT calculations further elucidated that the irreversible reaction of diborane with surface hydroxyls is only possible in the presence of a second nucleophile (such as adventitious water). On the metal-organic framework (MOF) UiO-66 NH2, unique chemistry was observed in which diborane reacted with the –NH2 groups

Ammonium aminodiboranate (AADB) and diammoniate of diborane (DADB) are two isomers of ammonia borane (AB), which have been intensively studied for hydrogen storage. Their high hydrogen contents give them the high potential to serve as hydrogen storage materials. To explore their dehydrogenation pathways, molecular dynamics (MD) simulations with a reactive force field (ReaxFF) were applied. Temperature ramping simulations of their thermolysis were carried out. For AADB, at low temperatures, its hydrogen release can be realized mainly via inter-molecular dehydrogenations. As the temperature of the simulated system increases, the formations of B-N bonds begin to occur. In the case of DADB, we found that this molecule could release hydrogen at a lower temperature with the cleavage of B-N bond. The compositional analysis of the simulated systems was also conducted to monitor the potential intermediates along their dehydrogenation pathways. Our current work provides a detailed picture of the initial dehydrogenation steps of AADB and DADB, and highlights the difference in their respective dehydrogenation processes.

textversion:author

Three novel potassium-zinc borohydrides/chlorides are described. KZn(BH 4) 3 and K 2Zn(BH 4) xCl 4-x form in ball-milled KBH 4:ZnCl 2 mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K 3Zn(BH 4) xCl 5-x forms only in the 2:1 mixture after longer milling times. The new compounds have been studied by a combination of in situ synchrotron powder diffraction, thermal analysis, Raman spectroscopy, and the solid state DFT calculations. Rhombohedral KZn(BH 4) 3 contains an anionic complex [Zn(BH 4) 3] - with D 3 (32) symmetry, located inside a rhombohedron K 8. KZn(BH 4) 3 contains 8.1 wt % of hydrogen and decomposes at ∼385 K with a release of hydrogen and diborane similar to other Zn-based bimetallic borohydrides like MZn 2(BH 4) 5 (M = Li, Na) and NaZn(BH 4) 3. The decomposition temperature is much lower than for KBH 4. Monoclinic K 2Zn(BH 4) xCl 4-x contains a tetrahedral complex anion [Zn(BH 4) xCl 4-x] 2- located inside an Edshammar polyhedron (pentacapped trigonal prism) K 11. The compound is a monoclinically distorted variant of the paraelectric orthorhombic ht-phase of K 2ZnCl 4 (structure type K 2SO 4). K 2Zn(BH 4) xCl 4-x releases BH 4 starting from 395 K, forming Zn and KBH 4. As the reaction proceeds and x decreases, the monoclinic distortion of K 2Zn(BH 4) xCl 4-x diminishes and the structure transforms at 445 K into the orthorhombic ht-phase of K 2ZnCl 4. Tetragonal K 3Zn(BH 4) xCl 5-x is a substitutional and deformation variant of the tetragonal (I4/mcm) Cs 3CoCl 5 structure type possibly with the space group P4 2/ncm. K 3Zn(BH 4) xCl 5-x decomposes nearly at the same temperature as KZn(BH 4) 3, i.e., at ∼400 K, with the formation of K 2Zn(BH 4) xCl 4-x and KBH 4, indicating that the compound is an adduct of the two latter compounds. © 2011 American Chemical Society.

The new double-cation AlLi-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (-70°C) combined with a high hydrogen density (17.2 wt%). It was synthesised by high-energy ball milling of AlCl3 and LiBH4. The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group = P-43n, a = 11.3640(3) Å). The unexpected composition Al3Li4(BH 4)13 can be rationalized on the basis of a complex cation [(BH4)Li4]3+ and a complex anion [Al(BH 4)4]-. The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at -70°C, forming LiBH4. The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Successful isolation and characterization of Me3NB4H8 revealed an aspect of the base catalysis of borane condensation, and triggered the discovery of new hypho class compounds B4H8(2PMe3) and B3H5(3PMe3). Formation of a new class (klado) of compound B4H6(4PMe3) was confirmed also. These higher class compounds are promising reagents for borane expansion. A triborane cation, (B3H6(PMe3)2)+, was prepared from B2H4(2PMe3). The diborane(4) adduct can be used as a reagent for borane expansion. An attempted conversion of B5H9 to B10H14 is described. (Author)

A direct and economical process for the manufacture of high purity diborane is provided by the silver-catalyzed hydrogenation of boron trichloride and the disproportionation of the dichloroborane formed. Pilot plant operating data and experience were accumulated for the design of a large scale plant.

Three ligand-stabilized Mg(BH4)2–based complexes have been synthesized and evaluated as potential hydrogen storage media for portable fuel cell applications. The new borohydrides: Mg(BH4)2 × 0.5Et2O and Mg(BH4)2 × diglyme (diglyme – CH3O(CH2)2O(CH2)2OCH3) have been synthesized and examined by X-ray single crystal diffraction method. Hydrolysis reactions of the compounds liberate hydrogen in quantities ranging from 46 to 96% of the theoretical yield. The hydrolysis of Mg(BH4)2 and other borohydrides is also accompanied by the diborane formation. The amount of liberated diborane depends on the Mg-coordination environment. To explain this fact quantum-chemical calculations have been performed. It is shown that formation of Mg-O-Mg-bridges enables the side process of diborane generation. It means that the size and denticity of the ligand directly affects the amount of released diborane. In general, the larger the ligand and the higher its denticity, the smaller is amount of diborane produced. The new compound Mg(BH4)2 × diglyme decomposes without diborane formation that allows one to be considered as a new promising chemical hydrogen storage compound for the practical usage. © 2017 Elsevier B.V.