Borislav Bogdanović

Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials

Abstract New reversible hydrogen storage systems are proposed, based on catalyzed reactions (Eqs. 4–6). The catalytic acceleration of the reactions in both directions is achieved by doping alkali metal aluminium hydrides with a few mol% of selected Ti compounds. The PCI diagrams of the Ti catalyzed systems show an absence of hysteresis and nearly horizontal pressure plateaus. The PCI of the NaAlH4 system reveals two temperature-dependent pressure plateaus, corresponding to the two-step reversible dissociation of NaAlH4. The PCI of the Na3AlH6 system shows only one pressure plateau; the latter can be lowered by partial substitution of Na by Li. In cyclic tests, reversible H2 capacities of 4.2–3.1 and 2.7–2.1 wt% H have been achieved.

Metal-doped sodium aluminium hydrides as potential new hydrogen storage materials

Abstract Thermodynamics and kinetics of the reversible dissociation of metal-doped NaAlH 4 as a hydrogen (or heat) storage system have been investigated in some detail. The experimentally determined enthalpies for the first (3.7 wt% of H) and the second dissociation step of Ti-doped NaAlH 4 (3.0 wt% H) of 37 and 47 kJ/mol are in accordance with low and medium temperature reversible metal hydride systems, respectively. Through variation of NaAlH 4 particle sizes, catalysts (dopants) and doping procedures, kinetics as well as the cyclization stability within cycle tests have been substantially improved with respect to the previous status [B. Bogdanovic, M. Schwickardi, J. Alloys Comp. 253–254 (1997) 1]. In particular, using combinations of Ti and Fe compounds as dopants, a cooperative (synergistic) catalytic effect of the metals Ti and Fe in enhancing rates of both de- and rehydrogenation of Ti/Fe-doped NaAlH 4 within cycle tests, reaching a constant storage capacity of ∼4 wt% H 2 , has been demonstrated. By means of 57 Fe Mossbauer spectroscopy of the Ti/Fe-doped NaAlH 4 before and throughout a cycle test, it has been ascertained that (1) during the doping procedure, nanosize metallic Fe particles are formed from the doping agent Fe(OEt) 2 and (2) already after the first dehydrogenation, the nanosize Fe particles with NaAlH 4 present are probably transformed into an Fe–Al-alloy which throughout the cycle test remains practically unchanged.

Thermodynamic investigation of the magnesium–hydrogen system

Abstract Thermodynamic properties of the magnesium hydride–magnesium system have been investigated using both calorimetric and equilibrium pressure measurements. Based on calorimetric measurements, on average a satisfactory agreement between experimental and calculated enthalpy and entropy values for the formation of MgH2 is achieved; systematic deviations are found only at higher temperatures (∼480°C). Experimental determination of PCIs (absorption and desorption mode) has been carried out in the temperature range 403–520°C, using Mg powders of different origin and particle size. According to measurements at 526°C, a minor dependence of H2 equilibrium pressure over MgH2 on the particle size of Mg powders used exists.

Investigation of the perovskite related structures of NaMgH3, NaMgF3 and Na3AlH6

Abstract Two perovskite related metal hydrides, NaMgH 3 and Na 3 AlH 6 were structurally investigated using powder diffraction techniques. Single crystal X-ray data was also used to for the first time confirm that the structure of NaMgF 3 is analogous to the orthorombically distorted perovskite structure of GdFeO 3 space group Pnma (no. 62) . Looking for new ternary hydrides in the NaH–MgH 2 system, the only new compound found was NaMgH 3 which is isotypic with NaMgF 3 . The positions of the D atoms in Na 3 AlD 6 , isotypic with the low temperature phase of Na 3 AlF 6 , space group P2 1 /n (no. 14), could for the first time be determined from neutron diffraction data. The degree of distortion was discussed from the point of bonding distances and angles in the octahedra of the hydrides and fluorides. In the case of NaMgH 3 and NaMgF 3 , the angles of which the octahedra are rotated ( φ ) and atomic coordinates were calculated. Both NaMgH 3 and Na 3 AlH 6 appear to be less tilted but more deformed than their corresponding fluorides.

High Temperature Metal Hydrides as Heat Storage Materials for Solar and Related Applications

For the continuous production of electricity with solar heat power plants the storage of heat at a temperature level around 400 °C is essential. High temperature metal hydrides offer high heat storage capacities around this temperature. Based on Mg-compounds, these hydrides are in principle low-cost materials with excellent cycling stability. Relevant properties of these hydrides and their possible applications as heat storage materials are described.

Investigation of hydrogen discharging and recharging processes of Ti-doped NaAlH4 by X-ray diffraction analysis (XRD) and solid-state NMR spectroscopy

Abstract The processes occurring in the course of two sequential hydrogen discharging and recharging cycles of Ti-doped sodium alanate were investigated in parallel using XRD analysis and solid-state NMR spectroscopy. Both methods demonstrate that in hydrogen storage cycles ( Eq. (1) ) the majority phases involved are NaAlH 4 , Na 3 AlH 6 , Al and NaH. Only traces of other, as yet unidentified phases are observed, one of which has been tentatively assigned to an Al–Ti alloy on the basis of XRD analysis. The unsatisfactory hydrogen storage capacities heretofore observed in cycle tests are shown to be due entirely to the reaction of Na 3 AlH 6 with Al and hydrogen to NaAlH 4 ( Eq. (1) , 2nd hydrogenation step) being incomplete. Using XRD and NMR methods it has been shown that a higher level of rehydrogenation can be achieved by adding an excess of Al powder.

The development, testing and optimization of energy storage materials based on the MgH2Mg system

Abstract A systematic investigation was conducted detailing the kinetics, extent of hydrogen loading and cycling stability for both the Ni-doped and undoped MgH2-Mg systems as heat and hydrogen storage materials. Several methods for doping Mg powders with Ni were tested, in which commercial Ni compounds are used as doping agents. The storage properties of these Ni-doped materials are comparable with those of a so-called standard material, Mg doped via bis(1,5-cyclooctadiene)Ni in solution. The mechanical mixing of Mg powder with Ni powder in the dry state turned out to be by far the simplest and least expensive doping method. The storage materials obtained by mechanical mixing showed, besides satisfactory kinetics and hydrogen loading capacity, the highest cycling stability in long-term tests. The results of the cycling tests reveal a different behavior for the Ni-doped and undoped MgH2-Mg storage materials at low vs high H2 pressures and temperatures; an increase of hydrogen loading when cycling is carried out under close-to-equilibrium hydrogenation conditions; and reversibility of loading losses. The complete observations and results presented are of practical utility for the operation of heat or hydrogen stores on the MgH2-Mg basis.

Ni-doped versus undoped Mg–MgH2 materials for high temperature heat or hydrogen storage

Abstract A comparative study of the behavior of various Ni-doped and undoped Mg–MgH 2 materials to be utilized for reversible (thermochemical) high temperature heat or hydrogen storage has for the first time been conducted over a broad range of hydrogenation/dehydrogenation (cycling) conditions (see Fig. 5 ). The storage capacity losses observed in the course of cyclic tests are found to be sensitively dependent to all the details of the applied cycling conditions and can be of temporary (reversible) or persistent (irreversible) nature. Based upon investigations via optical microscopy, the reversible capacity losses appear to be associated with an excessively high formation rate of MgH 2 -nucleation sites on the surface of Ni-doped Mg particles under intensified cycling conditions; irreversible capacity losses, especially pronounced in the case of Ni-doped materials, are the result of sintering of the material particles in the dehydrogenated (metallic) form upon prolonged cycling at higher temperatures. Ni-doped Mg–MgH 2 materials have excellent cyclic stability and high hydrogenation rates even under very mild pressure/temperature cycling conditions (so-called standard cycling conditions or below them [B. Bogdanovic, Th. Hartwig, B. Spliethoff, Int. J. Hydrogen Energy 18 (1993) 575; Final Report of Project No. 0328939 C, Federal Ministry for Research and Technology of the F.R.G., Bonn (1992)]) suitable for applications such as solar generation of heat and cold, heat pumps, hydrogen storage, and the like. On the other hand, based on their cyclic stability and sufficient reaction rates under severe reaction conditions, neat Mg powders produced by brushing can be used as cheap materials for the purpose of reversible thermochemical high temperature heat storage in the temperature range of 450–500°C with heat storage capacities amounting to 0.6–0.7 kWh/kg Mg, applicable for solar power generation via Stirling engines or storage of industrial heat in the above temperature ranges.

A process steam generator based on the high temperature magnesium hydride/magnesium heat storage system

As a first pilot project application of the reversible thermochemical high temperature heat storage system magnesium hydride/magnesium a process steam generator has been built and tested. It draws the heat for the generation of superheated steam from a magnesium hydride/magnesium (MgH2Mg) heat store and is primarily meant for the storage of high grade industrial waste heat which can be made available as superheated process steam during peak load hours. In addition, other areas of application, for instance in high temperature solar engineering, are also opened.

A comparative study of the McMurry reaction utilizing [HTiCl(THF) –0.5]x, TiCl3(DME)1.5Zn(Cu) and TiCl2 · LiCl as coupling reagents☆

Abstract An investigation of the reaction course and stoichiometry of the McMurry reaction of acetophenone utilizing [HTiCl(THF) – 0.5]x (THF = tetrahydrofuran), TiCl3(DME)1.5Zn(Cu) (DME = 1,2-dimethoxyethane) and TiCl2 · LiCl as coupling reagents has been undertaken. The detection of 1-phenylethanol (3a) or dideutero-1-phenylethanol (3b) (Schemes 1 and 3) as hydrolysis or deuterolysis products in the early stage of reactions gave the first direct experimental evidence for the occurrence of the “side-on” bonded ketones 3 and 3″ as possible precursors of the pinacolates 4 and 7. This result supports the nucleophilic rather than the radical mechanism for the CC coupling step of aromatic ketones. Contrary to the current opinion, upon refluxing TiCl3(DME)1.5Zn(Cu) mixtures in DME, no reduction of Ti3+ to low valence Ti species could be detected. The reduction of Ti3+ by Zn(Scheme 2) only starts in the presence of the carbonyl substrate which is coordinated to the Ti (the “instant method”); both the ketone → pinacolate and the pinacolate → alkene steps (Scheme 2) apparently involve a transient reduction of Ti3+ to Ti2+ by Zn. This view is supported by experiments in which TiCl2 · LiCl is used as a reagent and in which it behaves as a one-electron reductant (Scheme 3). On the basis of these results, the overall stoichiometry of the McMurry reaction utilizing TiCl3(DME)1.5Zn(Cu) as a reagent can be represented by Eq. (4). High yields (95–97%) of the alkene 2 in acceptable reaction times can already be achieved with an acetophenone: TiCl3(DME)1.5 : Zn(CU) molar ratio of 1:2:2. A conclusion which can be drawn from the results is that the McMurry reaction when performed with two of the most commonly applied reagents, namely TiCl3LiAlH4-THF (in fact HTiCl(THF)0.5!) and TiCl3(DME)1.5Zn(Cu)-DME, is mainly associated with changes in the (formal) oxidation state of titanium between Ti2+ and Ti3+.