Magnesium Hydride

Magnesium, although capable of storing hydrogen up to the composition MgH₂ presents a considerable surface activation barrier to hydrogen (50 kJ/mol) and this means that the hydrogenation reaction usually requires temperatures above 500 K, and pressures of tens to hundreds of bars. In a recent publication by A. Krozer and B. Kasemo, of Chalmers University of Technology, Sweden, a palladium coating was used to extend the available information on the magnesium-hydrogen phase diagram to pressures 10⁵ times lower than was previously possible, and to temperatures down to 290 K (1). Their approach involved the deposition of a palladium layer (5-20 nm thick) on a magnesium sample which was vacuum evaporated onto a piezo-electric quartz crystal microbalance. Hydrogen uptake was measured by the weight increase of the sample registered as a change in the resonant frequency of the quartz crystal.

The palladium film on the magnesium sample removed the kinetic barrier for converting gas phase H₂ to atomic hydrogen prior to bulk absorption, thus allowing the establishment of equilibrium at pressures below 20 Torr and temperatures in the range 330 to 370 K. At lower temperatures, kinetic limitations appeared due to hydride formation at the interface between the palladium and the magnesium, whereas at higher temperatures intermixing/ alloying of these two metals began to occur.

Hydrogen uptake was characterised by Sievert’s law behaviour up to hydrogen: magnesium ratios <0.01 Slow kinetics and deviations from Sievert’s law at higher hydrogen contents pointed to the increased importance of H-H attractive interactions as more sites in the metal are filled.

Photo-assisted Hydrogen Storage

In another recent paper H. Imamura, M. Futsutara and S. Tsuchiya of Yamaguchi University, Japan, reported photo-assisted hydrogen storage in the rare earth intermetallic compounds RCO₅ (R = lanthanum or samarium) using alcohol as a hydrogen source (2). The work relies on the transfer hydrogenation of a metal hydride as a method of producing useful hydrogen from industrial waste alcohol, and presents an interesting possibility for the conversion and storage of solar energy. The reaction is:

Their standard procedure involves a solution containing 0.3 g of RCO₅ and 0.01-0.1 cm³ carbon tetrachloride in 220 cm³ of alcohol. In one experiment irradiation of LaCO₅ in 2-propanol for four hours resulted in the formation of the hydride LaCO₅H_1.4 and acetone.

In order to assess the possibility of enhanced surface reactions contributing to the effect, some LaCo₅ was treated with chloroplatinic acid to produce an electroless surface deposit of platinum. When placed in the reactor, platinum coated alloys showed faster hydriding rates and remarkable ease of conversion into the metal hydride when compared with non-coated material, under the same conditions.

It was concluded that, since there were no differences in the LaCo₅ used, the rate limiting processes must be exterior to the bulk, with the deposited platinum being active as an alcohol dehydrogenation catalyst.

The use of the catalytic and other special properties of platinum group metals in association with metal hydrides has long been known, with coatings of palladium being effectively employed to activate metals such as tantalum, niobium and vanadium (3). Both the above recent papers illustrate the continued use of platinum group metals as modifiers of the surface properties of metal hydrides, opening up the possibility of developing useful metal hydride systems capable of operating in new, more favourable conditions of temperature and pressure.