Magnesium and magnesium alloy hydrides

Hydrogen is stored atomically in metal hydrides. Dissociative chemisorption and associative desorption are therefore important steps in the hydrogen absorption and desorption processes. We summarize the theoretical and experimental knowledge of hydrogen chemisorption on clean and precovered metal surfaces and correlate it with the techniques for preparing metal hydrides. At the surface of hydride-forming intermetallics, precipitates of d metals and a metallic subsurface are produced by surface segregation and decomposition. The subsurface and the precipitates are able to dissociate H2. Our recent work on the surface analysis of LaNi5, FeTi, Mg2Ni and ErFe2 is reviewed.

Surface effects and the formation of metal hydrides

The proposition that the apparent differences between the reactivities of magnesium and hydrogen are partially due to a lack of appreciation of the effects of various parameters on the kinetics is developed. The results of investigations of the formation-decomposition kinetics and morphology of CeMg12 and Mg-10wt.%Ni alloys performed at pressures and temperatures where only the formation of the MgH2 hydride is reversible support the hypothesis that the high reactivity of such alloys with hydrogen is due, at least partially, to the effects of morphological parameters such as mechanical stresses and strains and incoherent grain boundaries which increase hydrogen diffusion and hydride nucleation and which are produced by the formation of finely divided precipitates of CeH3 or Mg2Ni. The decrease in the hydrogen capacity on cycling is interpreted as a consequence of the removal of defects.

The mechanism and kinetics of hydride formation in Mg-10wt.%Ni and CeMg12

Hydriding and dehydriding characteristics of Mg-LaNi5 composite materials prepared by mechanical alloying

Activation characteristics of graphite modified hydrogen absorbing materials

LEED and AES studies of LaNi5 single crystals cleaved in UHV and exposed to oxygen and hydrogen show that surface segregation of La is induced at room temperature by oxygen, but not by hydrogen. This segregation is driven by the chemical exchange reaction (La‐Ni)+O→ (La‐O)+Ni. A surface‐layer model is derived supporting the important implications of the segregation for the excellent kinetics in hydrogenation and the resistance to contaminants.

P. Zürcher

Papers

Surface segregation in LaNi5 induced by oxygen

Mg-based hydrogen storage materials: Surface segregation in Mg2Cu and related catalytic effects