A SURFACE ANALYTICAL STUDY OF SO2 STABILIZATION OF LANI5HX SURFACES

The stabilisation of LaNi5H6 by SO2 poisoning, under conditions approx imating those expected in real engineering applications of the hydride , has been investigated with several complementary techniques - SEM, T EM, XPS, XAES and in-situ XRD. The effects of hydrogen loading and SO2 exposure, and of the ambient conditions include: the formation of a 1 -2 monolayer thick oxide film at the interface between the gaseous amb ient atmosphere and the solid; the formation of a 25-100 nm thick deco mposition layer below the oxide film; and the retention of the alloy b elow the decomposition layer. The decomposition layer is found to be d epleted in Ni, according to XPS analyses, while TEM reveals the presen ce of nano-crystallites (most likely Ni) embedded in an amorphous matr ix; the latter consists of mixed oxide, hydroxide and carbonate specie s. The relationships between the data have resulted in the formulation of a model which proposes that the nano-crystallites promote dissocia tive/associative H-2 chemisorption/desorption at surface sites which u nder normal circumstances are protected against poisoning by the surro unding amorphous matrix, which is permeable to H-2-. The effect of SO2 exposure is apparently for the S to react at the surface sites of the nano-crystallites, to form a sulphide, thus stabilising the fully loa ded hydride by prevention of associative desorption. There is little, if any, penetration by S of the metal hydride substructure. Also, we f ind that longterm stability is promoted by the presence of trace amoun ts of SO2 in the ambient atmosphere; this suggests that the stabilisat ion is, at least in part, a dynamic process.