A SURFACE ANALYTICAL STUDY OF SO2 STABILIZATION OF LANI5HX SURFACES

The stabilisation of LaNi5H6 by SO2 poisoning, under conditions approximating those expected in real engineering applications of the hydride, has been investigated with several complementary techniques - SEM, TEM, 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 ambient atmosphere and the solid; the formation of a 25-100 nm thick decomposition layer below the oxide film; and the retention of the alloy below the decomposition layer. The decomposition layer is found to be depleted in Ni, according to XPS analyses, while TEM reveals the presence of nano-crystallites (most likely Ni) embedded in an amorphous matrix; the latter consists of mixed oxide, hydroxide and carbonate species. The relationships between the data have resulted in the formulation of a model which proposes that the nano-crystallites promote dissociative/associative H-2 chemisorption/desorption at surface sites which under normal circumstances are protected against poisoning by the surrounding 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 loaded hydride by prevention of associative desorption. There is little, if any, penetration by S of the metal hydride substructure. Also, we find that longterm stability is promoted by the presence of trace amounts of SO2 in the ambient atmosphere; this suggests that the stabilisation is, at least in part, a dynamic process.