Pneumatochemical impedance spectroscopy. 2. Dynamics of hydrogen sorption by metals

In this paper, pneumatochemical impedance spectroscopy is used to analyze multistep reaction mechanisms such as those observed in solid solution domains of LaNi5-H2(g) systems. It is shown that hydrogen sorption is a two-step mechanism including (i) dissociative surface chemisorption of molecular hydrogen and (ii) atomic hydrogen bulk transport by diffusion. Data fitting of experimental transfer functions with model equations yields the value of the kinetic parameter associated with each individual reaction step, i.e., surface sorption resistances and hydrogen bulk diffusion coefficients. The technique is used to follow the activation procedure of the sample as well as the degradation of sorption properties in oxygen-containing hydrogen atmospheres. A decrease in sorption kinetics is attributed to surface oxidation, whereas bulk properties remain unchanged. The perspectives offered by the technique which potentially can be used to optimize surface and bulk composition of IMC for increased sorption rates are discussed.