Thermal desorption spectra of hydrogen from LaNi 5

Abstract

Thermal desorption of hydrogen from LaNi 5H x shows spectra which consist of a single peak for x ≲ 6 and develop into a two-peak structure for x ≈ 6.7. The desorption of hydrogen starts at a low temperature of about 160 K and terminates at about 420 K. Analysis of the spectra leads to a conclusion that the rate-determining step of the desorption is a diffusion process for temperatures lower than about 175 K and either a diffusion or a phase transformation for higher temperatures. This is dissimilar to the PdH x system, where the desorption that occurs in the temperature range 150–380 K is a surface-controlled process. The activation energy obtained for hydrogen desorption from the β phase of the LaNi 5H x system was 32 ± 4 kJ (mol H 2) −1.