Kinetics of hydrogen absorption-desorption by niobium

The kinetics of hydrogen absorption and desorption by clean, recrystallized Nb foils, (110) orientation, were measured as a function of temperature. The results are discussed on the basis of a previously published model for the kinetics. It was found that, contrary to the original assumption, the initial sticking coefficient of hydrogen on the clean Nb(110) surface is temperature dependent and can be described by the expression ${S}_{i}={S}_{0}\mathrm{exp}[\ensuremath{-}(\frac{2{E}_{I}}{\mathrm{RT}})]$, where ${S}_{0}$ was found to be close to unity and ${E}_{I}=(1.27\ifmmode\pm\else\textpm\fi{}0.01)$ kcal/mol H. This led to a modification of the theoretical model by the inclusion of an activation energy for chemisorption. Furthermore, an analysis of the temperature-dependent data yielded values for the chemisorption energy of hydrogen ${E}_{D}=(14.8\ifmmode\pm\else\textpm\fi{}0.2)$ kcal/mol H, the vibrational entropy at the surface, the surface coverage, as well as other parameters related to the energy barrier at the surface.