Kinetics of hydrogen absorption by Pd(110)

A coupled high-pressure–ultrahigh-vacuum technique is employed to make the first kinetic measurements of high-pressure hydrogen absorption through an atomically clean palladium surface, Pd(110). The hydrogen uptake kinetics in the α-phase region are found to be rate limited by diffusion of atomic H into the bulk rather than by dissociative chemisorption of ${\mathrm{H}}_2$ at the surface as previously reported. Our findings strongly suggest that previous high-pressure measurements of hydrogen absorption by palladium may be suspect due to uncharacterized surface contamination. The bulk diffusivity of hydrogen in palladium determined in this study has an activation barrier of 5.39±0.30 kcal/mol with a preexponential factor of (2.83±0.05)×${10}^{\mathrm{-}3}$ ${\mathrm{cm}}^2$/s.