Kinetics of dissociative absorption of hydrogen through Nb surface covered by Oxygen

The sticking coefficient of H₂, α, on the surface of polycrystalline Nb containing 0.1 at% of O was examined by means of permeation experiments in a wide temperature range between 510 and 1260 K. The temperature dependence of surface coverage of O was measured by Auger electron spectroscopy. Arrhenius plots of α showed a clear bend at around 900 K. The activation energy was estimated to be 5 kJ mol^-1 in the low temperature region and 44 kJ mol^-1 in the high temperature region. On the other hand, the surface coverage of O, θ_OX, was almost independent of the temperature below 900 K, but it decreased with increasing temperature above 900 K. Therefore, it was concluded that the activation energy of 5 kJ mol ^-1, obtained in the lower temperature region at constant θ_OX, corresponded to the potential barrier against H ₂ dissociation, E_C, while the value of 44 kJ mol ^-1, obtained in the higher temperature region, was an apparent activation energy determined by the combination of E_C and the heat of segregation of O controlling the temperature dependence of θ _OX. The obtained value of E_C was comparable to that on a clean Nb surface, although the dissociation of H₂ was radically impeded by the presence of O. These observations indicate that the surface of the polycrystalline Nb covered by O was not homogeneous: a majority of surface sites was poisoned by O, while a small portion kept the intrinsic activity for H₂ dissociation.