Penetration mechanisms of surface-adsorbed hydrogen atoms into bulk metals: Experiment and model

Abstract

The conditions enabling the transition of surface-adsorbed hydrogen atoms into bulk metals are explored by comparing the response of chemisorbed H on Pd(100) and Ti(0001) single crystals to thermal activation in vacuum. Thermal desorption spectroscopy and $^{1}\text{H}(^{15}\text{N},\ensuremath{\alpha}\ensuremath{\gamma})^{12}\text{C}$ nuclear reaction analysis reveal that heating causes ${\text{H}}_{2}$ desorption from Pd(100), whereas H atoms on Ti(0001) reversibly exchange between the surface and the Ti bulk with negligible desorption loss of ${\text{H}}_{2}$. A general model is proposed in which the competition between desorption and bulk absorption of surface hydrogen is, on one hand, kinetically determined by the activation energy for associative ${\text{H}}_{2}$ desorption relative to the effective energy barrier for H absorption. On the other hand, the thermodynamic possibility to dissolve the surface H atoms into the metal must be considered to consistently explain the opposite behavior of H on Pd(100) and Ti(0001). The first experimental estimation of the energy of surface adsorption, ${\ensuremath{\epsilon}}_{s}=\ensuremath{-}0.92\text{ }\text{eV}$, for H/Ti(0001) is presented, in good agreement with theoretical calculations.