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

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 ¹H(¹⁵N,αγ)¹²C nuclear reaction analysis reveal that heating causes H₂ desorption from Pd(100), whereas H atoms on Ti(0001) reversibly exchange between the surface and the Ti bulk with negligible desorption loss of H₂. 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 H₂ 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, εₛ=-0.92 eV, for H/Ti(0001) is presented, in good agreement with theoretical calculations.