HYDROGEN AND OXYGEN AT METAL-OXIDE INTERFACES

Using mostly electrochemical methods the chemical potential, diffusion coefficient, partial molar volume and the resistivity increment of hydrogen were determined at room temperature in internally oxidized Pd-Al, Pd-Mg, Pd-Zn and Pd-Zr alloys. In all samples hydrogen is trapped at the formed metal/oxide interfaces, where irreversible trapping (with respect to anodic polarization) can be distinguished from reversible one. In the cases of Pd/Al2O3 and Pd/MgO phase boundaries the oxide precipitates were characterized by electron microscopy with regard to their size and their orientation and, therefore, the coverage of the phase boundaries with irreversibly bound hydrogen could be calculated as 1.5.10(15) H/cm2 or 7.10(14) H/cm2, respectively, which is about one monolayer of the densest packed oxygen planes of the corresponding oxides. This equivalence and the irreversible nature of the bonding indicates the formation of OH bonds at the phase boundaries. During irreversible trapping a larger volume change is observed as compared to dissolution of hydrogen in regular octahedral sites of palladium. The measured partial molar volumes correspond to an increase of the phase boundary "thickness" of 0.3 to 0.9 angstrom between Pd and oxide. The irreversible component of hydrogen trapping disappears after annealing in the vapor of the alloying addition (Al, Mg,...) or in vacuum and appears again after a second annealing treatment in air. As an explanation of this effect we propose that annealing in metal vapor or in vacuum, respectively, changes the composition at the phase boundary. At high oxygen activities oxygen segregates to the phase boundaries. In comparison to the oxygen in the oxide the segregated one is less strongly bound and, therefore, can be desorbed at low O-activities. Furthermore the missing bonds to the stable oxide allow the formation of O-H bonds.