Equilibrium and kinetic measurements on thin Pd-H layers

Abstract

Equilibrium and kinetic measurements on a thin (approximately 3000 Å) Pd-H layer at various temperatures (39–96 °C) and pressures (α phase region) are reported. The experimental data were obtained using the piezoelectric quartz crystal microbalance technique under dynamic conditions. They exhibit two successive steps during both the adsorption and the desorption processes: a second-order kinetic step in the earlier stage which is ascribed to a dissociation (recombination) and transfer process, and a first-order kinetic step in the last stage which is ascribed to a surface migration process. The rate constants and the activation energies for the two steps were calculated according to kinetic relations derived theoretically. The potential energy diagram of the second-order step shows that the weakly bound hydrogen is a metastable form, slightly endothermically adsorbed on the permeation surface (Δ H w = +7 kJ (g atom H) -1). The corresponding diagram for the first-order step is discussed in comparison with adsorption data for strongly adsorbed hydrogen on a clean palladium surface in ultrahigh vacuum conditions.