Hydrogen permeation through copper-coated palladium

The rate of hydrogen uptake and release by metals can be strongly affected by surface barriers for adsorption and desorption. The rate of hydrogen permeation through a Pd membrane was measured for both incident molecules (10−3–10−4 Pa) and neutral atoms (1016–1019 H0/m2⋅ s) for membrane temperatures of 300–570 K. The pressure dependence of H2-driven permeation was used to identify regimes where the permeation was controlled by bulk processes (diffusion-limited) and surface processes (surface-limited). The dependence of the H0-driven permeation rate on the direction of permeation was used to separate the contribution of each surface to the overall surface-limited permeation rate. One of the membrane surfaces was coated in situ with copper evaporated from a hot source. This same surface could be monitored in situ by Auger electron spectroscopy. At temperatures below 450 K, stable copper coatings were made with thicknesses ranging from ∼3 to 25 nm. The thin Cu coatings led to a decrease in the H2-driven permeation rate. The permeation rate was found to increase, however, for H0 atoms incident on the Cu-coated surface. This is consistent with a barrier for H2 dissociation and H recombination at the Cu/vacuum interface. Membranes with such a barrier, in conjunction with a source of H0 atoms, have applications as hydrogen pumps.