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 (10 1 6 –10 1 9 H 0 /m 2 ⋅ s) for membrane temperatures of 300–570 K. The pressure dependence of H 2 ‐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 H 0 ‐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 H 2 ‐driven permeation rate. The permeation rate was found to increase, however, for H 0 atoms incident on the Cu‐coated surface. This is consistent with a barrier for H 2 dissociation and H recombination at the Cu/vacuum interface. Membranes with such a barrier, in conjunction with a source of H 0 atoms, have applications as hydrogen pumps.
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