Microscopic Insights into Hydrogen Permeation Through a Model PdCu Membrane from First-Principles Investigations

Palladium-based
alloys are commonly used in industry as a membrane
material for the purification of hydrogen. In this work, we report
a systematic theoretical study of all of the processes associated
with permeation of H through a model PdCu membrane. The surface of
the membrane is modeled using the most stable (110) surface. Our calculations
show that the nuclear quantum effects due to the light mass of the
H atom can significantly affect the stability and reaction rates.
On the basis of a microkinetic model of the permeation process, we
find that the permeation can be limited by diffusion of H in the membrane
bulk or the reassociation of atomic H to form H2 on the
permeate side of the membrane depending on the operation temperature
and membrane thickness. At typical operating conditions, for membranes
thinner than 0.5 μm, the permeation at high temperature (T > 500 K) is limited by surface processes, whereas at
lower
temperatures it can be either diffusion-limited or reassociation-limited.