Structural
and Permeation Kinetic Correlations in PdCuAg Membranes
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Abstract
Addition
of Ag is a promising way to enhance the H<sub>2</sub> permeability
of sulfur-tolerant PdCu membranes for cleanup of coal-derived hydrogen.
We investigated a series of PdCuAg membranes with at least 70 atom
% Pd to elucidate the interdependence between alloy structure and
H<sub>2</sub> permeability. Membranes were prepared via sequential
electroless plating of Pd, Ag, and Cu onto ceramic microfiltration
membranes and subsequent alloying at elevated temperatures. Alloy
formation was complicated by a wide miscibility gap in the PdCuAg
phase diagram at the practically feasible operation temperatures.
X-ray diffraction showed that the lattice constants of the fully alloyed
ternary alloys obey Vegard’s law closely. In general, H<sub>2</sub> permeation rates increased with increasing Ag and decreasing
Cu content of the membranes in the investigated temperature range.
Detailed examination of the permeation kinetics revealed compensation
between activation energy and pre-exponential factor of the corresponding
H<sub>2</sub> permeation laws. The origin of this effect is discussed.
Further analysis showed that the activation energy for H<sub>2</sub> permeation decreases overall with increasing lattice constant of
the ternary alloy. The combination of these correlations results in
a structure–function relationship that will facilitate rational
design of PdCuAg membranes