(569e) Isothermal Solid-State Transformation Kinetics Applied to Pd/Cu Alloy Membrane Fabrication

Hydrogen selective Pd/Cu membranes can improve the efficiency and cost effectiveness of the coal gasification process by recovering high purity hydrogen from the syngas. In addition, the fcc phase of the Pd/Cu alloy provides more sulfur resistance than pure Pd membranes. The lack of a co-deposition bath for Pd and Cu necessitates plating sequential layers of Pd and Cu and followed by high temperature annealing in order to form the fcc alloy. The objective of this study was to investigate the annealing conditions required to form the fcc Pd/Cu alloy from deposited bi-layers and apply that knowledge to membrane fabrication and the examination of membrane permeation characteristics.

Samples of 316L PSS (0.5 micron media grade) were oxidized at 800°C for 12 hours to form an intermetallic diffusion barrier and then plated with Pd and Cu by the electroless deposition method. The total thickness of the layers was approximately 15 µm and the total Cu concentration was roughly 15 wt%. The samples were annealed in pure H₂ at 500, 550 and 600°C while the solid-state transformations were monitored with time resolved, in situ high temperature X-ray diffraction (HT-XRD).

The high temperature annealing in H₂ caused the Cu to diffuse into the Pd layer, forming the Cu rich fcc phase, the bcc phase and the Pd rich fcc phase. The Cu rich fcc phase, pure Cu phase and pure Pd phase eventually disappeared while the bcc phase increased and decreased in wt% over time. The kinetics of the sulfur tolerant fcc phase formation were successfully described by the Avrami nucleation and growth model. The Avrami exponents were 1.16, 1.08 and 1.16 at 500, 550 and 600°C showing that the nucleation process was athermal, the growth of the new phase was one-dimensional and the solid state transformation of the Pd/Cu bi-layers to the Pd rich fcc alloy was diffusion controlled. The Avrami rate constant dependence on temperature yielded an activation energy of 175 kJ/mol which was similar to the value of the activation energy of the bulk interdiffusion of a low Cu content Pd/Cu alloy.

A Pd/Cu membrane with 12.5 µm Pd and 1.5 µm Cu was characterized between the temperatures of 250 - 500ºC. The increases in permeance seen at 400 and 450°C could be explained by the formation of the more permeable bcc phase. At 500°C, the permeance decreased slightly over a period of 200 hours before stabilizing indicating that the amount of the more permeable bcc phase decreased and the less permeable fcc phase increased. The time predicted by the Avrami model for the fcc formation to complete at 500°C was 225 hours, showing that the Avrami model successfully described the permeation changes of the Pd/Cu alloy membrane during characterization both qualitatively and quantitatively.