Pd composite membranes can be used to improve H₂ production in membrane reactors by removing the H₂ from product gases during the reaction. This lowers the operating temperature required to achieve a high conversion. However, Pd is readily poisoned by the presence of small concentrations of H₂S in the product gases. Although the FCC phase of Pd/Cu alloys has been shown to have resistance to H₂S poisoning, alloying Pd with Cu lowers the permeance of the membrane in the FCC phase. However, if a Pd/Cu alloy in the FCC phase was only present on the surface of the membrane, the total permeance would be greater than that of a homogeneous alloy. In addition, annealing time is greatly reduced if a homogeneous alloy is not desired. It is the objective of this work to produce Pd/Cu membranes with a Cu gradient on the surface and understand the mechanism of their permeation behavior.

Before fabrication of the membranes, a coupon study was performed to determine how long a membrane would need to be annealed to provide the necessary FCC structure on the surface. Several coupons of porous stainless steel were prepared by depositing Pd by electroless plating followed by a thin layer of Cu (less than 2 μm), also by electroless plating. The coupons were annealed at various times and temperatures and then analyzed with XRD. It was seen that annealing at 500 ^oC in H₂ for 5 hours gave the necessary Cu phase on the surface of the coupon. A Pd/Cu composite membrane was constructed by first oxidizing a porous metallic support (supplied by Mott, Inc.) at 700 ^oC for 12 hours to form an intermetallic diffusion barrier between the membrane and the support. Afterwards, Pd was deposited by electroless plating until the membrane was impermeable to He followed by a deposition of Cu also by electroless plating. The membrane had a final thickness of 12.5 μm Pd + 1.5 μm Cu. To ensure an FCC Pd/Cu phase on the surface, the membrane was annealed for 5 hours in H₂ at 500 ^oC in accordance with the results of the coupon studies.

The H₂ permeance of the membrane was found to be 10.8 m³/m²*h* bar^0.5 at 450 ^oC and did not decrease even after 120 hours of testing at 450 ^oC. The activation energy was calculated to be 14.7 kJ/mol and the ideal separation factor (J_H2 /J_He at ΔP = 1 bar) at 450 ^oC was 3300. Testing at 500 ^oC caused a 20% decrease in the permeance.

From the stability of the permeance at 450 ^oC and the decrease of the permeance at 500 ^oC, it can be concluded that the Cu gradient was stable at 450 ^oC and did not diffuse further into the Pd. However, the decrease in permeance seen at 500 ^oC showed that the Cu gradient was not stable and that the Cu might have diffused further into the Pd.