(285d) Surface Modification of Pd Membranes with Cu

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 top layer of the membrane, the total permeance would be greater than that of a homogeneous alloy of the same surface concentration, and have the same H₂S resistance. In addition, annealing time is greatly reduced if a homogeneous alloy is not desired. It was the objective of this work to produce Pd/Cu membranes with a Cu gradient on the top layer and to characterize their H₂ permeation properties.

Before fabrication of the membranes, a coupon study was performed to study the Cu plating deposition and annealing time necessary to produce the top layer FCC structure. 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 in H₂ at 500 ^oC and then analyzed with XRD. It was seen that annealing at 500 ^oC in H₂ for 5 - 10 hours gave the FCC Pd/Cu phase on the surface.

Two Pd/Cu composite membranes were constructed on porous Inconel supports (supplied by Mott, Inc.). To form an intermetallic diffusion barrier between the membrane and the support, the first support was oxidized at 700 ^oC for 12 hours and the second was plated with 0.5 μm Ru. Pd was then plated until the membranes were impermeable to He followed by plating Cu. The total composition was 8 % wt Cu for the former and 19 % wt for the latter. Both depositions were 14 μm thick. To ensure an FCC Pd/Cu phase on the top layer, the 8 % wt and 19 % wt membranes were annealed for 5 and 10 hours respectively in H₂ at 500 ^oC in accordance with the results of the coupon studies.

The activation energy of the 8 % wt membrane decreased from 21.0 to 14.7 kJ/mol after 120 hours of testing at 450 ^oC. This was attributed to the instability of the Cu gradient on the top layer. The activation energy of the 19 % wt membrane was calculated to be 16.4 kJ/mol. At 500 ^oC, the H₂ permeance of the 19 % wt membrane started at 14.8 m³/m²*h*bar^0.5 and sharply decreased with time, most likely from intermetallic diffusion of the support metals into the Pd/Cu layer.

At 500 ^oC, the permeance of the 8 % wt membrane decreased slightly from 11.9 to 10.1 m³/m²*h*bar^0.5, but then remained stable for 1100 hours at this temperature. The He leak rates of the 8 and 19 % wt membranes were 4.9 and 3.7 m³/m²*h*bar respectively which is an order of magnitude less than most membranes tested in this lab. The total testing times for these membranes were 1900 and 1650 hours, respectively.