Rajkumar Bhandari, Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609 and Yi Hua Ma, Worcester Polytechnique Institute, Chemical Enginering, Worcester, MA 01609.
Because of their unique property of selective permeation for hydrogen, Pd and its alloys can be used to separate H2 from gaseous mixtures. Alloying Pd with metals like Ag, Au, and Cu improves its properties such as permeability and resistance to H2 embrittlement. In the case of alloying Pd with Ag, in addition to improved permeability and resistance to the H2 embrittlement casued by the α to β transformation, the less expensive Ag reduces the cost of the membranes. Thinner Pd-Ag membranes are desired for high fluxes. The important step in the synthesis of a Pd-Ag alloy composite membrane is the deposition of Pd and Ag. One method of the deposition is the sequential deposition method consisting of the deposition of alternate layers of Pd and Ag. This work is focused on obtaining uniform alternate layers of Pd and Ag on the porous metals surface using electroless and electroplating techniques. PSS coupons (0.2µm grade) of approximate 2cm2 area each were used for the study. For the deposition of Pd, electroless plating bath based on N2H4 and NH3 was used. The deposition of Ag was achieved by electroplating using succinimide based baths containing polyethyleneimine (PEI) as leveling agents. The SEM (JEOL, 840) equipped with EDX was used to characterize the deposits. The SEI images of the Pd deposits obtained on the PSS surface showed uniform growth in the lateral as well as perpendicular direction to the surface. The deposits also showed good penetration into the pores of the substrate. The obtained Ag deposits were also uniform in nature showing equal growth in all direction and good penetration into pores. However, the uniformity of the Ag deposits was sensitive to the PEI concentration. In the absence of PEI, deposits did not cover the PSS surface uniformly. After the addition of PEI into the bath, uniform and bright deposits were obtained. Furthermore, the sandwich of Pd and Ag alternate layers was nonporous and uniform in nature on the PSS surface. The Pd and Ag deposition was also tested on porous metal tube (0.1µm grade) of 0.5 inch diameter. The total of 17µm thick Pd-Ag sandwich was deposited with Ag between 6-10 wt%. The Pd-Ag deposited membrane was further tested for its H2 permeation characteristics at 500oC. The membrane showed permeance of 13.5 m3/m2-h-atm0.5 in the beginning to stable value of 21 m3/m2-h-atm0.5 after 330h with the final selectivity of 240 (H2/He at ΔP = 1bar) indicating increase in the permeance due to the annealing of the Pd and Ag layers. In summary, the uniform Pd-Ag sandwich was obtained with the combination of electroless and electroplating techniques on the porous metal surface. The annealing of the Pd-Ag sandwich could be achieved at 500oC in H2 atmosphere. The homogeneity of the formed alloy is characterized by examining the cross-section of the annealed membrane using the EDX. The homogeneity of the annealed alloy is discussed in terms of the annealing time and temperatures.