A new preparation technique for Pd alumina membranes with enhanced high-temperature stability

Pd/alumina composite membranes were fabricated using the generally practice d electroless plating process involving two-step activation of a symmetric 0.2 mu m alpha-alumina microfilter with tin (Sn) chloride sensitizer (conta ining SnCl2 and SnCl4) and palladium(II) chloride (PdCl2). Pd films were de posited on these activated supports with a hydrazine- (N2H4-) and PdCl2-con taining electroless plating bath. When these membranes were tested at 823 K for several days, the ideal H-2/N-2 separation factor (pure gas permeabili ty ratio) declined substantially, depending on the membrane thickness. Modi fications to the activation procedure minimized the amount of Sn chloride u sed in the sensitizing step. This reduced the selectivity decline, although the problem was not eliminated. The amount of Sn present at the Pd/ceramic interface was qualitatively related to the high-temperature performance. P ossible routes for pore formation and selectivity decline are suggested. Sn chloride was removed from the process entirely with a new activation techn ique utilizing palladium(II) acetate (Pd(O2CCH3)(2)). Prior to electroless plating, substrates were dip-coated in a chloroform solution of Pd acetate, dried, calcined, and then reduced in flowing H-2. At 973 K, nitrogen flux through these membranes remained constant for a period of at least a week. However, hydrogen permeability decreased at 873 K and above because of anne aling.