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

Pd/alumina composite membranes were fabricated using the generally practiced electroless plating process involving two-step activation of a symmetric 0.2 mu m alpha-alumina microfilter with tin (Sn) chloride sensitizer (containing SnCl2 and SnCl4) and palladium(II) chloride (PdCl2). Pd films were deposited on these activated supports with a hydrazine- (N2H4-) and PdCl2-containing electroless plating bath. When these membranes were tested at 823 K for several days, the ideal H-2/N-2 separation factor (pure gas permeability ratio) declined substantially, depending on the membrane thickness. Modifications to the activation procedure minimized the amount of Sn chloride used 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. Possible routes for pore formation and selectivity decline are suggested. Sn chloride was removed from the process entirely with a new activation technique 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 annealing.