Preparation of Pd coated anodic alumina membranes for gas separation media

Different procedures of Pd electroless deposition onto anodic alumina membranes were investigated to form a dense metal layer covering pores. The main difficulty was related to the amorphous nature of anodic alumina membranes, determining low chemical stability in solutions at pH>9, where Pd plating works more efficiently. As a consequence, it was necessary to find the operative conditions allowing Pd deposition without damaging the membrane: to reduce alumina dissolution, the plating bath was buffered at pH 8.5 by addition of either NaHC O3 or Na2 B4 O7 H2 O. Acceptable conversion of Pd was found after a deposition time of 3 min. Single and multiple deposition steps (each lasting 3 min) were accomplished. After five deposition steps, pores remained open in the plating bath containing NaHC O3, while they were almost totally occluded in the bath containing Na2 B4 O7 H2 O. In this last bath, a dense layer of Pd, ∼1 μm thick, was obtained after four deposition steps alternated with surface reactivation steps; the complete closure of pores was confirmed by gas permeability measurements. The different behavior of the two buffers can be attributed to a higher solubility of amorphous alumina in NaHC O3 solution.