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 Ͼ 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 NaHCO 3 or Na 2 B 4 O 7 •H 2 O. Acceptable conversion of Pd was found after a deposition time of 3 min.Single and multiple deposition steps ͑each lasting 3 min͒ were accomplished.After five deposition steps, pores remained open in the plating bath containing NaHCO 3 , while they were almost totally occluded in the bath containing Na 2 B 4 O 7 •H 2 O.In this last bath, a dense layer of Pd, ϳ1 m 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 NaHCO 3 solution.