Continuous palladium films were synthesized at controlled depths within porous alumina disks by H 2 reduction of organopalladium compounds dissolved in supercritical CO 2 at 60 °C using an opposing reactants deposition geometry. Film position was controlled by adjusting the relative concentrations of H 2 and the palladium precursor (π-2-methylallyl(cyclopentadienyl)palladium (II) or palladium(II) hexafluoroacetylacetonate) on opposite sides of the alumina substrate. Because of a disparity in the diffusivity of the metal precursor and H 2 in the support, a temporary barrier of poly-4-methyl-1-pentene on the H 2 side of the alumina substrate was used to reduce H 2 flux in a controlled manner. Guided by a simple mass transport model, Pd films between 2 and 80 µm thick were deposited at prescribed depths between 80 and 600 µm as measured from the precursor side. Electron probe microanalysis indicated complete pore filling of the porous alumina at the reaction zone and X-ray diffraction revealed that the structure of the deposit is nanocrystalline. The flux of N 2 through the alumina disk was reduced by over 4 orders of magnitude after deposition and annealing at 500 °C.

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