Structural characterization of catalytically active metal clusters in polymer membranes

Metal clusters were generated and stabilized in pore free, mechanically stable poly(amide imide) (PAI) polymer membranes in high dispersion and high concentration of typically 15wt-%. These membranes have been successfully tested for catalytic applications. Pure Pd-loaded and bimetallic Pd/Ag, Pd/Cu, Pd/Co, Pd/Pb PAI films were investigated by means of x-ray absorption spectroscopy, small and wide angle x-ray scattering and transmission electron microscopy to characterize the structure of the metal clusters in the protective polymer. The measurements consistently show a homogeneous distribution of metallic nanoclusters of 10-30 Å size within the membranes. Also, a smaller amount of larger aggregates up to 300 Å is observed in most of the films. The precise cluster size distribution critically depends on the solvents used as well as on other preparation parameters such as the stirring time of the metal precursor/polymer solution. In case of Pd/Ag and Pd/Pb bimetallic films no clear evidence for the formation of bimetallic clusters in the membrane, i.e. alloying of both metal components, is found. In Pd/Cu and Pd/Co membranes, chlorine from CuCl₂ and CoCl₂ precursors reacts with Pd which may influence the Pd catalytic behavior. Reduction and oxidation of the metal nanoclusters is quantitatively studied by means of x-ray absorption spectroscopy. Membrane properties are discussed with respect to catalytic applications.