STRUCTURAL CHARACTERIZATION OF CATALYTICALLY ACTIVE METAL NANOCLUSTERS IN POLY(AMIDE IMIDE) FILMS WITH HIGH METAL LOADING

Noble metal clusters were generated and stabilized in poly(amide imide ) (PAI) polymers in high dispersion and high concentration of typicall y 15 wt %. The loaded polymers were prepared as pore-free, mechanicall y stable membranes, which have been successfully tested for catalytic activity in membrane reactors. Pure Pd- and Ag-loaded as well as bimet allic Pd/Ag, Pd/Cu, Pd/Co, and Pd/Pb PAI films were investigated by me ans of X-ray absorption spectroscopy (XAFS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) to characterize the structu re and morphology of the metal clusters in the protective polymer. The measurements consistently show a homogeneous distribution of metallic nanoclusters of 1-3 nm size with a smaller amount of larger aggregate s up to 30 nm in some of the films. The precise cluster size and distr ibution critically depend on the solvents used (N-methyl-2-pyrrolidone , tetrahydrofuran) as well as on other preparation parameters such as the stirring time of the metal precursor/polymer solution. In the case of Pd/Ag and Pd/Pb bimetallic films no clear evidence for the formati on of bimetallic clusters in the membrane, i.e. alloying of both metal components, is found. In Pd/Cu and Pd/Co membranes, chlorine from the CuCl2 and CoCl2 precursors reacts with Pd, which may influence the Pd catalytic behavior. Reduction of the oxidized metal nanoclusters by H -2 at 300 K is quantitatively studied by means of XAFS and gas permeat ion. Optimum membrane preparation conditions are discussed with respec t to the cluster formation mechanism.