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 typically 15 wt %. The loaded polymers were prepared as pore-free, mechanically stable membranes, which have been successfully tested for catalytic activity in membrane reactors. Pure Pd- and Ag-loaded as well as bimetallic Pd/Ag, Pd/Cu, Pd/Co, and Pd/Pb PAI films were investigated by means of X-ray absorption spectroscopy (XAFS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) to characterize the structure 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 aggregates up to 30 nm in some of the films. The precise cluster size and distribution 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 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 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 permeation. Optimum membrane preparation conditions are discussed with respect to the cluster formation mechanism.