Local Structure of Catalytically Active Metal Clusters in Polymer Membranes

Metal clusters were stabilized in poly(amide imide) (PAI) polymers in high dispersion and large amounts of metal loading of typically 15 wt-%. The loaded polymers are prepared as pore-free, mechanically stable membrane films. Pure Pd-loaded and bimetallic Pd/Ag, Pd/Cu 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 of the metal clusters in the protective polymer. The reduction of nitrous oxide by Pd/Ag-loaded membranes in the membrane mode is demonstrated. Cluster sizes are calculated from the measured XAFS coordination numbers by use of an onion model which describes the clusters as spherical metal cores with uniform oxide surface layers. All measurements consistently show a homogeneous distribution of metallic nanoclusters of size 1-6 nm with a smaller amount of larger aggregates in most of the films. Indications of metal alloying in bimetallic films are generally weak. The precise cluster size distribution critically depends on the solvents used as well as on other preparation parameters. In Pd/Cu-loaded membranes which are judged to be amorphous from the XRD spectra, XAFS clearly demonstrates a reaction of Pd with chlorine from CuCl2 precursors which may influence the Pd catalytic behavior. Reduction behavior of the metal nanoclusters at 300K is linked by means of the onion model to microscopic quantities