EXAFS investigations of Pd-doped conductive polymers

Polyacetylene (PA) samples were prepared from unstretched PA films by the Naarmann method and doped with Pd(NO3)2 in acetonitrile solution. X-ray absorption spectra near the Pd K edge were measured in transmission mode at room temperature for Pd–(PA) foils, palladium metallic foil and palladium oxide powder. The EXAFS (extended X-ray absorption fine structure) analysis showed that C and Pd atoms are the nearest neighbours around Pd atoms in Pd–(PA) samples. The averaged Pd–C bond distance in Pd–(PA) specimens varies from 2.15 to 2.18 Å. In two Pd–(PA) specimens which are worse catalysts, a considerable part of Pd atoms remains in grains resembling Pd metal with the distance Pd–Pd smaller by about 0.02 Å, as compared to the Pd metal. In another Pd–(PA) specimen revealing a good catalytic character, a very weak EXAFS signal from the Pd–Pd distance being larger by 0.04 Å in comparison to Pd metal is observed. It is concluded that ultra-small Pd particles, the size of few atoms, exist in this particular catalyst, which can play a crucial role in catalytic activity and selectivity in reaction of the hydrogenation of the triple CC bond