Electrocatalytic hydrogenation activity of palladium and rhodium microparticles dispersed in alkylammonium- and pyridinium- substituted polypyrrole films

Abstract

Zerovalent palladium and rhodium catalysts have been dispersed in conducting anion-exchange polymers coated on carbon-felt electrodes by oxidative electropolymerization of pyrrole monomers containing alkyl-ammonium or pyridinium groups. Incorporation of metal catalysts in polymeric films was effected either by impregnation of PdCl₄^2– or RhCl₆^3– complexes followed by an electroreductive precipitation to Pd⁰ or Rh⁰(method A), or by electroreduction of polymer-coated electrodes immersed in K₂PdCl₄ or Na₃RhCl₆ aqueous solutions (method B). A study of the electrocatalytic activity in hydrogenation of isophorone of various electrodes differing in loading level and incorporation method of metal catalysts was performed. Their activity increased with increasing amounts of metal in the polymer. The most efficient electrodes were obtained by incorporation (method A) of rhodium in poly(pyrrole–alkylammonium) films. Scanning electron microscopy (SEM) showed a dispersion of metal particles from 100–200 nm (method A) up to 2000 nm (method B) on the polymer surface and a large accumulation of metal at the carbon substrate polymer interface. These results were confirmed by Auger electron spectroscopy (AES) depth profile analysis, which also showed that metal catalyst is dispersed throughout the polymer overlayer. Differences in catalytic activities of method A and method B cathodes could be due to a difference in metal particle sizes.