Polypyrrole–palladium systems prepared in PdCl2 aqueous solutions

In the work, reactions of a partially deprotonated polypyrrole doped with hydroxide ions (PPyOH) in various PdCl2 aqueous solutions which differed in acidity were studied. Using X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy it was established that in the PdCl2 solutions of lower acidity PPyOH was oxidatively doped and Pd0 and Pd2+ were incorporated into the polymer matrix. Pd2+ formed palladium(II) hydroxy-and/or aquochlorocomplex dopant anions and/or was coordinated by nitrogen atoms of the polymer (Pd–N bond). Additionally, deprotonation of PPyOH occurred in the PdCl2 solutions of lower acidity. It was proposed that deprotonation of PPyOH was caused by nucleophilic attack of [PdCl3(H2O)]− on the positively charged, doped polymer chain. By comparison of the PPyOH and chloride-doped polypyrrole (PPyCl)–palladium systems prepared in similar PdCl2 solutions of lower acidity it was shown that the type of the counterion in the starting polymer has a decisive effect on the deprotonation process. PPyOH was less reactive towards palladium species in the PdCl2 solutions of higher acidity where [PdCl4]2− was the dominant complex. PPy–palladium systems containing exclusively Pd2+ were obtained in this case. It was proposed that incorporation of palladium species in these conditions proceeded via an acid–base reaction or coordination of palladium ions by the polymer chain (Pd–N bond formation). Results of the studies may serve as the basis for the preparation of a variety of polypyrrole-supported palladium catalysts.