Reactive & Functional Polymers, Vol.37, No.1, 111-119, 1998
Promotion effect of lanthanoid ions on catalytic activity of polymer-immobilized palladium nanoparticles
Hydrogenation of acrylic acid was catalyzed by the Pd nanoparticles protected by lanthanoid polyacrylate (PAA-Pd-Ln). In comparison with the case without lanthanoid ions (Ln(3+)), the hydrogenation rate was accelerated by Ln(3+) and independent of the concentration of substrate, These results indicate that Ln(3+) ions were concentrated so fast around the Pd nanoparticles by ion exchanging of Na+ ions with the Ln(3+) ions in polyacrylate located near the Pd nanoparticles. This is also supported by the result that no promotion effect was observed when using PVP as a protective polymer, which has a weak coordination ability to Ln(3+) in place of polyacrylate. The promotion by Nd3+ ions was observed in the PAA-Pd-catalyzed hydrogenation of methyl acrylate and allyl amine, but not of 1-hexene. This fact again indicates that the coordination of Ln(3+) ions to the substrates containing an oxygen or a nitrogen atom(s) is an important factor for the promotion effect by Ln(3+). On the other hand, it was clarified from XPS and TPD measurements that the dissociative hydrogen adsorption on the surface of Pd nanoparticles can be accelerated by the presence of Ln(3+) ions, probably due to an electron transfer from Ln(3+) to Pd nanoparticles, although this is thought to contribute slightly to the present promotion effect.