Multilayered Polyelectrolyte Films Containing Palladium Nanoparticles: Synthesis, Characterization, and Application in Selective Hydrogenation

Abstract

Catalytic Pd nanoparticles in multilayer polyelectrolyte films can be easily prepared by alternating immersions of a substrate in PdCl42- and polyethylenimine (PEI) solutions followed by chemical reduction of Pd(II) with NaBH4. Transmission electron microscopy confirms that reduced [PdCl42-/PEI]3 films contain nanoparticles with diameters of 1−4 nm, and X-ray photoelectron spectroscopy indicates that ∼70% of the Pd in these films is Pd(0). Atomic emission analysis shows that little Pd is removed from PdCl42-/PEI films during deposition of PEI or rinsing, but about 60% of the Pd in the film is leached during reduction by NaBH4. In a system in which Pd(II) is deposited as a PEI complex, poly(acrylic acid) (PAA)/PEI−Pd(II) films, minimal leaching occurs during either reduction or deposition of PAA. Encapsulated nanoparticles in both [PAA/PEI−Pd(0)]nPAA and reduced [PdCl42-/PEI]n films exhibit selective catalysis, as shown by the fact that Pd-catalyzed hydrogenation of allyl alcohol can occur an order of magnitude faster than hydrogenation of 3-methyl-1-penten-3-ol. Interestingly, adsorption of only PdCl42- on alumina followed by reduction yields Pd that is 5-fold more active than commercial 5 wt % Pd on alumina. Without PEI, however, the catalyst is not selective. In [PAA/PEI−Pd(0)]nPAA films, turnover frequency decreases with the number of layers deposited, suggesting that the outer layer of the film is primarily responsible for catalysis. In contrast, turnover frequency increases with the number of deposited layers for reduced [PdCl42-/PEI]n films. Selective diffusion through [PdCl42-/PEI]n membranes and first-order kinetics with respect to substrate concentration suggest that hydrogenation selectivities are due to different rates of transport to catalytic sites on the nanoparticles.