Sonochemical preparation and catalytic behavior of highly dispersed palladium nanoparticles on alumina

Palladium nanoparticles dispersed on Al2O3 were prepared via the sonochemical reduction of tetrachloropalladate(II) (Pd(II)) in an aqueous solution. The reduction of Pd(II) to metallic Pd successfully proceeded, even in the presence of Al2O3 powder, by ultrasonic irradiation at 200 kHz. The rates of Pd(II) reduction strongly depended on the type of alcohol additive, which acts as an effective accelerator for Pd(II) reduction. The dispersion of metallic Pd particles on the Al2O3 surface was appreciably enhanced by increasing the rate of reduction. By UV-visible, pH, and transmission electron microscopy measurements, the major pathway in the formation of Pd nanoparticles on Al2O3 was speculated to proceed via the following three steps: (I) the reduction of Pd(II) ions proceeds with reducing radicals formed by sonolysis of water and alcohol molecules, resulting in the formation of Pd nuclei, (2) the growth and/or the agglomeration of the Pd nuclei rapidly occurs to form Pd nanoparticles, and (3) the Pd particles are simultaneously adsorbed onto the Al2O3 surface. The catalytic activities of the Pd nanoparticles prepared by sonochemical reduction were investigated for the hydrogenation of olefins at room temperature. The rates of hydrogenation over these catalysts were much higher than those over a conventionally prepared Pd/Al2O3 catalyst and a commercially available Pd black.