Study On The Catalytic Activity Of Icosahedral Metal Palladium Nanoparticles

Much attention has been paid to the effect of morphology of metal nanoparticles on its catalytic properties. In recent yeas, though the shape-controlled synthesis of metal nanoparticles has made a progress, the catalytic properties of metal nanoparticles with a certain morphology has been scarcely reported. As one of noble metals, palladium has also attracted great interests because of its extraordinary catalytic properties, and it can be widely used as the primary catalyst. So, in this paper, the catalytic properties of icosahedral Pd nanoparticles were investigated.Uniform and well-dispersive icosahedral Pd nanoparticles with an average diameter of about 22.90nm were synthesized with H2PdCl4 as a precursor, tetraethylene glycol (TEG) as both a solvent and a reducing agent, polyvinylpyrrolidone (PVP) as a stabilizer in the presence of a little amount of KOH under microwave irradiation. The as-prepared Pd nanoparticles were characterized by by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectrometry (XPS). The Pd icosahedra supported onγ-Al₂O₃ was examined by Flame Atomic Absorption Spectrometry (FAAS), N2 adsorption-desorption isotherms and TG-DTA.The catalytic activities of icosahedral Pd nanoparticles supported onγ-Al₂O₃ were examined by means of the hydrogenation reaction of cyclohexene to cyclohexane. The effects of temperature, hydrogen pressure, supports on the catalytic activities of icosahedral Pd nanoparticles were investigated. The results showed that the hydrogenation of cyclohexene finished in 30 min with 1.0 MPa of hydrogen pressure at ambient temperature, and the conversion rate and TOF reached 100% and 1347 h-1, respectively. In addition, icosahedral Pd/γ-Al₂O₃ remained a high catalytic activity after being used for 7 times.The catalytic propertiy of icosahedral Pd/γ-Al₂O₃ was also investigated with the hydrogenation of o-chloronitrobenzene (o-CNB) to o-chloroaniline (o-CAN). The effects of temperature, hydrogen pressure and metallic ions on the catalytic activities of Pd nanoparticles were examined. Under the condition of 1 MPa, 50°C and reaction time for 2 h, o-chloroaniline (o-CAN) with 65.4% selectivity at 100% conversion of o-CNB was obtained. With the addition of appropriate metal ions, the hydrogenation time was shortened and the selectivity of o-CAN increased to 88.1% at 100% conversion without dehalogenated product. Moreover, the Pd/γ-Al₂O₃ catalyst was still active after reaction for 5 times.The catalytic performance of icosahedral Pd/γ-Al₂O₃ catalyst for Heck reaction was investigated by a model reaction of iodobenzene or bromobenzene with acrylic acid or styrene. The results showed that the yield of cinnamic acid and 1,2-stilbene reached 95.1% and 92.5%, respectively, in the condensation reaction of iodobenzene with acrylic acid or styrene for 6 h at 90°C. and the Pd/γ-Al₂O₃ catalyst remained a high activity after being used for 5 times.