Study On The Controlled Synthesis Of Palladium Nanoparticles And Their Catalytic Properties

Synthesis of size- and shape-controlled metal nanoparticles (NPs) has been found to be an active area of research because the size and shape of most nanostructures have a great effect on their potential applications in chemistry, physics, biology, medicine, and electronics. NPs, due to their quantum effects and extremely high surface-to-volume ratio, act as excellent catalysts in organic synthesis reactions. The catalytic activity and selectivity of the NPs largely depend on the size and shapes of the particles. With the change in size and shapes, the catalytic property also changes. This can be attributed in part to the potential binding sites presented by the atoms situated at the corners and the edges of NPs. It is reported that the rate of a catalyzed reaction increases exponentially with the percentage of these atomic sites. It is a major challenge for researchers to understand how to control NP shape in order to maximize the number of coordinatively unsaturated surface sites come to improving the catalytic property. In the paper, the palladium cubic NPs, icosahedral NPs, snowflakelike NPs and dendrimer encapsulated NPs were successfully prepared in the presence of different precursor,surfactants,polymers or reducing agents. The shape-controlled palladium nanostructures are prepared through a polymer-mediated polyol process by microwave (MW) irradiation. The solution of K2PdCl4 is reduced by NaBH4 in the presence of PAMAM for dendrimer encapsulated Pd NPs. The catalytic properties of the nanocatalysts are evaluated by the characterizations of TEM,HR-TEM,XRD,XPS,GC-MS.The catalytic activity and selectivity of p-chloroaniline of the Pd NPs with different shapes and Pd/C for hydrogenation p-chloronitrobenzene are systematically investigated in high pressure reaction pot. Careful observation reveals that the catalytic activity and selectivity of the reaction is best with snowflakelike NPs and worst with cubic NPs, whereas the reaction is intermediate with icosahedral NPs when keeping the molar mass of particles the same. The catalytic activity of Pd/C is the highest but the selectivity is lowest. The different coversitions and selectivities with the different catalysts are analyzed under the identical condition. The catalysts of the different shapes after the reactions are tested again by TEM, the results show that the shapes of the catalysts are same as the original ones.P-nitroaniline and the Pd NPs of the different shapes act the reaction substrate and the catalysts of the reaction of catalytic hydrogenation in the presence of NaBH4.To compare the catalytic activity and apparent activation energy through the reaction time and the rate constant determinated by UV-vis spectrum. The order of apparent activation energy is Ea(icosahedral NPs)﹤Ea(cubic NPs)﹤Ea(snowflakelike NPs). The reduction of the nitro aromatic compounds with Pd NPs as catalysts may follow two different pathways. To find out the exact pathway we carried out our reduction reaction of 4-NA with the intermediate compounds.Monodisperse Pd nanoparticles as small as～1.5nm were synthesized determinated by UV-vis spectrum within a fourth generation polyaminoamide (PAMAM) dendrimer, a hyperbranched polymer, in aqueous solution and reducing agent NaBH4 and successfully immobilized by depositing onto a high-surface-area SBA-15 mesoporous support and SiO2 support. Pd/SBA-15,Pd/ SiO2 and Pd/C as catalysts for catalytic hydrogenation nitrobenzene to compare the catalytic activity. The results show that Pd/SBA-15,Pd/ SiO2 show more excellent catalytic activity for the hydrogenation compared with Pd/C and the superior efficiency of the recyclable use. From the TEM figures of the catalysts after the reactions, the agglomeration phenomenon of small Pd NPs of Pd/SBA-15,Pd/ SiO2 recyclable used is observed.