Preparation Of Bimetallic Colloids Of Plati Num Group And Their Catalytic Properties

Metal colloid is a mesoscopic system with the unique physical and chemical properties.Owning to the size of nanoscale particles, the electron distribution and electronic states of metal colloids are different from microscopic metal atoms and macroscopic metals. Due to their high surface area and high surface energy, metal colloids have significant potential as a new type of catalyst with high activity and selectivity. Metal colloids can be arranged to three types according to the stabilizer in the colloids, namely solvent or anions stabilized metal colloids, surfactant or coordinations ligands stabilized metal colloids and polymer stabilized metal colloids. The most attractive polymer protective metal nanoclusters are Poly(N-vinyl-2-pyrrolidone) (PVP) protected colloids. Many investigations have clarified that the catalytic behavior of bimetallic colloidal catalysts is better than monmetallic colloids in various hydrogenation reactions. Thus, PVP-stabilized bimetallic colloids of platinum group as nanocatalysts were prepared and their catalytic behavior in the the hydrogenation of chlorobenzene and chloronitrobenzene was studied. This thesis focuses on the following aspects:Firstly, Poly(N-vinyl-2-pyrrolidone)-stabilized Pd, Pt, Pd-Pt nanocatalysts were prepared by reducing the corresponding metal salts in methanol and water mixed solvents. The metal colloids were characterized by transmission electron microscopy (TEM). TEM measurements showed that PVP-Pd/Pt nanoparticles with varieties of the molar ratio of Pd to Pt were well dispersed and no aggregation of the metallic particles could be detected. It was found that all the metallic nanoparticles were in the size range of 2.0-5.5 nm. When the platinum content increased, no significant change on particle size (dav =3.18-3.69 nm) was observed. The average particles size of PVP-Pd and PVP-Pt nanocatalysts were 3.18 and 3.65 nm, and standard deviations (σ) were 0.52 and 0.56 nm, respectively. The average diameters of the PVP-Pd/Pt bimetallic colloidal particles were ranged from 3.38 to 3.69 nm, which were all larger than that of PVP-Pd nanoparticles.Secondly, hydrogenation of chlorobenzene was carried out over these PVP-Pd, PVP-Pt and PVP-Pd/Pt colloidal nanocatalysts under ambient conditions. The catalytic properties for the hydrogenation of chlorobenzene depended on the composition of the bimetallic nanocatalysts. The conversion of chlorobenzene over PVP-Pd (83.64%) was higher than that of PVP-Pt (66.69%), which indicated that the activity of Pd was higher than that of Pt. In 10 hrs. the conversions of all the bimetallic nanocatalysts were higher than that of PVP-Pt (66.67%) monometallic nanocatalysts, and the maximum conversion of chlorobenzene (95.34%) was achieved using PVP-Pd/Pt = 1/1 catalytic system, which was much higher than that of the physical mixture of monometallic nanocatalysts (PVP-Pd and PVP-Pt) at the same Pd/Pt ratio as the PVP-Pd/Pt bimetallic nanocatalysts used. The selectivity to benzene and cyclohexane of the bimetallic nanocatalysts (with≤40 mol% Pt) was similar to that of PVP-Pd monometallic nanocatalysts, and nearly ~100% selectivity to benzene could be obtained, the selectivity to cyclohexane increased slowly with increasing of platinum content in bimetallic nanocatalysts.Thirdly, PVP stabilized ruthenium-platinum (PVP-Ru/Pt) bimetallic colloids were used as catalysts for the selective hydrogenation of meta-chloronitrobenzene (m-CNB) in methanol at 303 K under 0.1 MPa of hydrogen. The influence of substrate concentration, catalyst concentration and reaction temperature on the activity and selectivity has been investigated. When the concentration of m-CNB in the system was below 0.33 mol·L-1, the reaction rate increased with the increasing of the reactant concentration. The hydrogenation rate of m-CNB over PVP-Ru/Pt bimetallic colloidal nanocatalysts was observed to be approximately zero order respect to the concentration of the reactant when the concentration of chloronitrobenzene was higher than 0.33 mol·L-1. Kinetic results show that the reaction rate of hydrogenation is observed to be the first order dependence with catalyst concentration. The relationship between reaction rate and reaction temperature obeys the Arrehinus equation. The activation energy of the m-CNB hydrogenation reaction is 20.1 KJ/mol.Finally, PVP stabilized ruthenium-platinum (PVP-Ru/Pt) bimetallic colloids were also used as catalysts for the selective hydrogenation of para-chloronitrobenzene (p-CNB) in methanol at 303 K under 0.1 MPa of hydrogen. The results show that the catalytic properties in the hydrogenation of p-CNB are similar with that of m-CNB. The hydrogenation reaction rate of p-CNB over PVP-Ru/Pt bimetallic colloidal nanocatalysts can be approximately first order respect to the substrate concentration in the low concentration of reactant. Kinetic results show that the reaction rate of hydrogenation is observed to be the first order dependence with catalyst concentration. The activation energy of the p-CNB hydrogenation reaction derived from the Arrhenius plot is 13.7 KJ/mol.