Preparation Of Noble Metal Nanoparticles And Their Catalytic Properties

It is known that the properties of nanomaterials change with their size and shapes.So the synthesis of new nanoparticles of different sizes and new shapes is beneficial tomake full use of nanomaterials. In this paper, platinum colloids were used as model forthe study of the synthesis and properties of nanoparticles.Firstly, using the glycol solution of NaOH as solvent and reducing agent,“unprotected” platinum colloids were prepared via thermal injection method. Theaverage diameters of the stable metal colloids determined by means of TEM were in arange from2to3nm with narrow size distributions. Platinum colloids were preparedvia thermal injection method by varying the concentration of metallic precursor andthe reaction temperatures. At lower concentration of the metallic precursor, the averagediameters tended to increase with increasing the concentration of metallic precursor;but the increasing stoped when the concentration of metallic precursor reaches acertain value. But, in contrast, the standard deviations tended to increase all along.With the same concentrations of metallic precursor, the average diameters showed alinear dependency on reaction temperatures. In the range from140to198C, whileincreasing each20C of reaction temperature, the average diameters would increase0.13nm.Secondly,“unprotected” Pt colloids were prepared via thermal injection method;some other solvents and protectors were added to the system to further verify theeffects of solvents and protectors on the Pt nanoparticle. It showed that the addition ofwater increased the average diameters of Pt nanoparticle, but the colloid obtained wasunstable. Acetone had a little effect on the diameters of Pt nanoparticle, but the shapeof Pt nanoparticle tended to be sphericity. The Pt colloids obtained was more stableand more spherical with the addition of N-methyl-2-pyrrolidone （NMP）, and theaverage diameters of Pt nanoparticle was increased to1.5times of that preparedwithout NMP. The addition of polyvinylpyrrolidone （PVP） to the system improved the monodispersity and stability of Pt colloids. The average diameter of Pt nanoparticlewas doubled with the addition of a little PVP to the system without any NaOH.Finally, PVP-stabilized platinum/ruthenium colloids （Pt:Ru=2:1）, prepared inmethanol-water mixed solvents, were immobilized on tin dioxide （SnO₂） to make asupported Pt/Ru/SnO₂catalyst with a total loading of0.454%w. The average diameterof the Pt/Ru/SnO₂catalyst determined by means of TEM was2.31nm. This catalystwas used to catalyze the selective hydrogenation of o-chlorobenzene （o-CNB） too-chloroaniline （o-CAN） without using any solvent. It can be seen that at50oC and1.0MPa H2pressure, there was a best reaction time: at60minutes, the conversion ofo-CNB was>99.5%and the selectivity to o-CAN reached to⁹9.5%. It wasrepeatedly used for4times under the standard reaction condition; they all exhibitedvery similar selectivity and activity. The activity only reduced about a third, and theselectivity to o-CAN>98%. When the H2pressure was higher, the activity increased.For the Pt/Ru/SnO₂catalyst, the activity can be reduced substantially by the absorptionof water. Then the catalyst should be vacuum dried at50oC to remove the water andrestore its activity.