Study On Synthesis Of Carbon Nanotubes Catalysts And Application In Catalytic Hydrogenation

Temperature programming one-step synthesis for carbon nanotubes(CNTs) using Chemical Vapor Deposition (CVD) method was investigated in this paper. CNTs of purification and modification were done in refluxing nitric acid. After surface modification, Pt and Pd can be supported on the surface of CNTs. These novel catalysts were successfully used in the selective hydrogenation of cinnamaldehyde and halogenated aromatic nitro compounds.CNTs are mainly produced by Chemical Vapor Deposition (CVD) method, in which the yield is high, equipments are simple and technology parameters are easy to control. The CVD method is also one of the most common preparation methods of CNTs. The preparation condition of catalyst, ascending velocity of temperature, and rate of carrier gas were studied in this paper. The composition of carbon nanotubes was confirmed by XRD, TG-DTA and TEM. Comparing with catalyst Co/SiO2, Co/Al2O3 is better for the growth of CNTs. The CNTs prepared by temperature programming CVD method were characterized with diameters of 8~12 nm, surface purity, good graphitic and a high yield.CNTs prepared by CVD method include catalyst, amorphous carbon, carbon fiber, and graphite particles et al. from TEM images. Catalyst was removed by dipping it into 6.0M hydrochloric acid and then ultrasonic treatment for 30min in surfactant (SDS) in order to exfoliate with impurities. After refluxed in nitric acid for 24 hours, oxidized CNTs can be obtained. The factors of reaction temperature and time in purifying CNTs were studied. The oxidation of CNTs created many functional groups and a large number of defects, which were very important to support metal on the surface of CNTs.CNTs supported platinum catalysts were prepared through impregnation and ion-exchanged with modified and crude CNTs. Then reduced by hydrogen at 300℃, and they were characterized by TEM and XRD. Compared with impregnation, the ion-exchanged can obtained smaller metal particles on the surface of modified CNTs. It's very difficult to support metal on the surface of crude CNTs. Compared with Pt/C,Pt/CNTs have higher performances in term of efficiency and selectivity. It can be concluded that the interaction between metal and CNTs favors both the high activity and selectivity in cinnamaldehyde hydrogenation.Palladium catalysts on the surface of CNTs were prepared through impregnation and ion-exchanged method with modified CNTs, and ion-exchanged method can obtain smaller metal granules. When pH is 9 and reduction temperature is 300℃, Pd/CNTs catalyst can be obtained which not only have some certain metal particles on the surface of carbon nanotubes, but the diameter of the metal particles was small. The Pd/CNTs catalyst was then characterized by TEM and XRD, and the diameter of Pd was about 15nm. This novel catalyst was successfully used in the selective hydrogenation of halogenated aromatic nitro compounds. Good activity and selectivity (a conversion of 99.9% and selectivity of 99.8%) were obtained. Compared with 15% dehalogenation in Pd/C catalyst, only less than 0.2% hydrodehalogenation of halogenated aromatic nitro compounds was occurred by using Pd/CNTs catalyst in liquid phase conditions. The activity and life of this novel catalyst were much higher and longer than Pd/C catalyst. The results show that Pd/CNTs catalyst has good quality for the hydrogenation of halogenated aromatic nitro compounds.