Catalytic Performances Of Carbon Nanotubes Supported Transition Metal In Hydrogenation Reaction

Multi-walled carbon-nanotubes (MWCNTs) possesses a series of unique features, such as well-defined tubular morphology, good electrical conductivity, sp2-C-constructed surface, and excellent performance for adsorption and spillover of hydrogen, all of which make the MWCNTs full of promise to be a novel catalyst support or promoter. It has been demonstrated that CNTs can improve activity and selectivity in hydrogenation, hydroformylation and electrocatalysis when it is used as catalyst support. In the present work, the catalysts that transition metal supported CNTs were synthesized, and their redox property, catalytic performance as well as confinement effect of CNTs were studied.1. The tuning effect of CNTs channel on the redox property of Ni and Co catalysts was investigated. The results of temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES) and thermogravimetric analysis (TGA) indicate that the Ni and Co species within the channel of CNTs is not only facilely reduced but also oxidized easier in air than that deposited on outer surface of CNTs.2. The catalytic performance of Ni filled inside or deposited outside of CNTs was evaluated in hydrogenation of benzene. The conversion of benzene on the inside Ni catalyst is 4.6 times as that of outside Ni catalyst. The research results indicate that the enhanced catalytic activity can be attributed to the confinement of CNTs with more defects which provides facile reduction, reinforced reactivity, increased reactants concentrations, special interaction between metal and CNTs surfaces, deficient electron in tubular micro-reactor, and the gaps formed on the sidewall of CNTs during the treatment process also play an important role for decreasing the diffuse resistant kinetically.3. Ni and Co catalyst inside low defect of CNTs exhibit lower catalytic activities than that of outside catalyst. Results indicate that the structural defects of support CNTs act as more important factor which influences the confinement effect mostly. We obtained different defect density of CNTs by ultrasonication treatment and investigated the relationship between the defect and confined catalytic activities in detail. It is revealed that the catalytic activity of transition metal confined in CNTs increases along with the defect density of CNTs, only the defect density of CNTs reaches to adequate lever could the confinement effect exhibit perfectly, otherwise, the channel of CNTs could limit the taking place of catalytic reaction.4. Ni catalyst was supported on three types of CNTs which produced by different technique process, and the catalytic performance of Ni supported CNTs was evaluated in hydrogenation of benzene. Result showed that there are distinct differences in graphitization, structure defect, BET surfaces, thermal stability, function groups and redox property between the three types of CNTs. Structure defect of CNTs is the essential factor which affects the catalytic performance. The high structure defect of CNTs leads thermal stability decrease, reducibility increase, electrical conductivity enhance and function groups add, all of which bring about the high catalytic activity of Ni supported CNTs in hydrogenation of benzene.5. A series of CNTs supported bimetallic Pd-Ni and Pt-Ni catalysts were prepared base on 6% Ni loading. TEM morphologys displayed that the high dispersed catalysts were obtained for the bimetallic catalysts. Results of H2-TPR, Raman spectroscopy, thermogravimetric analysis and cyclic voltammetry revealed that the reducibility and electrical conductivity are enhanced for the bimetallic Pd-Ni and Pt-Ni catalysts, the defect formed on CNTs is due to the strong interaction between the different metals, which facilitate the decomposition of CNTs in treatment process. All of which make the bimetallic Pd-Ni and Pt-Ni catalysts exhibit excellent catalytic performances compared to corredponding monometallic in hydrogenation of benzene.