Study On The Catalytic Properties Of Nickel-phosphorus Amorphous Alloy Supported On Carbon Nanotubes

Carbon nanotubes (CNTs) were synthesized from decomposition of methane over three nickel-based catalysts prepared by co-precipitation method. Experimental results showed that the nature of catalysts could affect the yield, microstructures, morphology and properties of CNTs that were characterized by XRD, TEM, BET and TG. The addition of sodium carbonate into Ni-Cu-Al catalyst brought about slight decreases in the yield of CNTs and led to the morphology change from bamboo-shaped CNTs to large-inner-diameter CNTs. The sodium carbonate added in system played an important role in controlling the morphology of CNTs. Chemical vapor deposition technique based on porous anodic aluminum oxide (AAO) template was applied for synthesizing highly aligned carbon nanotube arrays. The channel diameter of AAO, thereby the diameter of CNTs inside the channels, could be easily regulated by altering anodization voltage.Amorphous NiP/CNTs catalyst was prepared by induced reduction, in which H₃PO₂ was used as the reducing agent, and amine was used to adjust the pH value of the solution. Benzene hydrogenation was used as a probe reaction for the study of effect of different treatment to CNTs and preparation conditions on catalytic activity of amorphous NiP/CNTs catalyst. Catalysts were characterized by XRD, TEM, ICP, TPR, XPS, H₂-TPD and DTA. The effect of support-CNTs and rare earth elements on catalytic activity and the thermal stability were discussed. Experimental results showed that CNTs oxidized by HNO₃ or activated by KOH resulted in improvements in catalytic activity of amorphous NiP/CNTs catalyst, and preparation conditions could affect its catalytic activity to some extent. NiP supported on CNTs belonged to amorphous structure. The particle size of NiP alloy supported on CNTs became smaller. Its dispersions and the specific surface area increased remarkably. In amorphous NiP/CNTs catalyst, both phosphorus and CNTs could donate electrons to nickel, making Ni electron-rich. The reduction of the electron-rich Ni became easy and the strength of hydrogen adsorptions on it was weakened. The catalytic activity of benzene hydrogenation for NiP/CNTs catalyst was lower than that for amorphous NiP alloy, but its specific activity was higher, which attributed to the dispersing effect of support, the electronic interaction between support and alloy, the relatively weak Ni-H bond and hydrogen-storage ability of CNTs. The amorphous NiP alloy supported on CNTs could improve its thermal stability, owing to the dispersing effect of support, heat sink of the support and interaction between support and alloy. The addition of a small amount of rare earth elements could increase the content of nickel in NiP/CNTs catalyst, the bulk content of nickel, specific area of catalyst as well as activate metal surface area. The small amounts of rare earth elements added in amorphous NiP/CNTs catalyst promoted the catalytic activity of benzene hydrogen because of structure effect and electronic effect. However, higher content of rare earth elements led to the coverage of surface Ni atoms by rare earths, therefore decrease of catalytic activity. The addition of rare earths enhanced the thermal stability of catalyst due to the possibility that a larger atom of rare earths compared with Ni and P partly displaced Ni in the lattice, resulting in less free volume and a lower rate of diffusing, moreover, such addition might prevent the gathering of Ni atom due to the dispersing effect of rare earth.