Effect of anodic alumina pore diameter variation on template-initiated synthesis of carbon nanotube catalyst supports

Multi-walled carbon nanotubes (MWCNTs) are a potential alternative to commonly used heterogeneous catalyst supports due to their high surface area, structural integrity, and controllable dimensional characteristics. Synthesis of MWCNTs with specific pore diameters can be achieved by chemical vapor deposition (CVD) of a carbon source onto an anodic aluminum oxide (AAO) template. AAO films consist of pore channels in a uniform hexagonal arrangement that run parallel to the surface of the film. These films are created by the passivation of an aluminum anode within an electrolysis cell consisting of certain weak acid electrolytes. A two-step anodization method has been developed to produce these templates. Changing the concentration of the oxalic acid electrolyte and the electrical potential of the electrolysis cell altered the structural characteristics of these AAO films, specifically the diameter of their pore channels. The AAO characteristics were examined by SEM analysis. Controlling the pore diameter of these templates enabled the pore diameter of MWCNTs synthesized by CVD to be controlled as well. After considering their thermal and structural stability, the produced MWCNTs were characterized by SEM, TEM, TGA-DTA, BET N-2 adsorption analysis, and DRIFT spectroscopy. Anodizing conditions of 0.40 M oxalic acid concentration and 40.0 V maximum anodizing potential were found to produce AAO films that resulted in MWCNTs with optimum surface characteristics for a catalyst support application. These MWCNTs with optimum pore diameters were synthesized for the purpose of being used as a support for NiMoS hydrotreating catalyst. This optimum grade of MWCNTs was found to produce the highest HDS and HDN activities when applied in the packed-bed processing of coker light gas oil. (C) 2009 Published by Elsevier B.V.