(454f) Control of Multi-Walled Carbon Nanotube Structure by Adjusting Metal-Support Interactions in the Active Catalysts

The production of carbon nanotubes (CNT) with known and predictable structure has become an important field for different nanotechnology applications [1-3]. Among the production techniques, chemical vapor deposition (CVD) is not only the least expensive but also the easiest to scale up. In several of our previous studies [4-7] we have shown that the structure of single-walled carbon nanotubes (SWNT) can be controlled by varying either conditions of the catalyst synthesis or by varying the reaction conditions. For multi-walled carbon nanotubes (MWNT) there is now also the demand for application-specific structures, and so there is interest in the development of a degree of control in MWNT growth that is similar to that reached with SWNT. In this study, we have investigated the effects of varying, in a controlled way, the interactions between the active metal and the supports on the resulting CNT morphologies and yield. In particular, the catalytic systems investigated are cobalt and cobalt-molybdenum supported on different aluminas [8-13], with special attention to the phenomena that occur on these catalysts during the various steps in the CNT growth: during calcination and pre-reduction, as well as during contact with the carbonaceous gas feed. The catalyst and catalyst precursors have been characterized using TPR, XPS, XRD, and UV-Vis spectroscopy and the products have been characterized by TEM and TPO. Supporting cobalt on high surface area aluminas results in low carbon yield due to the formation of cobalt aluminate. However, use of low surface area aluminum hydroxide controls the formation of the aluminate, and the dispersion of the cobalt to form a catalyst which has higher productivity of a more uniform product. Elucidation of the formation of the cobalt active species on different alumina supports is presented.

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