(521dd) Catalyst Design for Kinetic Modeling of High Yield CNT Synthesis

Carbon nanotubes (CNT) possess astounding physical properties that make them ideal materials for a variety of industrial products. Yet, the expenses associated with CNT production and purification have prevented widespread incorporation of CNTs into many fields. That cost can be drastically reduced if CNTs can be grown in such high yields that the catalyst may remain among the nanotubes as a minor impurity that does not warrant costly purification. The ideal catalyst support for this is vermiculite, a cheap naturally occurring clay that consists of two-dimensional nanosheets, while the ideal catalyst metal is iron, a common and non-toxic metal. Exfoliation of vermiculite with species such as citric acid (CA) and dodecyltrimethylammoniumbromide (DTAB) can dramatically increase the surface area of the support and consequently CNT yields. Figure 1a shows effect of exfoliants, and subsequent exfoliations, to dramatically increase carbon nanotube yields via chemical vapor deposition in a fluidized bed reactor. Figure 1b shows on-line monitoring of hydrogen production via a mass spectrometer, which provides insight to the kinetics of carbon nucleation, CNT growth, and catalyst deactivation to better understand the mechanisms at play during the reaction. Catalyst deactivation was modified by including water as a reactant, which has been shown to increase catalyst lifetime by decreasing carbon concentration on the catalyst surface. Additionally, the procedure was studied with both ethylene and methane. These results were fit to a kinetic model to compare the effects of nanotube growth and catalyst deactivation to design a catalyst that yields over 25 grams of CNT per gram of catalyst, which surpasses many yields presented in literature.

Figure 1a. CNT yields on vermiculite treated with dodecyltrimethylammoniumbromide (DTAB) and citric acid (CA) and subsequent exfoliations. Figure 1b. Representative mass spec results for CNT synthesis on treated vermiculite