Controlled growth of carbon nanotubes (CNTs) is of interest for numerous applications due to the superlative properties they exhibit as consequence of their chirality. Chiral specific growth may be possible using uniform critically-sized catalysts under tightly controlled reaction conditions. The uniform molecular cluster [HxPMo12O40⊙H4Mo72Fe30(O2CMe)15O254(H2O)98] (FeMoC) was the first polyoxometalate catalyst to show potential for controlled CNT growth. However, it was found necessary to add catalyst promoters to FeMoC at elevated growth temperatures (T &gt; 900 °C) to enable CNT growth. To understand these disappointing results the “activation” of FeMoC for CNT growth was studied by systematic investigation of H2 gas concentration and growth temperature. The mechanism for “activation” of FeMoC occurs through the sufficient reduction of both metal oxide components in the pro-catalyst. By ensuring pro-catalyst reduction prior to introduction of growth gases, we demonstrate for the first-time growth of CNTs at temperatures as low as 600 °C without the use of catalyst promoters using the single molecular precursor, FeMoC. To understand the role of catalyst promoters used in prior work, thermogravimetric analysis (TGA) experiments were performed. The addition of an iron catalyst promoter is observed to play two key roles in the “activation” of FeMoC: 1) to replenish sublimated metal atoms, and 2) to reduce the reduction temperature required for reduction of FeMoC into an “active” catalyst. In addition, a kinetic study is undertaken to provide a predictive tool for FeMoC reduction, and thus FeMoC “activation”. These results caution the conditions employed in earlier work, and provide a framework for optimizing process conditions for FeMoC and related catalysts in the aim of controlled CNT growth.

Understanding single molecule pro-catalyst "activation" for growth of carbon nanotubes

255th National Meeting (2018)

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