The influence of physicochemical reaction parameters on the synthesis of multi-walled carbon nanotubes for use as catalyst supports.
Oosthuizen, Rachel Suzanne.
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Multi-walled carbon nanotubes (MWCNTs) and other shaped carbon nanomaterials (SCNMs) were synthesized by the floating catalyst chemical vapour deposition (CVD) method, using either ferrocene  as the catalyst at 2.5 or 5 wt.%, or a synthesized heteroatom-containing ferrocene derivative, in toluene, in the range 750 to 950 °C. The derivatives used were ferrocenoyl imidazolide  (a source of N and O) at 2.5 and 5 wt.%, (N-phenylcarbamoyl)ferrocene  (a source of N and O) at 1.25 wt.% and S,S-bis(ferrocenylmethyl)dithiocarbonate  (a source of S and O) at 2.5 wt.%, which was synthesized from ferrocenylmethanol . These were characterized by melting point, 1H- and 13C-NMR spectroscopy, IR spectroscopy and mass spectrometry (MS). The effects of variations in the CVD physicochemical reaction parameters, namely temperature, catalyst employed (and the effect of its heteroatoms, where applicable) and catalyst concentration, on the CVD products were investigated. These materials were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive Xray spectroscopy (EDX), Raman spectroscopy, thermogravimetric analysis (TGA) and some by the Brunauer, Emmett and Teller method (BET). The best temperature range, in terms of high yields of MWCNTs with relatively high thermal stabilities and surface areas, in general, was identified as being 800 to 900 °C, from results obtained with . This temperature range was used for further experiments. Among other results, it was shown that  and , at 2.5 wt.%, and at 800 and 850 °C respectively, produced the best materials. Catalysts  and  produced primarily carbon spheres, however, in general, all experiments using N-containing catalysts produced bamboo-shaped MWCNTs. For , at 2.5 wt.%, smaller bamboo compartment lengths correlated with decreasing temperature and decreasing crystallinity, suggesting a larger incorporation of nitrogen with lowered temperature. Catalyst  at 2.5 wt.% also produced very “clean” MWCNTs and this was attributed to optimal levels of oxygen being able to convert amorphous carbons to gas. Certain MWCNT properties were shown to be dependent on the combined, or synergistic, effects of catalyst concentration and temperature. The best undoped MWCNTs that were synthesized and commercially produced MWCNTs were loaded with Pd nanoparticles using a metal organic CVD (MOCVD) method. Results revealed well dispersed metal nanoparticles of narrow size distribution.