(503b) Sol-Gel Synthesis and Characterization of Co-Mo/Silica Catalysts for Single-Walled Carbon Nanotube Production

Previous studies by our group [1-3] have demonstrated that Co-Mo/silica catalysts exhibit high selectivity to the growth of single-walled carbon nanotubes (SWNT) by CO disproportionation. It has been seen that different parameters such as the extent of interaction of different Co-Mo species with the support, metal loading, Co-Mo molar ratio, and reaction conditions have an important effect on the yield and structural properties of SWNT.

In this context, the present study focuses on the synthesis of catalysts by using different methods such as sol-gel and incipient wetness impregnation.

The sol-gel method has been extensively used in the preparation of supported metal catalysts because it typically results in homogeneous materials with a high metal dispersion [4,5]. In particular, this method has been used to obtain active phases of Ni-Mo and Co-Mo [6] bimetallic catalysts supported on silica for the oxidative dehydrogenation of alkanes. It has been observed that catalysts prepared by the sol-gel method can stabilize the bimetallic active phases much more efficiently than those prepared by other methods. Therefore, sol-gel appears as a suitable method to produce Co-Mo/SiO2 catalysts, which are highly selective towards for single-walled nanotubes (SWNT) when the gaseous feed is CO [7]. In this study, a series of silica-supported Co-Mo samples prepared by the sol-gel method has been compared as catalysts for the synthesis of single-walled carbon nanotubes (SWNT). The ratio of ammonium hydroxide to the silica precursor tetraethoxysilane (TEOS) has an important effect on the resulting morphology of the silica support [8] and, consequently, on the nature of the Co-Mo catalytic species. In turn, these morphology changes have significant effects on carbon yield, quality, and type of the single-walled carbon nanotubes obtained by the disproportionation of CO at 750 °C. In addition, a catalyst with an open microscale structure has been prepared by using carbon fibers as burnable sacrificial templates. This open structure results in several-fold enhanced carbon yield, while keeping the same nanotube quality as those obtained on conventional powder catalysts.

On the other hand, a series of Co-Mo/Silica catalysts were prepared by using incipient wetness impregnation. This method has been widely recognized, as the previous one, to prepare supported metal catalysts [1-3,9]. These catalysts were prepared using silica supports with different surface properties (surface area, point of zero charge, residual impurities, particle size, crystallinity, etc.) in order to analyze the effect that these parameters may have on the yield and quality of the nanotube product. The results have been compared with the aim of obtaining structure/properties relationships for this interesting application of heterogeneous catalysts.

References

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