The nature of cobalt species in carbon nanotubes and their catalytic performance in Fischer–Tropsch reaction

The nature of cobalt species in the catalysts supported by multi-wall carbon nanotubes and their catalytic performance in Fischer–Tropsch synthesis were investigated using nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy, in situ magnetic method, X-ray absorption and temperature programmed reduction. The catalysts were prepared by incipient wetness impregnation using solutions of cobalt nitrate assisted by sonochemical process followed by calcination in nitrogen. The characterization techniques uncovered that acid pretreatment oxidized the carbon nanotube surface and removed impurities. Small cobalt oxide particles of 8–10 nm diameter and irregular shape anchored to the outer surface of carbon nanotubes were detected in the calcined samples by several characterization techniques. The catalysts displayed high cobalt reducibility, which was slightly affected by the pretreatment with nitric acid and nanotube outer diameter. Cobalt catalysts supported on carbon nanotubes exhibited high catalytic activity in Fischer–Tropsch synthesis. Pretreatment with nitric acid leads to a 25% increase in hydrocarbon yield, while carbon nanotube diameter does not seem to significantly affect the Fischer–Tropsch performance of the resulting catalysts.