Catalytic growth of macroscopic carbon nanofiber bodies with high bulk density and high mechanical strength

Carbon nanofibers (CNF) are non-microporous graphitic materials with a high surface area (100-200 m²/g), high purity and tunable surface chemistry. Therefore the material has a high potential for use as catalyst support. However, in some instances it is claimed that the low density and low mechanical strength of the macroscopic particles hamper their application. In this study we show that the bulk density and mechanical strength of CNF bodies can be tuned to values comparable to that of commercial fluid-bed and fixed-bed catalysts. The fibers were prepared by the chemical decomposition of CO/H₂ over Ni/SiO₂ catalysts. The resulting fibers bodies (1.2 μm) were replicates of the Ni/SiO₂ bodies (0.5 μm) from which they were grown. The bulk density of CNF bodies crucially depended on the metal loading in the growth catalyst. Over 5 wt% Ni/SiO₂ low density bodies (0.4 g/ml) are obtained while 20 wt% Ni/SiO₂ leads to bulk densities up to 0.9 g/ml with a bulk crushing strength of 1.2 MPa. The 20 wt% catalysts grow fibers with diameters of ∼22 nm, which grow irregularly in space, resulting in a higher entanglement and a concomitant higher density and strength as compared to the thinner fibers (∼12 nm) grown from 5 wt% Ni/SiO₂.