Structure-Selective Preparation of Carbon Nanofibers under Gaseous CO ₂ Mixed with Other Carbon Sources

Carbon nanofibers (CNFs) are fibrous materials formed via layering of hexagonal, carbon network planes. CNFs are used to prepare energy devices, catalytic supports, and adsorbents. CNF shapes, diameters, layers, and surface structures vary by the synthetic conditions. Usually, CNFs are prepared via high-temperature catalytic reactions involving high-purity gases such as CO and C2H4, which are expensive. This renders CNF use challenging. We here present a low-cost and environmentally friendly CNF preparation method. A gas that models the exhaust gas of chemical plants serves as the CNF feedstock. The gas contains not only the hydrocarbon feedstock but also CO2, which may sometimes inhibit CNF growth. We previously showed that, under certain conditions, CO2 in fact promoted CNF growth and that some CO2 was converted to CNF by a specific iron-based catalyst. However, CNFs thus produced may vary in terms of structure and shape. Platelet CNF (PCNF), herringbone CNF (HCNF), and tubular CNF (TCNF) differ in terms of the stacking directions of the hexagonal carbon network planes. The various CNF structures exhibit distinct physical properties that affect their applications. Structure-selective CNF preparation methods are urgently required. Here, we synthesized structurally defined CNFs using CO2 as the carbon source. This is novel, and will aid future developments in the field. CNF structure depends on the shape of the catalytic metal particles. We therefore prepared homogeneous catalysts: Fe–Ni alloys, such alloys on MgO, and Fe–Co alloys on MgO, and used these to synthesize structurally selective CNFs from an H2/C2H4/CO2 mixture. CNF structural selectivity and yield greatly improved when Fe was alloyed with Ni on an MgO support. The CNF fibers were uniform. When the preparation conditions varied, such catalysts grew three types of structure-selective CNFs with homogeneous shapes. This renders it possible to produce structure-selective CNFs using the exhaust gases of factories. Furthermore, it was found that the electrical conductivity of CNF synthesized using CO2 as a carbon source with a catalyst supported on MgO was higher than that of CNF obtained by conventional preparation methods. This study reveals new phenomena, including the selective synthesis of structurally distinct CNFs from CO2 and the fabrication of CNFs with enhanced electrical conductivity.