Formation of fine Fe-Ni particles for the non-supported catalytic synthesis of uniform carbon nanofibers

The morphological changes of Fe-Ni catalyst for the preparation of carbon nanofiber (CNF) were examined at 5 steps; (1) the precipitation of Fe-Ni carbonate from Fe-Ni nitrate solution, (2) the calcination of Fe-Ni carbonate into Fe-Ni oxide, (3) the reduction of Fe-Ni oxide, (4) the second reduction of Fe-Ni metal before the growth of CNF, and (5) the reaction with CO/H₂ for the growth of CNF. The Fe-Ni fine particle was formed from the Fe-Ni aggregate through the second reduction and successive CNF growth from CO/H ₂. The temperature of these two steps is the most important factor which determines the size and shape of the Fe-Ni fine particle as a catalyst for CNF growth. The lower temperature of 580°C provided hexagonal particles with very smooth surface sized around 100-200 nm which allowed the growth of platelet CNFs of the same diameter and cross-sectional shape of the formed catalyst particle. At the higher temperature of 630°C, the Fe-Ni aggregate was found to give the very fine Fe-Ni particles by the two steps; the first step did the Fe-Ni particle sized around 100-500 nm which was successively degraded into smaller particles sized around 20-40 nm, thinner tubular CNFs growing with the contact of CO/H₂. Such smaller particles definitely originated from as-precipitated Fe-Ni carbonate through the steps. The metal particle on the top of CNF was almost exclusively composed of Fe although the catalyst particle before the growth of CNFs carried around 65% of iron and 35% of nickel. The preferential activity of Fe to CO gas may cause such the selectivity. The major role of Ni in the present reaction should be limited to provide the uniform particle of Fe. Controlling the size of the Fe-Ni particle through the reduction and reaction steps was proved to be a key factor to determine the dimension and structure of resultant CNF.