Preparation of carbon spheres by low-temperature pyrolysis of cyclic hydrocarbons

Abstract

Low-temperature pyrolytic decomposition of xylene, benzene, toluene, and naphthalene was investigated as a low-cost method for synthesis of a large quantity of perfectly shaped pure carbon spheres. The reaction occurred in a closed iron container at temperatures in the range of 500–700 °C and under pressure of 20 MPa. Some of the experiments were carried out in the presence of distilled water in vapor state near to its critical point, under which conditions it reacted as a very strong oxidant. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis were used to investigate the properties of the material obtained. The results demonstrated that synthesis had proceeded in the following sequence: vaporization of hydrocarbon, hydrocarbon transformation to condensing drops of heavy hydrocarbons, then drop growth accompanied by continuous alteration of the heavy carbon compounds until their final carbonization. Synthesis in a closed space stimulated particle growth, and particle diameters reached 1 to 12 μm. Continuous condensation of vapor onto the resulting drops caused agglomeration of some of the spheres to form pearl-necklace-like chains when the spheres docked to one another in the course of growth. Carbon atoms in the spheres were arranged in concentric, incompletely closed graphitic shells, which made them stable up to 600 °C in air. Above this temperature, rapid sphere degeneration occurred. Oxygen reacted with carbon atoms situated at the shell edges during heating in air, with some remaining in the spheres; oxygen penetration increased with treatment temperature.