Observation of carbon nanostructure and evolution of chemical structure from coal to graphite by high temperature treatment, using componential determination, X-ray diffraction and high-resolution transmission electron microscope

Coal can be a promising feedstock to prepare graphite. Due to its heterogeneity, confusing viewpoints of graphitization still exist, which hinders the utilization of coal resources. Herein, we demonstrate coal graphitization from perspective of carbon nanostructure by componential determination, high-resolution transmission electron microscopy and X-ray diffraction. The results showed that the amorphous, graphite-like, polygonal concentric, and pyrolytic graphitic nanostructures coexist in coal-based graphite. The pyrolytic graphitic carbon nanostructure was vapor-deposited from gaseous sources from coal. The formation of graphite-like carbon nanostructure was a successive process with three stages. In carbonization (from ambient to 1000 °C) the majority of heteroatomic elements (H, N, S, O) were eliminated and local molecular orientation was formed. In secondary carbonization and pre-graphitization (1000 ∼ 2000 °C), heteroatomic content (S) was further eliminated. Porous aromatic structure coalesced into larger-diameter polygonal concentric nanostructure. Graphitization (﹥2000 °C) took place with growth in crystalline size and ordering in orientation. De-wrinkling and re-orientating lattice fringes contributed to form the graphite-like structure. The four forms of carbon nanostructures (amorphous, polygonal concentric, graphite-like, pyrolytic graphitic nanostructures) made it clear to understand coal graphitization. Moreover, the finding of pyrolytic graphite provided crucial information for preparing high-quality graphite from coal, which also provided a new insight for clean utilization of coal resources.