Atomic-Scale Understanding about Coke Carbon Structural Evolution by Experimental Characterization and ReaxFF Molecular Dynamics

Atomic-scale structural transformation of coke carbon in the thermal annealing process was investigated with four coke carbon samples using X-ray diffraction, Raman spectroscopy, X-ray photoemission spectroscopy, and ReaxFF molecular dynamics. Microcrystal graphite was confirmed to be the basic structural unit in coke carbon structure, and its average size increased from (Lc: 23-27 angstrom; L-a: 48-50 angstrom) to (L-c: 53-78 angstrom; L-a: 53-63 angstrom) with increasing annealing temperature. The coke carbon structural evolution in 1300-1500 degrees C is not Raman-sensitive because most coke carbon bonding types were found to be sp(2) with a stable content (>60%). ReaxFF simulation repeated the experimental results very well and indicates that it is relatively hard to form a well-structured order in the graphene plane (L-a) direction compared with that in the vertical direction (L-c). Five-membered carbon rings, especially when two five-membered rings are connected together, make the graphene layers curved to some extent, leading to the formation of defects as well as disorder in the graphene plane. The increase of carbon structural order decreases the carbon reactivity with CO2, but a clear correlation cannot be constructed in the present study since some other factors such as nanopores and residue ash may also influence coke reactivity.