Mechanisms of N₂ formation from zigzag configurations with different dinitrogen active sites during coal pyrolysis

To have a deep insight into the microscopic mechanism of N₂ formation during coal pyrolysis, the density functional theory was employed to compute and investigate the formation path of N₂. Seven-membered ring containing two pyridine nitrogen is selected as carbonaceous models. The results reveal that different dinitrogen active sites induce varied electron distribution on the surface of models. It makes varying energy barriers for the breakage of the C-N. In the pyrolysis process of R1 (two N atoms numbered 4 and 6) and R2 (two N atoms at positions 6 and 8), the energy barriers of the cleavage of the first N (the nitrogen first stripped in the structure) are 387.49 and 357.47 kJ/mol, respectively, and the energy barriers of the cleavage of the second N (the nitrogen stripped later in the structure) are 142.43 and 316.89 kJ/mol, respectively. However, different dinitrogen active sites have no effect on the main pyrolysis properties of the structure. Both of the two dinitrogen char models experienced the same process: the stripping of the first N atom and the formation of five-membered ring, the formation of dinitrogen five-membered ring, the stripping of the second N atom, the formation of C(NN) structure, N₂ desorption. According to the analysis of the reaction energy barriers, dinitrogen zigzag char model to N₂ is consistent with the experimental results.