The C(N)-NO reactions with isolated and contiguous active sites leading to the release of N-2 are determined by density functional theory (DFT) calculations. Possible transition states and intermediates involved in these reactions are characterized because these influence the directed conversion of C(N) to N-2. The thermodynamic results encompassing these processes are obtained, and they are used as a good candidate to distinguish the difference in reaction mechanism. The contiguous active sites are more favorable than the isolated active site for chemisorption but less favorable for N-2 separation due to the large coulomb attractive force. The overall N-2 separation from carbonaceous surface with contiguous active sites can take place with the largest energy penalty of 209.6 kJ/mol and an exothermicity of 436.5 kJ/mol. Energetically, the C(N)-NO reactions with isolated active site are also possible where an energy of 410.5 kJ/mol will release and 219.0 kJ/mol energy barrier is required. According to our calculated results, contiguous active sites responsible for surface migration and rearrangement are important in C(N)-NO reactions. Further comparisons with the published works indicate that the N-2 separation is dependent on (1) the location of the active site, (2) the form of surface nitrogen and (3) the nearby groups. (C) 2017 Elsevier Ltd. All rights reserved.