The effect of CO during the NO heterogeneous reduction on the carbonaceous surface has been studied using density functional theory (DFT) at the M06-2X16-311G(d) level of theory. The zigzag model with pyridinic-N embedded in the aromatic ring clusters was selected to model the nitrogen-containing char (char(N)). The rate coefficients for the key reaction steps were computed by the canonical variational transition-state theory (CVT), providing a deeper understanding of the NO reduction process. The calculations show that the most stable intermediates for NO chemisorption involve interactions with the previously adsorbed CO moiety. Schematic energy profiles for each favorable adsorption route were obtained in order to elucidate the mechanisms for N-2, CO2 and N2O releases. In the presence of CO, the emission of CO2 in the NO reduction is available at low temperatures, indicating that CO enhances CO2 release. The promoting action of additional CO is mainly reflected in a reduction of the energy barrier with its concomitant increasing of reaction rate. The different results drawn from CVT provide direct insight into the limitation of the conventional transition state theory (TST), which deserves more attention in further theoretical research. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.