Heterogeneous reaction mechanisms of the reduction of nitric oxide on carbon surfaces: a theoretical analysis

The mechanism of reaction between NO and two models of carbonaceous materials with active sites was investigated at the UB3LYP/6-31 + G(d) and UM06-2X theory levels. The small model is the anthracene radical and the large one is also a monoradical built with ten benzene rings. The mechanistic routes found with both models lead to a satisfactory justification of the experimental data and showed the important role of the temperature and the oxygen and nitrogen surface complexes, generated in the carbonaceous material at intermediate steps of the mechanism, in the global process. The computational results presented in this work revealed that, at low temperatures, the high Gibbs energy barrier that appears after N-2 release from the (NO)(2) dimer, initially chemisorbed on the char surface, prevents the subsequent evolution of the system with the result that CO2 emission does not take place. On the other hand, at high temperatures, the mean energy available to the reactants may be sufficient to overcome this energy barrier giving rise to the formation of N-2 and CO2 as reduction products. The N-2 may come from two sources depending on the approach of the NO molecule at different points of the reaction coordinate. The best description of the carbonaceous surface through a larger model confirms the absence of N2O release in the reduction of nitric oxide on carbon surfaces.