Theoretical study on the reduction reactions from solid char(N): The effect of the nearby group and the high-spin state

Highlights: • The one-step reduction reactions from solid char(N) are proposed. • The multi-step reduction reactions from solid char(N) are proposed. • The effect of nearby group on the energy property is investigated. • The role played by the high spin state is determined. -- Abstract: A fundamental understanding of the reduction reactions from solid char(N) is proposed with the aim of providing useful information to aid in minimising NO_x emission. The direct reduction with (a) nearby radical; (b) nearby oxygen and (c) nearby vacancy is calculated. The results show that the nearby oxygen will increase the activation energy by 51.8 kJ/mol and the nearby vacancy will reduce the exothermicity by 269.7 kJ/mol. This leads us to the firm conclusion that the nearby radical site will provide a favorable route. The indirect reduction reactions from surface migration are also determined. According to the calculated results, the high-spin singlet reactions are more complicated than the low-spin triplet reactions. The barrier height involved in the singlet reactions is much lower than that encountered in the triplet reactions, indicating that the reaction with singlets holds potential of being an important channel for N₂ production. Our qualitative analysis of the density functional theory (DFT) results confirms that the high-spin excited-state changing the electronic properties of the carbonaceous surface is much more favorable for the reduction than the low-spin ground-state. Much more emphasis therefore should be placed on the high-spin states during the mechanism study of the heterogeneous reactions.