DFT study on role of OH radical in selective non-catalytic reduction of NO by NH₃

Nitrogen oxides (NO_x) is one of the main pollutants produced by coal-fired power plants. Selective non-catalytic reduction (SNCR) denitration technology has been widely used due to its advantages of simple structure, low cost and no need for catalyst. A large number of OH radicals are produced in the process of SNCR denitration. It is necessary to explore the role of OH radical to improve the efficiency of SNCR denitration. Based on density functional theory (DFT), the role of OH radical in the selective non-catalytic reduction of NO by NH₃ was explored. The results showed that there are three reaction pathways for the reaction of NH₃ and NO, and the optimal reaction pathway is that NH₃ and NO first react to form the intermediate NH₂NO and an isolated H atom, which attacks the O atom of NH₂NO to form OH and NH₂N, followed by the formation of NNH and H₂O. The process of removing the first hydrogen atom from NH₃ requires the highest energy barrier (265.0 kJ/mol), which is the rate-limiting step. The energy barrier of rate-limiting step of NH₂+NO reaction (140.2 kJ/mol) is 124.8 kJ/mol lower than that of NH₃+NO reaction (265.0 kJ/mol), indicating that the denitration reaction rate of NH₂+NO is faster. The energy barrier of rate-limiting step of NH₃+NO+OH reaction (198.7 kJ/mol) is 66.3 kJ/mol lower than that of NH₃+NO reaction (265.0 kJ/mol). The OH radical improves the denitration efficiency of NH₃+NO mainly by promoting the removal of the first hydrogen atom from NH₃ to produce NH₂. The reaction mechanism of NH₃+NO+OH is similar to that of NH₂+NO. The OH radical participating in NH₂+NO denitration reaction can form a six-element complex structure and promote denitration reaction. The OH radical can not only significantly improve the denitration reaction rate of NH₃+NO, but also promote the denitration reaction rate of NH₂+NO at a lower temperature (673−1 273 K). The OH radical can reduce the activation energy of NH₃+NO and NH₂+NO reactions, promoting the denitration reaction. For the selective non-catalytic reduction of NO with NH₃, the removal of the first hydrogen atom from NH₃ to NH₂ is the rate-limiting step, and accelerating this process is the key to improve the denitration efficiency of SNCR.