钙和羟基影响焦炭还原N_(2)O的量子化学研究

Quantum chemistry study on effect of calcium and hydroxyl group on reduction of N_(2)O by char

基于密度泛函理论(DFT),在M06-2X/6-311G(d)基组水平下,研究了钙和羟基在焦炭还原N_(2)O过程中的作用。采用了Zigzag焦炭模型及钙和羟基修饰后的焦炭模型对N_(2)O还原反应过程进行模拟。结果表明:整个反应过程分为三个阶段,N_(2)O的吸附、N_(2)的析出以及C(O)配合物的解吸,三种模型的决速步均位于C(O)配合物解吸阶段;动力学结果表明,随着反应温度升高,反应速率常数增大,温度升高对N_(2)O-C反应有促进作用;根据传统过渡态理论(TST)计算得到,原始Zigzag焦炭模型及钙和羟基修饰后的焦炭模型异相还原N_(2)O的反应活化能分别为450.1 kJ/mol,420.4 kJ/mol和406.3 kJ/mol,表明钙和羟基对焦炭还原N_(2)O均有促进作用;钙和羟基促进焦炭还原N_(2)O反应的机理不同,钙吸附到焦炭表面,向焦炭转移电子使焦炭带有更多的负电荷,增加了焦炭边缘活性位点的反应活性,决速步的能垒降低,整个反应的放热增加,羟基修饰后整个反应的放热减少,并且N_(2)O—C反应路径发生改变,解吸产物由CO转变为CO_(2),降低了焦炭表面含氧配合物的解吸能垒,促进了焦炭对N_(2)O的还原。

The effects of Ca and OH on the heterogeneous reduction of N_(2)O by char were investigated based on density functional theory(DFT)at the M06-2X/6-311G(d)level.The pris-tine Zigzag model and Ca-or OH-decorated Zigzag model were selected in this study.The entire reaction process is divided into three stages:adsorption of N_(2)O,desorption of N_(2),and desorption of C(O)complexes.The rate-limiting steps of the three models all located in the desorption of C(O)complexes.The kinetic results show that the reaction rate constant increases with increas-ing temperature,indicating that the increasing temperature promotes the N_(2)O-C reaction.Ac-cording to the traditional transition state theory(TST),the reaction activation energies of heter-ogeneous reduction of N_(2)O by the pristine Zigzag model,Ca-or OH-decorated Zigzag model are 450.1 kJ/mol,420.4 kJ/mol and 406.3 kJ/mol respectively,indicating that Ca and OH facili-tates the reduction of N_(2)O.The mechanisms of the promotional effect of Ca and OH are differ-ent.For the Ca-decorated Zigzag char model,the adsorption of Ca on the surface of char results in more negative charge due to the electron transfer from Ca to char,thereby increasing the reac-tivity of the active sites at the edge of the char.The energy barrier of the rate-limiting step after Ca-decoration decreases,but the heat release in the overall reaction process increases.However,after OH decoration,the heat release reduces in the overall reaction process and the N_(2)O-C re-action pathway is changes.For the OH-decorated Zigzag model,the desorption product is CO_(2)rather than CO,reducing the desorption energy barrier of the oxygen-containing complexes on the char surface and promoting the reduction of N_(2)O by char.