N_2O在焦炭表面异相生成和分解机理的密度泛函理论研究

A density functional study of heterogeneous formation and decomposition of N_2O on the surface of char

选用合理简化的焦炭模型,对煤焦燃烧过程中N2 O的异相生成和分解机理进行了分子水平上的研究。采用UB3LYP/6-31G(d)密度泛函理论方法优化得到了反应路径上反应物、产物、中间体和过渡态的几何构型和各中间反应的活化能和反应焓变。NO与其预先吸附在焦炭表面解离生成的表面氮组分反应生成N2O的路径有两个,需要克服的势垒分别为69.3 kJ/mol和200.0 kJ/mol;NO亦可直接与焦炭中的吡啶氮结合释放出N2 O,该反应路径所需克服的最大势垒为418.0 kJ/mol。N2O可在焦炭表面分解释放出N2,异相分解反应为一步反应,计算所得活化能为100.8 kJ/mol。N2 O的异相生成和异相分解反应均为放热反应。采用经典过渡态理论计算得到了各路径中速率控制步骤的反应速率常数。低温条件下,N2O的异相分解反应速率略低于其异相生成速率,随着温度的升高,两者逐渐接近,说明高温条件有利于N2O的异相分解。

A comprehensive density functional study was carried out to get an insight into the mechanisms of heterogeneous formation and decomposition of N2O on the char surface by using a char model with zig-zag configuration.The geometry optimizations of reactants,intermediates,transition states and products were made by using density functional theory at the UB3LYP/6-31G（d） level.On the basis of the reaction pathways analysis,the energies of optimized geometries were calculated and corrected with zero point energy;the relative energies and the change of enthalpy were then worked out.Two different reaction pathways are found for N2O formation from reactions between gaseous NO and another pre-adsorbed NO on the surface of char,with the energy barriers of 69.3kJ/mol and 200.0kJ/mol,respectively.N2O may also be released through direct attacking pyridinic nitrogen by NO;the highest energy barrier is 418.0kJ/mol.N2O can be decomposed to N2 on the surface of char;the energy barrier is 100.8kJ/mol.Both the heterogeneous formation and decomposition of N2O are exothermic.Reaction rate constants for rate-determining steps were calculated with transition state theory.The rate of N2O destruction is slightly lower than that of N2O formation on the char surface at low temperature;two rates become close at higher temperature.High temperature is favorable for the heterogeneous decomposition of N2O.