Microscopic effect mechanism of Ca on NO heterogeneous reduction by char: A DFT study
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摘要: 基于密度泛函理论、结合电子结构分析和Mayer键级变化研究了钙对焦炭非均相还原NO的微观作用机理。对焦炭模型进行电子定域化函数和静电势极值点分析发现，焦炭边缘未饱和的碳原子周边表现出高的电子定域性，体系静电势最小值为-101.1 kJ/mol，边缘碳活性位存在孤对电子。钙的添加可促进第一个NO分子在焦炭边缘的吸附，但对第二个NO分子的吸附影响不大。钙的添加不改变NO在焦炭边缘的非均相还原反应路径，但可将决速步的活化能由124.4 kJ/mol降至91.9 kJ/mol。动力学分析发现添加钙后，非均相还原反应的指前因子增大，焦炭边缘的活化位点增多，有利于加快NO非均相还原反应的进行。Abstract: The effect mechanism of Ca on nitric oxide (NO) heterogeneous reduction by char was investigated using density functional theory (DFT). The electronic structure of char model was analyzed to predict reactive sites. Mayer bond orders were used to quantify formation and breaking of chemical bonds in the reactions. There is a region with high electron localization function values in the extended outer region of unsaturated carbon atoms at the edge. The minimum electrostatic potential of char model, -101.1 kJ/mol, also exists at the edge, indicating the presence of lone pair electrons on edge carbon atoms. The doping of Ca could promote adsorption of the first NO molecule, but has little effect on that of the second NO molecule. The activation energy of rate-determining step is 124.4 kJ/mol for heterogeneous reduction of NO at the edge of char, whereas it is 91.9 kJ/mol at the Ca-decorated char edge. The kinetic analysis shows that the anterior factor increases after doping of Ca, meaning more sites are activated. The promotion of Ca to NO heterogeneous reduction is attributed to combination of the above two aspects.
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Key words:
- calcium /
- char /
- nitric oxide /
- heterogeneous reaction /
- density functional theory
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图 2 NO非均相还原反应中间组分的几何构型(键长: nm)
Figure 2 Geometric configurations of intermediate components in NO-char heterogeneous reaction (bond length unit: nm)
图 4 反应过程中重要原子间的键级变化曲线
Figure 4 Mayer bond orders along IRC of transition states
(a): mayer bond orders along IRC of TS1; (b): mayer bond orders along IRC of TS2; (c): mayer bond orders along IRC of TS3
图 5 修饰Ca的焦炭模型及其电荷分布
Figure 5 Geometric configuration of Ca-decorated char (a) and distribution of Hirshfeld atomic charges (b) (bond length: nm; atomic charge: a.u.)
图 6 Ca修饰焦炭非均相还原NO反应中间组分几何构型(键长单位:nm)
Figure 6 Geometrical parameters for stable species and transition states in NO heterogeneous reduction by Ca-decorated char (bond length: nm)
图 8 反应过程中重要原子间的键级变化曲线
Figure 8 Mayer bond orders along IRC of transition states
(a): mayer bond orders along IRC of CaTS1; (b): mayer bond orders along IRC of CaTS2; (c): mayer bond orders along IRC of CaTS3
表 1 拟合所得反应动力学参数
Table 1 Fitted kinetic parameters of Arrhenius expressions
Reaction A/s-1 Ea/(kJ·mol-1) IM4→P+N2 2.63×1013 128.3 CaIM4→CaP+N2 4.16×1013 96.9 
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