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摘要: 基于密度泛函理论和过渡态理论，在分子水平上对焦炭异相还原NO以及碱金属钠的作用机理进行探究。结合单点能的零点能校正以及过渡态的虚频验证，发现钠能够有效促进焦炭对于第一个NO分子的吸附。尽管钠不能改变反应步骤，但可将焦炭异相还原NO决速步的活化能由121.04 kJ/mol降至100.62 kJ/mol；钠的存在使焦炭异相还原NO的指前因子增大且反应速率加快，增加了焦炭边缘的活性位点，强化了焦炭对于NO的异相还原性能。Abstract: A molecular modeling study based on density functional theory (DFT) and transition state theory (TST) was performed to investigate the effect of Na on the NO heterogeneous reduction by char; zero point energy correction was considered and the transition states was confirmed by frequency analysis. The results show that Na can effectively promote the adsorption of first NO molecule on to the char. The presence of Na cannot change the reaction steps, but reduce the activation energies of rate-determining steps from 121.04 kJ/mol to 100.62 kJ/mol. Moreover, the presence of Na can increase the pre-exponential factors as well as the reaction rate, meaning more active sites and enhanced catalytic performance of char in NO reduction.
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Key words:
- biomass reburning /
- heterogeneous reduction /
- sodium /
- molecular simulation /
- kinetic analysis
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图 1 Zigzag型焦炭模型示意图
Figure 1 Char model with zigzag edge (carbon atoms at the edge were labeled as: C1, C2, C3, C4)
图 2 NO异相还原反应中间产物及过渡态几何构型(键长：nm)
Figure 2 Geometric parameters for stable species and transition states (bond length: nm)
图 3 Zigzag型焦炭与NO异相还原反应的反应势能面
Figure 3 Reaction potential energy surface of NO heterogeneous reduction at the edge of char
图 4 Na修饰的焦炭模型及其电荷分布
Figure 4 Geometric configuration of Na-decorated char (a) and distribution of Hirshfeld atomic charges (b) (bond length, nm; atomic charge, a. u.)
图 5 Na修饰焦炭异相还原反应中间产物及过渡态几何构型(键长：nm)
Figure 5 Geometric parameters for the stable species and transition states in the NO heterogeneous reduction by Na-decorated char model (bond length: nm)
图 6 Na修饰焦炭与NO异相还原反应过程的反应势能面
Figure 6 Reaction potential energy surfaces of NO heterogeneous reduction at the edge of Na-decorated char
表 1 反应路径各驻点能量及过渡态振动虚频
Table 1 Energies of various compounds and imaginary frequency of transition states
Species E/(kJ·mol-1) Frequency/cm-1 Species E/(kJ·mol-1) Frequency /cm-1 IM -2856326.30 - NaIM2 -3624341.19 - IM1 -3197558.53 - NaIM3 -3624339.89 - IM2 -3197908.82 - NaP+N2 -3624293.46 - IM3 -3197907.32 - NaTS1 -3623920.36 -263.48 P+N2 -3197839.39 - NaTS2 -3624328.91 -233.86 TS1 -3197483.39 -475.11 NaTS3 -3624234.95 -165.53 TS2 -3197895.36 -264.72 NO -341168.95 - TS3 -3197768.49 -131.19 N2 -287649.68 - NaIM -3282727.29 - R -2514653.09 - NaIM1 -3623928.51 - Na -426013.72 - 
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表 2 模型平均键长的计算值和文献值对比
Table 2 Calculated average bond length for the model used in this work, compared with those reported in the literature
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表 3 NO在焦炭及Na修饰焦炭边缘吸附的Mulliken电荷布局分析
Table 3 Mulliken atomic charge populations for NO adsorption on the edge of char and Na-decorated char
Atom Charge /(a.u.) NO/R IM NaIM N1 0.041 -0.170 -0.186 O1 -0.041 -0.346 -0.503 C2 -0.029 0.302 0.328 C3 -0.024 0.172 0.178
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表 4 反应动力学参数
Table 4 Reaction kinetic parameters
Reaction Pre-exponential factor A/s-1 Activation energy Ea/(kJ·mol-1) Arrhenius equation IM3→P+N2 1.50×1014 130.29 k=1.50×1014e-15670.84/T NaIM3→NaP+N2 7.32×1014 100.20 k=7.32×1014e-12050.92/T
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