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摘要: 采用量子化学密度泛函理论结合热力学和动力学分析研究了含氮煤焦还原NO的途径；从微观角度探究了含氮煤焦还原NO的间接还原和直接异相还原两种途径，分析了NO还原过程中的能量变化。结果表明，含氮煤焦先产生中间体NH2再还原NO（间接还原）的过程决速步能垒值较直接异相还原NO的决速步能垒值高183.76 kJ/mol；由能垒角度分析，含氮煤焦与NO直接发生异相还原的过程更为有利。从热力学角度分析，含氮煤焦直接异相还原NO为可自发进行的单向放热反应，较间接还原过程有利。动力学分析结果表明，含氮煤焦间接还原NO的过程决速步速率常数较直接异相还原至少低10个数量级，说明含氮煤焦直接异相还原NO的路径更容易发生。Abstract: Density functional theory was used to investigate the reaction pathways for the reduction of NO by nitrogen-containing char with char-bound nitrogen, viz., char(N); the reaction paths of heterogeneous reduction of NO by char(N) were analyzed from the thermodynamic and kinetic point of view. The results show that the energy barrier of the rate-determining step via the indirect NO reduction path by char(N), viz., first producing NH2 intermediate and then reducing NO, is 183.76 kJ/mol higher than that via the direct heterogeneous NO reduction path, suggesting that the later direct heterogeneous NO reduction path is more favorable. Thermodynamically, the direct heterogeneous NO reduction by char(N) is a spontaneous and exothermic process in the coal combustion system. Kinetically, the rate constant of the rate-determining step in indirect NO reduction path by char(N) is at least 10 orders of magnitude lower than that in direct heterogeneous reduction, also illustrating the superiority of the direct heterogeneous NO reduction path by char(N).
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Key words:
- char(N) /
- indirect reduction /
- direct heterogeneous reduction /
- NO /
- thermodynamic /
- kinetic
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图 2 间接还原过程中各驻点结构
(键长单位为nm)
Figure 2 Geometrical structures of stationary points in indirect NO reduction
(numbers are selected bond lengths in nm)
图 5 直接异相还原NO过程中各驻点结构
(键长单位为nm)
Figure 5 Geometrical structures of stationary points in the direct heterogeneous reaction of NO
(Numbers are selected bond lengths in nm)
图 6 直接异相还原NO的能量示意图
Figure 6 Schematic energy profiles for the direct heterogeneous reactions of NO
图 8 不同温度下的热力学参数
Figure 8 Thermodynamic parameters at different temperatures
K indirect NO reduction; K direct heterogeneous NO reduction; —●—: ΔH direct heterogeneous NO reduction; —○—: ΔH indirect NO reduction; —■—: ΔG direct heterogeneous NO reduction; —□—: ΔG indirect NO reduction
图 9 800-1800 K两种途径的各步反应速率常数
Figure 9 Rate constants for elementary reactions over the temperature range of 800-1800 K for two reaction pathways
图 10 800-1800 K温度条件下限速步速率常数
Figure 10 10 Rate constants of all rate-limiting steps over the temperature range of 800-1800 K
表 1 由NBO计算的一些重要原子的自然居群
Table 1 Natural population of some important atoms calculated by NBO
Atom Species Charge Valence H1 TS6 0.37298 0.62531 IM8 0.38078 0.61231 N2 TS6 0.15304 4.80537 IM8 -0.03578 5.00058 
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