Reaction mechanism of hydrocyanic acid with calcium oxide in sludge pyrolysis: A density functional theory study
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摘要: 采用密度泛函理论对污泥热解中CaO与HCN在低温段的反应进行了研究。在B3LYP/6-311++（3df，2p）水平上计算得到了反应路径上各驻点的几何构型与频率，并在此构型上使用CCSD（T）/cc-pVQZ进行单点能计算。结果表明，两个HCN分子吸附于CaO后，质子发生转移时出现反应路径中最大能垒（310.33 kJ/mol）。使用经典过渡态理论拟合了反应中各步骤的阿累尼乌斯公式，计算了三种典型温度下各步骤的反应速率，发现质子转移为该反应的决速步骤，且温度越高CaO对HCN的作用效果越好。Abstract: The reaction mechanism of CaO with HCN during low temperature sludge pyrolysis was investigated by density function theory. The geometric optimization and frequency calculations of reactants, products, intermediates and transition state in the reaction pathway were performed at B3LYP/6-311++(3df, 2p) level; single point energy calculation was performed at CCSD (T)/cc-pVQZ level and the total energy was corrected by zero-point energy at B3LYP/6-311++(3df, 2p) level. The results indicate that largest energy barrier (310.33 kJ/mol) appears in proton transition process after 2 HCN molecules are adsorbed on CaO. Arrhenius equation for each step was fitted by classical transition state theory and the reaction rate was calculated at 3 typical temperatures. The results suggest that proton transition is the rate-determining step; moreover, the promoting effect of CaO on HCN is enhanced with the increase of temperature.
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Key words:
- calcium oxide /
- hydrocyanic acid /
- density functional theory /
- transition state theory
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图 1 B3LYP/6-311++G (3df, 2p) 水平下优化后各驻点的几何构型(键长/nm，键角/(°))
Figure 1 Optimized geometries of the reactants, intermediates (IM), transition states (TS) and products in the reaction between HCN and CaO at B3LYP/6-311++G (3df, 2p) level (bond length/nm, bond angle/(°))
图 2 CCSD (T)/cc-pVQZ//B3LYP/6-311++(3df, 2p) 水平下CaO与HCN反应能垒图
Figure 2 Potential energy surface of the reaction CaO+HCN calculated at the CCSD (T)/cc-pVQZ//B3LYP/6-311++(3df, 2p) level
表 2 反应各驻点的总能量E、相对能量Erel及过渡态的振动虚频
Table 2 Energies and relative energies of various compounds and the imaginary frequency of transition states
Species B3LYP/6-311++G (3df, 2p) CCSD (T)/cc-pVQZ ZPE/(a.u.) E/(a.u.) Erel/(kJ·mol-1) ν/cm-1 E/(a.u.) Erel/(kJ·mol-1) CaO 0.001 8 -752.828 0 -751.888 HCN 0.016 3 -93.444 1 -93.265 6 IM1 0.019 2 -846.290 2 245 289.963 0 -845.172 3 244 819.736 0 IM2 0.044 2 -939.831 6 -302.982 7 -938.516 8 -256.248 8 IM3 0.043 4 -939.787 6 -187.460 7 -938.467 -125.498 9 IM4 0.019 4 -938.891 8 2 164.462 2 -937.290 4 2 963.664 4 IM5 0.018 4 -938.578 9 2 985.981 2 -937.295 9 2 949.224 2 IM6 0.018 0 -938.575 8 2 994.120 2 -937.296 8 2 946.861 2 TS1 0.037 1 -939.721 4 -13.652 6 -1 912.48 -938.398 6 54.085 3 TS2 0.031 0 -939.679 8 95.568 2 -2 043.50 -938.363 5 146.240 4 TS3 0.018 2 -938.578 9 2 985.981 2 -134.99 -937.293 6 2 955.262 8 TS4 0.018 0 -938.574 9 2 996.483 2 -42.07 -937.295 6 2 950.011 8 CaCN2 0.012 2 -825.216 1 -824.125 2 CO 0.005 0 -113.351 7 -113.164 4 H2 0.010 1 -1.169 9 -1.163 7 
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表 3 各中间反应的阿累尼乌斯公式参数
Table 3 Parameters of Arrhenius equation in the intermediate reaction
Step A ΔE/(kJ·mol-1) IM2→IM3 5.17×1013 318.65 IM3→IM4 9.56×1013 279.83 IM5→IM6 7.45×1013 6.58 the remaining steps could occur spontaneously
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表 4 典型温度下的反应速率常数
Table 4 Reaction rate constants at typical temperature
Step 298 K 523 K 723 K IM2→IM3 7.21×10-43 7.72×10-19 4.92×10-10 IM3→IM4 8.48×10-36 1.07×10-14 5.79×10-7 IM5→IM6 5.24×1012 1.64×1013 2.49×1013
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