Study on CO/CO2 formation mechanism of Zigzag model coke with high oxygen coverage based on DFT theory
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摘要: 碳资源在能源、材料及化工等领域的清洁高效利用日益重要，而焦炭氧化特别是脱附产生CO2/CO的机理研究并不充分。其中较高焦炭表面氧覆盖率相应于较低温度或较高压力的反应条件，对此，本研究基于第一性原理研究讨论了该情况下焦炭Zigzag结构碳环簇氧化脱附过程的反应路径。计算表明，表面吸附氧热解生成CO2过程需要经过重排形成含O−C−O团簇的结构，最终至CO2完成脱附需多个中间反应步，与对比文献中形成碳氧六元环再依次断掉两个C−O 键而脱附 CO2 不同，本研究得到了相关的两种路径，分别为形成 CO2−C−官能团再断掉 C−C 而脱附CO2以及基于碳氧六元环结构直接断裂两个C−O 键而脱附CO2的可能反应路径。另外，研究了CO脱附过程的不同反应路径。模型计算结果与相关文献理论和实验结果具有良好的符合。Abstract: The clean and efficient utilization of carbon resources is becoming more and more important, in energy, material, and chemical engineering field, but the mechanism of coke oxidation, especially that of the CO2/CO desorption is not fully studied yet. In this paper, density functional theory was used to study the oxidation mechanism of Zigzag char structure with high coverage of O2, which is related to an oxidation under lower temperature or high pressure. Based on the corresponding quantum chemistry calculation, it is shown that there are several possible pathways for the CO2 desorption process, which may need rearrange to form the structure containing O−C−O clusters. And successively, multiple intermediate reaction steps are required to complete the desorption of CO2. Other than in the literature that the COO–O–C functional group formed first, with then the C–O bond broken and CO2 desorbed respectively, a novel pathway with two C–O bonds broken simultaneously to generate CO2 was found. It results from a functional group of COO–char formed, and certain alternative pathways via C–C bonds breaking were also dealt with, as well as related CO desorptions. The reaction model built was validated by theoretical and experimental results from literature satisfactorily.
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Key words:
- char /
- oxidation /
- desorption /
- reaction path /
- DFT theory
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图 1 Zigzag与Armchair焦炭表面模型示意图
Figure 1 Zigzag (left) and Armchair (right) models of coke molecule
图 2 高覆盖度Zigzag表面氧吸附模型示意图
Figure 2 Oxygen adsorption model with high coverage along the Zigzag coke edge
图 4 模型边缘活性位CO/CO2的生成与脱附过程对应构型变化（所框为路径最终产物）
Figure 4 Corresponding configuration changes of the desorption of CO/CO2 at the edge of the coke molecule
图 5 边缘活性位CO/CO2的脱附过程相对能量变化
Figure 5 Diagram of relative energy with CO/CO2 desorption at the edge active sites
图 6 初始构型X分别到B4及U+CO（图中所框为过程末端构型）过程的构型变化
Figure 6 Configuration diagram of the initial configuration X to B4 and CO, respectively, with the frames denoting the final configurations (the frame is the final configuration)
图 7 由B4到CO2脱附的各路径构型变化（框内为相应路径产物）
Figure 7 Configuration changes of each path from B4 to CO2 desorption (the products of each path in the color boxes)
图 8 初始吸附构型重排至B4过程相对能量变化
Figure 8 Diagram of relative energy change from initial configuration rearrangement to B4
图 9 碳氧六元环结构B4重排脱附CO2过程的相对能量变化
Figure 9 Relative energy diagram of six-membered ring rearrangement to CO2 desorption
图 10 上述六元环结构直接脱附CO2过程构型变化
Figure 10 Configuration diagram of direct desorption of CO2 from a six-membered ring structure
图 11 图10反应过程相对能量变化
Figure 11 Diagram on relative energy of direct desorption of CO2 related to Figure 10
图 12 各脱附反应路径速控步反应速率常数随温度的变化
Figure 12 Diagram of rate constant of rate-controlled step vs temperature in each desorption pathway
表 1 边缘活性位CO/CO2的生成与脱附过程的能垒、焓变及过渡态虚频
Table 1 Energy barrier, enthalpy change and imaginary frequency of each step in the CO/CO2 formation and desorption process
Reaction ΔE/(kJ·mol−1) ΔH/(kJ·mol−1) Imaginary frequency/cm−1 X → Yts → Y+CO 562.96 20.56 91.42 i X→ Ats → A1 196.65 159.41 411.10 i A1→ A1ts → A2 14.40 2.04 271.17 i A2→ A2ts → A3 2.34 −27.06 155.45 i A2 → Zts → Z+CO 8.28 −21.45 308.18 i A3→ A3ts → A4 112.38 17.58 377.67 i A4→ A4ts →A5 52.97 −293.34 336.70 i A5→ A5ts →A6 323.71 309.16 275.81 i A6→ A6ts →A7 6.65 −57.24 395.01 i A7→ A7ts →A8+CO2 34.73 −35.15 351.6 i 
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表 2 中间位点重排及脱附过程中各基元步的能垒、焓变以及虚频
Table 2 Energy Barrier, enthalpy change and imaginary frequency of each step in the processes of rearrangement and desorption
Reaction ΔE
/(kJ·mol−1)ΔH
/(kJ·mol−1)Imaginary frequency
/(kJ·mol−1)X → Yts → Y+CO 562.96 20.56 91.42 i X → Bts → B1 306.48 271.29 368.64 i B1→ B1ts →B2 20.08 −103.93 208.84 i B2→ B2ts → B3 30.62 −38.95 217.17 i B3→ Uts → U+CO 48.37 −71.72 727.24 i B3 → B3ts → B4 21.42 −247.52 712.15 i B4→ B4ts → B5 259.37 86.15 116.25 i B4 → αts → ɑ1 281.83 234.81 547.88 i α1 → α1ts → α2 8.41 −10.29 351.53 i α2 → α2ts → α2+CO2 25.90 −46.19 366.38 i B4 → βts → β1 295.26 198.44 582.07 i β1 → β1ts → β2 0.13 −12.26 207.88 i β2 → β2ts → β3+CO2 35.73 −22.64 352.48 i B5 → θts → θ1 228.19 162.46 551.45 i θ1 → θ1ts → θ2 24.86 13.89 325.85 i θ2 → θ2ts → θ3+CO2 20.62 −46.15 383.07 i B4→ Vts→ V+CO2 286.18 163.54 822.10 i
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