Theoretical study on the catalysis mechanism of sulfur-doped carbon nanotubes in CO2 desorption from monoethanolamine solution
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摘要: 醇胺法吸收CO2是目前最成熟的碳捕集技术，虽然吸收效率高、稳定性好，但过高的解吸能耗限制其大规模工业推广应用。催化解吸提供了降低CO2解吸能耗的可能性。本工作利用基于密度泛函理论（DFT）的量子化学模拟方法，探索了硫掺杂碳纳米管（S-CNTs）催化单乙醇胺（MEA）溶液吸收-解吸CO2反应机理。通过过渡态搜索发现，以S-CNTs为催化剂的解吸过程，决速步骤的反应能垒降低了1.15 kcal/mol。局部态密度分析表明（PDOS），产物氨基甲酸酯吸附质子化胺MEACOO-_MEAH+和吸收中间产物MEA+COO−中的C、N、O原子在CNTS和S-CNTs表面吸附时PDOS差距较大。此外，与未改性CNTs相比，S-CNTS上电荷密度增加，掺杂的硫原子附近碳原子具有明显的电负性。相比于CNTs，吸收中间产物MEA+COO−和吸收产物MEACOO−-MEAH+均向S-CNTs转移了更多的电荷，表明更多的电荷转移有利于CO2的释放。本工作旨在通过CO2催化解吸机理的研究为催化剂的设计提供一定的理论依据。
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关键词:
- 掺杂调控 /
- 碳纳米管 /
- 催化MEA解吸CO2机理 /
- DFT计算
Abstract: The CO2 absorption by alkanolamine solution has been applied industrially because of its excellent efficiency. However, the energy consumption for CO2 desorption is high. To reduce the energy consumption, the catalyst is introduced into the alkanolamine capture system. In this study, the catalysis mechanism of sulfur-doped carbon nanotubes (S-CNTs) in CO2 desorption from monoethanolamine (MEA) solution is explored by simulation based on density-functional theory (DFT) calculations. It was found that compared with the single wall carbon nanotubes (CNTs), the adsorption performance of the key substances in the absorption-desorption process on S-CNTs was different, and the adsorption energy of the reactant MEA was reduced by 1.50 kcal/mol, and the adsorption energy of the absorption intermediates MEACOO-_MEAH+ was increased by 2.32 kcal/mol, and the adsorption energy of the absorption product carbamate (MEACOO-)increased significantly. By transition state searching, energy barrier for the rate-determining step was reduced by 1.15 kcal/mol in the desorption process with S-CNTs as the catalyst, suggesting that S-CNTs contributes to amine regeneration. By observing the Mulliken charge of C atoms in the vicinity of S atoms, it was found that the charge of C atoms changed from electroneutral (0.001 eV) to electronegative (−0.325 eV) . The partial density of states (PDOS) of C, N and O atoms from the absorption intermediate MEA+COO− and the absorption product MEACOO−-MEAH+ changes greatly when they adsorbed on CNTs and S-CNTs. In addition, compared with CNTs, the charge density on S-CNTs increases, and the C atoms near the doped S atoms attain obvious electronegativity. Compared with CNTs, the absorption intermediate MEA+COO− and the absorption product MEACOO−-MEAH+ transfer more charges to S-CNTs. This paper is of guiding significance for protecting the environment, maintaining the sustainable development of the energy industry, improving the utilisation rate of raw materials, and reducing the production cost of desorbed CO2. It aims to provide some theoretical basis for the design of catalysts through the study of the catalysis mechanism of S-CNTs in CO2 desorption from monoethanolamine solution. -
图 1 S-CNTS结构示意图
Figure 1 Schematic diagram of S-doped CNTS structure
yellow ball represents Satoms, gray represents Catoms.
图 2 （a）MEA-CO2和MEA-CO2-CNTS/ S-CNTS催化反应路径的能量变化（b）对应的结构示意图
Figure 2 (a) Energy changes in the catalytic reaction path of MEA-CO2 and MEA-CO2-CNTS / S-CNTS (b) Schematic diagram of the corresponding structure
图 3 （a） MEA-CO2-CNTS/S-CNTS催化反应路径结构的静电势图 和（b） 电荷密度图 （c） 差分电荷密度图
Figure 3 (a) Electrostatic potential diagram of the structure of the catalytic reaction pathway of MEA-CO2-CNTS/S-CNTS and (b) Charge density diagram (c) Differential charge density diagramR1 / R refers to the reactant monoethanolamine solution and CO2 in the catalytic desorption reaction, IM 1 refers to the intermediate zwitterion ZW in the catalytic desorption reaction, IM 2 refers to the intermediate ZW and single ethanolamine solution in the catalytic desorption reaction, P1 / P refers to the products carbamate and protonation amine, TS1 and TS2 are transition states.
图 4 （a）CNTS/S-CNTS结构中C、S等原子的PDOS图（b）CNTS/S-CNTS_R1催化反应结构中C、N、O等原子的PDOS图
Figure 4 PDOS plots of C, N, O and other atoms in the structure of the CNTS/S-CNTS_R1 catalytic reaction and PDOS maps of C and S and other atoms in the CNTS / S-CNTS structure(The red ball represents oxygen atoms, the gray ball represents C atoms, the yellow ball represents S atoms,the white ball represents H atoms, and the blue ball represents N atoms)
图 7 MEA-CO2-S-CNTS催化反应路径结构中C、S、O原子的PDOS图和 精确分子模型系空间充满式模型（Corey-Pauling-koltun，CPk）模式
Figure 7 The PDOS diagram of C, S and O atoms in structure of MEA-CO2-S-CNTS catalytic reaction pathway and the exact molecular model of S-CNTS adsorption structure are space-filled model (Corey-Pauling-koltun, CPk) mode (Red balls represent oxygen atoms, gray balls represent carbon atoms,yellow balls represent sulfur atoms, and white balls represent hydrogen atoms)
表 1 不同反应物/吸收产物/中间态吸附在碳纳米管/S掺杂碳纳米管上的吸附能
Table 1 Adsorption energy of different reactants/desorption products/intermediate states adsorbed on carbon nanotubes/S-doped carbon nanotubes
Materials MEA CO2 MEACOO− MEAH+ MEA_CO2 MEA+COO− MEA+COO−_MEA MEACOO−_MEAH+ Ead(CNTs)/(kcal·mol−1) −3.728 −1.489 − −57.228 −5.582 −3.528 −2.700 −3.531 Ead(S-CNTs)/(kcal·mol−1) −2.229 −0.242 −26.322 −56.306 −2.484 −2.748 −2.748 −5.846 注：碳纳米管吸附了吸收产物MEACOO-之后，然后经过结构优化发现MEACOO−分解成了MEA和CO2。 
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