铂掺杂单壁碳纳米管对新型环保气体CF3I典型分解产物吸附特性研究

曹政钦1 李 威1 魏 钢1 姚 强2 胡 刚1

(1. 重庆科技学院电气工程学院 重庆 401331 2. 国网重庆市电力公司 重庆 400015)

摘要 环保型绝缘气体CF3I会在局部放电下发生分解并生成C2F6、I2、C2F5I、C2F4和CH3I等产物。为此,该文基于密度泛函(OFT)理论,构建和优化了Pt掺杂(8, 0)单壁碳纳米管(Pt-SWCNT)及两种典型CF3I局部放电分解产物C2F6和CH3I的分子模型。在此基础上,计算和分析了C2F6和CH3I气体在Pt-SWCNT上的吸附距离、吸附能量、电荷转移量、态密度和前沿分子轨道等吸附特性。研究发现:Pt-SWCNT和C2F6分子间的吸附能及电荷转移量均很小,并且吸附前后的态密度和能隙变化并不明显,表明Pt-SWCNT并不适用于检测C2F6气体。而Pt-SWCNT和CH3I分子之间有着较强的相互作用,并且该吸附以化学吸附为主,吸附CH3I后的能隙显著减小,表明Pt-SWCNT对CH3I气体具有较好的气敏特性。这为环保型气体CF3I在气体绝缘设备中的监测提供理论依据。

关键词:CF3I 吸附特性 单壁碳纳米管 密度泛函理论

0 引言

SF6绝缘气体因其良好的绝缘和灭弧性能,已被广泛应用于多种高压电气设备中[1-4]。但SF6气体绝缘设备在安装、运行、调试和检修过程中,不可避免地会有SF6气体泄漏到大气中。然而,SF6是一种温室效应极强的气体,其温室效应是CO2的23 900倍,是CH4的1 140倍[5-6],并且在大气中极难降解。随着《巴黎协定》的签署,为了实现在21世纪后半叶温室气体的净零排放目标,采用电气性能和优良的气体替代SF6,已成为电力工业亟待解决的问题。

从20世纪70年代开始,各国学者就开始了新型SF6环保型代气体的研究,并发现C3F7CN、C5F10O、C6F12O和CF3I等环保气体具有良好的绝缘性能,在一定程度上可替代SF6[7-13]。其中,电负性气体CF3I因具有稳定、绝缘能力强、热传导和灭弧水平接近SF6等突出优势,受到了广泛的关注[14-15]。但CF3I会在局部放电(Partial Discharge,PD)情况下发生分解,并进一步与气室中不可避免存在的微量H2O和微量O2反应,生成C2F6、I2、C2F5I、HF、C2F4和CH3I等[16-18]。其中,C2F6的含量最多,CH3I则在放电达到一定程度时才会产生[16]。一方面这些分解产物含量过多会降低气体绝缘设备的绝缘水平;另一方面它们也能有效反映PD程度和故障情况。因此,有必要对典型PD产物含量进行监测。

由于基于单壁碳纳米管(Single Walled Carbon Nanotube, SWCNT)的气敏传感器具有响应好、体积小和灵敏度高等突出优点,近年来被广泛关注。此外,通过在其表面修饰过渡金属,SWCNT的气敏特性将在原有基础上显著提高。其中,掺杂对分子具有较高催化活性的Pt可实现对某些不常见气体的检测[19-24]。Cui Hao等学者通过仿真发现,由于吸附材料与目标气体之间存在较强的轨道相互作用和化学吸附,在表面修饰有Pt、Pd和Rh的SWCNT对SOF2、CO、CH4和H2S具有良好的气敏特性[25-27]。M.Yoosefian[28]则研究了SO2在Pt和Au掺杂的(5, 5)SWCNT上的吸附特性,并发现SO2在Pt-SWCNT上的吸附能隙比在Au-SWCNT上的变化更大。

为此,本文基于密度泛函理论(Density Functional Theory,DFT),探究了C2F6和CH3I两种典型CF3I的PD分解产物在Pt修饰(8, 0) SWCNT上的吸附参数、态密度和前沿分子轨道等吸附特性,为环保型气体CF3I在气体绝缘设备中的最终运用和监测提供理论依据。

1 计算参数

本研究采用Dmol3量子化学模块进行DFT计算[29]。采用广义密度近似方法(Generalized Gradient Approximation, GGA)的Perdew-Burke-Ernzerho(PBE)函数处理电子交换关联作用[30]。采用双数值p极化(Double Numerical plus Polarization, DNP)作为原子轨道基组。最大原子位移、能量收敛精度、能量梯度和轨道拖尾效应分别设置为5×10-3Å(1Å=10-10m)、1.0×10-5Ha(1Ha=27.211eV)、0.002Ha/Å和0.005Ha[31]。此外,为保证总能量的计算精度,自洽场误差和全球轨道截止半径选为1.0×10−6Ha和5.0Å[32]。布里渊k点网格空间设置为1×1×8[33]。使用DFT-D(Tkatchenk and Scheffler, TS)方法处理色散力[34]。吸附过程中的电荷转移量 TQ通过Mulliken法获得[35]。若电荷转移量 TQ>0,表示电子从气体分子转移到Pt-SWCNT表面;反之,若电荷转移量TQ<0,则表示电子从Pt-SWCNT表面转移到气体分子。此外,定义吸附能Eg

式中,Egas为单个气体分子的所具有能量;EPt-SWCNT为未吸附气体分子时的本征Pt-SWCNT的能量;Egas/Pt-SWCNT为气体分子吸附在Pt-SWCNT表面后的总的能量。若Eg>0,则代表该吸附过程中,整个体系从外界吸附能量;反之,若Eg<0,则代表该吸附过程中,整个体系向外界释放能量。

2 结果分析

在进行吸附计算前,本研究首先优化了CH3I、C2F6和Pt-SWCNT的结构。其中,选择具有20Å×20Å×8.5Å的超胞作为Pt掺杂(8,0)SWCNT的纳米载体。Pt原子则与SWCNT上两个相邻的C原子同时相连,在SWCNT外形成桥位[35]。Pt掺杂后并未改变SWCNT的结构,C-C键键长仍保持为1.439Å,而Pt-C键键长为2.265Å,如图1所示。Pt掺杂前后的总态密度(Total Density of State,TDOS)如图2所示。Pt掺杂后,在费米能级左面的-1.25~-0.5eV范围内出现了一个波峰,这主要由掺杂的Pt原子所贡献。同时,在掺杂过程中,Pt向SWCNT转移了0.200e电子。优化后的C2F6分子和CH3I分子如图3所示。C2F6分子中C-C键和C-F键的键长分别为1.560Å和1.348Å;F-C-F和F-C-C的键角分别为109.037°和109.894°。对于CH3I分子,C-H键和C-I键的键长分别为1.092Å和2.187Å;H-C-H和H-C-I的键角则分别为111.664°和107.185°。

图1 Pt掺杂SWCNT优化结构
Fig.1 The optimized structure of Pt-doped SWCNT

图2 Pt掺杂SWCNT前后的总态密度
Fig.2 The TDOS of Pt-SWCNT and intrinsic SWCNT

图3 气体分子优化结构
Fig.3 The optimized structure of gas molecules

2.1 C2F6分子和CH3I分子在Pt-SWCNT表面的吸附特征参数

C2F6分子和CH3I分子分别吸附在Pt-SWCNT表面的最优化结构如图4和图5所示,吸附能等吸附参数见表1。由于C2F6分子呈轴对称及两端对称性,仅考虑一种初始吸附结构,即C2F6分子的一个F原子靠近Pt原子。由图4优化后的C2F6分子吸附于Pt-SWCNT表面的最优结构可知,该体系的吸附能为0.223eV,即在吸附过程中整个体系会吸收0.223eV的能量。吸附距离为2.769Å,且在吸附过程中有0.016e的电子从C2F6分子转移到Pt-SWCNT表面。此外,Pt-SWCNT和C2F6分子的键长键角在吸附后也未发生变化。考虑到较小的吸附能和电荷转移量,可以认为Pt-SWCNT和C2F6分子之间的相互作用较弱。

表1 C2F6分子和CH3I分子在Pt-SWCNT表面上的吸附参数
Tab.1 Adsorption parameters of C2F6 and CH3I molecules on the surface of Pt-SWCNT

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图4 C2F6吸附在Pt-SWCNT表面的最优结构示意图Fig.4 The optimized configuration of C2F6 adsorbed on the surface of Pt-SWCNT

图5 CH3I吸附在Pt-SWCNT表面的最优结构示意图
Fig.5 The optimized configuration of CH3I adsorbed on the surface of Pt-SWCNT

对于呈轴对称的CH3I分子,本文考虑了两种初始吸附结构:一种为CH3I分子的一个H原子靠近Pt原子(简写为CH3I-H);另一种为CH3I分子的I原子靠近Pt原子(简写为CH3I-I)。由图5优化后的C2F6分子吸附于Pt-SWCNT表面的最优结构可知,对于CH3I-H吸附结构,该体系在吸附过程中吸收了0.132eV能量,吸附距离为2.275Å,同时在吸附过程中有0.070e的电子从CH3I分子转移到Pt-SWCNT表面上。吸附后的Pt-SWCNT和CH3I分子的结构(键长和键角)未发生改变。对于CH3I-I吸附结构,其中一个Pt-C键断裂并与CH3I分子的I原子形成键长为2.728Å的Pt-I键。但相较于吸附前的CH3I分子,吸附后的CH3I分子结构并未发生显著变化。与此同时,该体系的吸附能远远大于CH3I-H吸附结构的吸附能,达到了0.772eV,并有0.192e的电子在Pt-I键的成键过程中从CH3I分子转移到Pt-SWCNT表面。

2.2 态密度分析

C2F6分子吸附体系的TDOS和分波态密度(Partial Density of State, PDOS)如图6所示。其中,“Pt-SWCNT”代表未吸附气体分子的本征Pt-SWCNT的TDOS,“C2F6吸附”则代表C2F6在Pt-SWCNT表面达到稳定状态时整个体系的TDOS。由图6可知,相较于本征Pt-SWCNT,吸附C2F6后的TDOS在-16eV、-13 eV、-11.5 eV、-10.5 eV和-6eV处显著增大,但在费米能级处的TDOS变化很小。此外,根据PDOS可知,Pt-SWCNT吸附C2F6分子后,Pt原子的5d轨道和C2F6分子中C和F原子的2p轨道的重合区较小,这进一步表明Pt-SWCNT和C2F6分子间的相互作用可能较弱。

图6 C2F6在Pt-SWCNT表面的稳定体系的态密度
Fig.6 The density of states of C2F6 absorbed on the surface of Pt-SWCNT

CH3I分子吸附体系的TDOS和PDOS如图7和图8所示。由图7可知,对于CH3I-H吸附结构,吸附CH3I分子后,整个体系的TDOS与本征Pt-SWCNT的差异并不显著;吸附CH3I分子后的PDOS,Pt原子的5d轨道仅与CH3I分子中I原子的5p轨道在-1.25eV处有一定的重叠,吸附前后体系的导电性变化不大。由图8可知,相较于本征Pt-SWCNT的TDOS,CH3I-I的稳定体系的TDOS发生显著变化,带隙和费米能级处的TDOS也明显减小。在-3.75~-2.0eV处,CH3I中I原子的5p轨道和Pt-SWCNT中Pt原子的5d轨道面积有一定的重叠。这表示Pt-SWCNT材料按CH3I-I吸附结构吸附CH3I分子后的导电性有所增强。

图7 CH3I-H的稳定体系的态密度
Fig.7 The density of states of CH3I-H

图8 CH3I-I的稳定体系的态密度
Fig.8 The density of states of CH3I-I

2.3 前沿分子轨道理论分析

基于前沿分子轨道理论,本文得到了本征Pt-SWCNT表面、C2F6分子吸附体系及CH3I分子吸附体系的最高占据轨道(Highest Occupied Molecular Orbital, HOMO)和最低未占据轨道(Lowest Unoccupied Molecular Orbital,LUMO)的能量数值及分布,如图9~图11所示。其中能隙Ea=LUMOHOMO。在本征Pt-SWCNT中,LUMO和HOMO分别为-4.377eV和-5.083eV,能隙为0.706eV。在C2F6吸附于Pt-SWCNT表面后,该体系的LUMO和HOMO分别变为-4.335eV和-5.048eV,均略大于本征Pt-SWCNT的;而该体系能隙仅略微增大至0.713eV;此外,LUMO和HOMO未发生明显的变化。在CH3I分子吸附于Pt-SWCNT表面后的LUMO和HOMO则如图11所示。对于CH3I-H吸附结构,吸附CH3I分子后体系的LUMO、HOMO和能隙分别为-4.434eV、-5.151eV和0.717eV。LUMO和HOMO均略小于本征Pt-SWCNT的,但能隙略微大于本征Pt-SWCNT的;此外,LUMO和HOMO的分布在吸附前后也未发生明显的变化。对于CH3I-I吸附结构,吸附CH3I分子后体系的LUMO、HOMO和能隙分别为-4.557eV、-5.002eV和0.445eV,能隙在吸附后减小了0.261eV,并且HOMO基本分布于Pt原子和CH3I分子上。若能隙变大,表示体系的导电性变弱,而能隙变小则体系的导电性会变强[36]。因此,在宏观上,按CH3I-I吸附结构吸附CH3I分子后的Pt-SWCNT的导电性可能会显著增大;吸附C2F6分子后的Pt-SWCNT和按CH3I-H吸附结构吸附CH3I分子后的Pt-SWCNT的导电性均可能会发生略微的减小。

图9 Pt-SWCNT的HOMO和LUMO分布及能量
Fig.9 HOMO and LUMO distribution and relative energies for Pt-SWCNT

图10 C2F6吸附体系的HOMO和LUMO分布及能量
Fig.10 HOMO and LUMO distribution and relative energies of C2F6 adsorbed on Pt-SWCNT

图11 CH3I吸附体系的HOMO和LUMO分布及能量
Fig.11 HOMO and LUMO distribution and relative energies of CH3I adsorbed on Pt-SWCNT

3 结论

本文基于DFT,研究了C2F6和CH3I两种典型CF3I PD分解产物在Pt-SWCNT上的吸附特征参数具体结论如下:

1)Pt-SWCNT和C2F6分子间的吸附能及电荷转移量均很小,并且Pt-SWCNT在C2F6分子吸附前后的态密度、LUMO、HOMO及能隙变化并不明显,表明Pt-SWCNT和C2F6分子之间相互作用很弱,Pt-SWCNT并不适用于检测C2F6气体。

2)对于CH3I,本文考虑了两种初始吸附结构,综合分析吸附参数、态密度和前沿分子轨道,Pt-SWCNT更可能按CH3I-I吸附结构吸附CH3I分子。此外,Pt-SWCNT和CH3I分子之间有着较强的相互作用,并且该吸附以化学吸附为主。由于吸附CH3I后能隙减小了0.261eV,使得Pt-SWCNT的导电性的增加,可认为Pt-SWCNT对CH3I有着较好的气敏特性。

简而言之,Pt-SWCNT可用于检测CH3I气体,但对C2F6的气敏特性较差。这为环保型气体CF3I在气体绝缘设备中的监测提供理论依据。

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Adsorption Behavior of Typical Decomposition Components of New Environmental Protection Gas CF3I on Single Walled Carbon Nanotube Doped With Pt

Cao Zhengqin1 Li Wei1 Wei Gang1 Yao Qiang2 Hu Gang1
(1. School of Electrical Engineering Chongqing University of Science and Technology Chongqing 401311 China 2. State Grid Chongqing Electric Power Company Chongqing 400015 China)

Abstract Environmental friendly insulated gas CF3I would decompose and generate to C2F6, I2,C2F5I, C2F4 and CH3I, etc. under partial discharge. In this paper, the molecular models of Pt doped (8,0)single-walled carbon nanotubes (Pt-SWCNT) and two typical CF3I partial discharge decomposition components, namely C2F6 and CH3I, were constructed and optimized based on the density functional theory (DFT). Then, the adsorption properties of C2F6 and CH3I adsorbing on Pt -SWCNT, including adsorption distance, adsorption energy, charges transfer, density of states and frontier molecular orbital,were calculated and analyzed. The results shown that the adsorption energy and charge transfer of the system C2F6 adopting on Pt-SWCNT are very small, and the change of density of states and energy gap before and after adsorption is unobvious, which indicates that Pt-SWCNT is not suitable the detection of C2F6. However, there is a strong interaction between Pt-SWCNT and CH3I where the chemisorption is priority. The energy gap of adsorption system decreases significantly after CH3I adsorption, which indicates that Pt-SWCNT have has a good sensitivity to CH3I. All the work provides a theoretical basis for the monitoring of environmentally friendly gas CF3I in gas insulated equipment.

Keywords:CF3I, adsorption characteristics, single-walled carbon nanotubes, density functional theory

中图分类号:TM213

DOI:10.19595/j.cnki.1000-6753.tces.201701

重庆市自然科学基金面上资助项目(cstc2020jcyj-msxmX0267)。

收稿日期 2021-01-19 改稿日期 2021-03-29

作者简介

曹政钦 男,1989年生,博士,讲师,研究方向为电气设备绝缘状态检测及诊断。E-mail:caozhengqin@vip.sina.com

魏 钢 男,1976年生,博士,教授,研究方向为电气设备绝缘状态检测及诊断。E-mail:eecqweig@163.com(通信作者)

(编辑 郭丽军)