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工程科学与技术:2019,51(6):28-35
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化学链燃烧中Co掺杂改性Fe2O3(104)载氧体反应特性
(1.西北大学 化工学院 国家碳氢资源清洁利用国际科技合作基地 陕北能源先进化工利用技术教育部工程研究中心 陕西省洁净煤转化工程技术研究中心 陕北能源化工产业发展协同创新中心, 陕西 西安 710069;2.华北电力大学 可再生能源学院 生物质发电成套设备国家工程实验室, 北京 102206)
Modification of Co-doping on Reaction Properties of Fe2O3(104) Oxygen Carrier During Chemical Looping Combustion
(1.International Sci. & Technol. Cooperation Base of MOST for Clean Utilization of Hydrocarbon Resources, Chemical Eng. Research Center of the Ministry of Education for Advanced Use Technol. of Shanbei Energy, Shaanxi Research Center of Eng. Technol. for Clean Coal Conversion, Collaborative Innovation Center for Dept. of Energy and Chemical Industry in Northern, School of Chemical Eng., Northwest Univ., Xi'an 710069, China;2.National Eng. Lab. for Biomass Power Generation Equipment, School of Renewable Energy, North China Electric Power Univ., Beijing 102206, China)
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投稿时间:2019-07-31    修订日期:2019-09-20
中文摘要: 基于密度泛函理论建立金属Co掺杂的铁基载氧体的微观模型,探究掺杂Co后模型表面的电子结构及反应特性的变化。首先,采用Material Studio软件中CASTEP模块构建并优化Fe2O3(104)的平板模型;其次,以Co原子分别替换模型表面不同配位数的Fe原子(Fe5f,Fe6f和Fe7f),构建Co在表面不同Fe原子位的掺杂模型(Co-Fe2O3(104));最后,计算纯净模型和掺杂模型的表面能、掺杂结合能、态密度以及掺杂位点原子的键长、键角和原子间距离等参数,考察CO在Fe2O3(104)和Co掺杂的Fe2O3(104)表面的等温吸附特性,并以CO分子为探针测试Co掺杂模型和纯净模型表面的氧化反应特性,获取反应路径、过渡态和反应活化能等信息。几何优化结果得到Co掺杂模型的稳定性顺序是:Co5f-Fe2O3(104)> Co6f-Fe2O3(104)> Co7f-Fe2O3(104),对应的结合能分别为-0.399 eV、-0.215 eV和0.487 eV,Co在Fe5f和Fe6f位的掺杂是放热过程,并且在Fe5f原子位的掺杂时放热较多,而在Fe7f原子的掺杂属于是吸热反应;Co掺杂改变了掺杂位点相邻O原子的平均键长LO-M(M代表Fe或Co),其中Co替换Fe7f后相邻O原子的LO-M增加了0.004 4 nm;掺杂Co后模型的总态密度(DOS)均向费米能级(0 eV)方向移动,在-8 eV~0 eV能量范围内离域性增强,而且Co5f-Fe2O3(104)模型体系靠近费米能级左边的填充态能量高于其他模型。等温吸附表明Co掺杂可以提高CO在模型表面的吸附量,并且存在吸附两种方式:-2.0 eV附近的峰为CO模型表面碱性位点的吸附峰,-0.75 eV附近的峰为CO在非碱性位点的吸附峰。CO在Co5f-Fe2O3(104)表面的吸附能(-0.851 eV)最大,而在Co7f-Fe2O3(104)表面的吸附需要外加能量(0.386 eV),CO在Co6f和Co7f掺杂位吸附的键长(LCO)比纯净模型表面的分别增加了0.000 4 nm和0.001 1 nm,表明Co掺杂表面对CO分子的活化作用较大;过渡态分析表明CO在Co掺杂表面氧化生成CO2的反应活化能均明显下降,其中CO在Co5f-Fe2O3(104)表面生成CO2的活化能最低,比在Fe2O3(104)表面的减少了0.518 eV,且相应的反应能增加了0.445 eV。研究表明,Co与Fe在其氧化物中成键结构不同,导致掺杂后模型表面的悬键增多,表面能增大,态密度向费米能级方向移动,提高了Fe2O3(104)表面活性,并且Co在低配位数Fe原子位的掺杂更有利于降低氧化CO的反应活化能。因此,通过掺杂金属Co提高铁基载氧体反应活性是可行的,其改性效果与掺杂活性成分的特性和掺杂方式有密切的关系。
Abstract:The modified mechanism of iron-based OC doped by Co were studied. Three microscopic Co-doped models of iron-based OC were established based on density functional theory (DFT) to research the changes of electronic structure and reaction characteristics of doping model surfaces. Firstly, the periodic slab models of Fe2O3(104) were built using CASTEP, one of the module of Material Studio software; Secondly, the Co-doped models (Co-Fe2O3(104)) were built via replacing different coordinate Fe atoms (Fe5f, Fe6f and Fe7f) with Co atoms; Finally, the surface energy, binding energy and density of state (DOS) of all models were calculated. Moreover, the bond length, angle and distance of doping sites atoms with other atoms were calculated. Besides the isothermal adsorption features of CO on Fe2O3(104) and Co-Fe2O3(104) were detected. CO molecules were selected as a probe to test the oxidation reaction properties of Co-doped and pure Fe2O3(104) models and get the information of the reaction path, transition state and reaction activation energy. The stability order of Co-doped models obtained after geometric optimization as follows: Co5f-Fe2O3(104) > Co6f-Fe2O3(104) > Co7f-Fe2O3(104), corresponding to the binding energy is -0.399 eV, -0.215 eV and 0.487 eV (1 eV=96.485 3 kJ/mol), respectively. The doping of Co at Fe5f and Fe6f sites were exothermic process, and released much more energy at Fe5f atom site. However, the doping of Fe7f atoms was an endothermic reaction. Co doping changed the average bond length (LO-M) of adjacent O atoms at doping site, where the M represents Fe or Co atoms. Especially, the LO3 f-M increased 0.004 4 nm after Co atom replacing Fe7f atom. The total state density (DOS) of the models all moved towards Fermi level (0 eV), and the delocalization was enhanced in the range of -8 eV~0 eV energy, and the filling state energy of the Co5f-Fe2O3(104) model system near Fermi level was higher than that of other models. The isothermal adsorption of CO indicated that Co doping improved the adsorption amount of CO on the surface of the model. It shown two ways of adsorption: the peak near -2.0 eV was the adsorption peak of CO formed at the alkaline site on the surface of the model, and the peak near -0.75 eV was formed at the non-alkaline site. The adsorption energy of CO on the surface of Co5f-Fe2O3(104) was largest (-0.851 eV), while the adsorption on the surface of Co7f-Fe2O3(104) required additional energy (0.386 eV). The bond length (LCO) of Co6f and Co7f doped sites were increased by 0.000 4 nm and 0.001 1 nm compared with the pure surface model, indicating that Co-doped surfaces had a greater activation effect on CO molecules. The transition state analysis illustrated that the reaction activation energy of CO oxidized to CO2 at Co-doped surfaces decreased significantly. It also shown that the activation energy of CO on the surface of Co5f-Fe2O3(104) was the lowest, decreased 0.518 eV (49.974 kJ/mol) less than that on the surface of Fe2O3(104), and the corresponding reaction energy increased by 0.445 eV. It is indicated that the different bonding structures in their oxides of Co and Fe lead to an increase in suspended bonds and surface energy on the doped model surface, and a shift in state density toward Fermi level, which improved the surface activity of Fe2O3(104). In addition, Co doping at the low coordination of Fe atom sites was more conducive to reducing the reaction activation energy of oxidized CO. Therefore, it was feasible to improve the reactivity of iron-base OC by doping metal Co, and its modification effect was closely related to the characteristics of the components and the doping mode.
文章编号:201900769     中图分类号:    文献标志码:
基金项目:国家自然科学基金重点项目(21536009);国家重点研发计划项目(2018YFB0604603)
作者简介:梁志永(1985-),男,博士后.研究方向:煤化学链燃烧及气化.E-mail:junjinxing@163.com
引用文本:
梁志永,覃吴,石司默,马晓迅.化学链燃烧中Co掺杂改性Fe2O3(104)载氧体反应特性[J].工程科学与技术,2019,51(6):28-35.
LIANG Zhiyong,QIN Wu,SHI Simo,MA Xiaoxun.Modification of Co-doping on Reaction Properties of Fe2O3(104) Oxygen Carrier During Chemical Looping Combustion[J].Advanced Engineering Sciences,2019,51(6):28-35.