Fe_2O_3(1-12)作用下CO化学链燃烧反应机理研究

Theoretical analysis of the CO reaction mechanism on Fe_2O_3(1-12) surface during chemical looping combustion

在化学链燃烧(CLC)过程中,载氧体表面的原子结构和电子特性决定了其化学反应活性.本文以Fe_2O_3为载氧体,探讨了其自然条件下主要裸露的高米勒数指表面(1-1 2)的结构性质,研究发现表面不同配位数的氧和铁原子(包括O2f、O3f、O4f、Fe4f和Fe5f)的键参数、电子态密度及电荷布居等存在明显差异.为探究这种差异对Fe_2O_3反应活性的影响,对比分析了CO在表面5种氧和铁原子位生成CO_2的吸附-反应机理.CO在表面低配位O原子O2f和O3f首先形成物理吸附,然后被晶格氧氧化生成CO_2,反应需要克服能垒分别为3.657 e V和3.401 e V;然而,CO在O4f位吸附时,首先克服1.864 e V能垒形成二齿形碳酸盐物种,之后克服1.097 e V的能垒形成CO_2.当CO在Fe4f和Fe5f位吸附时,CO与Fe原子成键,后经过活化与表面O原子成键,形成二齿形碳酸盐物种,能垒分别为0.416和0.219 e V,最终碳酸盐物种分别克服0.500和1.462e V的能垒生成CO_2.因此,可以推断表面高配位数的O4f、Fe4f和Fe5f原子,由于其较高的氧化态,在化学链燃烧过程中充当活性位的作用.本研究有助于了解铁基载氧体表面化学链燃烧反应的微观机理,并为载氧体表面结构性能调控制备提供理论借鉴.

In chemical looping combustion（CLC） process,the reactivity of oxygen carrier（OC） is largely dependent on the atomic structure and electronic property of the surface of oxygen carrier（OC）.In this work,Fe2O3 was selected as the OC,and the property of the dominant natural growth high index facet（1-1 2） was detected.Results showed the bond property,the density of state,and the charge population of different coordinate O and Fe atoms（O2 f,O3f,O4 f,Fe4fand Fe5f） on the surface differed from each other.Effect of these various properties on the reactivity of OC was detected by comparatively studying the adsorption and reaction mechanism of CO oxidation into CO2 over different O and Fe atomic sites of the surface of OC.CO was firstly physisorbed on the low coordinated oxygen atom（O2 fand O3f）,then oxidized by lattice oxygen atom changing into CO2,which crossed the barrier of 3.657 e V and 3.401 e V,respectively.Afterwards,CO was chemisorbed on O4 f,and turned into bidentate carbonate species with the barrier of 1.864 e V,which further crossed the barrier of 1.097 e V converting into CO2.However,while CO was chemisorbed on Fe4 fand Fe5f,the hybrid between CO and Fe activated the adsorbed CO,which interacted to a lattice oxygen atom to form bidentate carbonate species with the barrier energy of 0.416 e V and 0.219 e V,respectively.The carbonate species crossed the barrier energy of 0.500 e V and 1.462 e V and changed into CO2,respectively.Results implied that high coordinate O and Fe atoms（O4 f,Fe4fand Fe5f）,due to their high oxidation state,acted as active adsorption sites for CO adsorption and oxidation during CLC process.This fundamental works improved understanding of the detailed CLC reaction mechanism over the surface of iron-based OC,and provided theoretical guide for modifying the electronic structure and properties of OC.