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CeO_2是一类使用非常广泛的稀土氧化物催化材料,在许多重要的催化反应过程,如机动车尾气净化、水汽转换、石油裂解等,表现出很高的活性.大量研究表明,CeO_2的高活性来源于其表面晶格氧,正是由于这些晶格氧能够直接参与氧化反应,同时反应留下的氧空位又能够被气相氧分子吸附填补,因而体现出很好的储放氧催化性能.目前多数研究采用CO氧化为模型反应,研究了CeO_2常见的(111)和(110)晶面的晶格氧活性,但对于其另外一种重要低指数晶面(100)的结构和活性研究却非常有限.需要指出的是,CeO_2(100)是一种极性表面,这给该表面的模型构建和理论研究带来了困难.为了深入了解这种极性表面的结构稳定性和催化活性,本文运用在位库仑力校正的密度泛函理论(DFT+U)方法系统研究了CeO_2(100)极性面的可能结构及相关稳定性,并且深入分析了CO在该表面上的吸附和反应.本文首先利用板层模型尝试构建稳定的CeO_2(100)极性面结构,方法是在保证整个板层化学计量配比完整的前提下,在表层或体相去除氧原子,同时使得整个板层上下对称不存在极性以利于计算.通过计算发现,在CeO_2(100)表层分布氧空位的结构比体相中分布氧空位的结构要稳定,同时,氧空位的分布越接近表面,CeO_2(100)面的结构稳定性就会越高,其最稳定的结构是将表层满覆盖氧离子移除一半.对CeO_2(100)面不同结构的稳定性及相关电子结构分析表明,CeO_2(100)表层满覆盖的氧离子间存在很强的相互排斥作用,因此倾向于降低表面氧浓度来提高表面的稳定性.另外,这种相互作用会降低相邻氧离子的价态,并能引起体相铈离子在整体表面维持完整的化学计量比的情况下,仍能出现局域4?电子而被还原为三价铈.随后我们研究了CO在CeO_2(100)最稳定和次稳定表面上的氧化反应.发现CO在不同CeO_2(100)表面的氧空位处吸附较强,另外,CO在CeO_2(100)最稳定结构上可与表面晶格氧反应形成吸附的CO_2中间物种,中间物种可直接解离成气相CO_2,也可以继续与表面晶格氧反应形成碳酸盐.而在CeO_2(100)次稳定表面上,CO很难与表面晶格氧形成吸附的CO_2中间态,而直接产生气态CO_2.
CeO 2 is a very widely used rare earth oxide catalytic material and exhibits high activity in many important catalytic reactions such as vehicle tail gas purification, water vapor shift, petroleum cracking, etc. A large number of studies have shown that the high activity of CeO 2 Oxygen from its surface lattice, it is precisely because these lattice oxygen can be directly involved in the oxidation reaction, while leaving the oxygen vacancies and oxygen molecules can be adsorbed to fill the gas phase, which shows a good oxygen storage catalytic performance. Most studies have used CO oxidation as a model reaction to study the lattice oxygen activity of the (111) and (110) planes common to CeO 2, but the structure and activity of the other important low index plane (100) It is important to point out that CeO 2 (100) is a polar surface that poses difficulties in model building and theoretical studies of this surface. In order to gain insight into the structural stability and catalytic activity of this polar surface, The possible structure and the relative stability of the CeO_2 (100) polar face were systematically investigated by in-situ Coulomb-force-corrected density functional theory (DFT + U) method and the CO adsorption and reaction on the surface were investigated. At first, we try to build a stable CeO_2 (100) polar surface structure by using the slab model. The method is to remove oxygen atoms in the surface layer or bulk phase while keeping the stoichiometry of the entire slab intact, It is found that the structure of oxygen vacancies distributed on the surface of CeO_2 (100) layer is more stable than the distribution of oxygen vacancies on the surface of CeO_2 (100), and the distribution of oxygen vacancies is closer to the surface. The CeO_2 (100) And the most stable structure of the CeO_2 (100) surface is covered by the most stable structure. The stability of the CeO_2 (100) surface and the related electronic structure analysis show that the CeO_2 (100) Of the oxygen ions have a strong mutual exclusion, and therefore tend to reduce the surface oxygen concentration to improve the surface stability.In addition, this interaction will reduce the valence of adjacent oxygen ions, and can cause the bulk cerium ions in When the entire surface maintains the complete stoichiometric ratio, local 4? Electrons still appear to be reduced to trivalent cerium, and then we study the oxidation of CO on the most stable and sub-stable surface of CeO 2 (100) CO is not In the most stable structure of CeO_2 (100), CO can react with surface lattice oxygen to form adsorbed CO_2 intermediate species, while intermediate species can dissociate directly into gaseous CO_2 Can continue to react with the surface lattice oxygen to form carbonate, while on the CeO 2 (100) stable surface, it is difficult for CO to form an adsorbed CO 2 intermediate state with the surface lattice oxygen and produce gaseous CO 2 directly.