CeO2(100)极性面结构与活性的密度泛函理论研究

CeO2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,CeO2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CeO2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,CeO2(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CeO2(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CeO2(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CeO2(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,CeO2(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CeO2(100)面不同结构的稳定性及相关电子结构分析表明,CeO2(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CeO2(100)最稳定和次稳定表面上的氧化反应.发现CO在不同CeO2(100)表面的氧空位处吸附较强,另外,CO在CeO2(100)最稳定结构上可与表面晶格氧反应形成吸附的CO2中间物种,中间物种可直接解离成气相CO2,也可以继续与表面晶格氧反应形成碳酸盐.而在CeO2(100)次稳定表面上,CO很难与表面晶格氧形成吸附的CO2中间态,而直接产生气态CO2.

A DFT+U study of the structures and reactivities of polar CeO2(100) surfaces

Density functional theory calculations corrected by on-site Coulomb interactions were carried out to study the structures of polar CeO2(100) surfaces as well as activities during catalytic CO oxida-tion. The stabilities of various CeO2(100) termination structures are discussed, and calculated ener-getics are presented. The most stable CeO2(100) surface was obtained by removing half the outer-most full layer of oxygen and the surface stability was found to decrease as the exposed oxygen concentration was increased. Assessing the reaction pathways leading to different final products during CO oxidation over the most stable CeO2(100) surface, we determined that the formation of carbonate species competed with CO2 desorption. However, during CO oxidation on the less stable CeO2(100) surfaces having more exposed oxygen, the CO is evidently able to react with surface oxygen, leading to CO2 formation and desorption. The calculation results and electronic analyses reported herein also indicate that the characteristic Ce 4?orbitals are directly involved in deter-mining the surface stabilities and reactivities.