CeO2/Pt(111)反向催化剂结构和活性的密度泛函理论研究

Structures and reactivities of the CeO2/Pt(111)reverse catalyst:A DFT+U study

铈基材料因其独特的Ce^4+/Ce^3+转化性质而广泛运用于非均相催化反应中.尽管在实验和理论上对纯净二氧化铈表面的物理和/或化学性质进行了深入研究,但是与二氧化铈有关的界面结构和反应性能引起了人们的极大兴趣.其中,已有报道表明,氧化铈/金属反向催化剂相较于氧化铈、金属或者金属/氧化铈负载材料能明显提高CO催化氧化和水汽转化等反应活性.然而多数前期研究并没有从理论上给出合理解释,同时也并未说明反向催化剂中氧化铈结构(层数)和性质的关系.可以预见,因受到金属基板的影响,二氧化铈表面的物化性质,如氧空位形成能、电子分布、催化活性等必然会发生变化.本文通过库伦作用校正的密度泛函理论(DFT+U)计算,系统地研究了不同厚度的Ce O2/Pt(111)反向催化剂几何结构和电子性质,催化CO氧化的性能.本文首先在Pt(111)载体上明确了单层Ce O2(111)的最佳结构,然后研究随着二氧化铈厚度增加,各复合结构界面热力学稳定性、几何结构和电荷性质的变化.计算结果表明:首先,单层Ce O2/Pt(111)比双层和三层Ce O2/Pt(111)复合结构在界面处表现出更强的相互作用,并且其强度与界面结合结构密切相关,如界面O–Pt键的数量及其长度等;其次,氧化铈板层和Pt基板之间的接触会显著影响界面处一个氧化铈层和两个金属层内的电子分布,使氧化铈外暴露表面的氧空位形成能降低~0.3 e V,而界面氧空位形成能则显著降低1.3-1.8 e V,并且当表面上沉积≥2个氧化铈层时,氧化铈/铂复合材料的物理性能会趋向收敛;最后,通过计算单层Ce O2/Pt(111),单层Ce O2和模拟体相结构的三层Ce O2(111)表面上的CO氧化过程,结果表明三者均遵循Mvk机理,并且关键步骤OC…Os偶联的反应能垒分别是0.45,0.33和0.61 e V,表明三者的活性趋势为ML Ce O2>ML Ce O2/Pt(111)>TL Ce O2(111).综合考虑到单层Ce O2/Pt(111)界面处适度的二氧化铈-铂相互作用,一方面可以极大提高复合材料热力学稳定性,另一方面还成功保留了单层二氧化铈的优异催化活性,因此单层Ce O2/Pt(111)复合材料从理论上认为是一种优异的CO氧化催化剂.

For heterogeneous catalysts,the build-up of interface contacts can influence markedly their activities.Being different from the conventional supported metal/oxide catalysts,the reverse type of oxide/metal structures,e.g.the ceria/Pt composite,have emerged as novel catalytic materials in many fields.However,it remains challenging to determine the optimal interface structure and/or the metal-oxide synergistic effect that can boost catalytic activities.In this work,we conducted density functional theory calculations with on-site Coulomb interaction correction to determine the optimal structures and investigate the physical as well as catalytic properties of various Ce O2/Pt(111)composites containing Ce O2(111)monolayer,bilayer,and trilayer at Pt(111).We found that the interaction strength between Ce O2(111)and Pt(111)substrate first reduces as the ceria slab grows from monolayer to bilayer,and then largely gets converged when the trilayer occurs.Such trend was well rationalized by analyzing the number and distances of O–Pt bonds at the interface.Calculated Bader charges uncovered the significant charge redistribution occurring around the interface,whereas the net electron transfer across the interface is non-significant and decreases as ceria thickness increases.Moreover,comparative calculations on oxygen vacancy formation energies clarified that oxygen removal can be promoted on the Ce O2/Pt(111)composites,especially at the interface.We finally employed CO oxidation as a model reaction to probe the surface reactivity,and determined an intrinsic activity order of monolayer Ce O2(111)>monolayer Ce O2(111)/Pt(111)>regular Ce O2(111).More importantly,we emphasized the significant role of the moderate ceria-Pt interaction at the interface that endows the Ce O2/Pt reverse catalyst both good thermostability and high catalytic activity.The monolayer Ce O2(111)/Pt(111)composite was theoretically predicted highly efficient for catalyzing CO oxidation.