VO3/CeO2(111)催化剂上丙烷氧化脱氢反应活性和选择性的密度泛函理论研究

低碳烯烃一直以来都是化工行业非常重要的基础原料,一般采用烷烃直接热裂解制得,但该方法耗能很大,经济价值有限.近年来,人们开始尝试利用氧化脱氢反应(ODH)方法制备低碳烯烃,并取得了巨大的研究进展,其中稀土氧化物负载钒氧化物催化剂具有良好的低碳烷烃氧化脱氢性能.本文分析了前人对于钒氧化物负载在CeO2表面的计算研究结果,并选取了最具代表性的VO3/CeO2(111)作为烷烃ODH制烯烃的模型催化剂,详细研究了丙烷在该催化剂体系中发生ODH反应机理.通过使用密度泛函理论,对丙烷在VO3/CeO2(111)催化剂上断裂第一根和第二根碳氢键的反应过程进行了理论模拟,并对比了丙烷制丙烯中碳氢键断裂先后的活化能及VO3/CeO2(111)催化剂材料自身的电子性质.结果表明,该催化剂的电子结构在丙烷氧化脱氢反应中扮演关键角色.在丙烷分子断裂第一根碳氢键的反应过程中,会产生两个自由电子,对其电子结构分析发现,其中的一个自由电子会局域在由VO3/CeO2(111)催化剂中五个相关氧原子的2p轨道所形成的新发生局域空轨道(NELS)上,这个独特的新发生局域空轨道只能接受一个电子,另一个电子则会通过丙基在CeO2表面发生吸附将电子传递到CeO2表面的Ce原子上;当丙烷分子进一步发生第二根碳氢键断裂反应时,同样会产生两个新的局域电子,其中一个电子局域在Ce的4f轨道上,此时CeO2表面存在两个局域电子,相互排斥,导致该催化剂上丙烷断裂第二根碳氢键所需的活化能远高于第一根碳氢键.综上,本文对VO3/CeO2(111)催化剂上低碳烷烃ODH反应独特的催化活性和选择性给出了较为细致的分析和解释.

Activity and selectivity of propane oxidative dehydrogenation over VO3/CeO2(111) catalysts: A density functional theory study

The oxidative dehydrogenation (ODH) of propane on monomeric VO₃ supported by CeO₂(111) (VO₃/CeO₂(111)) is studied by periodic density functional theory calculations. Detailed energetic, structural, and electronic properties of these reactions are determined. The calculated activation energies of the breaking of the first and second C–H bonds of propane on the VO₃/CeO₂(111) catalyst are compared, and it is found that both the unique structural and electronic effects of the VO₃/CeO₂(111) catalyst contribute to the relatively easy rupture of the first C–H bond of the propane molecule during the ODH reaction. In particular, the so-called new empty localized states that are mainly constituted of O 2p orbitals of the ceria-supported VO₃ species are determined to be crucial for assisting the cleavage of the first C–H bond of the propane molecule. Following this they become occupied and the remaining C–H bonds become increasingly difficult to break owing to the increasing repulsion between the localized 4f electrons at the Ce cations, resulting in the adsorption of more H and other moieties. This work illustrates that CeO₂-supported monomeric vanadium oxides can exhibit unique activity and selectivity for the catalytic ODH of alkanes to alkenes.