Methane Activation at the Metal–Support Interface of Ni₄–CeO₂(111) Catalyst: A Theoretical Study

Methane activation is usually assumed to take place on top of metal surfaces or metal clusters. It can also occur at the metal–support interface in metal-supported catalysts with reducible oxides, such as CeO 2 . In the present work, we exploit density functional theory with an additional Hubbard-like parameter (DFT + U ) to calculate the activation of methane at an O site interfacing a Ni 4 metal cluster on a support, CeO 2 (111) surface. Two reaction routes, namely, radical and nonradical routes, are taken into account. We show that the nonradical route is favored with an apparent activation energy of 18.1 kcal/mol, which is lower than that for the radical route by 15.0 kcal/mol. In the nonradical route, the formation of a four-centered transition-state structure is observed while a C–H bond of methane is being cleaved to form an OH moiety and a CH 3 fragment that is being bound to the interfacial Ni atom. It is also found that the interfacial O atoms are out of the CeO 2 surface plane with Ce–O bond distances being much longer than those in the crystalline bulk CeO 2, which allows them to be easily reduced, and hence, the interfacial O atoms become more reactive toward methane, as compared to the surface O atoms. The interactions between Ni 4 cluster and the CeO 2 (111) surface result in the reduction of two Ce 4+ ions to Ce 3+, improving the reducibility of the interfacial O atoms. This should be an important key to the facile methane activation.