Methane Activation at the Metal-Support Interface of Ni ₄ -CeO ₂ (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.