Thermal Reactions of NiAl₃O₆⁺ and Al₄O₆⁺ with Methane: Reactivity Enhancement by Doping

Investigation of the reactivity of heteronuclear metal oxide clusters is an important way to uncover the molecular-level mechanisms of the doping effect. Herein, we performed a comparative study on the reactions of CH₄ with NiAl₃O₆⁺ and Al₄O₆⁺ cluster cations at room temperature to understand the role of Ni during the activation and transformation of methane. Mass spectrometric experiments identify that both NiAl₃O₆⁺ and Al₄O₆⁺ could bring about hydrogen atom abstraction reaction to generate CH₃^• radical; however, only NiAl₃O₆⁺ has the potential to stabilize [CH₃] moiety and then transform [CH₃] to CH₂O. Density functional theory calculations demonstrate that the terminal oxygen radicals (O_t^–•) bound to Al act as the reactive sites for the two clusters to activate the first C–H bond. Although the Ni atom cannot directly participate in methane activation, it can manipulate the electronic environment of the surrounding bridging oxygen atoms (O_b) and enable such O_b to function as an electron reservoir to help O_t^–• oxidize CH₄ to [H–O–CH₃]. The facile reduction of Ni³⁺ to Ni⁺ also facilitates the subsequent step of activating the second C–H bond by the bridging “lattice oxygen” (O_b^2–), finally enabling the oxidation of methane into formaldehyde. The important role of the dopant Ni played in improving the product selectivity of CH₂O for methane conversion discovered in this study allows us to have a possible molecule-level understanding of the excellent performance of the catalysts doping with nickel.