Thermal Reactions of NiAl 3 O 6 + and Al 4 O 6 + 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 CH 4 with NiAl 3 O 6 + and Al 4 O 6 + cluster cations at room temperature to understand the role of Ni during the activation and transformation of methane. Mass spectrometric experiments identify that both NiAl 3 O 6 + and Al 4 O 6 + could bring about hydrogen atom abstraction reaction to generate CH 3 • radical; however, only NiAl 3 O 6 + has the potential to stabilize [CH 3 ] moiety and then transform [CH 3 ] to CH 2 O. 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 CH 4 to [H-O-CH 3 ]. The facile reduction of Ni 3+ 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 CH 2 O 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.