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

Abstract

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.

Reaction of size-selected iron-oxide cluster cations with methane: a model study of rapid methane loss in Mars’ atmosphere

“…65,71 In particular, reaction with methane has been examined by experiment only for Fe 2 O 2 + . 77 Note that, because methane activation has been attracting strong attention, a significant number of methane-reaction experiments have been reported for metal and metal-compound clusters, including Fe n + ( n = 2–15), 78 Ni n + ( n = 2–15), 79 Rh n + , 80 Pd n + , 81–83 Ta n + , 84 Au 2 + , 81,82,85,86 FeC 6 − , 87 W n C m + ( n =1–5, m ≤ 5), 88 W n N m + ( n = 1–6, m ≤ 2), 88 Al 2 O 7 + , 89 Al 4 O 6 + and NiAl 3 O 6 + , 90 as well as vibrational spectroscopy of methane complexes with Fe n + , 91–93 Cu n + , 94 Ta 4 + , 95 Pt n + 96 and Au 2 + . 97…”