The gas-phase reactions of [Ta x O y ]+ (x = 4, 5; y = 0, 1) nanoclusters with methane have been explored in a ring-electrode ion trap under multicollision conditions and theoretically with the use of first-principles quantum simulations. At room temperature, Ta4 + dehydrogenates consecutively two methane molecules with the concurrent elimination of H2, whereas Ta5 + is found to be unreactive. Both of the corresponding mono-oxides, [Ta4O]+ and [Ta5O]+, demonstrate significantly increased reaction rates. Binding of the methane molecule to the tantalum clusters is found to occur through Pauli-repulsion-driven polarization of the electronic charge distribution in the metal cluster, induced by the closed-shell methane molecule. The subsequent dehydrogenation reaction is found to entail active participation of up to four tantalum atoms, whereas the doping oxygen atom does not form bonds to the methane molecule or the reaction intermediates and acts merely as a cluster-charge-polarizing ligand spectator. Clusters exhibiting such enhanced reactivity, influenced by oxo-ligand modification of the local electronic charge distribution, with consequent tuned local Lewis acid–base-pair balancing, may serve as potent models for active centers in small particle and surface metalorganic chemistry or heterogeneous nanocatalysis.

Mass-selected Ta8O2 + cluster ions catalyze the transformation of methane in a gas-phase ion trap experiment via nonoxidative coupling into ethane and H2, which is a prospective reaction for the generation of valuable chemicals on an industrial scale. Systematic variation of the reaction conditions and the isotopic labeling of methane by deuterium allow for an unambiguous identification of a catalytic cycle. Comparison with the proposed catalytic cycle for tantalum-doped silica catalysts reveals surprising similarities as the mechanism of the C–C coupling step, but also peculiar differences like the mechanism of the eventual formation of molecular hydrogen and ethane. Therefore, this work not only supplies insights into the mechanisms of methane coupling reactions but also illustrates how the study of trapped ionic catalysts can contribute to the understanding of reactions, which are otherwise difficult to study.

The energetics of small cationic tantalum clusters and their gas-phase adsorption and dehydrogenation reaction pathways with methane are investigated with ion-trap experiments and spindensity-functional-theory calculations. Ta n + clusters are exposed to methane under multicollision conditions in a cryogenic ring electrode ion-trap. The cluster size affects the reaction efficiency and the number of consecutively dehydrogenated methane molecules. Small clusters (n = 1−4) dehydrogenate CH 4 and concurrently eliminate H 2 , while larger clusters (n > 4) demonstrate only molecular adsorption of methane. Unique behavior is found for the Ta + cation, which dehydrogenates consecutively up to four CH 4 molecules and is predicted theoretically to promote formation of a [Ta(CH 2 −CH 2 − CH 2 )(CH 2 )] + product, exhibiting C−C coupled groups. Underlying mechanisms, including reaction-enhancing couplings between potential energy surfaces of different spin-multiplicities, are uncovered.

Cationic tantalum carbenes [TaCH2]+ and [Ta4CH2]+, products of methane dehydrogenation, are reacted with dioxygen in an ion trap. Detected products suggest a formation of value-added molecules originating from C–O coupling reactions.

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