The reaction of a highly reactive cluster anion Ta₂N₃ ⁻ toward methane was characterized using cryogenic photoelectron velocity-map imaging spectroscopy. Well resolved vibrational modes (finger prints) for Ta₂N₃ and Ta₂N₃CH₄ were observed, which unambiguously determined the structures of the reactant Ta₂N₃ ⁻ and the reaction intermediate Ta₂N₃CH₄ ⁻ that was formed through C−H cleavage by the Lewis acid-base pair in Ta₂N₃ ⁻.

Abstract

Activation and transformation of methane is one of the “holy grails” in catalysis. Understanding the nature of active sites and mechanistic details via spectroscopic characterization of the reactive sites and key intermediates is of great challenge but crucial for the development of novel strategies for methane transformation. Herein, by employing photoelectron velocity-map imaging (PEVMI) spectroscopy in conjunction with quantum chemistry calculations, the Lewis acid-base pair (LABP) of [Ta^δ+−N^δ−] unit in Ta₂N₃ ⁻ acting as an active center to accomplish the heterolytic cleavage of C−H bond in CH₄ has been confirmed by direct characterization of the reactant ion Ta₂N₃ ⁻ and the CH₄-adduct intermediate Ta₂N₃CH₄ ⁻. Two active vibrational modes for the reactant (Ta₂N₃ ⁻) and four active vibrational modes for the intermediate (Ta₂N₃CH₄ ⁻) were observed from the vibrationally resolved PEVMI spectra, which unequivocally determined the structure of Ta₂N₃ ⁻ and Ta₂N₃CH₄ ⁻. Upon heating, the LABP intermediate (Ta₂N₃CH₄ ⁻) containing the NH and Ta−CH₃ unit can undergo the processes of C−N coupling and dehydrogenation to form the product with an adsorbed HCN molecule.