Gas‐Phase Mechanism of O^.−/Ni²⁺‐Mediated Methane Conversion to Formaldehyde

Abstract

The gas-phase reaction of NiAl 2 O 4 + with CH 4 is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl 2 O 4 H + and NiAl 2 O 3 H 2 + , are identified in the mass spectra. The DFT calculations predict that the global minimumenergy isomer of NiAl 2 O 4 + contains Ni in the + II oxidation state and features a terminal AlÀ O *À oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH 3 * ) or formaldehyde (CH 2 O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O *À site, followed by either CH 3 * loss or CH 3 * migration to an O 2À site next to the Ni 2 + center. The CH 3 * attaches as CH 3 + to O 2À and its unpaired electron is transferred to the Ni-center reducing it to Ni + . The proposed mechanism is experimentally confirmed by vibrational spectroscopy of the reactant and two different product ions.