Gas-Phase Mechanism of O .- /Ni 2+ -Mediated Methane Conversion to Formaldehyde.

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 minimum-energy isomer of NiAl 2 O 4 + contains Ni in the +II oxidation state and features a terminal Al-O .- 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 .- site, followed by either CH 3 ⋅ loss or CH 3 ⋅ migration to an O 2- site next to the Ni 2+ center. The CH 3 ⋅ attaches as CH 3 + to O 2- 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.