Gas-Phase Mechanism of O.- /Ni2+ -Mediated Methane Conversion to Formaldehyde.

Abstract

The gas‐phase reaction of NiAl2O4 + with CH4 is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl2O4H+ and NiAl2O3H2 +, are identified in the mass spectra. The DFT calculations predict that the global minimum‐energy isomer of NiAl2O4 + 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 (CH3⋅) or formaldehyde (CH2O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O.− site, followed by either CH3⋅ loss or CH3⋅ migration to an O2− site next to the Ni2+ center. The CH3⋅ attaches as CH3 + to O2− 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.