Formation of Gas-Phase Methyl Radicals over MgO

Article abstract of DOI:10.1021/ja00287a011

When methane was passed over MgO at temperatures of approximately 500 deg C, methyl radicals were produced on the surface, released into the gas phase, and trapped downstream in a solid argon matrix where they were analyzed by EPR spectroscopy.Significant differences in initial activity were observed, depending on whether the MgO was pretreated under vacuum or a flow of oxygen.Vacuum conditioning led to essentially no activity while oxygen conditioning resulted in substantial radical production.The oxidant of choice was also critical.Nitrous oxide resulted in a continuous decline of activity while in the presence of oxygen the formation of radicals was at a steady state.Doping of MgO with lithium, sodium, or iron was also examined.Lithium was found to greatly increase the activity up to a doping level of approximately 15.0 wt percent.Two pathways are believed to be responsible for the radical formation.Over pure MgO, intrinsic cation vacancies react with molecular oxygen to give an O(1-) center which can abstract a hydrogen atom from methane to produce the methyl radical.For the lithium-doped samples, substitutional Li(1+) ions react with molecular oxygen to form a center which is also capable of abstracting a hydrogen atom from methane.