Arrested Formation of Carbon Oxides in Programmed Oxidation of Methane on Nonstoichiometric Samaria

CO_x species evolved during transient exposure of Sm_2O_3 catalysts to very low pressure CH_4/O_2 mixtures are monitored in real time by molecular beam mass spectrometry at 1100 K. The rise of m/z = 28 and 44 signals is significantly delayed relative to the onset of CH_4 decay only on substoichiometric Sm_2O_(3-γ) samples. This observation rules out CO and CO_2 as primary products of methane heterogeneous oxidation and implies that their immediate precursor, most probably CH_3(g), is slowly oxidized during the initial stages of Sm_2O_(3-γ) surface reconstruction. Present results suggest the possibility of suppressing carbon oxides in the oxidative dimerization of methane by dynamic control of selective site populations.