3.2. The key role of methoxy intermediates in methane conversion
The absence of gas phase methanol at the reactor outlet during both the methane-nitrous oxide co-feed step as well as the ensuing purge step under inert, but its detection upon exposing the sample to water vapor during the extraction step suggests one of two possibilities: a) water displaces adsorbed methanol through competitive adsorption, or b) reacts with a persistent intermediate formed upon exposure to methane and N2O to form gas phase methanol. Exposure to D2O (as opposed to H2O) allows for a differentiation between these two scenarios. Whereas the displacement of adsorbed methanol by water should result in the exclusive detection of non-deuterated methanol, incorporation of deuterium into the methanol product would be indicative of the formation of a persistent intermediate that undergoes steps involving the exchange of deuterium from water. Exposure to D2O was found to result exclusively in the formation of mono-deuteromethanol (Figure 4a), consistent with the formation of methoxy intermediates that then undergo reaction with water to form methanol and reform the hydroxyl anion that was eliminated to create the Fe2+ site in the first place. The fraction of mono-deuteromethanol in the product tracks with the fraction of D2O in H2O-D2O mixtures that the methoxy-covered surface is exposed to (Figure 4b), suggesting a lack of preferential incorporation of hydrogen versus deuterium into the methanol product. Exposure to H218O yielded exclusively CH316OH (Figure 4c), consistent with the formation of methoxy intermediates that desorb subsequent to bond formation between methoxy oxygens and hydrogens/deuteriums in water. These data suggest that a significant fraction of the methane converted form methoxy intermediates which are then extracted using water vapor, and are inconsistent with the formation of adsorbed methanol that is subsequently displaced by water.