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.