Figure 2. (a) Cumulative moles of methanol (left axis) and
CO2 (right axis) formed per total Fe in the absence of
any titrant and in the presence of NO (0.5 kPa) or H2O
(0.9 kPa). (b) Under 0.5 kPa NO, the quantity of H2O
adsorbed in MIL-100(Fe) over a range of partial pressures (0.1 - 0.9
kPa) and the cumulative moles of CO2 formed per total Fe
under that condition. Reaction conditions: 423 K, 14.5 kPa
N2O, 1.5 kPa CH4, t = 2 h, activated at
523 K for 12 h in He.
Thermal treatment protocols can be used to access increasing densities
of Fe2+ and Fe3+sites,35, 43,52 with activation temperatures below 423
K yielding a majority of Fe3+ sites, and those above
423 K resulting in the formation of Fe2+ sites in
addition to Fe3+ sites (Table S3). Cumulative
CO2 yields normalized by the density of
Fe3+ sites remain nearly invariant in activation
temperature, unlike those normalized by total iron content which
increase monotonically with activation temperature (Figure 3),
consistent with the involvement of the former in CO2formation. Also consistent with the sole involvement of
Fe3+ sites is the monotonically decreasing trend of
CO2 formation rates per Fe2+ site with
activation temperature, expected to result from greater relative
contributions of Fe3+ sites to the total open-metal
site density at higher temperatures (Figure S9). Overall,
CO2 cumulative yields that increase linearly with time
and are unaffected by decreasing Fe2+ site densities,
the insensitivity of CO2 formation rates to NO pressure,
a linear correlation between the amount of water adsorbed in the
presence of NO and cumulative CO2 yields, the precise
correspondence between the moles of water required to saturate
Fe3+ sites and those required for the complete
elimination of CO2 formation, and the invariance in
Fe3+-normalized cumulative CO2 yields
with increasing thermal activation temperature are all consistent with
and suggestive of the involvement of Fe3+ sites in
CO2 formation, even though these data do not help
definitively exclude the possibility that a minute amount of
extraframework iron may be responsible for the same.