Figure 1. Cumulative moles of N2,
CO2, and CH3OH formed as a function of
time of exposure to CH4 and N2O. The
quantity of CH3OH reported is that formed upon
extraction with water subsequent to
CH4/N2O exposure. The estimated quantity
of N2 formed was determined by the balance of
CH3OH and CO2 formed [(mol
N2) = (mol CH3OH) + 4(mol
CO2)] Reaction conditions: 2.9 kPa
N2O, 1.5 kPa CH4, 0.35 kPa
H2O, 473 K, MIL-100(Fe) activated at 523 K.
The invariance in CO2 formation rates with time despite
the consumption of Fe2+ sites converting methane to
methanol indicates that Fe2+ site densities that allow
for the rigorous normalization of methanol formation, do not do so for
CO2 formation, and suggests an independence between
sites responsible for the formation of these two products. Moreover,
unlike methanol formation, which can be completely inhibited by the
presence of gas phase NO under reaction conditions, the formation of
CO2 is unaffected by the presence of NO, as reflected by
the insensitivity of cumulative CO2 formation to the
presence of NO co-feeds (Figure 2a). Reported cumulative moles of
CO2 formed are corrected for those measured when NO was
flown over MIL-100(Fe) in the absence of methane and N2O
(0.0035 mol (mol total Fe)-1). Such NO-induced
oxidation (presumably of the MIL-100 framework) accounts accurately for
the slight increase in CO2 formation upon introduction
of NO with methane and N2O (0.0039 mol (mol total
Fe)-1, Table S4), and suggests that linker oxidation
rates are unaffected by the presence of methane and N2O.
The insensitivity in cumulative CO2 formation rates to
Fe2+ site densities both in the presence and absence
of NO suggest that a significant fraction of CO2formation may occur over a distinct set of sites compared to those
identified for methanol formation.
Given the near complete absence of NO adsorption onto
Fe3+ sites under reaction conditions and the
insensitivity of CO2 formation rates to the presence of
NO in the gas phase, their involvement in CO2 formation
warrants further evaluation- a question that is challenging to
definitively address in the absence of titrants that bind exclusively to
Fe3+ sites (and not Fe2+ sites). A
clue as to the involvement of Fe3+ sites in
CO2 formation is provided by water titrations (0.9 kPa
H2O at 423 K) that bind unselectively to open-metal
sites regardless of their oxidation state, as indicated by the
adsorption of one mole water per mol iron under conditions of interest
(Figure S8). Whereas co-feeding NO eliminates (solely)
CH3OH formation, introduction of H2O
with methane and N2O (14.5 kPa N2O, 1.5
kPa CH4, 0.9 kPa H2O) results in the
complete elimination of both oxidation products (Figure 2a).
Additionally, the presence of 0.5 kPa NO in the gas phase causes the
introduction of increasing H2O partial pressures (0.1 -
0.9 kPa) to result in a systematic increase in the total quantity of
water adsorbed, and a concurrent linear decrease in cumulative moles of
CO2 formed with increasing amount of water adsorbed
(Figure 2b). The linear relationship between the cumulative moles of
CO2 formed and those of water adsorbed reflects a
constant ratio between the number of sites that adsorb water and those
eliminated from participation in CO2 formation.
Moreover, the quantity of water adsorption required to completely
suppress CO2 formation was found to be 0.62 mol (total
mol Fe)-1- a value approximately equal to the
concentration of Fe3+ open-metal sites (0.65 mol
(total mol Fe)-1) measured independently using
D2O adsorption measurements (Section S2.5, SI).