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.