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).