2.1. Synthesis and characterization
MIL-100(Fe) and MIL-100(Cr) were synthesized according to previously reported protocols50,51 that are described in detail in Sections S1.1 and S1.2. of the SI. Comparison of X-ray diffraction patterns, nitrogen physisorption isotherms, and thermogravimetric analysis profiles confirm the crystal structure, porosity, and composition, respectively, of these materials as being consistent with previous reports (Figure S1 and S2).50,51 Infrared (IR) spectroscopy measurements were conducted to track the evolution of H2O and OH- coordinated species in MIL-100(Fe) and MIL-100(Cr) to estimate M2+/M3+ open-metal site densities under various thermal activation conditions (as described in detail in Section S1.6 of the SI). Fe2+ and Fe3+ open-metal site densities were quantified in-situ using a combination of breakthrough measurements with NO (0.5 kPa, 423 K) and D2O (0.9 kPa, 373 K), as described in Sections 2.4 and 2.5 of the SI, respectively.