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.