Raman Mapping as a Tool for Evaluating I2 and I3– Diffusion Over Single-Crystal UiO-67_NH2(M) (M = Zr, Zr/Hf, or Hf)

The capture of gaseous iodine has been deeply studied for trying to mitigate the dangers of nuclear power energy. The UiO family of metal–organic framework (MOF) materials is considered as one of the best candidates for such purposes since it couples high specific surface areas, facility to be chemically modified, great iodine adsorption capacity, and good stability under nuclear accidents conditions. UiO-66 was profoundly evaluated in several works for trapping I2 by using different linkers and metal contents. A transformation of the I2 molecule into I3– inside such porous systems was verified in other studies and is yet to be better elucidated. The comprehension of this transformation can improve the materials used to capture iodine species and guarantee a better stabilization of such pollutants in the long term. For this reason, three UiO-67_NH2 samples with different metal contents (Zr, Zr/Hf, and Hf) were employed to capture iodine, and the signature of the different species was evaluated using Raman spectroscopy mappings in and out of resonance conditions (λex = 515, 633, and 785 nm). The UiO-67_NH2(Hf) compound demonstrated the best adsorption capacity after 48 h of contact with gaseous I2 under room temperature, capturing 3428 g·mol–1 of iodine. The other two samples, UiO-67_NH2(Zr/Hf) and UiO-67_NH2(Zr), adsorbed 2835 g·mol–1 and 1658 g·mol–1 in the same conditions, respectively. The I2 transformation into I3– was confirmed by the presence of bands related to "perturbed" I2 and I3– at about 170 and 107 cm–1, respectively. The Raman mapping demonstrated that both the monometallic UiO-67_NH2 samples displayed a homogeneous distribution of the two species after 48 h of contact with the iodine gas flow, whereas the bimetallic sample exhibited zones with different concentrations of I2 and I3–. This effect was related to the I2 diffusion process through the UiO-67_NH2 crystallites, which could be faster in the monometallic UiO-67_NH2 samples because of their smaller crystal size (ϕ ≈ 44 μm and ϕ ≈ 51 μm for UiO-67_NH2(Hf) and UiO-67_NH2(Zr), respectively) when compared to the UiO-67_NH2(Zr/Hf) sample (ϕ ≈ 140 μm). This paper shows the spatial distribution of I2 and I3– along the crystals of UiO-67_NH2 materials and correlates this data with the diffusion process of both species, improving the comprehension of the mechanism responsible for iodine conversion and stabilization in UiO materials.