Many works reported the encapsulation of iodine in metal–organic frameworks as well as the I2 → I3 – chemical conversion. This transformation has been examined by adsorbing gaseous iodine on a series of UiO-66 materials and the different Hf/Zr metal ratios (0–100% Hf) were evaluated during the evolution of I2 into I3 –. The influence of the hafnium content on the UiO-66 structure was highlighted by PXRD, SEM images, and gas sorption tests. The UiO-66­(Hf) presented smaller lattice parameter (a = 20.7232 Å), higher crystallite size (0.18 ≤ Φ ≤ 3.33 μm), and smaller SSABET (818 m2·g–1) when compared to its parent UiO-66­(Zr)  a = 20.7696 Å, 100 ≤ Φ ≤ 250 nm, and SSABET = 1262 m2·g–1. The effect of replacing Zr atoms by Hf in the physical properties of the UiO-66 was deeply evaluated by a spectroscopic study using UV–vis, FTIR, and Raman characterizations. In this case, the Hf presence reduced the band gap of the UiO-66, from 4.07 eV in UiO-66­(Zr) to 3.98 eV in UiO-66­(Hf). Furthermore, the UiO-66­(Hf) showed a blue shift for several FTIR and Raman bands, indicating a stiffening on the implied interatomic bonds when comparing to UiO-66­(Zr) spectra. Hafnium was found to clearly favor the capture of iodine [285 g·mol–1, against 230 g·mol–1 for UiO-66­(Zr)] and the kinetic evolution of I2 into I3 – after 16 h of I2 filtration. Three iodine species were typically identified by Raman spectroscopy and chemometric analysis. These species are as follows: “free” I2 (206 cm–1), “perturbed” I2 (173 cm–1), and I3 – (115 and 141 cm–1). It was also verified, by FTIR spectroscopy, that the oxo and hydroxyl groups of the inorganic [M6O4(OH)4] (M = Zr, Hf) cluster were perturbed after the adsorption of I2 into UiO-66­(Hf), which was ascribed to the higher acid character of Hf. Finally, with that in mind and considering that the EPR results discard the possibility of a redox phenomenon involving the tetravalent cations (Hf4+ or Zr4+), a mechanism was proposed for the conversion of I2 into I3 – in UiO-66based on an electron donor–acceptor complex between the aromatic ring of the BDC linker and the I2 molecule.

Hg concentration as a function of temperature for a Hg(0) standard using a direct Hg analyzer.

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 I 2 by using different linkers and metal contents. A transformation of the I 2 molecule into I 3 − 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_NH 2 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 (λ ex = 515, 633, and 785 nm). The UiO-67_NH 2 (Hf) compound demonstrated the best adsorption capacity after 48 h of contact with gaseous I 2 under room temperature, capturing 3428 g•mol −1 of iodine. The other two samples, UiO-67_NH 2 (Zr/Hf) and UiO-67_NH 2 (Zr), adsorbed 2835 g•mol −1 and 1658 g•mol −1 in the same conditions, respectively. The I 2 transformation into I 3 − was confirmed by the presence of bands related to "perturbed" I 2 and I 3 − at about 170 and 107 cm −1 , respectively. The Raman mapping demonstrated that both the monometallic UiO-67_NH 2 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 I 2 and I 3 − . This effect was related to the I 2 diffusion process through the UiO-67_NH 2 crystallites, which could be faster in the monometallic UiO-67_NH 2 samples because of their smaller crystal size (ϕ ≈ 44 μm and ϕ ≈ 51 μm for UiO-67_NH 2 (Hf) and UiO-67_NH 2 (Zr), respectively) when compared to the UiO-67_NH 2 (Zr/Hf) sample (ϕ ≈ 140 μm). This paper shows the spatial distribution of I 2 and I 3 − along the crystals of UiO-67_NH 2 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.