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

Abstract

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.

Electron-Donor Functional Groups, Band Gap Tailoring, and Efficient Charge Separation: Three Keys To Improve the Gaseous Iodine Uptake in MOF Materials

“…Interestingly, a similar feature was observed in previous studies with I 2 @UiO-66­(Zr, Hf) and I 2 @UiO-6n­(Zr, Hf)_NH 2 systems ( n = 6 or 7). ,, These studies revealed using EPR spectroscopy that the metal centers of UiO-6n materials (Zr 4+ or Hf 4+ ) cannot be oxidized nor reduced, even upon UV light irradiation (λ < 420 nm). , Also, there were no evidences of a C–I bond formation in these compounds. , Yet, the I 2 into I 3 – redox conversion was verified in both I 2 @UiO-66­(Zr, Hf) and I 2 @UiO-6n­(Zr, Hf)_NH 2 , materials through Raman spectroscopy. In light of this, the transformation of I 2 to I 3 – in the cavities of UiO-66­(M) (M = Hf or Zr) porous structure was ascribed to a CTC between the BDC organic linker and the iodine molecule.…”

“…In this case, the Hf-based UiO-67_NH 2 was found to capture 13.5 mol mol –1 of I 2 after 48 h of contact with the gas flow, 2.08 times more than the UiO-67­(Zr)_NH 2 material. At this point, it is important to mention that the bimetallic UiO-67­(Zr/Hf)_NH 2 compound adsorbed 11.2 mol mol –1 under the same conditions . However, contrary to that observed for the monometallic UiO-67­(Zr)_NH 2 and UiO-67­(Hf)_NH 2 , the UiO-67­(Zr/Hf)_NH 2 did not reach a plateau after 48 h I 2 loading.…”

“…This feature was also considered during the iodine adsorption and evolution into I 3 – in UiO-66 systems, whereas a higher acidity was ascribed to improve the I 3 – stabilization in the hafnium-based UiO-66, boosting the iodine uptake by 1.22 times . On the other hand, a UiO-67­(Hf)_NH 2 MOF also exhibited greater iodine adsorption than its zirconium parent . In this case, the Hf-based UiO-67_NH 2 was found to capture 13.5 mol mol –1 of I 2 after 48 h of contact with the gas flow, 2.08 times more than the UiO-67­(Zr)_NH 2 material.…”

“…In light of this, the transformation of I 2 into I 3 – and I n – favors the iodine uptake by creating specific adsorption sites and stabilizing these anionic species through an electrostatic interaction with the organic linker radical ( · BDC + or · NH 2 -BDC + ). , For this reason, the effective charge separation in the Ti-based compounds (due to the LMCT mechanism) in addition to the hole stabilizer −NH 2 group in MIL-125­(Ti)_NH 2 guarantees a stronger adsorption of both I 3 – and I n – anions, as it attenuates the recombination process that would annihilate the positive charges located over the linker. This process is advantageous for the adsorption kinetics and also for the maximum iodine uptake capacity. , …”

“…Ultimately, the I 3 – was found to be the last and most stable species in UiO-66 materials, which was ascribed to an electrostatic interaction between the I 3 – and the formed cation radical · BDC + . This proposal was later verified by a spectroscopy study over single crystal UiO-67_NH 2 materials, which demonstrated the poorest diffusion of I 3 – compared to that of I 2 in the pores of the MOF after its formation …”