Ab initio exploration of materials for the detection and selective capture of iodine species and nitrogen oxide

Résumé

Radioactive isotopes of iodine, such as ¹²⁹I and ¹³¹I, are likely to be disseminated in the environment after a serious nuclear accident or a leak in fuel reprocessing facilities, under the form of highly volatile gaseous species, I₂ and ICH₃, resulting in dramatic consequences. It is therefore necessary to accurately detect these molecules and develop passive filters for trapping them. The challenge today is to find an effective solution that can be applied in nuclear conditions. In particular, the presence of other gaseous species called contaminants, such as CO, H₂O and O₂ can affect the performance of materials used to detect or trap these iodine volatile species in a perennial manner. Molecular scale simulation methods provide a fundamental understanding of the observed phenomena, providing in-depth knowledge at the atomic level that is often difficult to obtain by experimental methods. In this work, density function theory (DFT) calculations and grand canonical Monte Carlo (GCMC) simulations have been used to identify promising materials for the detection and capture of gaseous molecules. Regarding sensing, graphene, and two-dimensional carbon materials (BC₃, C₃N, BCN₆-2) are promising candidates. For graphene, our results reveal that in terms of thermodynamic selectivity, PG (pristine graphene), Cu_PG (copper-doped pristine graphene) and to a lesser extent Ag_MG (silver-doped monolayer graphene) are clearly the most interesting graphene monolayers for the selective capture of I₂, ICH₃ in the presence of CO, H₂O and O₂. As far as 2D carbon materials are concerned, results of adsorption on C₃N seem very promising insofar the difference between the adsorption energies of (I₂, ICH₃) vs (CO, H₂O) is very significant. These findings are strengthened by simulations at finite temperatures. In addition, a discussion of electronic structure calculations is also provided. For trapping, we have selected a class of porous materials named Metal-Organic Frameworks (MOFs). Our systematic evaluation of the adsorption performance of M-MOF-74 where M = Mg, Zn, Cu, Fe, Co, Ni and Mn showed that from a thermodynamic point of view, Fe-MOF-74 and Cu-MOF-74 are clearly the most interesting structures for the selective capture of iodine compounds. A second application addressed in this thesis, still in the context of noxious gases, is the adsorption of NOx emissions. These emissions in a confined work environment without ventilation or treatment represent a major concern. Recent studies have revealed that zeolites can provide effective capture of NOx. In this context, our results reveal that from a series of divalent cations-exchanged zeolite (Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Fe²⁺, Cu²⁺, Zn²⁺, Pd²⁺, Pt²⁺), Faujasite Y-Pt²⁺, is an interesting material for the selective adsorption of NOx from diesel engine exhaust in the presence of water vapor. We then have extended our explorations to MOFs by integrating the same cations as metals into the catecholate ligand prior to its incorporation into the cage-like UiO-66. GCMC simulations implementing a new NOx/MOF force field were deployed to gain an in-depth understanding of the microscopic mechanism involved. Our molecular simulations indicate that the nanoporous UiO-66-CatFe(II) would be an excellent adsorbent for NOx capture, even at very low concentrations of a few ppm. This complements the portfolio of porous materials that, to date, have been almost exclusively tested under operating conditions involving higher NOx concentrations (>1000 ppm).

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