Impact of the Si/Al ratio on the selective capture of iodine compounds in silver-mordenite: a periodic DFT study
Résumé
Silver modied zeolithes with a mordenite structure can capture volatile iodine compounds (I2 and ICH3) as released during a severe nuclear accident. However, in these particular conditions, molecules such as CO and H2O present in the containment atmosphere are expected to inhibit the adsorption of iodine compounds. In the present work, periodic DFT calculations have been carried out to investigate the adsorption of I2, ICH3, H2O and CO molecules in silver-exchanged mordenite for various Si/Al ratios aiming to find values that favor a selective adsorption of I2 and ICH3. It is seen that the adsorption energies of CO and H2O remain at the same order of magnitude (from -120 to -140 kJ/mol for CO and from -90 to -120 kJ/mol for H2O) for all the investigated Si/Al ratios. In contrast, ICH3 is increasingly strongly adsorbed as the Si/Al ratio decreases, from around -145 kJ/mol when Si/Al = 47 to -190 kJ/mol for Si/Al = 5. The same trend is observed for I2 with a larger amplitude: from -135 kJ/mol for Si/Al = 47 to -300 kJ/mol for Si/Al = 5. Therefore, the use of silver-exchanged mordenite with Si/Al ratios of 5 or 11 would limit drastically the inhibiting effect of contaminants on the adsorption of volatile iodine species. Also for the same ratios, a spontaneous dissociation of I2 during its adsorption is observed leading to the formation of AgI complexes which are prerequite to the immobilization of iodine at the long term.