A systematic DFT screening of cationic faujasite-type zeolites for the adsorption of NO, NO2 and H2O

Abstract

The limitation of NO and NO₂ (NO_x) emissions out of exhaust gases released from diesel engines in confined environments requires efficient adsorbents. Since NO_x species are present in trace amounts (50–1000 ppm) in exhaust gases, and always co-exist with a large content of H₂O (2–12 wt%), adsorbents need to be highly selective to trap NO_x over H₂O. To this end, periodic density functional theory (DFT) calculations in combination with dispersion corrections have been used for a systematic screening of monovalent and divalent cation-exchanged faujasite zeolites. The present work investigates the effect of the cation nature and Si/Al ratios (1.4; 2.43; 23; 47), on the adsorption selectivity of faujasite towards NO and NO₂ against H₂O. Alkali and alkali-earth metals Li(I), Na(I), K(I), Rb(I), Cs(I) and Ca(II), Ba(II), as well as monovalent and divalent transition metals Cu(I), Ag(I), and Zn(II), Pt(II), Pd(II), Cu(II), Fe(II), Co(II), Ni(II) embedded in faujasites, have been explored for their ability to capture NO and NO₂. Bond activation of adsorbed gases has also been checked for the most promising materials to assess the tendency of these gases to further react with the adsorption site. Bader charges and charge density difference calculations were carried out for the most effective faujasite structures to assess the bond formation between materials and adsorbed gases. Much weaker interaction energies were predicted for Y vs. X faujasites, which is in favour of the material's regeneration. Cu(I) and Fe (II) based Y zeolites (Si/Al = 2.43) were identified as the most attractive candidates. Nevertheless, iron strongly activated the bonds of NO₂ upon adsorption raising doubts about its implementation with faujasite. This is the first time that such a large screening of cationic zeolites has been performed for a separation topic using DFT calculations. In the specific case of NO_x/H₂O separation, the present work helped to exclude most of the zeolites explored from future theoretical or experimental investigations, highlighting the potential of Cu(I)Y and the promising selectivity that Fe(II) can bestow on a zeolite.