ONIOM2-DFT study of N2, O2 and NO interaction with Ce-MOR: Active sites and thermodynamics

A theoretical study was carried out for the adsorption of N2, O2, NO and NO+ with Cerium (Ce) modified mordenite (MOR). A two-layer ONIOM2 (two layer Integrated molecular Orbital + molecular Mechanics) methodology was employed, combining UFF (Universal Force Field) and DFT (Density Functional Theory) calculations for the low and high level, respectively. The formation of active Ce species based on the adsorption of CeO+ on the crystallographic positions T1, T2 and T4 in the H-MOR was studied. The geometrical, vibrational and thermodynamic results indicate that the Ce atom of CeO+ binds exothermically and spontaneously to two of the non-equivalent (Om) crystallographic oxygens of MOR (TnOm1Om2) according to: T1O1O4, T1O2O3, T2O4O7, T2O5O7 and T4O7O10 (T: Al or Si). The results of the interaction of N2, O2, NO and NO+ with Ce-MOR indicate that only exothermic and spontaneous adsorptions occur on the active sites located over the main channel of the 12-membered ring (12-MR) according to T1O1O4, T2O5O7 and T4O7O10. In general, the Ce-MOR system stabilizes electrophilic [CeO(NO+)] species, with possible activity deNOx reactions in the presence of nucleophilic reductants such as NH3; while for CeO(NO) species adsorbed on MOR a dynamic equilibrium between κ1NO, κ1ON, κ2NO adsorptions is reported which could be applicable for deNOx catalysis in the absence of reductants. On the basis of thermodynamic reaction functions, it is proposed that the most likely site for the location of active CeO+ is T2O5O7.