The Initial Stages of NH3 and NO Adsorption On (Mo2O5)2+/HZSM-5 with Two Adjacent Unsaturated fiveFold Mo Sites in SCR Reaction: A Cluster DFT Study

Abstract

The density functional theory (DFT) has been employed to investigate the initial stages of NH3 and NO adsorption on both Lewis and Bronsted acid sites of (Mo2O5)2+/HZSM-5 by using cluster models. The calculated results reveal that NH3 can strongly adsorb on both Lewis and Bronsted acid sites in the form of coordinated NH3 and NH4 + and the corresponding adsorption energies are − 48.16 and − 37.28 kcal/mol, respectively. Compared with NH3, NO represents much poorer adsorption ability on both Lewis and Bronsted acid sites. Upon NH3 adsorption on Lewis acid site, its connected Mo site converts to sixfold coordination and octahedral structure while the other Mo site still remains fivefold coordination and tetrahedral structure and electronic structure. However, NH3 adsorption on Bronsted acid site leads to the decrease of stability and the increase of reactivity of the two fivefold coordinated Mo sites. Along the proposed reaction mechanisms, the results of NO adsorption on (Mo2O5)2+/HZSM-5 with adsorbed NH3 indicate that NO can be adsorbed on the second unsaturated fivefold Mo site for both Lewis and Bronsted acid sites and be activated, which weaken the interaction between adsorbed NH3 species and (Mo2O5)2+/HZSM-5.