Selective catalytic reduction of NO with NH3 over Cu-exchanged CHA, GME, and AFX zeolites: a density functional theory study

Abstract

Density functional theory calculations have been applied to study the selective catalytic reduction of NO by NH₃ over the Cu-exchanged zeolites with cha, gme, and aft cages. The Cu^I, Cu^II, and [Cu^II(OH)]⁺ ions are considered as the active sites to study both the reduction and oxidation processes during the catalytic cycle. In the case of the reduction process, the NH₂NO formation at the [Cu^II(OH)]⁺ site possesses high barriers in the three frameworks, while the lower barriers are found at the Cu^II site. Importantly, it is found that the barriers are largely decreased at the solvated [Cu^II(NH₃)₄]²⁺ site for the cha and aft frameworks, while the barrier is only slightly decreased for the gme cage. As for the oxidation, the nitrate formation has similar reaction barriers at the Cu^I site of the three frameworks, which are lower than the following nitrite formation. In particular, the smallest gme cage possesses the highest barrier for the nitrite formation. Calculations on the O₂ activation by the NH₃-solvated Cu dimer revealed that the cha and aft cages have better performance than the gme cage, and the much smaller adsorption energy of O₂ in the gme cage indicates the unfavorable O₂ insertion. Therefore, the selectivity caused by the cage size is identified during the reaction process, and the cha and aft cages are more favorable.