Cu speciation in CHA framework and its impact in selective catalytic reduction of NO

The selective reduction of NO with NH3 (SCR) is an important reaction for emission control. The mobility of a metal cation in a confined space is a classic problem in transport phenomena. When the metal cation is an active centre of a chemical reaction, its mobility will be decisive to its catalytic performance, such as the Cu in chabazite (CHA) zeolite catalyst for selective catalytic reduction of NO (SCR) in diesel exhaust. Here we identify electron paramagnetic resonance (EPR) as an effective tool to quantify Cu2+ at different locations of CHA, and study the mobility of hydrated Cu2+. The Cu2+ transfers from one adjacent Al3+ site (1AlCu) to two adjacent Al3+ site (2AlCu) during dehydration. In the hydrothermal aging process, the transition is recorded from CuOx clusters to isolated Cu2+ sites and from Cu at 8-O member ring to 6-O member ring. The result indicates that the Cu – framework (fw) O interaction is thermodynamic drive for the migration of Cu species. For SCR, this means that the reduced Cu-Ofw interaction will lead to mobile Cu species in the catalytic circle.