Recent Understanding of Low-Temperature Copper Dynamics in Cu-Chabazite NH3-SCR Catalysts

Dynamic motion of NH3-solvated Cu sites in Cu-chabazite (Cu-CHA) zeolites, which are the most promising and state-of-the-art catalysts for ammonia-assisted selective reduction of NOx (NH3-SCR) in the aftertreatment of diesel exhausts, represents a unique phenomenon linking heterogeneous and homogeneous catalysis. This review first summarizes recent advances in the theoretical understanding of such low-temperature Cu dynamics. Specifically, evidence of both intra-cage and inter-cage Cu motions, given by ab initio molecular dynamics (AIMD) or metadynamics simulations, will be highlighted. Then, we will show how, among others, synchrotron-based X-ray spectroscopy, vibrational and optical spectroscopy (diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) and diffuse reflection ultraviolet-visible spectroscopy (DRUVS)), electron paramagnetic spectroscopy (EPR), and impedance spectroscopy (IS) can be combined and complement each other to follow the evolution of coordinative environment and the local structure of Cu centers during low-temperature NH3-SCR reactions. Furthermore, the essential role of Cu dynamics in the tuning of low-temperature Cu redox, in the preparation of highly dispersed Cu-CHA catalysts by solid-state ion exchange method, and in the direct monitoring of NH3 storage and conversion will be presented. Based on the achieved mechanistic insights, we will discuss briefly the new perspectives in manipulating Cu dynamics to improve low-temperature NH3-SCR efficiency as well as in the understanding of other important reactions, such as selective methane-to-methanol oxidation and ethene dimerization, catalyzed by metal ion-exchanged zeolites.

Automated summary

The dynamic motion of NH3-solvated Cu sites in Cu-chabazite (Cu-CHA) zeolites, a unique phenomenon linking heterogeneous and homogeneous catalysis, has been reviewed with recent advances in theoretical understanding, including evidence of intra-cage and inter-cage Cu motions.Various spectroscopic techniques have been combined and complemented to track the evolution of coordinative environment and local structure of Cu centers during low-temperature NH3-SCR reactions. Insights into the essential role of Cu dynamics in tuning low-temperature Cu redox and preparing highly dispersed Cu-CHA catalysts by solid-state ion exchange method have been discussed, along with new perspectives in manipulating Cu dynamics for improved NH3-SCR efficiency and understanding of other important reactions catalyzed by metal ion-exchanged zeolites.