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

Huarong Lei,Peirong Chen,Valentina Rizzotto,Anqi Guo,Daiqi Ye,Ulrich Simon

doi:10.3390/catal11010052

Abstract

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.

Highlights

Nitrogen oxide emissions from power plants and automobiles harm the human respiratory system and participate in atmospheric reactions to form, among others, fine particulate matter (PM), ground-level ozone (O3), and photochemical smog [1]
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
To explore the nature of SSIE-introduced Cu ions and their effects in NH3-SCR catalysis, Clemens et al synthesized a series of Cu-SSZ-13 with different Cu loadings and characterized them using multiple techniques such as HD-XRD, XPS, and DRUVS [118]

Summary

Introduction

Nitrogen oxide emissions (mainly NO and NO2) from power plants and automobiles harm the human respiratory system and participate in atmospheric reactions to form, among others, fine particulate matter (PM), ground-level ozone (O3), and photochemical smog [1]. Under typical NH3-SCR conditions, Cu ions can be “solvated” by molecules, which bear free electron pairs to form strong coordinative bonds, such as H2O and NH3, and migrate from the original equilibrium position to a location closer to the cage center at low temperatures (i.e., 150–200 ◦C) [44,53,54,55]. The inter-cage diffusion of NH3-solvated Cu species is often considered as the rate-limiting step in cyclic reaction paths for low-temperature NH3-SCR, for Cu-CHA catalysts with low Cu density [57,70].

Impedance Spectroscopy

Monitoring NH3 Storage and Conversion

Findings

Summary and Perspectives