Computational and spectroscopic characterization of key intermediates of the Selective Catalytic Reduction cycle of NO on zeolite-supported Cu catalyst

Abstract

Chabazite supported Cu is a promising catalyst platform for implementing a NH3/urea-based Selective Catalytic Reduction (SCR) system to remove hazardous NOx gases from lean-burn engine exhaust. Whereas in-depth spectroscopic and other studies have attempted to identify key features of the catalytic cycle previously, a deep understanding of the SCR mechanism amenable to systematic improvement of catalyst performance remains elusive. For example, neither the precise Cu coordination geometry at the active site nor the substrate binding affinities to the catalytic center are known. To establish a more rational approach to catalyst optimization based on the thermodynamics and kinetics of the key steps of the underlying NOx-transformations we developed a quantum chemical model and benchmarked it to match vibrational data from Diffuse Reflectance Infrared Fourier Transform spectroscopy resulting in plausible assignments of each observable intermediate to specific oxidation states of Cu and NO-binding properties. Among these intermediates, we identified the structure of a lattice supported NO+ cation species, expected to be reactive towards NH3, corresponding to a high frequency IR-absorption at 2170cm−1. This approach enables a more precise assignment of the experimental vibrational data to key intermediates potentially involved in the catalytic cycle in order to develop a micromechanistic proposal for the catalysis that is chemically meaningful and is logically consistent.