Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx

Abstract

Cu-exchanged zeolites have demonstrated widespread use as catalyst materials in the abatement of NOx, especially from mobile sources. Recent studies focusing on Cu-exchanged zeolites with the CHA structure have demonstrated them to be excellent catalysts in the ammonia-assisted selective catalytic reduction (NH3-SCR) of NOx. Thorough characterization of these materials using state-of-the-art techniques has led to a significant improvement in the understanding of active sites present, which contributes toward a fundamental understanding of the catalytic processes and the rational design of new materials; however, the availability of multiple techniques at our disposal has led to various observations and conclusions on the nature of the active sites. This article begins with a brief introduction to exhaust emission control in the mobile sector, followed by an overview of hydrocarbon-SCR and NH3-SCR; the former technology having found common use in light duty passenger vehicles, whereas the latter are applied for medium (or heavy) duty vehicles, such as trucks and busses. This is followed by an overview of zeolite-based catalysts, especially for NH3-SCR reaction with a focus toward zeolites known to possess high activity. They include zeolites Y (FAU framework), ZSM-5 (MFI framework), SSZ-13 (CHA framework), and (briefly) zeolite Beta (BEA framework). A few common techniques used for the characterization of zeolites and the information that they bring to help determine the salient structural and mechanistic aspects of the NH3-SCR process are introduced. The combination and comparison of the information obtained from the approaches have resulted in an accurate elucidation of the local geometry and environment of Cu within zeolites, thus forming the active site. The article further focuses on three main aspects: (a) the crystallographic cation location of Cu within the structures as compared to results from techniques more sensitive to the local environment; (b) the interaction of Cu at these sites with reactant or probe molecules, which illustrates their (potential) mobility and accessibility; and (c) the proposed active sites within the zeolites ZSM-5, Y, and SSZ-13 as evident in literature. The discussion is focused toward the influence of the zeolite structure, from both a long-range perspective and that of the local structure around the active Cu species, on the thus formed active sites and their implications toward the NH3-SCR reaction.