NH3-Selective Catalytic Reduction (NH3-SCR) is a widely used technology for NO x reduction in the emission control systems of heavy duty diesel vehicles. Copper-based ion exchanged zeolites and in particular Cu-SSZ-13 (CHA framework) catalysts show both exceptional activity and hydrothermal stability for this reaction. In this work, we have studied the origin of the SCR activity of Cu-SSZ-13 as evidenced from a combination of synchrotron-based and laboratory techniques. Synchrotron-based in situ XAFS/XRD measurements were used to provide complementary information on the local copper environment under realistic NH3-SCR conditions. Crucial then to the catalytic activity of Cu-SSZ-13 is the local environment of the copper species, particularly in the zeolite. Cu-SSZ-13 contains mononuclear Cu2+ species, located in the face of the double-6-ring subunit of the zeolite after calcination where it remains under reaction conditions. At lower temperatures (with low activity), XAFS and XRD data revealed a conformational change in the local geometry of the copper from a planar form toward a distorted tetrahedron as a result of a preferential interaction with NH3. This process appears necessary for activity, but results in a stymieing of activity at low temperatures. At higher temperatures, the Cu2+ possess a local coordination state akin to that seen after calcination.

Cu-exchanged zeolites have demonstrated widespread use as catalyst materials in the abatement of NO x , 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 NO x . 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.

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Three different types of NH3 species can be simultaneously present on Cu(2+)-exchanged CHA-type zeolites, commonly used in Ammonia Selective Catalytic Reduction (NH3-SCR) systems. These include ammonium ions (NH4(+)), formed on the Brønsted acid sites, [Cu(NH3)4](2+) complexes, resulting from NH3 coordination with the Cu(2+) Lewis sites, and NH3 adsorbed on extra-framework Al (EFAl) species, in contrast to the only two reacting NH3 species recently reported on Cu-SSZ-13 zeolite. The NH4(+) ions react very slowly in comparison to NH3 coordinated to Cu(2+) ions and are likely to contribute little to the standard NH3-SCR process, with the Brønsted groups acting primarily as NH3 storage sites. The availability/reactivity of NH4(+) ions can be however, notably improved by submitting the zeolite to repeated exchanges with Cu(2+), accompanied by a remarkable enhancement in the low temperature activity. Moreover, the presence of EFAl species could also have a positive influence on the reaction rate of the available NH4(+) ions. These results have important implications for NH3 storage and availability in Cu-Chabazite-based NH3-SCR systems.