Solvation and Mobilization of Copper Active Sites in Zeolites by Ammonia: Consequences for the Catalytic Reduction of Nitrogen Oxides

Abstract

Conspectus Copper-exchanged chabazite (Cu-CHA) zeolites are catalysts used in diesel emissions control for the abatement of nitrogen oxides (NO x ) via selective catalytic reduction (SCR) reactions with ammonia as the reductant. The discovery of these materials in the early 2010s enabled a step-change improvement in diesel emissions aftertreatment technology. Key advantages of Cu-CHA zeolites over prior materials include their effectiveness at the lower temperatures characteristic of diesel exhaust, their durability under high-temperature hydrothermal conditions, and their resistance to poisoning from residual hydrocarbons present in exhaust. Fundamental catalysis research has since uncovered mechanistic and kinetic features that underpin the ability of Cu-CHA to selectively reduce NO x under strongly oxidizing conditions and to achieve improved NO x conversion relative to other zeolite frameworks, particularly at low exhaust temperatures and with ammonia instead of other reductants. One critical mechanistic feature is the NH3 solvation of exchanged Cu ions at low temperatures (<523 K) to create cationic Cu–amine coordination complexes that are ionically tethered to anionic Al framework sites. This ionic tethering confers regulated mobility that facilitates interconversion between mononuclear and binuclear Cu complexes, which is necessary to propagate SCR through a Cu2+/Cu+ redox cycle during catalytic turnover. This dynamic catalytic mechanism, wherein single and dual metal sites interconvert to mediate different half-reactions of the redox cycle, combines features canonically associated with homogeneous and heterogeneous reaction mechanisms. In this Account, we describe how a unified experimental and theoretical interrogation of Cu-CHA catalysts in operando provided quantitative evidence of regulated Cu ion mobility and its role in the SCR mechanism. This approach relied on new synthetic methods to prepare model Cu-CHA zeolites with varied active-site structures and spatial densities in order to verify that the kinetic and mechanistic models describe the catalytic behavior of a family of materials of diverse composition, and on new computational approaches to capture the active-site structure and dynamics under conditions representative of catalysis. Ex situ interrogation revealed that the Cu structure depends on the conditions for the zeolite synthesis, which influence the framework Al substitution patterns, and that statistical and electronic structure models can enumerate Cu site populations for a known Al distribution. This recognition unifies seemingly disparate spectroscopic observations and inferences regarding Cu ion structure and responses to different external conditions. SCR rates depend strongly on the Cu spatial density and zeolite composition in kinetic regimes where Cu+ oxidation with O2 becomes rate-limiting, as occurs at lower temperatures and under fuel-rich conditions. Transient experiments, ab initio molecular dynamics simulations, and statistical models relate these sensitivities to the m...

17O-EPR determination of the structure and dynamics of copper single-metal sites in zeolites

“…Here we show that the 17 O isotopic labelling of the framework provides a unique source of information about the local binding environment of the active site enabling the rationalization of structure-property relationships in Cu-loaded zeolites under, for instance, different hydration conditions. Water has been shown to play a strong effect on the reactivity of Cu species in zeolites promoting specific reaction pathways 29 – 31 , promoting dynamic catalytic mechanisms at the cross road between homogeneous and heterogeneous catalysis 32 – 34 .…”

Experimental and Theoretical Evidence for the Promotional Effect of Acid Sites on the Diffusion of Alkenes through Small‐Pore Zeolites

“…Recently,e vidence was given that controlling the acid site distribution may be an important design parameter for various catalytic processes. [11][12][13] Nevertheless,t he precise influence of the acid site density and distribution on the transport properties in small-pore zeolites is still unresolved.…”

In situ X-ray absorption study of Cu species in Cu-CHA catalysts for NH3-SCR during temperature-programmed reduction in NO/NH3

“…The structure of Cu-CHA is characterized by a very high symmetry. The fact that all T-atom sites of the CHA structure are equivalent, combined with the excellent catalytic properties, makes it an ideal material for modeling studies for NH 3 -SCR [6][7][8][9][10][11]. The characteristic cage in the CHA structure is surrounded by 6-and 8-member rings, where the exchanged Cu ions, stabilized by the framework Al atoms, are located.…”