Determining the storage, availability and reactivity of NH3 within Cu-Chabazite-based Ammonia Selective Catalytic Reduction systems

Abstract

Three different types of NH₃ species can be simultaneously present on Cu²⁺-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 Cu²⁺ 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 Cu²⁺ 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 Cu²⁺, 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.