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

摘要

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 Bransted 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 Bransted 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.