I. Lezcano-Gonzalez

Ammonia-selective catalytic reduction (NH3-SCR) using Cu zeolites is a well-established strategy for the abatement of NOx gases. Recent studies have demonstrated that Cu is particularly active when exchanged into the SSZ-13 zeolite, and its location in either the 6r or 8r renders it an excellent model system for fundamental studies. In this work, we examine the interaction of NH3-SCR relevant gases (NO and NH3) with the Cu2+ centers within the SSZ-13 structure, coupling powder diffraction (PD), X-ray absorption spectroscopy (XAFS), and density functional theory (DFT). This combined approach revealed that, upon calcination, cooling and gas exposure Cu ions tend to locate in the 8r window. After NO introduction, Cu ions are seen to coordinate to two framework oxygens and one NO molecule, resulting in a bent Cu–nitrosyl complex with a Cu–N–O bond angle of ∼150°. Whilst Cu seems to be partially reduced/changed in coordination state, NO is partially oxidized. On exposure to NH3 while the PD data suggest the Cu2+ ion occupies a similar position, simulation and XAFS pointed toward the formation of a Jahn–Teller distorted hexaamine complex [Cu(NH3)6]2+ in the center of the cha cage. These results have important implications in terms of uptake and storage of these reactive gases and potentially for the mechanisms involved in the NH3-SCR process.

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 Bronsted 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 Bronsted 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.