Origins of the Hydrothermal Stability of Cu-Chabazite Zeolites for the Selective Catalytic Reduction of NO x

Cu-exchanged chabazite zeolites are used industrially as catalysts for abatement of NO_x pollution from diesel engines; however, catalyst activity is adversely impacted by exposure to high temperature steam. This occurs due to both dealumination of the zeolite framework and aggregation of Cu²⁺ cations into larger oxide agglomerates that are less active. Under oxidation-limited conditions in the presence of coadsorbed NH₃, hydrothermal aging of Cu-chabazite leads to a surprising increase in the rate of NO_x reduction per redox-active Cu cation at low temperatures (200 °C). A combination of electron microscopy, electron diffraction, NMR spectroscopy, and reaction kinetics analyses reveals that, although a portion of Cu species sinter into large agglomerates during aging, activity is maintained by the remaining Cu²⁺ cations that are stabilized by pairs of framework aluminum sites. These sites exhibit lower activation enthalpies for ammonia exchange dynamics, manifesting enhanced mobility of Cu²⁺ ions that persist as the extent of aging increases. The results yield insights into the complicated physicochemical processes and ramifications associated with deactivation of technologically important Cu-CHA zeolite catalysts, including the dynamics of adsorbed intermediates and the macroscopic reaction properties of the selective catalytic reduction of NO_x over active Cu species.