H₂O-Mediated CuO_x Redispersion and Hydroxyl Reactivity for Enhancing NO_x Reduction over Cu-SSZ-13

Traditional high-temperature hydrothermal aging commonly inhibits selective catalytic reduction of NO_x with ammonia (NH₃-SCR) over zeolite catalysts, but a comprehensive understanding of the water vapor effect remains elusive. Herein, by combining the experiment and ab initio molecular dynamics (AIMD) simulations, a promoted mechanism for NO_x conversion by water molecules at moderate temperatures is proposed over the Cu-SSZ-13 zeolite. Upon introducing 10 vol % H₂O, NO_x conversion is enhanced by approximately 20% at 400 °C, while under 5 vol %H₂O at 180 °C, the conversion increased from 70% to 80%. Acting as a reactant in the SCR and a ligand for active Cu sites, water vapor drives CuO_x redispersion to isolated framework Cu²⁺ species on the zeolite, creating Lewis acid sites for NH₃ activation and avoiding NH₃ overoxidation in the high temperature, while its dissociation produces bridge and terminal hydroxyl groups for NH₃ adsorption. Meanwhile, preferential coordination of H₂O molecules at Cu sites triggers a shift in hydroxyl reactivity, from free H atom attacks on the coordinated hydroxyl in Cu(H₂O)(OH) to proton transfer between the NNH⁺ transition state and free hydroxyl groups. This transformation effectively avoids deep energy wells and decreases the overall energy barrier. This research elucidates a distinct water-mediated mechanism over a Cu-exchanged zeolite for NH₃-SCR.