H 2 O-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 (NH 3 –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 % H 2 O, NO x conversion is enhanced by approximately 20% at 400 °C, while under 5 vol %H 2 O 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 Cu 2+ species on the zeolite, creating Lewis acid sites for NH 3 activation and avoiding NH 3 overoxidation in the high temperature, while its dissociation produces bridge and terminal hydroxyl groups for NH 3 adsorption. Meanwhile, preferential coordination of H 2 O molecules at Cu sites triggers a shift in hydroxyl reactivity, from free H atom attacks on the coordinated hydroxyl in Cu(H 2 O)(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 NH 3 –SCR.