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

Area of Science:

• Catalysis
• Environmental Chemistry
• Materials Science

Background:

• High-temperature hydrothermal aging typically hinders ammonia-selective catalytic reduction (NH3-SCR) of NOx over zeolite catalysts.
• The specific role of water vapor in this process, especially at moderate temperatures, requires comprehensive understanding.

Purpose of the Study:

• To elucidate the mechanism by which water vapor promotes NOx conversion over Cu-SSZ-13 zeolite at moderate temperatures.
• To investigate the dual role of water as a reactant and ligand for active copper sites.

Main Methods:

• Combined experimental studies with *ab initio molecular dynamics* (AIMD) simulations.
• Investigated the effect of varying water vapor concentrations (5-10 vol %) at different temperatures (180 °C and 400 °C).

Main Results:

• Water vapor significantly enhanced NOx conversion, increasing it by approximately 20% at 400 °C (10 vol % H2O) and from 70% to 80% at 180 °C (5 vol % H2O).
• Water promotes the redispersion of copper oxide species to isolated Cu2+ sites, creating Lewis acid sites for ammonia activation and preventing overoxidation.
• Water dissociation forms hydroxyl groups crucial for ammonia adsorption, and its coordination shifts hydroxyl reactivity, lowering the overall energy barrier for NOx conversion.

Conclusions:

• A distinct water-mediated mechanism for NH3-SCR over Cu-exchanged zeolites has been elucidated.
• Water vapor plays a critical role in enhancing catalytic activity at moderate temperatures by stabilizing active sites and facilitating reaction pathways.