Identifying Spatially Segregated Ir Sites Within ZSM-5 for Enhanced Redox Cycle in NO_x Reduction by CO

Developing high-performance Ir-based catalysts for selective catalytic reduction of NO_x by CO (CO-SCR) under low temperatures remains challenging. This study presents an Ir-based catalyst encapsulated in Zeolite Socony Mobil-5 (Ir@ZSM-5), with Ir species partially confined in micropores (Ir^δ+) and partially aggregated on the surface (Ir⁰), achieving ∼88% NO_x conversion at 230 ^°C in the presence of 5% O₂ and 100 ppm SO₂. The confined Ir^δ+ species exhibit enhanced stability and oxidation states, whereas surface-aggregated Ir⁰ species, with weaker oxygen coordination, remain in a metallic state. The dynamic equilibrium between Ir^δ+ and Ir⁰ significantly improves the balance of CO oxidation and NO reduction. O₂ promotes the oxidation of Ir⁰ to Ir^δ+, whereas SO₂ facilitates the reverse, forming a reversible cycle that sustains catalytic efficiency. This work underscores the strategic interplay of Ir valence states and highlights a pathway for designing stable, high-performance Ir-based catalysts tailored for CO-SCR under complex reaction conditions.