Identifying Spatially Segregated Ir Sites within ZSM–5 for Enhanced Redox Cycle in NOx Reduction by CO

Developing high–performance Ir–based catalysts for selective catalytic reduction of NOx by CO(CO–SCR) under low temperatures remains challenging. This study presents an Ir–based catalyst encapsulated in ZSM–5 (Ir@ZSM–5), with Ir species partially confined in micropores (Irδ+) and partially aggregated on the surface (Ir0), achieving ~88% NOx conversion at 230 °C in the presence of 5% O2 and 100 ppm SO2. The confined Irδ+ species exhibit enhanced stability and oxidized states, while surface–aggregated Ir0 species, with weaker oxygen coordination, remain in a metallic state. The dynamic equilibrium between Irδ+ and Ir0 significantly improves the balance of CO oxidation and NO reduction. O2 promotes the oxidation of Ir0 to Irδ+, while 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.