Promoting effect of hydroxyl groups on the CO-SCR activity of Ir-Mo bimetallic catalysts under O₂ and SO₂

The incorporation of promvoters to form bimetallic clusters is a feasible strategy for boosting catalyst performance in the selective catalytic reduction of nitrogen oxides (NO_x) by carbon monoxide (CO) (CO-SCR); however, developing bimetallic catalysts suitable for complex flue gas conditions remains challenging. This study reports a highly dispersed, sub-nanometer iridium-molybdenum (Ir-Mo) cluster catalyst predominantly confined within ZSM-5 (MFI-type zeolite) crystals (denote as Ir-Mo@Z5), in which hydroxyl groups (-OH) serve as active sites, thereby preserving high catalytic performance under oxygen (O₂)- and sulfur dioxide (SO₂)-containing conditions. Remarkably, this catalyst achieves ∼ 62.4% NO_x conversion at 275 ℃ in the presence of 5% O₂ and 200 ppm SO₂. The crucial role of the -OH groups is revealed by comparison with an Ir‑Mo/Z5 catalyst prepared via a conventional impregnation method. On the one hand, the -OH groups are generally occupied by Ir species, thereby reducing the extent of CO oxidation and promoting NO reduction by CO. On the other hand, the oxidation of SO₂ on the catalyst and the resulting consumption of -OH groups, especially the bridging silanol-aluminum group (Si-OH-Al), constitute the intrinsic mechanism that reduces the nitrate-mediated “ineffective reaction pathway” in the presence of O₂. This work elucidates how the structural state of bimetallic clusters influences the SO₂-promoted -OH mechanism, guiding the rational design of high-performance bimetallic CO‑SCR catalysts tailored for complex reaction conditions.