Unlocking Mixed-Metal Oxides Active Centers via Acidity Regulation for K&amp;SO₂ Poisoning Resistance: Self-Detoxification Mechanism of Zeolite-Confined <i>de</i>NO<i>_x</i> Catalysts

Abstract

Selective catalytic reduction of nitrogen oxides (NO<i>_x</i>) with ammonia (NH₃-SCR) is an efficient NO<i>_x</i> reduction strategy, while the denitrification (<i>de</i>NO<i>_x</i>) catalysts suffer from serious deactivation due to the coexistence of multiple poisoning substances, such as alkali metal (e.g., K), SO₂, <i>etc</i>., in industrial flue gases. It is essential to understand the interaction among various poisons and their effects on the <i>de</i>NO<i>_x</i> process. Herein, the ZSM-5 zeolite-confined MnSmO<i>_x</i> mixed (MnSmO<i>_x</i>@ZSM-5) catalyst exhibited better <i>de</i>NO<i>_x</i> performance after the poisoning of K, SO₂, and/or K&SO₂ than the MnSmO<i>_x</i> and MnSmO<i>_x</i>/ZSM-5 catalysts, the <i>de</i>NO<i>_x</i> activity of which at high temperature (H-T) increased significantly (>90% NO<i>_x</i> conversion in the range of 220-480 °C). It has been demonstrated that K would occupy both redox and acidic sites, which severely reduced the reactivity of MnSmO<i>_x</i>/ZSM-5 catalysts. The most important, K element is preferentially deposited at -OH on the surface of ZSM-5 carrier due to the electrostatic attraction (-O-K). As for the K&SO₂ poisoning catalyst, SO₂ preferred to be combined with the surface-deposited K (-O-K-SO_2ads) according to XPS and density functional theory (DFT) results, the poisoned active sites by K would be released. The K migration behavior was induced by SO₂ over K-poisoned MnSmO<i>_x</i>@ZSM-5 catalysts, and the balance of surface redox and acidic site was regulated, like a synergistic promoter, which led to K-poisoning buffering and activity recovery. This work contributes to the understanding of the self-detoxification interaction between alkali metals (e.g., K) and SO₂ on <i>de</i>NO<i>_x</i> catalysts and provides a novel strategy for the adaptive use of one poisoning substance to counter another for practical NO<i>_x</i> reduction.