Design Strategy of the MnOx Catalyst for SCR of NO with NH₃: Mechanism of Lead Poisoning and Improvement Method

The conventional Mn-based catalysts suffer from lead toxicity and require other transition-metal oxides to enhance their resistance in the selective catalytic reduction of NOx with ammonia (NH₃-SCR). Herein, we found that the incorporation of inert silica into pure MnOx effectively improved the Pb resistance. The NOx conversion of the MnOx-SiO₂-Pb catalyst was nearly 55% higher than that of the MnOx-Pb catalyst, exhibiting enhanced activity at lower temperatures (150-225 °C). To reveal the essential roles at the molecular level, the types and numbers of surface acidity, nitrate species, and catalytic cycle were established through experimental analysis and theoretical calculations of catalysts. The presence of PbCl₂ occupied the active Mn sites, resulting in an obvious decline in the Brønsted acid sites (B-NH₄⁺) and the oxidation performance, and the NH₃-SCR cycle was energetically less favorable on the MnOx-Pb catalyst. Conversely, SiO₂ played a crucial role in preserving the activity of Mn sites on the MnOx-SiO₂-Pb catalyst by preferentially bonding with PbCl₂, generating more active intermediates. Significantly, this work provided mechanistic insights into the role of SiO₂ in regulating the surface acidity, oxidation performance, and stability of active Mn sites, which is helpful for the design of Mn-based catalysts with high Pb resistance for the NH₃-SCR reaction.