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

Abstract

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.

Enabling Wide-Temperature Selective Catalytic Reduction of NOX via Modulating Redox Ability of Mn-Based Catalysts

“…The redox capacity of the four catalysts was further investigated by the H 2 -TPR. As shown in Figure C, only a wide reduction peak could be observed on the MnOx precursor between 200 and 500 °C, which was attributed to the continuous reduction of MnO 2 to Mn 3 O 4 and MnO with the increase of temperature. − After being modified by ligands and calcined in NH 3 atmosphere, the reduction peak of MnOx-350-NH 3 shifted to 537 °C, resulting from the reduction of Mn 3 O 4 to MnO, indicating its weakened redox ability caused by the reduction of Mn 4+ to Mn 3+ in NH 3 atmosphere . When calcined in air atmosphere, MnOx-350-air catalysts displayed two obvious reduction peaks at 309 and 438 °C, which were attributable to the reduction of MnO 2 to Mn 3 O 4 and the simultaneous reduction of Mn 3 O 4 to MnO, respectively. , The phenomenon indicated that the introduction of air was conducive to the formation of Mn 4+ and Mn 3+ , thus improving the redox ability of the catalyst.…”