Spherical Mn<i><sub>x</sub></i>Co<sub>3-<i>x</i></sub>O<sub>4-</sub><i><sub>ƞ</sub></i> Spinel with Mn-enriched Surface as High-Efficiency Catalysts for Low-Temperature Selective Catalytic Reduction of NO<i><sub>x</sub></i> by NH<sub>3</sub>

Abstract: Currently, there is an urgent need to develop an efficient, non-toxic, and stable catalyst for the removal of NOx via selective catalytic reduction using NH3 (NH3-SCR) that is effective at low temperatures. Mn-based catalysts are particularly representative and have been widely studied. An investigation of the collaborative participation of Mn and Co can be of great importance for improving the catalytic activity and SO2 resistance of Mn-Co oxides with a spinel structure. Therefore, in this study, we prepared MnxCo3-xO4 spherical particles with high surface area using a co-precipitation method and investigated their ability to remove NOx via NH3-SCR. Mn-Co bimetal oxides mainly possess a spinel structure and undergo a tetragonal-to-cubic phase transformation with increasing Co-content. A high concentration of surface oxygen and strong effective electron transfer between the variable valence elements (Co3+ + Mn3+ ↔ Co2+ + Mn4+) improves the redox ability of typical MnxCo3-xO4 (x = 1.0, 1.5, 2.0) spinel catalysts. In addition, Mn-enrichment leads to more oxygen vacancies and abundant surface-active sites, which further promotes the SCR catalytic performance. The investigated MnxCo3-xO4 catalysts exhibit > 91% NOx conversion at 75 ℃, almost reaching 100% conversion with increasing reaction temperature. Notably, the NOx conversion rate remained above 80% during the test time of 15 h under 150×10-6SO2 at 175 ℃. It was found that the coordination structure likely formed into a Cotet(CoMn)octO4 spinel structure in which Mn ions (Mn3+ and Mn4+, mainly in trivalent manganese) and partial Co ions are configured into octahedral sites. These species were identified as the activity descriptor for probably owing to their strong electronic transfer interactions that were directly correlated with SCR activity. Furthermore, the Cotet(CoMn)octO4 configuration was important for promoting low-temperature de-NOx activity and highly conducive to protecting Mn active sites from poisoning by SO2. The active sites in this particular spinel structure with the micro-coordination structure were effectively built and maintained to ensure the smooth circulation of electronic interactions in the core octahedron. The reaction of adsorbed NH3 and gaseous NO (or NO2) mainly occurred on the surface of Mn-Co spinel following the Eley-Rideal mechanism. Additionally, the NH4NO3 intermediate was likely first transformed into NH4NO2 and then to N2 with increasing reaction temperature. Herein, we successfully synthesized a spinel-structured Mn-Co oxide catalyst comprising a Mn-enriched surface of (MnCo)3O4-ƞ spinel oxides that exhibited high NH3-SCR catalytic activity and good resistance to SO2 poisoning.