Abstract:

Currently, there is an urgent need to develop an efficient, non-toxic, and stable catalyst for the removal of NO_x 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 Mn_xCo_3−xO₄ spherical particles with high surface area using a co-precipitation method and investigated their ability to remove NO_x 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 Mn_xCo_3−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 Mn_xCo_3−xO₄ catalysts exhibit > 91% NO_x conversion at 75 ℃, almost reaching 100% conversion with increasing reaction temperature. Notably, the NO_x conversion rate remained above 80% during the test time of 15 h under 150 × 10⁻⁶ SO₂ at 175 ℃. It was found that the coordination structure likely formed into a Co_tet(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 Co_tet(CoMn)_octO₄ configuration was important for promoting low-temperature de-NO_x 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)₃O_4−ƞ spinel oxides that exhibited high NH₃-SCR catalytic activity and good resistance to SO₂ poisoning.

Key words: Mn-Co oxides, Spinel structure, Mn-enriched surface, Selective catalytic reduction, Synergistic effect, Reaction mechanism