A comparison of bifunctional MnO_x catalysts prepared via different precipitants for simultaneous removal NO and CO

In this work, ammonium carbonate, sodium hydroxide, and ammonium hydroxide were selected as the precipitants to prepare bifunctional MnO_x catalysts via the PEG-assisted co-precipitation method. SEM analysis revealed that MnO_x catalysts prepared with ammonium carbonate (AC) displayed a spherical morphology with relatively large size, whereas the catalysts synthesized with sodium hydroxide (SH) and ammonium hydroxide (AH) exhibited polyhedron shapes with smaller size. The results demonstrated that Mn-AC catalyst presented superior catalytic activity for both NO reduction and CO oxidation, achieving 89% NO conversion at 100 ºC and over 80% CO conversion at 200 ºC, outperforming Mn-SH and Mn-AH catalysts. Mn-AC catalyst possessed more abundant Mn⁴⁺ species and surface chemisorbed oxygen than Mn-AH and Mn-SH catalysts, which served as a significant motivation for the difference in catalytic activity. Additionally, Mn-AC catalyst exhibited stronger surface acidity, which facilitated the adsorption and activation of ammonia species in the NH₃-SCR reaction. Furthermore, in situ DRIFTS experiment suggested that the pre-desorption of CO promoted the adsorption and activation of NO. On this basis, the possible mechanism model of three precipitants on bifunctional MnO_x catalysts was proposed.