Structure-activity strategy comparison of (NH₄)₂CO₃ and NH₄OH precipitants on MnO_x catalyst for low-temperature NO abatement

Herein, the structure-activity strategy comparison of two different precipitants on MnO_x catalyst for low-temperature NH₃-SCR was investigated in detail. XRD results confirmed that ammonium carbonate (denoted as AC) could transfer manganese precursor from MnCO₃ to Mn₂O₃ phase with temperature from 80 to 450 °C, while ammonium hydroxide (denoted as AH) facilitated the phase transition of MnO_x from Mn₃O₄ to the final Mn₅O₈ phase. Further morphology defined that Mn-AC catalyst possessed a peculiar spherical feature while Mn-AH catalyst presented an accumulation of abundant nano-particles. The activity results suggested that Mn-AC catalyst possessed higher activity compared with Mn-AH catalyst at lower temperature, with over 60% conversion achieved from 100 to 225 °C and the maximum NO conversion reached circa (denoted as ca.) 85% at 175 °C. In addition, Mn-AC catalyst possessed stronger surface acidity and redox properties than Mn-AH catalyst, which served as an important motivation for the difference in activity. More manganese ions with higher valence and surface oxygen favored the superior redox circle of Mn-AC catalyst. Transient reaction revealed that the catalysts followed both Eley–Rideal (E-R) and Langmuir–Hinshelwood (L-H) pathways while the L-H pathway was harder to occur than the former due to the intense NO_x transformation over catalysts. On account of these analyses, the possible structure-activity strategy comparison model of (NH₄)₂CO₃ and NH₄OH precipitants on MnO_x catalyst was proposed.