Two-stage catalyst system for selective catalytic reduction of NO_x by NH₃ at low temperatures

Three catalysts, 1 wt.% Pt/Al₂O₃, 20 wt.% Cu/Al₂O₃, and 1 wt.% Pt-20 wt.% Cu/Al₂O₃ were evaluated for the oxidation of NO by O₂ and the selective catalytic reduction (SCR) of NO_x by NH₃ in the absence and presence of H₂O. The Pt/Al₂O₃ catalyst exhibited the largest values in NO oxidation efficiency and NO_x removal efficiency among the three catalysts. The presence of H₂O caused a drop in the NO oxidation and the SCR activities. The SCR performance was significantly improved when Pt/Al₂O₃ and Cu/Al₂O₃ catalysts were employed sequentially in a reactor. The highest SCR performance was achieved using two separate stages: oxidation of NO by O₂ over the Pt/Al₂O₃ catalyst followed by reduction of NO₂ by NH₃ over the Cu/Al₂O₃ catalyst. Under the condition with H₂O, the two-stage catalyst system showed NO_x removal efficiency over 80% at low temperatures below 200 °C, which was the highest value among the catalyst systems used in this study. The physical and chemical properties of the catalysts were determined using nitrogen adsorption, XRD, TPR, and pulse CO chemisorption experiments.