Two-stage catalyst system for selective catalytic reduction of NOx 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 NOx by NH3 in the absence and presence of H2O. The Pt/Al2O3 catalyst exhibited the largest values in NO oxidation efficiency and NOx 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 NOx 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.