SCR of NO by propene over nanoscale LaMn1−xCuxO3 perovskites
Nanoscale LaMn1−xCuxO3 perovskites with high specific surface areas were prepared by reactive grinding and characterized by N2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), H2-temperature programmed reduction (TPR), O2-, NO + O2- and C3H6-temperature programmed desorption (TPD) and NO + O2-temperature programmed surface reduction (TPSR) under C3H6/He flow. The samples were then submitted to activity tests in the selective catalytic reduction (SCR) of NO by C3H6 with or without O2. The catalytic performances over unsubstituted LaMnO3 is observed with maximum N2 yield of 62% and a C3H6 conversion of 80% at 550 °C at a space velocity of 50,000 h−1 (3000 ppm NO, 3000 ppm C3H6, 1% O2 in helium). The N2 yield is however significantly improved by Cu incorporation into the lattice, achieving a remarkable N2 yield of 86% at 500 °C at 20% Mn substitution by Cu. The content of α-oxygen over lanthanum manganite is enhanced by Cu substitution, but the opposite occurs for excess oxygen. The better performance of Cu-substituted samples is likely to correspond to the facility in the formation of adsorbed nitrate species via the oxidation of NO by α-oxygen in addition to the intrinsic effect of Cu in NO transformation. However, the excessive α-oxygen content observed over LaCo0.8Cu0.2O3 accelerated the unselective hydrocarbon oxidation and suppressed the formation of organo nitrogen compounds, which led to a poor N2 yield with respect to Mn-based perovskites. A mechanism involving the formation of an organic nitrogen intermediate, which further converts into N2, CO2 and H2O via isocyanate, was proposed. The gas phase oxygen acts as a promoter when its concentration is lower than 1000 ppm because of the promotion of nitrate formation and organo nitrogen compounds transformation. O2 acts however as an inhibitor when its concentration is higher than 5000 ppm due to the heavily unselective combustion of C3H6 by O2, in the reaction of NO and C3H6 over LaMn0.8Cu0.2O3 at 400 °C.
Journal: Applied Catalysis A: General - Volume 307, Issue 1, 12 June 2006, Pages 85–97