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 N-2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), H-2-temperature programmed reduction (TPR), O-2(-), NO + O-2(-) and C3H6-temperature programmed desorption (TPD) and NO + O-2-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 O-2. The catalytic performances over unsubstituted LaMnO3 is observed with maximum N-2 yield of 62% and a C3H6 conversion of 80% at 550 degrees C at a space velocity of 50,000 h(-1) (3000 ppm NO, 3000 PPM C3H6, 1% O-2 in helium). The N-2 yield is however significantly improved by Cu incorporation into the lattice, achieving a remarkable N2 yield of 86% at 500 degrees C at 20% Mn substitution by Cu. The content of alpha-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 alpha-oxygen in addition to the intrinsic effect of Cu in NO transformation. However, the excessive alpha-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 N-2, 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. O-2 acts however as an inhibitor when its concentration is higher than 5000 ppm due to the heavily unselective combustion Of C3H6 by O-2, in the reaction of NO and C3H6 over LaMn0.8Cu0.2O3 at 400 degrees C. (c) 2006 Published by Elsevier B.V.