SCR of NO by propene over nanoscale LaMn 1− xCu xO 3 perovskites

Abstract

Nanoscale LaMn 1− x Cu x O 3 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 C 3H 6-temperature programmed desorption (TPD) and NO + O 2-temperature programmed surface reduction (TPSR) under C 3H 6/He flow. The samples were then submitted to activity tests in the selective catalytic reduction (SCR) of NO by C 3H 6 with or without O 2. The catalytic performances over unsubstituted LaMnO 3 is observed with maximum N 2 yield of 62% and a C 3H 6 conversion of 80% at 550 °C at a space velocity of 50,000 h −1 (3000 ppm NO, 3000 ppm C 3H 6, 1% O 2 in helium). The N 2 yield is however significantly improved by Cu incorporation into the lattice, achieving a remarkable N 2 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 LaCo 0.8Cu 0.2O 3 accelerated the unselective hydrocarbon oxidation and suppressed the formation of organo nitrogen compounds, which led to a poor N 2 yield with respect to Mn-based perovskites. A mechanism involving the formation of an organic nitrogen intermediate, which further converts into N 2, CO 2 and H 2O 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 C 3H 6 by O 2, in the reaction of NO and C 3H 6 over LaMn 0.8Cu 0.2O 3 at 400 °C.