Effects of excess manganese in lanthanum manganite perovskite on lowering oxidation light-off temperature for automotive exhaust gas pollutants

Abstract

The effects of excess manganese on enhancement of oxidation activity of LaMn 1+ x O 3+ δ perovskite for CO and propane removal from a synthetic automotive exhaust gas are reported. Various LaMn 1+ x O 3+ δ ( x = 0.05–0.4) perovskite-type nanocatalysts were prepared by a microwave-assisted “gel combustion” method and calcined at 700 °C in air. Scanning electron microscopy (SEM) of the samples demonstrates highly porous and frothy particles. More than twice enhancement in the BET surface area of the catalyst samples is observed for the LaMn 1.2O 3+ δ catalyst, as compared to LaMnO 3. X-ray powder diffraction (XRD) analyses show only the perovskite structure for the catalysts. H 2-, CO-, and C 3H 8- temperature programmed reductions (TPR) indicate that the excess manganese oxides in LaMn 1+ x O 3+ δ significantly enhance the reducibility of the catalysts. The manganese oxides may be in the form of a separate phase of amorphous and/or small crystallites. Redox titration test results show that, as the amount of Mn in the structure increases, the average oxidation state of manganese reduces resulting in lowering cation vacancies. The light off temperature for CO and propane oxidation on LaMn 1.2O 3+ δ is decreased by 57 and 38 °C, respectively as compared to those of LaMnO 3 perovskite. LaMn 1.2O 3+ δ is stable under harsh conditions of the pollutants oxidation.