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

The effects of excess manganese on enhancement of oxidation activity of LaMn1+xO3+δ perovskite for CO and propane removal from a synthetic automotive exhaust gas are reported. Various LaMn1+xO3+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 LaMn1.2O3+δ catalyst, as compared to LaMnO3. X-ray powder diffraction (XRD) analyses show only the perovskite structure for the catalysts. H2-, CO-, and C3H8- temperature programmed reductions (TPR) indicate that the excess manganese oxides in LaMn1+xO3+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 LaMn1.2O3+δ is decreased by 57 and 38 °C, respectively as compared to those of LaMnO3 perovskite. LaMn1.2O3+δ is stable under harsh conditions of the pollutants oxidation.