Oxidation of soot on manganese oxide catalysts

Abstract

The present paper deals with the catalytic oxidation of soot by O2 on bare manganese oxides. The catalysts comprised MnO2, Mn2O3 and Mn3O4 as well as nano-sized Mn3O4 originated from flame spray pyrolysis (FSP-Mn3O4). The activity of the samples was evaluated by temperature programmed oxidation (TPO) using tight and loose contact catalyst/soot mixtures. The catalysts were characterised by powder X-ray diffraction, N2 physisorption, scanning electron microscopy, temperature programmed reduction by H2 and temperature programmed desorption of NH3. The correlation of physical–chemical characteristics with soot oxidation kinetics showed that the number of surface oxygen vacancies and particle size determine the catalytic performance by affecting the supply of oxygen. Isotopic TPO studies of FSP-Mn3O4/soot suggested that oxygen was transferred from the surface and bulk of the catalyst to the soot by physical contact points. A strong contribution of bulk oxygen (ca. 60%) occurred for tight as well as loose contact. The role of gas-phase O2 was found to refill the oxygen vacancies of the catalyst. Furthermore, FSP-Mn3O4 showed the best performance among the samples tested. The catalyst revealed high efficiency for tight contact yielding peak CO2 at 305 °C. Additionally, FSP-Mn3O4 exhibited only slight change in soot oxidation activity after hydrothermal exposure at 750 °C and thermal aging at 1050 °C. The practical efficiency of FSP-Mn3O4 was investigated by using a catalytic laboratory-scaled particulate filter. The TPO studies indicated that the major part of the soot deposited in the trap (ca. 80%) was weakly contacted to the catalyst causing oxidation above 400 °C only. Contrary, the minor fraction of tight contact soot evoked conversion already between 180 and 350 °C. This operation range indicated potential for the continuous regeneration of particulate filters in real diesel exhaust.