Study on the mechanism of the oxidation of soot on Fe2O3 catalyst

Abstract

The present paper deals with the mechanism of the soot oxidation on Fe2O3 catalyst. The catalytic oxidation of soot is essential for the regeneration of diesel particulate filters already applied for vehicles and working machines. A series of mechanistic studies were conducted including systematic variation of reaction conditions, isotopic labelling (18O2) and local temperature measurements by using an IR camera. These investigations provided knowledge on the transfer of oxygen from the gas-phase to catalyst and soot, the heat distribution on the surface of the catalyst/soot mixture and the effect of crucial parameters on the kinetics, i.e. heating rate, contact mode (tight vs. loose) and catalyst/soot ratio. From these examinations the following insights were derived: (1) Lattice oxygen of Fe2O3 is drastically involved in the soot oxidation by migration from the bulk to the soot. Therefore, the ratio of catalyst/soot directly affects the total amount of oxygen transported from catalyst to soot. (2) In another route, oxygen is “pumped” from the gas-phase via the Fe2O3 surface to the soot. (3) The oxygen transfer from Fe2O3 to soot mainly occurs by physical contact points. (4) The local temperature in the catalyst/soot mixture strongly depends on the amount and heat capacity of the catalyst, respectively, i.e. the catalyst acts as a temperature buffer. Thus, an optimum ratio of catalyst/soot exists reflecting a compromise of high number of contact points (high catalyst mass) and low heat capacity (low catalyst mass).Consequently, a global reaction mechanism of the catalytic soot oxidation on Fe2O3 was suggested. This mechanism implies the formation of oxygen defect sites as a result of the oxygen transfer to the soot. These defect sites are refilled either by migration of surface oxygen and re-oxidation by gas-phase oxygen, respectively, or by bulk oxygen. The resulting oxygen deficiency of the bulk is balanced by migration of oxygen from the surface or sub-surface.