Fundamental studies of catalytic systems for diesel emission control

Doctoral thesis, 2013

Due to global lean exhaust gas and new emission regulations, exhaust aftertreatment systems of diesel engines are more and more sophisticated and composed of a series of catalytic units. In the present work, two of these catalytic systems were studied with different approach. A model diesel oxidation catalyst (DOC), used to convert nitric oxide into nitrogen dioxide and hydrocarbons and CO into CO2, was examined in flow reactor experiments. A Cu-exchanged zeolite catalyst, devoted to the lean NOx reduction by ammonia was studied with SpaciMS in operating conditions. Since longevity and resistance to poisoning are two major challenges for automotive catalysts, the effect of thermal aging in reactive atmosphere on NO oxidation activity was addressed and correlated to platinum dispersion. Our experiments revealed the promotion of Pt sintering by SO2 as well as the improvement of oxidation ability. Sintering in argon to obtain similar Pt dispersion did not result in similar performance indicating the important role of aging atmosphere in subsequent activity. The DOC was subjected to SO2 treatment in order to characterize the sulfur species formed during SO2 poisoning and their impact on the oxidation of NO and C3H6. Two types of sulfur species that differ in stored amount and impact on the activity were distinguished by TPR experiment. However, both have a detrimental effect on the DOC performance. Finally, modification of the DOC formulation by incorporation of acidity enhancer groups was carried out. The introduction of chlorine and sulfate to increase the acidic nature of the support yielded suppression of catalyst deactivation due to platinum oxide formation. However, this effect disappeared after aging and subsequent TPR to 800°C, suggesting a loss of these acidity-promoters. The study of the NOx reduction catalyst was performed to evaluate its activity, NH3 storage capacity and NH3 and NO oxidation ability. Intra-catalyst measurements were achieved with SpaciMS at Oak Ridge National Laboratory. This technique provides insight of the reaction evolution throughout the monolithic catalyst and showed the diminishing of the zone used for SCR reaction as the temperature increased from 200 to 400°C. The intra-catalyst concentration profiles are valuable data, acquired in realistic flow and temperature conditions and was utilized to develop a kinetic model for standard NH3-SCR. The model accounts for the N2O production according to two routes and predicts well the transient phenomena resulting from changes in gas composition.