Vehicle emissions abatement: NO oxidation and ammonia SCR
Licentiate thesis, 2011
The design and the development of catalytic systems for reducing vehicle emissions is a complex task due to the variety of components in a wide range of gas flow and temperature. A complete system is, therefore, composed of a series of catalysts or catalytic systems, each of which is dealing with a particular aspect of the abatement process. A Diesel Oxidation Catalyst (DOC) is used for CO and hydrocarbon oxidation as well as the conversion of NO to NO2. The NO2 is used by the downstream processes, i.e. Diesel particulate filter (DPF) and NOx reduction catalyst. In the DPF the particulates are removed and the regeneration of the DPF is enhanced by the presence of NO2. One promising technique to remove the nitrogen oxides are urea selective catalytic reduction (SCR). Urea is decomposed to form ammonia, which reacts selectively with NOx over a catalyst. The SCR rate increases with 50% NO2 to NOx ratio, which again shows the importance of the NO oxidation process.
In the first study, a model Pt-based DOC was studied for NO oxidation. The effect of aging in various conditions was examined. More specifically, the impact of the aging on the NO oxidation activity and platinum dispersion was investigated. Thermal aging caused a decrease in dispersion and an increase in NO oxidation performance. However, the aging behavior was strongly correlated with the nature of the aging atmosphere. It was found that aging at low temperatures in O2 promotes the activity to a greater extent than after aging in Ar, even though the dispersions are similar for the two samples. Aging in SO2 and O2 led to a rapid dispersion drop to a minimum value and tremendously enhanced the activity. A long-term aging in presence of SO2 at 250°C confirmed the ability of SO2 to increase sintering rate and improved catalyst activity. The results clearly show that NO oxidation activity is controlled both by the dispersion as well as the atmosphere the platinum particles were aged in. The combination of SO2 and O2 during aging resulted in the highest NO oxidation activity.
In the second study that was focused on NH3-Selective Catalytic Reduction, intra-catalyst measurements of reaction and NH3 storage were performed using a unique tool: the SpaciMS. The spatial conversion rates at three temperatures (200, 325 and 400°C) were resolved showing a faster reaction at higher temperature. The same trend was observed for the direct oxidation of NO and NH3.The fraction of the catalyst, used to carry out the reaction until full conversion of NH3, was named “SCR zone” and became smaller at higher temperature and a higher reaction rate. During SCR, NH3 could store on the catalyst until complete saturation of the SCR zone. Surface NH3 was able to react with NO in the gas flow according to SCR reaction equation yielding production of N2 and the formation of a small amount of N2O. NH3 storage capacity during SCR (DC) was compared to the total NH3 storage capacity (TC). In the SCR zone, DC followed TC and no significant unused capacity (UC) was observed, indicating that, in the presence of NH3, storage sites are filled even during SCR operation in the SCR zone.