Kinetic Studies of NOx Storage and Reduction Catalysts - Flow Reactor Experiments and Microkinetic Modeling

Doctoral thesis, 2009

The NOx storage and reduction (NSR) technology is a promising technique to reduce NOx during oxygen excess e.g. in lean burn engine exhaust. A typical NSR catalyst consists of the following components: precious metals, such as platinum, palladium or rhodium, a storage component, usually barium, and a high surface area support, such as γ-alumina. By this method, NOx is stored in the catalyst for a relatively long lean period of time (minutes). To regenerate the catalyst, the engine operates in rich mode for a shorter time interval (seconds), during which the NOx stored is released and reduced to N2. The main objective of this work is to investigate the mechanism of storage and reduction of NOx over platinum, barium, and alumina based catalysts. This is accomplished through both flow reactor experiments and microkinetic modeling. In particular, the storage and reduction process when hydrogen is used as the reducing agent is examined. Additionally, the individual effect of the components in NSR catalysts and interactions between them are thoroughly investigated by studying Pt/Ba/Al, Pt/Al, and Pt/Si catalysts. Barium is the primary storage component in Pt/Ba/Al catalysts. However, it is shown that alumina contributes to the overall storage capacity. Thus, it is concluded that the storage of NOx occur on multiple storage sites. The storage capacity is influenced by the presence of H2O and CO2 in the feed. This work shows that byproducts such as N2O and NH3 can be formed when NOx is reduced with H2 and that the product selectivity is influenced by the temperature and the hydrogen concentration. Moreover, the performance of NSR catalysts is affected by the preparation procedure of the catalyst. The order of the impregnation steps influences the dispersion of barium, which impacts the utilization of storage components and the proximity between platinum and barium. Both the storage and regeneration processes are thereby affected by the preparation process. The reduction of NOx with H2 over platinum is investigated separately and a detailed kinetic model describing the process is developed in this work. This model is in turn used in a more comprehensive detailed model describing the complete NOx storage and reduction process in the presence of H2O and CO2, over a Pt/Ba/Al catalyst. The models are based on the results from the experimental investigations and can well describe the observed phenomena. Since the formation of significant amounts of NH3 is observed over Pt/Ba/Al catalysts the NOx reduction of a combined NSR and SCR system is investigated in this work. By placing an SCR catalyst downstream a Pt/Ba/Al catalyst the NOx reduction is improved and a decrease of the NH3 slip is achieved.