NOx Storage and Reduction Catalysts. Reduction with Hydrogen and the Influence of Water and Carbon Dioxide

Licentiate thesis, 2007

Environmental concern has increased over the past several years and resulted in stringent emission legislation on NOx, CO, and hydrocarbon emissions from automobile exhausts. Moreover, the demands to improve the fuel efficiency of engines and to lower the CO2 emissions have increased, and a way to meet this is by using lean burn engines which operate with oxygen excess. However, under oxygen excess the conventional three-way catalyst (TWC) does not reduce NOx efficiently. The NOx storage and reduction (NSR) technology is a potential technique to reduce NOx in lean exhaust. A typical NSR catalyst consists of the following components: precious metals, such as platinum or palladium and 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 as nitrites and nitrates 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 NOx storage and reduction performance was investigated by means of transient flow reactor experiments for three types of model catalysts: Pt/Ba/Al, Pt/Al, and Pt/Si. The reason for studying these samples was to clarify the characteristics of the individual components in a typical NSR catalyst. It was found that the Pt/Ba/Al catalyst offers the highest storage capacity of the catalysts examined, and that barium is the primary storage component in this sample. Since alumina also has the ability to store NOx, it can contribute to the overall storage capacity of a Pt/Ba/Al catalyst. Water and CO2 are two important components in the exhaust gas of lean burn engines; the effect of these species on the performance of NSR catalysts was studied. It was found that the storage capacity of the catalyst examined is higher in the absence of H2O and CO2. Carbon dioxide has a greater negative effect than H2O on the barium storage, while water has a greater negative effect than CO2 on the alumina storage. Microkinetic modeling was done, in addition to flow reactor experiments, to study the regeneration process on platinum when using H2 as the reducing agent. The results show that N2O, N2 and NH3 can be formed when NOx is reduced with H2. Furthermore, the NOx reduction, NH3 formation, and N2O formation depend on the H2 concentration. A good agreement was found between the model and the transient experiments. Carbon dioxide and H2O were found to have a promoting effect on the NH3 formation at lower temperatures. Furthermore, a spillover mechanism of NOx or H2 between barium and platinum was suggested as well as a similar spillover mechanism between alumina and platinum.