NOx storage and reduction over a lean-burn automotive catalyst

Nowadays, there is an increased interest in lean-burn technologies, i.e. diesel and lean-burn gasoline engines, mainly due to their higher fuel efficiency compared to conventional gasoline engines. Lean-burn engines work under excess oxygen and consequently produce oxygen-rich exhaust. This oxygen-rich environment favors the catalytic oxidation of CO and HC to water and CO2 over the reduction of NOx into N2 and the conventional threeway catalyst technology is not able to reduce NOx under these circumstances. Therefore, new catalytic systems have been developed. Among the most promising approaches is the so-called NOx storage reduction (NSR) catalyst along with mixed lean combustion, where the fuel combustion is altered between lean (oxygen excess) and rich (fuel excess) conditions. During the relatively long lean periods, NOx is stored on a storage component, e.g. barium. As the NOx storage component gets saturated, the catalyst needs to be regenerated. This is done during short rich operation of the engine resulting in oxygen-deficient exhaust. Consequently, the stored NOx is released and reduced over noble metals like Pt into the harmless N2. For reallife application, it is crucial to understand the NSR mechanism in order to minimize both fuel penalty and emissions. In this thesis, the NSR mechanism on an automotive catalyst is investigated, using both experimental and modelling methods. Lean/rich cycling experiments are performed in a packed bed reactor using an automotive Pt-Ba/¿-Al2O3 (1 wt.% Pt and 30 wt.% Ba) catalyst. Emphasis is given in understanding the catalyst behavior, the role of multiple storage sites, the influence of CO2 and H2O and the influence of different reducing agents (CO, H2 and C2H4) on NOx storage and reduction. Lean/rich cycling results show that in the absence of CO2 and H2O, almost 100% of barium is involved in NOx storage when the catalyst is exposed to 9 h lean conditions and both bulk and surface barium sites participate in NOx storage. The subsequent rich phase shows incomplete regeneration of stored NOx, even after 15 h rich exposure with H2. Due to this incomplete regeneration, less barium participates in the following lean phase. Additionally, the NO oxidation efficiency of the catalyst decreases upon lean exposure till steady cycling is achieved. Furthermore, the BET surface area, pore volume and Pt dispersion decrease by about 40%. Similar findings are observed for 30 min lean and 120 min rich cycling experiments. In the absence of added CO2 and H2O, NOx can be stored on ¿-Al2O3, BaO, BaCO3 and Ba(OH)2. However, in the presence of CO2, NOx is stored on ¿-Al2O3 and BaCO3 sites. Bulk barium sites are inactive in NOx trapping in the presence of CO2 and only 30% of total barium participates in NOx storage. Ba(NO3)2 is then always completely regenerated during the subsequent rich phase and the Pt dispersion, BET surface area and pore volume remain unchanged. A global reaction kinetic model is developed to describe the NOx storage/reduction process in the presence of CO2 with H2 as reducing agent at a temperature of 300 0C. The model considers that NOx storage occurs on three types of barium sites, viz. surface, semi-bulk and bulk barium sites. Fast NOx storage occurs at surface BaCO3 sites, determined by the reaction kinetics. Slow NOx storage occurs at semi-bulk sites, where diffusion plays a major role. Bulk sites are inactive in NOx trapping. It is assumed that surface and semi-bulk sites correspond with a dispersed barium phase and bulk barium sites with crystalline BaCO3 sites. The model elucidates that the initial complete NOx uptake can be mainly ascribed to NO storage on surface barium sites in the form of nitrites. As the surface coverage increases, NO breakthrough can be seen. The NO storage process continues with the involvement of semi-bulk barium sites but at a lower rate due to diffusion limitation. Meanwhile NO2 is consumed in oxidizing surface nitrites into nitrates with release of NO and by getting stored on semi-bulk barium sites. As a result, delay in NO breakthrough can be seen and the NO concentration passes through a maximum. In the presence of CO2, CO poisons the catalyst in the rich phase through carbon decomposition on the Pt sites and formation of strongly bound isocyanates at 300 0C. Under these conditions, stored NOx is not fully iii removed in the rich phase. C2H4 also poisons the catalyst, but to a lesser extent as all the stored NOx in the preceding lean phase is released and reduced. The type of reducing agent also affects the NOx breakthrough profile observed in the following lean phase. After catalyst regeneration with CO, the NOx breakthrough profile shows simultaneous NO and NO2 breakthrough and both NO and NO2 concentrations increase gradually in time. The NOx breakthrough profile with C2H4 as reductant, shows a combination of the profiles observed for CO and H2. H2O in the lean phase inhibits NO oxidation and no NO2 formation is observed. Less NO is stored in the presence of H2O, which supports the higher storage capacity towards NO2 than NO. Lean/rich cycling experiments with H2O and CO2 and H2 as reducing agent show that H2 is efficient in catalyst regeneration for temperatures between 200 and 300 0C. Experiments with CO show that at temperatures above 250 0C, the water gas shift reaction takes place and H2 acts as reductant instead of CO. At 200 0C, CO and C2H4 are not able to regenerate the catalyst. At higher temperatures, C2H4 is capable of reducing all stored NO, although C2H4 poisons the Pt sites at 250 0C. H2O prevents catalyst poisoning by C2H4 at 300 0C. The previous developed model is adapted to describe lean/rich cycling in the presence of CO2 and H2O for temperatures between 200 and 300 0C and for different reductants (CO, H2 and C2H4). The model can be used to simulate and optimize the catalyst at transient operating conditions.