Development of Highly Durable Zeolites as Hydrocarbon Trap Materials for Automotive Catalysts 2018-01-0947
Low-temperature activity is an important requirement for automotive catalysts. In particular, most of the tailpipe emissions occur right after the engine starts (cold emissions). These emissions can be effectively reduced by using a trap material such as zeolite for hydrocarbon (HC) adsorption [1, 2, 3, 4, 5, 6, 7, 8, 9]. However, using zeolite as a trap material in automotive catalyst is limited due to its low durability under hydrothermal aging conditions. That is the reason why zeolites can be often used for diesel engines which usually run at lower temperature than the gasoline engines during entire mode driving. In most cases, zeolites need to be placed away from large thermal loads in order to take advantage of their adsorption abilities. In general, the thermal endurance of close-coupled catalysts for gasoline powered vehicles proceeds at about 1000 °C in the presence of water. Under these conditions, the zeolite structure would be decomposed by the dissociation of aluminum from the zeolite frameworks . Through this study, we show that the hydrothermal durability of zeolite can be dramatically improved by chemical modification of zeolite with zirconium phosphate. This improvement strategy works well, especially for β-type zeolites (BEAs) with low SiO2/Al2O3 ratio, which can be easily decomposed by hydrothermal aging at around 1000 °C. We also found that this modified BEA worked well as HC trap material and showed an enormous reduction of cold HC emissions with Pd/Rh three-way catalyst (TWC). In the engine test evaluations with the close-coupled TWC + TWC and TWC + HC trap system as aged catalysts, effects of this HC trap catalyst on cold emissions were observed. It is found that HC emission decrease by up to 43% when compared to those from the corresponding TWC + TWC system which does not have any zeolite. In addition, a detailed analysis of this effect proved this improvement to be due to the adsorption-desorption process of zeolite and the purification process by TWCs.