NOx removal from exhaust gas from lean burn internal combustion engines through adsorption on FAU type zeolites cation exchanged with alkali metals and alkaline earth metals

The adsorption of NOx from gas mixtures containing 10% of oxygen, 5% of car bon dioxide, 5% of water and 1000 ppm of NOx of different compositions (500 ppm NO and 500 ppm NO2; 100 ppm NO and 900 ppm NO2; 400 ppm NO and 600 ppm NO2) is investigated in the temperature range 373-833 K on Y zeolites, cat ion exchanged with barium, caesium, sodium or Lithium. In a typical experim ent the gas mixture is conducted at a VHSV of 30 000 h(-1) over the adsorbe nt bed at 373 K, the temperature of which is increased at 4 K/min. A period of NOx adsorption at the lower temperatures is followed by NOx desorption at higher temperatures. The temperature window for NOx adsorption and desor ption and the NOx adsorption capacity are dependent on the nature of the ca tion exchanged into Y zeolite. The NOx adsorption capacity decreases in the order: Ba-Y > Cs-Y > Na-Y > Li-Y. With the present gas mixtures and at tem peratures of 373-450 K, nitrogen dioxide is selectively adsorbed, while nit ric oxide is inert to these adsorbents. Nitrogen dioxide contained in the g as stream reacts quantitatively with water to nitric acid and nitric oxide, the former being transformed into 'zeolitic' nitrate species, giving rise to an infrared band at 1384 cm(-1). The NO is released from the zeolite. Th is chemistry is observed independently of the NO/NO2 composition and of the nature of the cations in the Y zeolite. Accordingly, the maximum NOx uptak e level is limited to two-third of the NO2 present in the gas mixture. (C) 1999 Elsevier Science B.V. All rights reserved.