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 NO(x) from gas mixtures containing 10% of oxygen, 5% of carbon dioxide, 5% of water and 1000 ppm of NO(x) of different compositions (500 ppm NO and 500 ppm NO(2); 100 ppm NO and 900 ppm NO(2); 400 ppm NO and 600 ppm NO(2)) is investigated in the temperature range 373-833 K on Y zeolites, cation exchanged with barium, caesium, sodium or Lithium. In a typical experiment the gas mixture is conducted at a VHSV of 30 000 h(-1) over the adsorbent bed at 373 K, the temperature of which is increased at 4 K/min. A period of NO(x) adsorption at the lower temperatures is followed by NO(x) desorption at higher temperatures. The temperature window for NO(x) adsorption and desorption and the NO(x) adsorption capacity are dependent on the nature of the cation exchanged into Y zeolite. The NO(x) adsorption capacity decreases in the order: Ba-Y > Cs-Y > Na-Y > Li-Y. With the present gas mixtures and at temperatures of 373-450 K, nitrogen dioxide is selectively adsorbed, while nitric oxide is inert to these adsorbents. Nitrogen dioxide contained in the gas 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. This chemistry is observed independently of the NO/NO(2) composition and of the nature of the cations in the Y zeolite. Accordingly, the maximum NO(x) uptake level is limited to two-third of the NO(2) present in the gas mixture. (C) 1999 Elsevier Science B.V. All rights reserved.