date:Katholieke Univ Leuven, Centrum Oppervlaktechem & Katalyse, B-3001 Heverlee, Belgium
The adsorption of NOx from gas mixtures containing 10% of oxygen, 5% of carbon 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, 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 NOx adsorption at the lower temperatures is followed by NOx desorption at higher temperatures. The temperature window for NOx adsorption and desorption and the NOx adsorption capacity are dependent on the nature of the cation 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 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/NO2 composition and of the nature of the cations in the Y zeolite. Accordingly, the maximum NOx uptake level is limited to two-third of the NO2 present in the gas mixture. (C) 1999 Elsevier Science B.V. All rights reserved.