The interaction of NO, from lean exhaust gases with copper-exchanged ZSM-5 has been studied by combined adsorption and temperature-programmed desorption. Experiments with NO at 200 degreesC show that this molecule cannot be stored on Cu-ZSM-5. On the other hand, NO2. May be stored over a wide temperature range from both dry and humid feeds. Therefore, in order to obtain storage of NO, NO must first be oxidized to NO2. However, the oxidation reaction is inhibited by water in the exhaust gas. The adsorption of nitrogen dioxide involves a disproportionation reaction yielding two stored nitrate species and one molecule of NO released to the gas phase: 3NO(2) + [CuO] reversible arrow NO + Cu(NO3)(2) Water in the feed gas competes with the formation of nitrates, thus lowering significantly the amount of NO2 stored on Cu-ZSM-5. NO has also a negative effect on the storage of NO2. This effect is due to its reaction with the stored nitrates, causing a considerable shift in the equilibrium of the above reaction back to NO2. (C) 2002 Elsevier Science Inc. All rights reserved.
