TPD study of mordenite-type zeolites for selective catalytic reduction of NO by NH3

The effect of the adsorption of NO and NH3 on catalytic activity has been examined by temperature-programmed desorption over a series of cation-exchanged mordenite catalysts for selective catalytic reduction (SCR) of NO by NH3. The catalytic activity observed in a packed-bed flow reactor was well correlated with the cation content of the catalyst and its adsorption properties, making it possible to elucidate the role of metals and acidity in this reaction system. The amount of NH3 and NO adsorbed on the catalyst surface proportionally increased with the degree of cation exchange of the catalyst, especially at the Bronsted acid site (H+) and the metal site (Cu2+). SCR activity also gradually increased with the acidity of the catalyst and/or its degree of catalyst cation exchange, Surface acidity of the mordenite catalysts appears to be a dominant factor in the high performance of the SCR reaction system. The common activation energy is observed to be about 12 kcal/mole for NaHM and CuHM catalysts, independent of their cation content on the catalyst surface. The active sties of this catalytic system are both the Bronsted acid site and the metal site. Furthermore, the reaction occurred in a Langmuir-Hinshewood manner with a dual-site catalysis mechanism. (C) 1996 Academic Press, Inc.