Characterization and NO decomposition activity of Cu-MFI zeolite in relation to redox behavior

The redox behavior and states of Ca ions in Cu ion-exchanged MFI (Cu(n)-MFI , n: exchange level) have been investigated by means of temperature-program med desorption (TPD) of oxygen, diffuse reflectance (DR) W-VIS spectroscopy and Cu+ photoluminescence (PL) spectroscopy. TPD chromatograms of oxygen f rom Cu(n)-MFI were characterized by the appearance of three desorption peak s: or (below 200 degrees C), beta (300-500 degrees C) and gamma (above 500 degrees C). It has been suggested that alpha and beta oxygen are extra-latt ice oxygen adsorbed on Cu ions, while gamma oxygen is lattice oxygen coordi nated to Cu ions. The Cu+ emission was tremendously reduced once the cataly st contacted with O-2 and NO at elevated temperatures such as 500 degrees C , and it was almost invisible under the working state of the catalyst, sugg esting that PL-active Cu+ ions are not real active sites under the working state. The desorption of beta oxygen was intimately related to the creation of active centers for the NO decomposition reaction. DR measurements showe d that the desorption of beta oxygen caused tetragonal Cu2+ to decrease and trigonal Cu2+ to increase simultaneously. It has been proposed that both C u2+ and Cu+ are involved in the NO decomposition catalysis over Cu-MFI unde r the working state. (C) 2000 Elsevier Science B.V. All rights reserved.