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

Abstract

The redox behavior and states of Cu ions in Cu ion-exchanged MFI (Cu( n)-MFI, n: exchange level) have been investigated by means of temperature-programmed desorption (TPD) of oxygen, diffuse reflectance (DR) UV–VIS spectroscopy and Cu + photoluminescence (PL) spectroscopy. TPD chromatograms of oxygen from Cu(n)-MFI were characterized by the appearance of three desorption peaks: α (below 200°C), β (300–500°C) and γ (above 500°C). It has been suggested that α and β oxygen are extra-lattice oxygen adsorbed on Cu ions, while γ oxygen is lattice oxygen coordinated to Cu ions. The Cu + emission was tremendously reduced once the catalyst contacted with O 2 and NO at elevated temperatures such as 500°C, and it was almost invisible under the working state of the catalyst, suggesting that PL-active Cu + ions are not real active sites under the working state. The desorption of β oxygen was intimately related to the creation of active centers for the NO decomposition reaction. DR measurements showed that the desorption of β oxygen caused tetragonal Cu 2+ to decrease and trigonal Cu 2+ to increase simultaneously. It has been proposed that both Cu 2+ and Cu + are involved in the NO decomposition catalysis over Cu-MFI under the working state.