In situ X-ray photoelectron spectroscopy study of Cu/ZSM-5 in the selective catalytic reduction of NO by propene

The Cu 2P(3/2) photoelectron spectra were acquired to study the coordination state and oxidation state of Cu in ZSM-5 in the NO reduction by propene in the presence of excess oxygen. Two peaks at higher binding energy (937 eV) and lower binding energy (934.3 eV) were found to be changed in intensity depending on the treatment under oxidizing/reducing conditions. The oxidizing condition obtained by the presence of O-2 or NO/O-2 induces the formation of the peak at higher binding energy, indicating that Cu ions are coordinated to extralattice oxygen. However, the Cu-oxygen species began to be decomposed by thermal heating in He above 673 K. During propene oxidation in the gas mixture of O-2 and C3H6, the coordination of Cu to oxygen is achieved at 623 K where the surface carbonaceous species is completely oxidized. The same result was also observed in the NO reduction by propene with excess oxygen. However, the increase of reaction temperature up to 773 K leads to the decomposition of the Cu-oxygen species even in the oxidizing condition, as shown by the significant decrease in the intensity of the higher binding energy peak. From these results, it is suggested that the dependence of the selective catalytic reduction activity of NO on the temperature is correlated with the thermal stability of the Cu-oxygen species on which the reaction intermediates such as nitro species are stabilized.