Geometry of the Cu+ 540 nm luminescence centres in zeolites

Abstract

Cu⁺ ions, with characteristic emission at 540 nm, have been considered to be responsible for the high activity of Cu-ZSM-5 in NO decomposition. However, two types of Cu⁺ ions located in different structures, i.e. in ZSM-5 and Y, exhibit the 540 nm emission, but the Cu ions in Y zeolite do not exhibit such a unique activity in NO decomposition. Therefore, Cu⁺ emission spectra and luminescence decay of the Cu²⁺ ion exchanged and reduced in CuH-ZSM-5, CuH-, CuRb-, CuCs- and CuBa-mordenites and Cu-Y zeolites were used to identify cationic sites corresponding to the Cu⁺ luminescence centres in these zeolites emitting at 540 nm. The Cu⁺ ions with a single luminescence decay correspond to those with C_3v symmetry in Y zeolite, while the Cu⁺ ions possessing a double-exponential decay represent Cu⁺ cationic sites in ZSM-5 and mordenite, different from that described for Cu-Y, but the local arrangement of these Cu⁺ sites is similar in Cu-ZSM-5 and Cu-mordenite. The effect of the presence of Rb, Cs and Ba co-cations and pyridine adsorption in CuH-mordenites on the luminescence spectra was used to identify the cationic site represented by the 540 nm emission in mordenite. These Cu⁺ ions are located in site E of the mordenite main channel. The elongated six-ring of this site is composed of two five-rings sharing two oxygens, and the Cu⁺ ions are suggested to be coordinated close to the rectangle of the main channel wall. The 540 nm Cu⁺ emission of the Cu-ZSM-5 was attributed to the Cu⁺ ion in a site with a local arrangement similar to that of mordenite. The Cu⁺ ions are located in the main (straight) channels of the ZSM-5 matrix and are coordinated to the oxygen oblong of the channel wall. The cationic site is formed, as in mordenite, by an elongated six-ring composed of two five-rings. Thus, the structure of the Cu active site in Cu-ZSM-5 was suggested.