COORDINATION OF CU IONS IN HIGH-SILICA ZEOLITE MATRICES - CU+ PHOTOLUMINESCENCE, IR OF NO ADSORBED ON CU2+, AND CU2+ ESR STUDY

The Cu ion siting-coordination in MFI, mordenite, erionite, and Y zeolite matrices was investigated employing Cu+ luminescence, IR spectra of NO adsorbed on Cu2+, and ESR spectra of Cu2+ ions. Four discrete Cu+ emission bands (with decays) at 450 nm (35 mu s), 480 nm (55 mu s), 510 nm, and 540 nn (120 mu s) were identified and ascribed to four individual types of Cu ion sites. The Cu site distribution varied with the Cu loading and Si/Al framework ratio. It has been shown that the Cu siting in high-silica zeolites is controlled by the local Si-Al sequences, which affects the redox properties of the individual sites. The Cu+ emission bands were in sound agreement with characteristic shifts of the main LR bands of NO adsorbed on Cu2+ ions, found at about 1921, 1912, 1906, and 1895 cm(-1). This indicated different charges on the four Cu ion sites. The ESR spectra provide qualitative information on the existence of two square pyramidal and one square planar Cu2+ complexes. It is suggested that the Cu sites balanced by pairs of framework Al atoms (Cu+ emissions at 450 and 480 nm) exhibit square pyramidal coordination, while those adjacent to a single framework Al atom are coordinated close to square planar environment. The latter sites possess substantially higher reducibility compared to the other sites and are responsible for the Cu-ZSM-5 unique activity in NO decomposition.