LUMINESCENCE PROPERTIES OF CU+ IN ZEOLITES - INSITU STUDY OF THIN-LAYERS

Thin layers of zeolites A and X with different alkali cations partially exchanged by Cu2+ have been prepared on glass or quartz supports. X-ray powder diffraction of these samples yield surprisingly good patterns. A sample chamber for in situ luminescence spectroscopy is described which allows the monitoring of the luminescence of Cu+ during the reduction of Cu2+, as a function of the reducing gas, the pretreatment, the water content and other parameters. Such luminescence measurements have shown that Cu2+ is reduced with CO and with H-2 to some extent already at room temperature. The first electronic absorption observed in Cu+ zeolites occurs in the near UV and is attributed to a Cu+ (4s*) <-- zeolite-oxygen lone-pair LMCT transition. Out of this charge transfer state luminescence occurs with a large Stokes shift which is caused by structural relaxation. The position of the emission depends on the alkali co-cations. In zeolite A this shift is correlated with the size of the unit cell. No correlation has been observed in zeolite X. The luminescence intensity is remarkably influenced by the degree of hydration going through a pronounced maximum that depends on the co-cation and on the type o zeolite.