SITING AND REDOX BEHAVIOR OF CU IONS IN CUH-ZSM-5 ZEOLITES - CU+ PHOTOLUMINESCENCE STUDY

Sitting and redox properties of Cu ions in CuH-ZSM-5 zeolites differing in Cu content and Si/Al framework ratio were investigated employing the Cu+ luminescence spectra. It was shown that two main Cu sites exist in the ZSM-5 structure; one, reflected in the blue (480 nm) band, in close proximity to the two framework Al atoms and the second, yielding the yellow-green (540 nm) band, adjacent to one Al framework atom. The former sites are preferentially occupied at low Cu content. The reducibility of these sites is controlled by the content of the framework Al; with a lower framework charge, the reducibility (in both hydrogen and CO atmospheres) increases. The hydrogen reduction of Cu2+ passes from Cu+ to Cu-0, while reduction in CO yields practically only Cu+. In addition, the two Cu sites differ in reducibility. The Cu sites with the blue luminescence are reduced faster regardless of whether hydrogen is used; however, the degree of their reduction up to Cu-0 is slightly lower compared to the Cu sites characterized by the yellow-green luminescence.