Double 6-ring as a unique cation site for the Ce3+/Ce4+redox couple in zeolites

Luminescence spectroscopy reveals that the intrazeolitic redox process of Ce3+ <----> Ce4+ takes place only inside the double 6-ring (D6R) unit among the cation sites in seven different zeolite structures studied here. This unexpected phenomenon can be rationalized in terms of the spatial constraints in D6Rs imposed by the location of Ce3+ ions larger than Ce4+ ions, leading to a considerable decrease in the potential barrier of the Ce3+/Ce4+ couple.