DIFFUSE-REFLECTANCE SPECTROSCOPY OF DEHYDRATED COBALT-EXCHANGED FAUJASITE-TYPE ZEOLITES - A NEW METHOD FOR CO2+ SITING

The diffuse reflectance spectra of cobalt in X- and Y-type zeolites wi th varying Co2+ contents have been investigated after dehydration at 4 00 and 500 degrees C. A novel method for the determination and charact erization of Co sites in dehydrated zeolites is developed. The spectra were resolved with a decomposition program in five or six Gaussian ba nds in the visible region (12 500-22 500 cm(-1)) and three Gaussian ba nds and two Lorentz bands in the near infrared region (4000-10 000 cm( -1)). Interactive self-modeling analysis showed the presence of three independent Co species, called pure components. In the visible region component A appeared at 20 000-17 800 cm(-1), component B appeared in the 18 800-15 300 cm(-1) region with three band maxima, and component C appeared with main purity at 13 700-14 900 cm(-1). In the near infra red region component B appeared as a triplet in the 4600-7000 cm(-1) r egion together with two components of framework hydroxyls at 4500 and 7600 cm(-1). With the: aid of additional experiments on dehydrated lan thanum-, cesium-, and ammonium-exchanged Co zeolites, the three cobalt -related components (A, B, and C) were assigned as follows: A, (pseudo )octahedral cobalt in the hexagonal prisms (I); B, (pseudo)tetrahedral cobalt at I' (II',II); and C, trigonally coordinated cobalt at the sa me sites. These findings are discussed in relation with XRD results of Co siting.