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 with varying Co²⁺ contents have been investigated after dehydration at 400 and 500°C. A novel method for the determination and characterization of Co sites in dehydrated zeolites is developed. The spectra were resolved with a decomposition program in five or six Gaussian bands in the visible region (12 500-22 500 cm^-1) and three Gaussian bands 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 infrared region component B appeared as a triplet in the 4600-7000 cm^-1 region together with two components of framework hydroxyls at 4500 and 7600 cm^-1. With the aid of additional experiments on dehydrated lanthanum-, 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 same sites. These findings are discussed in relation with XRD results of Co siting.