Determining the Location of Co²⁺ in Zeolites by UV-Vis Diffuse Reflection Spectroscopy: A Critical View

UV–Vis spectroscopy as well as in situ FTIR spectroscopy of pyridine and CO adsorption were applied to determine the nature of Co species in microporous, mesoporous, and mixed oxide materials like Co–ZSM-5, Co/Na–ZSM-5, Co/Al–SBA-15, and Co/Al₂O₃–SiO₂. Because all sample types show comparable UV–Vis spectra with a characteristic band triplet, the former described UV–Vis band deconvolution method for determination and quantification of individual cationic sites in the zeolite appears doubtful. This is also confirmed by results of pyridine and CO adsorption revealing that all Co–zeolite samples contain two types of Co²⁺ species located at exchange positions as well as in oxide-like clusters independent of the Co content, while in Co/Al–SBA-15 and Co/Al₂O₃–SiO₂ only Co²⁺ species in oxide-like clusters occur. Consequently, the measured UV–Vis spectra represent not exclusively isolated Co²⁺ species, and the characteristic triplet band is not only related to γ-, β-, and α-type Co²⁺ sites in the zeolite but also to those dispersed on the surface of different oxide supports. The study demonstrates that for proper characterization of the formed Co species, the use of complementary methods is required.