Quantitative IR studies of the concentration of Co2+ and Co3+ sites in zeolites CoZSM-5 and CoFER

Abstract

Abstract Quantitative IR experiments were realized to determine the concentration of cobalt in a form of trivalent cations being in the oxide form as well as divalent ions in exchange positions, oxide-like clusters and cobalt oxide, present in a series of cobalt-exchanged zeolites. The first step of such quantitative studies was to elaborate the experimental conditions in which probe molecules NO and CO should react selectively with either of these sites. Subsequently, the values of absorption coefficients of the 1940 cm −1 band of Co 3+ –NO and the 2204 cm −1 band of Co 2+ –CO for Co 2+ in exchange positions, together with the 2194 cm −1 band of Co 2+ –CO for Co 2+ in oxide-like clusters were determined. The linear plots of the band intensities versus the concentration of NO and CO interacting with cationic sites showed the correlation coefficients higher than 0.99. Three zeolites were investigated: CoZSM-5 (Si/Al = 15) in which most of Co 2+ occupied the exchange positions, CoZSM-5 (Si/Al = 40) in which most of Co 2+ was in the form of oxide-like clusters and CoO, and CoFER prepared by the contact-induced ion exchange with nearly equal amount of Co 2+ in either type of the sites. The sum of the concentration of Co sites (Co 3+ and Co 2+ ) determined by IR spectroscopy was very close to the values obtained from the chemical analysis (AAS). The concentration ill-defined oxide form of Co 3+ was surprisingly high as in CoZSM-5 of Si/Al = 15 it constituted about half of the overall cobalt content, and in CoFER the contribution of Co 3+ reached almost 30%.