An IR spectroscopy study of Co sites in zeolites CoZSM-5

Abstract

Abstract We examined the status and properties of Co 2+ and Co 3+ in CoZSM-5 zeolites of various Si/Al and Co contents and activated under various conditions. CO, NO and pivalonitrile were used as probe molecules. Some information on the status of Co was also obtained by comparing the intensities of Si–OH–Al bands and the concentration of protonic sites measured with pyridine. IR studies showed that in zeolite with high concentrations of AlO 4 − (of low Si/Al) practically all Co takes the form of Co 2+ exchange cations (CO band at 2204 cm −1 ). However, in zeolites with low AlO 4 − concentrations (of high Si/Al) most Co occurs in the form of CoO (CO band at 2179 cm −1 ) or “oxide-like clusters” (bands at 2184 and 2194 cm −1 , respectively). Experiments with co-adsorption of pivalonitrile and CO prove that all cobalt species (both in exchange positions and in “oxide-like clusters”) are situated inside zeolitic channels. The spectra of NO revealed the presence of three kinds of Co 3+ (NO bands 1940, 1955 and 1970 cm −1 ), presence of Co 2+ mononitrosyls (NO band at 1850 cm −1 ) and of Co 2+ dinitrosyls (NO bands at 1815 and 1895 cm −1 ). Experiments on the co-adsorption of NO and CO suggested that NO can be bonded to Co 2+ in exchange cationic positions, in “oxide-like clusters” (characterized by CO bands at 2204 and 2194 cm −1 ), but it cannot interact with Co 2+ in “oxide-like clusters” (CO band at 2184 cm −1 ) or bulk CoO (CO band at 2179 cm −1 ).