Abstract
The redox chemistry of Co ions in cavities of zeolite MFI has been studied in materials prepared by solid-state ion exchange and displaying Co/Al ratios varying from 0.4 to 1.0. Fourier Transform Infrared (FTIR) spectroscopy, electron spin resonance (ESR), and diffuse reflectance UV−visible spectroscopy were used to identify the oxidation state and the coordination of the Co ions. In the dehydrated blue material, Co2+ ions in cationic exchange sites strongly interact with the zeolite framework; their coordination symmetry is tetrahedral. ESR reveals that the ions are in their high-spin state, detectable only below 60 K; no ESR signal was detected at 77 K. However, adsorption of ammonia ligating to the Co2+ cations gives rise to an ESR signal at 77 K, indicating a strong reconfiguration of the electronic states. After coadsorption of oxygen and ammonia, ESR reveals the presence of two kinds of cobalt: Co2+ in a high-spin state at g = 5.1, detected at 77 K, and low-spin (Co3+LxO2-)2+ adducts inside the zeo...
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