Coexistence of framework Co²⁺ and non framework Co⁰ in CoAPO-5

We report about the synthesis and characterization of a microporous CoAPO-5 catalyst. Diffuse Reflectance UV-Vis spectroscopy was employed to follow the changes upon oxidation and reduction treatments, showing that a small fraction of Co²⁺ ions can be reversibly oxidized to Co³⁺, responsible for the change of colour from blue to green. A large fraction of Co²⁺ ions are not oxidized in the employed conditions, in agreement with literature reports. FTIR spectroscopy of adsorbed probe molecules was employed to monitor the surface properties of Co ions. NO adsorption resulted in the formation of two stable Co²⁺(NO)₂ complexes (bands at 1890/1812 cm^-1 and at 1880/1798 cm^-1), assigned to two unoxidized framework Co²⁺ with an adjacent oxygen vacancy. A labile Co²⁺(NO) adduct adsorbing at 1845 cm^-1 could be assigned to Co²⁺ ions on small extraframework clusters. CO adsorption confirmed the presence of defective Co²⁺ ions with strong Lewis character that could not be oxidized in the present conditions. Upon reduction, highly dispersed Co⁰ clusters were formed within the microporous channels. In the presence of CO, surface Co⁰ atoms were extracted from the clusters to form volatile Co⁰(CO)_n (n = 2 or 4) complexes and bridged (Co⁰)_n(CO) species (bands at 2047 and 2008/1990 cm^-1, respectively). These results show that the insertion of heteroatoms in large pore AFI structure is strongly sensitive upon the synthetic conditions and metal loading, that have to be carefully tuned to get the desired catalytically active sites.