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.