Coexistence of framework Co 2+ and non framework Co 0 in CoAPO-5

Abstract

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 Co 2+ ions can be reversibly oxidized to Co 3+, responsible for the change of colour from blue to green. A large fraction of Co 2+ 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 Co 2+(NO) 2 complexes (bands at 1890/1812 cm −1 and at 1880/1798 cm −1), assigned to two unoxidized framework Co 2+ with an adjacent oxygen vacancy. A labile Co 2+(NO) adduct adsorbing at 1845 cm −1 could be assigned to Co 2+ ions on small extraframework clusters. CO adsorption confirmed the presence of defective Co 2+ ions with strong Lewis character that could not be oxidized in the present conditions. Upon reduction, highly dispersed Co 0 clusters were formed within the microporous channels. In the presence of CO, surface Co 0 atoms were extracted from the clusters to form volatile Co 0(CO) n ( n = 2 or 4) complexes and bridged (Co 0) 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.