Cobalt speciation in cobalt oxide-apatite materials : structure-properties relationship in catalytic oxidative dehydrogenation of ethane and butan-2-ol conversion

Impregnation of calcium hydroxyapatite by a solution of Co(II) nitrate followed by calcination at 823 K gives rise to various species, depending on the cobalt content. At low cobalt content (0.2 wt%), the cobalt species are isolated six-coordinated Co2+ ions. For Co content ≥0.4 wt%, the presence of tetrahedral Co2+ and octahedral Co3+ species is attested by UV–visible–NIR spectroscopy, magnetic measurements and XPS data. Magnetic data at low temperature suggest the formation of clustered CoxOy entities. For Co content ≥1.7 wt%, Co3O4 nanocrystals are generated, as evidenced by XRD and magnetic measurements.In the presence of oxygen, the butan-2-ol conversion produces only butan-2-one. The most active catalysts are the cobalt poorest samples which contain only isolated Co2+ ions. Oxidative dehydrogenation of ethane gives a similar trend. Upon increasing the cobalt loading above 0.9 wt%, the specific dehydrogenation activity of Co2+ ions decreases because the nature of the sites changes and the basic properties are lowered. Relationships between the nature of the sites and the catalytic performances are proposed.