Calcium-Deficient and Stoichiometric Hydroxyapatites Promoted by Cobalt for the Catalytic Removal of Oxygenated Volatile Organic Compounds

Abstract

Three series of hydroxyapatites (Hap) promoted by cobalt, have been synthesized using co-precipitation or impregnation methods, characterized by various techniques (XRD, UV-visible, Raman, and H2-TPR), and their catalytic properties were tested in the total oxidation of methanol. Characterization of the Co-promoted Hap solids showed that some Co2+ ions could incorporate the apatite structure and that bulk Co3O4 entities could be formed outside the apatite grains. The activity for methanol oxidation of the cobalt-coprecipitated samples was low because structural cobalt ions are difficult to reduce, whereas the activity of the Co-impregnated samples was found to be very high since the methanol was totally converted at low temperature. This was related to the presence of Co3O4 entities that are more easily reducible than structural Co ions and are found in higher quantity in the impregnated solids. This catalytic effect was highlighted using an Ca-deficient Hap as a support: cooperation of redox properties of Co3O4 and acid properties of the apatite support led to a very efficient catalyst since the corresponding light-off temperature was lowered by 50 °C with a catalyst containing 5 wt% of cobalt.