Cu-exchanged hydroxyapatites as green catalysts for CO oxidation

Hydroxyapatite (HA) is gaining attention as a sustainable and efficient green catalyst due to its environmental compatibility, non-toxicity and peculiar surface acid-base properties. In this study, we synthesized HA with needle-like or plate-like morphologies, characterized by Ca-rich or P-rich surface terminations, respectively. Then, we functionalized them with increasing amounts of Cu through an ion-exchange procedure. After a thorough characterization of their structural, morphological and redox properties, combining X-ray diffraction, electron microscopy and X-ray absorption spectroscopy, the materials were tested in the CO oxidation reaction. Pristine needle and plate HAs showed poor CO conversion (<5%), while all Cu-functionalized samples were catalytically active, reaching complete CO conversion at 573 K. The complete set of results highlighted that doped Ca-rich HAs exhibit better catalytic activity despite a lower copper content. This is explained by the easier reducibility of Cu2+ to Cu+ in these materials, as evidenced by IR measurements of CO adsorption.