Surface acidity of calcium phosphate and calcium hydroxyapatite: FTIR spectroscopic study of low-temperature CO adsorption

The surface properties of calcium phosphate precursor (CP) and crystalline calcium hydroxyapatite (HA) prepared biomimetically have been studied by IR spectroscopy of adsorbed CO. Both samples are characterized by the absence of Brønsted acidity. Low-temperature CO adsorption on CP evacuated at 523 K leads to formation of only one family of Ca2+–CO species (2168 cm− 1). The analysis indicates that the respective calcium ions on the surface are not isolated. Similar spectra were obtained with HA evacuated at 573 K. In this case, however, the Ca2+–CO band was detected at 2165 cm− 1 due to enhanced lateral interaction between the adsorbed CO molecules. Another family of Ca2+ sites (Ca2+–CO band at 2178 cm− 1) was created after evacuation of the HA sample at 673 K. These sites were assumed to be a result of sample dehydroxylation. The results demonstrate the absence of any protonic acidity of the samples (i.e. P–OH surface groups) and weak electrostatic Lewis acidity caused by coordinatively unsaturated Ca2+ cations.