Discrimination of infrared fingerprints of bulk and surface POH and OH of hydroxyapatites

Abstract

The identification of the nature of the surface acid base pairs present on hydroxyapatites surface is a key point to rationalize their catalytic properties. To investigate the possible involvement of acidic protonated phosphate (POH) and/or of basic OH emerging from the channels, the spectroscopic fingerprints of the surface sites have to be discriminated from bulk HPO42− and OH− groups. This was monitored by infrared spectroscopy (i) using isotopic H–D exchanges, either implemented in mild conditions (353–373K) or at higher temperature (573K) to selectively label the lone surface or to achieve deep deuteration, respectively (ii) following then the perturbation induced upon adsorption probe molecules. Two bulk contributions are observed in the νOH region, one at 3572cm−1 that is associated to OH− from the channels, and one at 3657cm−1 that was assigned to HPO42−. Five surface contributions perturbated upon CO adsorption at 77K can be ascribed to protonated forms of the various terminated phosphates. The formation of hydrogenocarbonate species upon CO2 adsorption evidences the presence of basic surface OH−. However, due to complex material restructuring occurring upon CO2 adsorption that greatly modifies the H-bonding interactions, the related νOH fingerprint could not be formally identified. Whatever, from the formation of low intense band at 2633 or 3572cm−1 under soft deuteration or protonation conditions respectively, it is inferred that the related bands could be ascribed to both surface and bulk OD or OH.