Hydroxyapatite/titanium dioxide nanocomposites for controlled photocatalytic NO oxidation

Abstract

Biphasic photocatalytic powders consisted of hydroxyapatite (HA) and titanium dioxide (TiO2) were prepared by precipitation of HA in presence of TiO2 (Evonik-Degussa P25). Depending on the volume ratio between HA and TiO2 in the initial solution the materials showed different textural properties. Aggregates consisted of spherical particles were formed at low HA/TiO2 ratio, while the decrease of the TiO2 amount resulted in formation of needle and lath like particles. The specific surface area (ssa) of the composites was higher than the ssa of the pure components and increased with the decrease of the TiO2 amount. The novel materials exhibited excellent activity in photocatalytic NO oxidation, while the activity in NO2 removal, either by oxidation or by adsorption to the HA, was extremely low for the composites with volume ratio VHA/VTiO2 more than 1. The selective behavior of these composites towards the two steps of the NOx oxidation process was related to the preparation procedure followed. The composite with 50% (v/v) TiO2 revealed higher OH formation ability and photocatalytic activity in overall NOx removal than the pure TiO2 component that was ascribed to the specific VHA/VTiO2 ratio and growth of HA on the TiO2 surface.