Enhancement mechanism of hydroxyapatite for photocatalytic degradation of gaseous formaldehyde over TiO₂/hydroxyapatite

Hydroxyapatite (HAP) was reported to promote photocatalytic degradation as a support for photocatalysts, but the enhancement mechanism is still unclear. In this study, the promotion role of HAP to TiO₂ in photocatalytic degradation is unraveled using experimental observations and kinetic modeling. TiO₂/HAP is successfully synthesized by a facile hydrothermal method, which is confirmed by SEM-EDS, TEM, ICP, XRD, and Raman. HAP exhibits a negative effect on the photo absorption ability of TiO₂/HAP, which is characterized by UV–vis reflectance spectra (Tauc plot). The formaldehyde-temperature programmed desorption results reveal an affinity between HAP and formaldehyde via a weak chemisorption, which is validated by the estimated adsorption enthalpy/entropy from Van't Hoff plot based on Langmuir–Hinshelwood model. The derived kinetic parameters, including reaction rate constant, Langmuir adsorption constant, apparent activation energy, and adsorption enthalpy and entropy, confirm the experimental results that TiO₂/HAP is more active than TiO₂. This work verifies the existence of superior chemisorption between HAP and VOCs leading to a better performance of TiO₂/HAP for photocatalytic degradation. It helps to understand the role of HAP as an effective support in designing novel HAP-based catalysts.