Photocatalytic Reduction of CO2 Using Titanium-Substituted and Fluorine-Doped Titanium-Substituted Hydroxyapatite as Photocatalysts

Abstract

Reduction of carbon dioxide (CO2) using semiconductor photocatalysts has aroused a great deal of attention, and most studies have applied titanium dioxide (TiO2) and modified TiO2 as photocatalysts in this process. In this study, we used Ti-substituted hydroxyapatite (TiHAP), an emerging photocatalytic material with a high adsorption capacity, as a novel photocatalyst to investigate the photocatalytic reduction of gaseous CO2 under ultraviolet (UV) light with water as the reductant. Powders of TiHAP and fluorine-doped TiHAP (TiFHAP) were prepared using a hydrothermal method, and their morphologies, structure, and composition were characterized via transmission electron microscopy, X-ray diffraction, UV–Vis spectrophotometry, photoluminescence fluorescent spectrometry, and X-ray photoelectron spectroscopy. Compared to commercial P25-TiO2, TiHAP provided enhanced photocatalytic reduction of CO2 into carbon monoxide (CO) and methane (CH4), due to its higher conduction band position. Fluorine doping further facilitated the reaction activity of TiFHAP, with 5.6 wt% fluorine providing the optimal results. The relative humidity of the reaction system had a substantial effect on the CO2 reduction process. Higher relative humidity ensured greater participation of protons, which was confirmed by the increasing formation rates of CO and CH4. Furthermore, a high CO formation rate always accompanied high CH4 formation selectivity, indicating that deep CO2 reduction can proceed rapidly over TiFHAP samples. This study supports the idea that TiHAP may be a novel and efficient photocatalyst for the reduction of CO2.