Toluene Oxidation by Photocatalytic Paint Under Different Indoor Environmental Conditions and Its Competitive Effect in the Presence of Another VOC

A photocatalytic paint based on carbon-doped titanium dioxide as a pigment replacement was tested for the oxidation of volatile organic compounds (VOCs) under visible-light illumination in indoor environments. The photocatalytic paint was characterized by X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, and diffuse reflectance and transmittance spectral measurements. The effects of environmental conditions, including inlet concentration, relative humidity (RH), incident radiation flux, and residence time, on the degradation of toluene, an aromatic VOC, were investigated. At certain operating conditions, toluene degradation efficiency increased, from 38% to 61%, with the rise in the incident radiation flux from 6.86 to 42.34 W/m², while it declined with increasing RH from 10% to 70%. A one-directional heterogeneous model predicted these trends, highlighting the opposing effects of incident radiation flux and humidity on toluene oxidation kinetics. In binary mixtures of toluene and acetaldehyde, representing aromatic and aliphatic VOCs, respectively, the presence of acetaldehyde had a negligible effect on toluene conversion, whereas acetaldehyde degradation was significantly reduced—by 36% and 65%—when co-fed with 3.5 and 6.5 ppmv of toluene, respectively, indicating a strong competitive effect between pollutants. This study highlights the benefit of using photocatalytic paint for indoor air decontamination and provides new insights into its performance under more realistic environmental conditions, particularly in the presence of mixed VOCs.