Optimization of process variables for the concurrent removal of aliphatic and aromatic volatile organic compounds over a copper impregnated titanium dioxide photocatalyst

Abstract

In this research, a copper-impregnated titanium dioxide (Cu@TiO₂) photocatalyst was prepared using a wet impregnation method and employed for the photocatalytic degradation of multicomponent gaseous systems (i.e., a mixture of seven aliphatic and four aromatic volatile organic compounds (VOCs)). The performance of Cu@TiO₂ was assessed in relation to the pretreatment conditions (i.e., nitrogen (N₂) + hydrogen (H₂) relative to N₂ only) and through the control of various process variables (e.g., VOC concentration, flow rate, and relative humidity) at a fixed photocatalyst mass (50 mg). The photocatalytic efficiency of Cu@TiO₂ was observed to improve with the increase in relative humidities (e.g., from 0% to 100%), although it was lowered with the increase of gas flow rates and VOC concentrations. The removal performance of Cu@TiO₂ was more apparent for aliphatic VOCs (e.g., isovaleraldehyde, valeraldehyde, butyraldehyde, and methyl isobutyl ketone) due to their enhanced surface interaction in the presence of active carbonyl groups. In contrast, the removal of aromatic VOCs was affected sensitively by the side alkyl (or alkene) substituents with the relative order of styrene (100%) > m-xylene (97%) > toluene (85%) > benzene (74%). Based on mass spectrometric analysis of treated gaseous samples, the degradation products of aliphatic VOCs were their corresponding acids, while those of aromatic VOCs were primarily alcohols and aldehydes with similarity indices >98% (and in concordance with Kovats retention indices). The photocatalytic performance of Cu@TiO₂, when assessed in terms of the figure of merit (FoM: molecules per photon per ppm per g) values, was distinguished between different groups such as the slightly enhanced propensity of aliphatic (1.89 × 10⁻⁵–3.41 × 10⁻⁶) over aromatic VOCs (1.53 × 10⁻⁵–1.41 × 10⁻⁶).