TiO2 mediated photocatalytic oxidation of volatile organic compounds: Formation of CO as a harmful by-product

Photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) including acetone, alcohols and hydrocarbons was investigated in a static reactor using an FTIR in situ method. Three commercially available TiO2 powders and one TiO2 sample synthesized via thermal hydrolysis of titanyl sulfate were employed in the kinetic experiments. PCO reactions were conducted under different UV light intensities, initial concentrations of substrates, humidities, and temperatures. The formation of the final oxidation products and the kinetics of their accumulation were investigated. Analysis of the FTIR spectra revealed the formation of CO along with CO2 and H2O as final products in the oxidation of VOCs over all the TiO2 samples. No other final products were detected. The method of spectral subtraction by minimizing the IR spectrum length was applied to accurately calculate the CO concentration. The conversion of organic substrates to CO did not exceed 5%. The differential selectivity of CO2 formation towards CO formation, which was defined as the initial CO2 rate divided by the sum of the initial CO2 and CO rates, was used to investigate the effect of the experimental parameters and the photocatalyst on the product distribution. The experimental conditions including light intensity, initial concentration and humidity as well as the photocatalyst and its pretreatment method did not exhibit a significant effect on the differential CO2 selectivity during acetone oxidation but an increase in the reaction temperature resulted in a decrease in the CO2 selectivity. A stronger influence was observed for the type of oxidizing substrate. The increase in the number of carbon atoms in the homologous series of C-2-C-4 alcohols and C-6-C-10 alkanes slightly decreased the CO2 selectivity. The most substantial effect was related to the extent of unsaturation in the C-6 hydrocarbons. The CO2 selectivity decreased from 98.9% for hexane to 93.3% for benzene. Benzene exhibited the highest conversion to CO (approximately 5%) among all the studied substrates. Only the deposition of Pt (1 wt.%) completely prevented the formation of CO during the photocatalytic oxidation of VOCs both at low and high UV intensity. Finally, the mechanism of CO formation is discussed. (C) 2016 Elsevier B.V. All rights reserved.