A QSAR approach for the understanding of photocatalytic oxidation mechanisms of two sets of VOCs: alkanes and C4- molecules
Résumé
With the COVID-19 pandemic, indoor air quality (IAQ) has received even more attention, particularly in terms of methods to prevent transmission of the coronavirus. One solution to provide a healthy indoor environment for occupants’ safety is based on adequate ventilation and air purification techniques. Along with particulate matter and microbial contaminants, volatile organic compounds (VOCs) represent the main common type of air pollutants encountered indoor. Among numerous possible treatments applicable to indoor air purification, gas-phase photocatalytic oxidation using TiO2 as photocatalyst is still a convenient innovative technology. However, the mechanism and the nature of the photocatalytic degradation reaction of the pollutant under consideration still have to be better understood. As for mixtures of VOCs, the reactivities of the individual molecules are also needed. We present here the results of a study performed several years ago in collaboration with much regretted Professor Dionysios D. Dionysiou (University of Cincinnati, USA) and Professor Bertrand Illien (Université de la Réunion, France). The aim of this study was to investigate the adsorption and photocatalytic oxidation mechanisms of several VOCs. This was achieved by the combination of an experimental work together with theoretical calculations. Nanocrystalline TiO2 photocatalysts with high photocatalytic activity developed by Professor Dionysios D. Dionysiou’s group were used. Several common indoor VOCs having different properties and reactivities were tested in dark conditions for adsorption isotherms and in a batch photocatalytic reactor for PCO kinetics. Two sets of VOCs were investigated: four VOCs containing 4 atoms of carbon (1-butanol, butyraldehyde, MEK and n-butane) and four alkanes (n butane, isobutane, pentane and hexane). The experimental results were modelled either by Langmuir equation or by Freundlich equation for adsorption isotherms of VOCs onto the TiO2 media and by Langmuir-Hinshelwood relation at the initial time t0. Equilibrium and kinetic constants were then calculated by linear regressions. In the same time, with the intention of determining the physical and chemical properties of the reactants, geometry optimizations of the compounds were performed with the B3LYP/6-31+G** hybrid density functional method. Then molecular descriptors as the molecular volume, the dipole moment µ, the HOMO and LUMO energies, the electrostatic potential, the solvation energy and the polarizability tensor were calculated. Finally, correlations between Langmuir, Freundlich and Langmuir-Hinshelwood constants and the determined molecular descriptors were examined using the QSAR methodology. Two main results could be highlighted from the obtained results. First, looking at the adsorption phenomenon of the tested molecules onto the mesoporous TiO2-anatase catalyst, it was suggested that the molecules’ adsorption is primarily governed by chemical adsorption onto the surface rather than physical adsorption in relation to the van der Waals forces. For high molecular concentration, dissolution or absorption process also occur. Secondly, it was shown that the reaction rate can be governed not only by the reactivity of the molecules with OH° radicals but also the affinity of the molecule with hydrophilic surface. It can be suggested that the Henry’s law constant, by extending the solvation free energy or the molecular electrostatic potential, may express the possible diffusion of the molecule in a water layer at the catalyst surface influencing the global kinetic rate.