Photocatalytic oxidation of acetonitrile in gas-solid and liquid-solid regimes

Photocatalytic degradation of acetonitrile was carried out in both gas–solid and liquid–solid regimes using two commercial TiO2 catalysts (Merck and Degussa P25). For the gas–solid regime, a continuous annular photoreactor was used. The influence on photodegradation kinetics of the gas flow rate and concentrations of acetonitrile, oxygen, and water was investigated. Acetonitrile degradation products detected in the gas phase included carbon dioxide and hydrogen cyanide. The same photoactivity was exhibited in the presence and in the absence of water vapour. The liquid–solid regime used a batch photoreactor with an immersed lamp (the same as for the gas–solid regime). The oxidation products detected in the solution were cyanide, cyanate, nitrite, nitrate, methanoate, and carbonate ions. The Langmuir–Hinshelwood kinetic model fit the photoreactivity data obtained in both regimes and allowed us to determine the rate constant and equilibrium adsorption constant values. The adsorption constant and kinetic constant value were lower in the liquid–solid regime than in the gas–solid regime. The Merck catalyst had higher values of these parameters for both regimes than the Degussa P25 catalyst. An evaluation of the possible competition between acetonitrile and water molecules for the surface sites of the photocatalyst (Ti4+ ions and hydroxyl groups) revealed that for high H2O/CH3CN ratios, as is typical for the photo-oxidation process carried out in a liquid/solid regime, acetonitrile molecules were not able to provide a specific interaction with the surface sites of TiO2, remaining dissolved in the interface water molecular layers. In contrast, for low H2O/CH3CN ratios, as is typical for the photo-oxidation process carried out in a gas–solid regime, acetonitrile could win the competition with water for surface hydroxyls.