Enhancement of gas–liquid mass transfer during the unsteady-state catalytic decomposition of ozone in water

Unsteady mass transfer and kinetic experiments were conducted in a gas–liquid reactor to study the catalytic and non-catalytic decomposition of ozone in water. In the absence of catalyst, first-order decomposition constants and volumetric mass transfer coefficients were determined at temperatures between 20 °C and 30 °C. Catalytic runs were performed on fumed colloidal TiO2, which primary particles had an average diameter of 20 nm. The evolution of the concentration of ozone in water after introducing a given amount of catalyst was fitted to a kinetic model that assumed two-stage adsorption–decomposition on the catalytic surface. Runs were performed under slightly acidic conditions allowing the adsorption of ozone molecules on Lewis acid sites. A significant enhancement of mass transfer has been linked to the presence of particles. This effect could be attributed to a physical shuttle mechanism involving the adsorption of ozone on surface adhering particles. For bulk concentrations of catalyst up to 0.65 kg m−3, the fraction of interface coverage did not reach saturation. The observed behaviour is consistent with the hydrophilic character of titanium oxide. Reaction constants and activation energies were determined for the two chemical steps describing catalytic ozone decomposition.