Kinetics of catalytic supercritical water oxidation of phenol over TiO₂

We oxidized phenol, a model pollutant, in supercritical water at 380-440 °C and 219-300 atm using bulk TiO₂ as a catalyst in a tubular flow reactor. The phenol conversion and CO₂ molar yield from this catalytic supercritical water oxidation (SCWO) are much higher than those from conventional noncatalytic SCWO of phenol under otherwise identical processing conditions. The selectivities to undesired phenol dimers decrease in the presence of TiO₂, and the catalyst is stable and maintains its activity during phenol oxidation. All of these features are desirable for a catalytic SCWO waste treatment process. The rate of phenol disappearance over TiO₂ was consistent with a power-law rate equation that is 0.69 order in phenol and 0.22 order in O₂. The rate of disappearance of total organic carbon (TOC) exhibited reaction orders of 0.51 for the TOC concentration and O for the oxygen concentration. Both rates are independent of the water concentration. The catalytic kinetics for phenol disappearance were also consistent with the Mars-van Krevelen mechanism and with a Langmuir-Hinshelwood dual-site mechanism comprising reversible adsorption of phenol on one type of catalytic site, reversible dissociative adsorption of oxygen on a different type of site, and irreversible, rate-determining surface reaction between adsorbed phenol and adsorbed oxygen. Our results show that the reactor volume for catalytic SCWO using TiO₂ would be about one-fourth that of the volume required for conventional, noncatalytic SCWO.