Catalytic supercritical water oxidation of model pollutants

We oxidized phenol, a model pollutant, in supercritical water using unsupported MnO₂, unsupported TiO₂, and CuO/Al₂O ₃ as catalysts in a tubular flow reactor. These catalysts have the desired effects of accelerating both phenol disappearance and CO₂ formation rates relative to conventional non-catalytic supercritical water oxidation (SCWO).; Rates of phenol disappearance and CO₂ formation are sensitive to phenol and oxygen concentrations, but independent of water density. A dual-site Langmuir-Hinshelwood-Hougen-Watson rate law and the Mars-van Krevelen rate law were used to correlate the catalytic kinetics for phenol disappearance over these catalysts. The results of comparative analyses using these rate relationships show that much smaller reactor volumes are needed for catalyzed than for non-catalyzed SCWO of aqueous solutions of phenol.; CuO exhibited a higher activity, on a unit-mass basis, for both phenol conversion and CO₂ formation than did MnO₂ or TiO ₂. Both MnO₂ and TiO₂ showed good stability and activity maintenance for periods of use up to 120 hours. CuO exhibited some initial deactivation, but otherwise maintained its activity throughout 100 hours of continuous use. Both Cu and AI were detected in the reactor effluent, which indicates the dissolution or erosion of the catalyst at reaction conditions. X-ray diffraction analyses revealed that phase or composition changes occurred for all three catalysts.; MnO₂ was also used as a catalyst for SCWO of acetic acid in aqueous solutions with complete oxidation to CO₂ occurring almost quantitatively. Rates of acetic acid disappearance exhibited saturation kinetics with respect to acetic acid, a maximum with respect to oxygen concentration, and were independent of water concentration. A global rate law that is both qualitatively and quantitatively consistent with these experimental results was identified. Based on comparisons of this catalytic rate law with literature rate laws for uncatalyzed SCWO of acetic acid, it is concluded that MnO ₂ can reduce reactor volumes required for oxidation of aqueous solutions of acetic acid by more than two orders of magnitude.