Kinetic studies in heterogeneous photocatalysis 4. The photomineralization of a hydroquinone and a catechol

The photocatalysed mineralization of methylhydroquinone (MHQ) and 4-methylcatechol (4-MCC) was examined in air-equilibrated, irradiated (UV-visible light) aqueous suspensions of TiO2 (Degussa P 25) at pH 3. The temporal course of the degradation was followed by high performance liquid chromatography and gas chromatography (for CO2 evolution), and also by diffuse reflectance spectroscopy for 4-MCC. Total disappearance of MHQ occurs in approximately 1 h via apparent first-order kinetics (k(app) almost-equal-to 0.041 +/- 0.003 min-1; t(1/2) = 17 min), followed by the concomitant stoichiometric evolution of CO2 which is complete after approximately 1 h of irradiation. 4-MCC is strongly adsorbed (approximately 53%) to the catalyst's surface, and its disappearance is complete within approximately 20 min of irradiation (confirmed by the diffuse reflectance spectra of the catalyst powder). Evolution of CO2 for 4-MCC occurs via first-order kinetics (k(app) almost-equal-to 0.025 +/- 0.005 min-1; t(1/2) = 28 min) and is complete after approximately 1.5 h of irradiation.