Photo-catalytic oxidation of cyclohexane over TiO2: a novel interpretation of temperature dependent performance

Abstract

The rate of cyclohexane photo-catalytic oxidation to cyclohexanone over anatase TiO₂ was studied at temperatures between 23 and 60 °C by in situATR-FTIR spectroscopy, and the kinetic parameters were estimated using a microkinetic model. At low temperatures, surface cyclohexanone formation is limited by cyclohexane adsorption due to unfavorable desorption of H₂O, rather than previously proposed slow desorption of the product cyclohexanone. Up to 50 °C, the activation energy for photocatalytic cyclohexanone formation is zero, while carboxylates are formed with an activation energy of 18.4 ± 3.3 kJ mol⁻¹. Above 50 °C, significant (thermal) oxidation of cyclohexanone contributes to carboxylate formation. The irreversibly adsorbed carboxylates lead to deactivation of the catalyst, and are most likely the predominant cause of the non-Arrhenius behavior at relatively high reaction temperatures, rather than cyclohexane adsorption limitations. The results imply that elevating the reaction temperature of photocatalytic cyclohexane oxidation reduces selectivity, and is not a means to suppress catalyst deactivation.