In Situ ATR-FTIR Study on the Selective Photo-oxidation of Cyclohexane over Anatase TiO2

Anatase-catalyzed photo-oxidation of cyclohexane was analyzed by in situ ATR-FTIR spectroscopy. A set of seven UV-LEDs (375 nm), with a photon flux of 9 × 10-9 Einstein·cm-2·s-1 (at the catalyst surface) was used to initiate the photoreaction. Surface-adsorbed cyclohexanone and water are the primary products of the photocatalytic reaction, formed with a photonic efficiency of 0.5 mmol·Einstein-1, through a cyclohexyl-hydroperoxide intermediate. Desorbed cyclohexanone and surface carboxylates and carbonates become dominant in the subsequent stages of the reaction, leading to deactivation of the catalyst. The carboxylates and carbonates are most likely formed through nonselective peroxide oxidation and consecutive oxidation of adsorbed cyclohexanone by hydroxyl radicals. In the photocatalytic oxidation of D12-cyclohexane, D10-cyclohexanone (in the adsorbed state and dissolved in D12-cyclohexane) was formed at rates comparable to those of cyclohexanone. The absence of a kinetic isotope effect suggests that the reaction is not limited by the activation of cyclohexane but rather by the activation of oxygen. Desorbed D10-cyclohexanone was observed at earlier stages and in higher quantities as compared to desorbed cyclohexanone. This is tentatively explained by a higher water content of the applied D12-cyclohexane compared to cyclohexane, inducing cyclohexanone desorption.