Photocatalytic oxidation of cyclohexane by titanium dioxide: Catalyst deactivation and regeneration

Two commercially available TiO2 catalysts were compared in the selective photocatalytic oxidation of cyclohexane: Hombikat UV100 (as received (H), and after calcination at 600 °C (H600)), and Solaronix, S450. Hombikat UV100 shows the highest initial activity on a g−1 catalyst basis, followed by H600 and S450 with very similar activity profiles. All catalysts suffer from deactivation. By in situ ATR and DRIFT spectroscopy, it is demonstrated that the extent, nature, and thermal stability of carboxylates and carbonates formed on the surface of the three catalysts are quite different. The extent of carboxylate formation was significantly smaller on the surfaces of H600 and S450, when compared to H. The thermal stability of the surface species at 400 °C decreased in the order H > H600 > S450. Complete removal of carboxylate and carbonate species from the surface was only achieved in the case of S450, which resulted in complete regeneration of activity, as demonstrated in both a slurry reactor (top illumination reactor) and internally illuminated monolith reactor (IIMR). The differences in surface chemistry and regenerability are discussed on the basis of lattice defects, affecting the opto-electronic properties, and defects/irregularities on the surface, affecting (thermal) stability of surface-adsorbed species. Solaronix S450 is the preferred catalyst for the desired conversion, the favorable surface properties being an important first step toward the practical application of photocatalysis in selective liquid-phase photo-oxidation processes and in particular toward employment of reactors with immobilized TiO2 in the photocatalytic oxidation of cyclohexane.