Identification of the role of surface acidity in the deactivation of TiO2 in the selective photo-oxidation of cyclohexane

Anatase-structured Ti_1−xZr_xO₂ materials with x = 0.00, 0.01 and 0.06, were prepared by a reverse microemulsion method, characterized, and tested as catalysts for the selective photo-oxidation of cyclohexane to cyclohexanone. In situ ATR-FTIR spectroscopy was used to evaluate the reaction. Zr incorporation into the anatase lattice enhances the surface acidity of TiO₂ without causing any significant structural or electronic modification. As expected, also the stability of surface adsorbed water, i.e. the hydrophilicity, was enhanced. The increase in the Brønsted acidity, together with the higher hydrophilicity, is shown to be detrimental for performance (selectivity and stability) in the selective photo-oxidation of cyclohexane. Apparently potential intrinsic catalytic advantages of having higher acidity are outweighed by (i) the enhanced number of water born OH radicals, inducing non-selective reactions, and (ii) enhanced hydrophilicity leading to slow desorption and consecutive oxidation of cyclohexanone.