Evidence for light-induced oxygen exchange in the oxidation of liquid hydrocarbons on oxygen 18-labelled titanium dioxide

Abstract

Titanium dioxide (TiO₂) nanoparticles labelled with ¹⁸O isotopes were prepared via a sol–gel method with titanium alkoxide and ¹⁸O water precursors. The presence of ¹⁸O in the labelled TiO₂ sample was confirmed using Raman spectroscopy and secondary-ion mass spectrometry. The photocatalysis of cyclohexane under UV light was investigated using ¹⁸O-labelled TiO₂ as the catalyst; this reaction yielded ¹⁸O-containing cyclohexanone as the major product. The ¹⁸O content in the cyclohexanone product dramatically decreased with increasing reaction time. This isotopic experiment demonstrates that the oxygen atom in cyclohexanone originates from surface oxygen in ¹⁸O-labelled TiO₂. Similar photoreactions performed with toluene and para-xylene as the hydrocarbon substrates afforded oxidation products containing ¹⁸O isotopes. The migration of oxygen atoms in ¹⁸O-labelled TiO₂ into liquid hydrocarbons provides direct evidence for the involvement of the solid oxide in the interfacial reaction between a liquid substrate and surface oxygen in a solid catalyst.