Oxygen-Isotope Exchange between CO₂ and Solid Ti¹⁸O₂

The light–induced oxygen-isotope exchange between gaseous CO₂ and solid Ti¹⁸O₂ (anatase) and the spontaneous thermal isotope exchange that takes place between the vacuum-calcined solid Ti¹⁸O₂ and CO₂ were studied by gas-phase high-resolution Fourier transform infrared absorption spectroscopy over a period of several days. The absorption ro-vibrational spectra of all the measured carbon dioxide isotopologues were assigned and served as the quantification of the time-dependent isotope exchange between the oxygen atoms from the Ti¹⁸O₂ solid and the oxygen related to the gaseous CO₂. The C¹⁸O₂ was formed as the dominating final product with a minor content of C¹⁶O¹⁸O. The rate of oxygen-isotope exchange is highly sensitive to the conditions of the titania pretreatment; vacuum-annealed Ti¹⁸O₂ at 450 °C exhibited a very high spontaneous oxygen exchange activity with gaseous C¹⁶O₂. A mechanism for the ¹⁸O/¹⁶O exchange process is discussed at the molecular level. The photocatalytic formation of methane, acetylene, and C¹⁶O released from the Ti¹⁸O₂ surface was observed after irradiation by an excimer laser.