Investigation of Photostimulated Oxygen Isotope Exchange on TiO2 Degussa P25 Surface upon UV-Vis Irradiation

The interaction of oxygen molecules with TiO2 (Degussa P25) surface under UV (lambda = 365 nm) and vis (lambda = 436 nm) irradiation at T = 293 K was investigated by means of mass-spectrometry and thermo-programmed desorption (TPD) spectroscopy. Oxygen chemisorbed on reduced TiO2 surface consists of molecular species O-2(-) and atomic ones. The adsorbed O-2 species which are stable at 293 K are not observed on oxidized TiO2. The UV or vis irradiation in O-18 enriched oxygen induces an intensive photostimulated oxygen isotope equilibration (POIEq) and exchange (POIEx) via a weakly bonded intermediate O-3(-) due to the interaction of O-18(2) with a hole center O-s(-) (which is an exchangeable anion O-s(2-) that captured a photogenerated hole h(+)). The number of surface exchangeable oxygen anions is similar to 2 x 10(11) cm(-2) for oxidized TiO2 and similar to 10(8) cm(-2) for reduced TiO2. There is a reason to consider the observed POIEq as a multiple interaction of O-s(-) with O-18(2) molecules (in fact by means of heteroexchange). Thus, the obtained POIEq rate corresponds to the number of O-s(-) centers, while the POLEx rate is proportional to the rate of O-s(-) formation. The kinetics of activation/deactivation of O-s(-) in POIEq was studied in the flow-through mode (at 10(-6)-10(-3) Torr). Under vis irradiation, the hole centers O-s(-) are formed in noticeable quantities and live for hundreds of seconds at T = 293 K, while the UV light transforms only a small part of all possible O-s(2-) into O-s(-). The lifetime of the latter is short. The lifetime of O-s(-) reduces with the increase of temperature or O-2 pressure. Two pathways of O-s(-) deactivation are supposed: the first one includes the recombination of O-s(-) with an electron with an estimated energetic barrier E-des = (0.35 +/- 0.06) eV; and the second one is a result of collision of intermediate O-3(-) with gaseous O-2.