PHOTOACTIVISED FORMS OF SURFACE OXYGEN OF OXIDENOUS CRYSTALS

Olga B. Kotova

INSTITUTE OF GEOLOGY, Russian Academy of Sciences, Ural Division, 54, Pervomaiskaya st., Syktyvkar, 167610, Russia
E-mail: secret@geo.Komi.ru

One of the methods of activation (modification) of surface of particles of raw minerals is selecting some active surface centres, which may be different surface oxygenous crystals.

Photoactivised forms of surface oxygen were investigated with the help of "marked" oxygen atoms. In order to determine the type of adsorbtion, we used the method, which consisted in revealing forms of oxygen adsorbed on photo-inducated hole centres ("anomalous" adsorbtion). It was done by comparison of two thermodesorbtion oxygen spectra, the spectrums obtained after irradiation of an oxygen-solid system; in one of the experiments the hole centres well blocked by preliminary hydrogen adsorbtion.

At T=77K the research was hampered by the presence of physically adsorbed oxygen, therefore irradiation of 18O2-sample system was carried out at T=110K. Photo-inducated heteroexchange was studied in the regime of thermodesorbtion. The exchange was tracked by the enrichment coefficient of a (a - the ratio of ¹⁸O₂ atoms to the total atom number N), by "heavy" oxygen in relation to the initial coefficient of the balanced gas mixture enrichment.

Well-balanced by heteroexchange gas mixture was pumped in a reactor, then cooled to 110K. At this temperature the experiments are not yet markedly complicated by physically adsorbed oxygen. The system was irradiated for two minutes, during which time adsorbtion of oxygen was observed stimulated by light; what's more observed, the time of irradiation was only extremely weak transitory heteroexchange, which was expressed in slight variations of isotopic composition of oxygen in the gas phase. After that, unadsorbed gas was pumped out of the reactor, and the sample was further heated up to the regeneration temperature.

We attempted revealing thermodesorbtion peaks, which appearance is connected with the presence of "anomalously" adsorbed oxygen on the hole centres. Admission of hydrogen in to the reactor must result in destroying the hole centres and, accordingly, removing the peaks connected with "anomalously" adsorbed oxygen from TD-spectrum, weakly affecting maximums connected with photosorbed oxygen. Exposure to H2 after the sample under study has been irradiated in isotopic oxygen at T=110K, considerably decreases the peaks of "heavy" oxygen in the spheres T1 and T2, practically not changing the peaks of "light" oxygen in the sphere T2, and "light" and "heavy" oxygen in the sphere T3.

The results allow to assert that except normal photosorbtion, there is "anomalous" photosorbtion in the investigated oxides, including TiO2. It should be noted, that in the work deals with temperatures ranging from nitrogen up to room ones, and reveals one type of hole centres which belongs to the sphere T1. Thus, irradiation gives rise to two types of hole centres on the surface of investigated samples, which differ by the energy of interaction with "anomalously" sorbed oxygen.