In the present paper, kinetics and mechanism of NO and oxygen coadsorption on TiO2 at room temperature, which is the key step of the catalytic removal of NOx pollutants from air, were studied. NO adsorption on TiO2 in the absence of oxygen is weak and reversible, but it is found to strongly increase in the presence of oxygen. The ratio between the amount of adsorbed NO and O2 in the course of adsorption is constant and close to three. A FTIR spectroscopic study reveals that the amount and composition of N-containing species on the TiO2 surface strongly depend on the contact time with the initial NO–O2 mixture and on its composition. At relatively small exposures, IR bands assigned to NO– and nitrosyl complexes Ti n+ –NO (n = 3–4) are predominant in the spectra. With increasing contact time, NO– disappears, and IR bands of NO3 – and possibly NO2 – appear and grow. The thermal stability of surface nitrates and nitrites correlates with their structure. IR spectra observed upon NO2 adsorption are similar to those after exposure to NO–O2 mixtures. Exposure of the sample with preadsorbed 14NO2 to gaseous 15NO results in a change in the IR spectra that suggests isotopic replacement of 14N with 15N in the adsorbed species. In the TPD profiles, after adsorption of NO–O2 and NO2, desorption peaks of NO and NO2 dominate which presumably arise from the thermal decomposition of NO3 – (NO2 –) and nitrosyls Ti n+ –NO. A multistep scheme for the interaction of NO and O2 with TiO2 is suggested which accounts for the results of both techniques applied.

This Article reports on the thermo-and photostimulated effects on the optical properties of rutile titania ceramic layers fabricated in an air atmosphere by hightemperature calcination of (technical grade) titanium substrates. The so-formed layers peeled off spontaneously during the cooling phase back to ambient temperature to reveal a yellow-colored upper surface and a cream-colored bottom surface that was in contact with the titanium plate. The two surfaces of the layers and a powdered specimen (formed from grinding the peeled-off layers) were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, electron dispersive spectroscopy, and diffuse reflectance spectroscopy. The upper surface demonstrates a strong photochromic effect. A pronounced increase of the amplitude of the absorption bands at 2.06 eV (AB3) and 1.56 eV (AB4) seen under irradiation in the UV or visible spectral region and a strong decrease of these bands during the heating of irradiated samples to 200−230 °C were characteristics of the upper layer's surface. A wide set of spectra resulting from the reversible absorption changes made possible the disclosure of higher-energy absorption bands at 2.91 eV (AB1) and 2.54 eV (AB2); the latter were not affected by irradiation and heating. An electronic mechanism based on known properties of intrinsic point defects of TiO 2 , F-type centers (two electrons trapped in oxygen vacancies) and Ti 3+ centers, is proposed to account for the optical changes that occurred through the photoinduced formation, photobleaching, and thermal bleaching of the absorption bands.

As a continuation of our previous work on the kinetics of photocatalytic reduction of NO by CO on titanium dioxide, interaction of a Degussa P-25 TiO2 photocatalyst with NO, CO, and NO−CO mixtures at ambient temperature has been studied by FTIR and TPD. Only reversible weak adsorption of NO on surface Ti4+ ions is found to occur in the dark. UV−vis irradiation greatly enhances the NO adsorption on Ti4+ and yields N2O, NO−, NO2 −, and NO3 − surface species. After irradiation of TiO2 in a CO atmosphere, IR bands of surface CO2 − and CO3 − species appear in addition to a continuous IR absorption tail towards lower wavenumbers due to free carriers in the reduced semiconductor. When TiO2 is exposed to a equimolar NO−CO mixture, N2O and CO2 − are formed without irradiation supposedly by the reaction 2 NO + 2 CO + O2 − → 2 CO2 − + N2O. Subsequent light irradiation is accompanied by the accumulation of NO− and Ti4 +...NO complexes. No TiO2 reduction occurs in this case. FTIR spectra show that NO− produced by the photoinduced adsorption of NO can be eliminated by the following reaction: NO− + CO → italich normalν CO2 − + (1/2) N2. It is believed that this reaction is a key step in the nitrogen production by the photocatalytic process. The data obtained enable us to refine the earlier proposed reaction mechanism and to directly prove some of its key steps.

The interaction of oxygen molecules with TiO 2 (Degussa P25) surface under UV (λ = 365 nm) and vis (λ = 436 nm) irradiation at T = 293 K was investigated by means of massspectrometry and thermo-programmed desorption (TPD) spectroscopy. Oxygen chemisorbed on reduced TiO 2 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 TiO 2 . The UV or vis irradiation in 18 O 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 18 O 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 ∼2 × 10 11 cm −2 for oxidized TiO 2 and ∼10 8 cm −2 for reduced TiO 2 . There is a reason to consider the observed POIEq as a multiple interaction of O s − with 18 O 2 molecules (in fact by means of heteroexchange). Thus, the obtained POIEq rate corresponds to the number of O s − centers, while the POIEx 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 .

We herein report a real-time optical reflectance/absorption study of the photochromic behavior of visible light absorbing (yellow) titania carried out using a newly designed novel accessory for a fluorescence spectrophotometer. Yellow rutile titania, thermochemically synthesized from technical grade titanium substrate, displays a fully reversible sequence of electronic processes controllable by optical reflectance/absorption photostimulated by UV or visible light; three absorption bands appeared in the range 2.16−1.52 eV that could be thermally annealed at temperatures up to 600 K. To carry out real-time studies of these processes, a special device was designed and constructed which when combined with the fluorescence spectrophotometer allowed for the measurement of the changes in the sample's absorption, ΔA λ (t), at wavelength λ that corresponded to the maximum of the photoinduced absorption spectral bands under monochromatic light irradiation or temperature-programmed heating as well as light irradiation at a desired constant temperature. The dependences of ΔA λ (t) obtained under heating at a constant rate and represented in differential form (temperature-programmed absorbance annealing (TPAA) spectra) determined the main advantage of the device developed, since these spectra permit probing the energy levels of electron and hole traps within the band gap of the yellow titania system. The results of the present work show that TPAA spectra provide a new (in addition to the absorption spectra) quantitative characterization of photoactive materials that display photochromic properties. It is also demonstrated that the TPAA spectra are convenient in the numerical modeling of charge carrier dynamics in such metal-oxide semiconductors.