The development of active and stable photocatalysts for the degradation of volatile organic compounds under visible light is important for efficient light utilization and environmental protection. Titanium dioxide doped with nitrogen is known to have a high activity but it exhibits a relatively low stability due to a gradual degradation of nitrogen species under highly powerful radiation. In this paper, we show that the combination of N-doped TiO₂ with bismuth tungstate prevents its degradation during the photocatalytic process and results in a very stable composite photocatalyst. The synthesis of Bi₂WO₆–TiO₂-N composites is preformed through the hydrothermal treatment of an aqueous medium containing nanocrystalline N-doped TiO₂, as well as bismuth (III) nitrate and sodium tungstate followed by drying in air. The effect of the molar ratio between the components on their characteristics and photocatalytic activity is discussed. In addition to an enhanced stability, the composite photocatalysts with a low content of Bi₂WO₆ also exhibit an enhanced activity that is substantially higher than the activity of individual TiO₂-N and Bi₂WO₆ materials. Thus, the Bi₂WO₆–TiO₂-N composite has the potential as an active and stable photocatalyst for efficient purification of air.

This paper describes the chemical engineering aspects for the preparation of highly active and stable nanocomposite photocatalysts based on N-doped TiO₂. The synthesis is performed using titanium oxysulfate as a low-cost inorganic precursor and ammonia as a precipitating agent, as well as a source of nitrogen. Mixing the reagents under a control of pH leads to an amorphous titanium oxide hydrate, which can be further successfully converted to nanocrystalline anatase TiO₂ through calcination in air at an increased temperature. The as-prepared N-doped TiO₂ provides the complete oxidation of volatile organic compounds both under UV and visible light, and the action spectrum of N-doped TiO₂ correlates to its absorption spectrum. The key role of paramagnetic nitrogen species in the absorption of visible light and in the visible-light-activity of N-doped TiO₂ is shown using the EPR technique. Surface modification of N-doped TiO₂ with copper species prevents its intense deactivation under highly powerful radiation and results in a nanocomposite photocatalyst with enhanced activity and stability. The photocatalysts prepared under different conditions are discussed regarding the effects of their characteristics on photocatalytic activity under UV and visible light.

Composite TiO₂/activated carbon (TiO₂/AC) and TiO₂/SiO₂ photocatalysts with TiO₂ contents in the 10 to 80 wt. % range were synthesized by the TiOSO₄ thermal hydrolysis method and characterized by AES, BET, X-ray diffraction and FT-IR ATR methods. All TiO₂ samples were in the anatase form, with a primary crystallite size of about 11 nm. The photocatalytic activities of the TiO₂/AC and TiO₂/SiO₂ samples were tested in the gas-phase photocatalytic oxidation (PCO) reaction of diethyl sulfide (DES) vapor in a static reactor by the FT-IR in situ method. Acetaldehyde, formic acid, ethylene and SO₂ were registered as the intermediate products which finally were completely oxidized to the final oxidation products – H₂O, CO₂, CO and SO₄²⁻ ions. The influence of the support on the kinetics of DES PCO and on the TiO₂/AC and TiO₂/SiO₂ samples’ stability during three long-term DES PCO cycles was investigated. The highest PCO rate was observed for TiO₂/SiO₂ photocatalysts. To evaluate the activity of photocatalysts the turnover frequency values (TOF) were calculated for three photocatalysts (TiO₂, TiO₂/AC and TiO₂/SiO₂) for the same amount of mineralized DES. It was demonstrated that the TOF value for composite TiO₂/SiO₂ photocatalysts was 3.5 times higher than for pure TiO₂.

The review summarizes the current knowledge about heterogeneous semiconductor photocatalysts that are active towards photocatalytic reduction of carbon dioxide and molecular nitrogen under visible and near-UV light. The main classes of these photocatalysts and characteristic features of their application in the target processes are considered. Primary attention is given to photocatalysts based on titanium dioxide, which have high activity and stability in the carbon dioxide reduction. For the first time, the photofixation of nitrogen under irradiation in the presence of various semiconductor materials is considered in detail.The bibliography includes 264 references.

A new method of spectral subtraction for gas-phase Fourier transform infrared (FT-IR) spectra was developed for long-path gas measurements. The method is based on minimization of the length of the spectrum that results from subtracting the spectrum of an individual component of a gas mixture (water, CO ₂ , etc.) from the experimental spectrum of the mixture. For this purpose a subtraction coefficient ( k _min ) is found for which the length of the resulting spectrum is minimized. A mathematical simulation with two Lorentzian absorption bands was conducted and the limits of application for the proposed method were determined. Two experimental examples demonstrate that a successful result could be achieved in the case when the subtrahend spectrum contains a number of narrow absorption bands (such as the spectrum of water vapor).