Novel photocatalysts for CO₂ reduction, which consist of a cadmium and zinc sulfide solid solution (Cd_1−xZn_xS), were successfully prepared by a simple two-step technique. The photocatalysts were characterized by X-ray diffraction, UV-VIS diffuse reflectance spectroscopy, and low-temperature N₂ adsorption techniques and were tested in the gas-phase photocatalytic reduction of CO₂ under visible light (λ = 450 nm). All the synthesized Cd_1−xZn_xS solid solutions were capable of enabling the chemical transformations of CO₂ under the conditions considered. Carbon monoxide was the major product during the CO₂ reduction over Cd_1−xZn_xS (x = 0–0.87). Methane and hydrogen were also detected in the gas phase in low amounts. The activity of the prepared samples and the distribution of the reduction products strongly depended on the actual cadmium to zinc ratio. The Cd_0.94Zn_0.06S photocatalyst showed the highest activity, 2.9 μmol CO per gram per hour, and selectivity, 95%, during CO₂ reduction under visible light in the presence of water vapor. The achieved values are very high for the sulfide-based photocatalysts.

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.