Purification of air from the organic contaminants by the photocatalytic process has been confirmed to be very perspective. Although many various photocatalysts have been prepared and studied so far, TiO₂ is still the most commonly used, because of its advantageous properties such as non-toxicity, relatively low cost and high stability. Surface modifications of TiO₂ were extensively proceeded in order to increase photocatalytic activity of the photocatalyst under both UV and visible light activations. The intention of this review paper was to summarize the scientific achievements devoted to developing of TiO₂-based materials considered as photocatalysts for the photocatalytic degradation of acetaldehyde in air. Influence of the preparation and modification methods on the parameters of the resultant photocatalyst is reviewed and discussed in this work. Affinity of the photocatalyst surfaces towards adsorption of acetaldehyde will be described by taking into account its physicochemical parameters. Impact of the contact time of a pollutant with the photocatalyst surface is analyzed and discussed with respect to both the degradation rate and mineralization degree of the contaminant. Influence of the photocatalyst properties on the mechanism and yield of the photocatalytic reactions is discussed. New data related to the acetaldehyde decomposition on commercial TiO₂ were added, which indicated the different mechanisms occurring on the anatase and rutile structures. Finally, possible applications of the materials revealing photocatalytic activity are presented with a special attention paid to the photocatalytic purification of air from Volatile Organic Compounds (VOCs).

The aim of the research was to determine how the admixture of nanosilica affects the structure and mechanical performance of cement concrete exposed to high temperatures (200, 400, 600, and 800 °C). The structural tests were carried out on the cement paste and concrete using the methods of thermogravimetric analysis, mercury porosimetry, and scanning electron microscopy. The results show that despite the growth of the cement matrix’s total porosity with an increasing amount of nanosilica, the resistance to high temperature improves. Such behavior is the result of not only the thermal characteristics of nanosilica itself but also of the porosity structure in the cement matrix and using the effective method of dispersing the nanostructures in concrete. The nanosilica densifies the structure of the concrete, limiting the number of the pores with diameters from 0.3 to 300 μm, which leads to limitation of the microcracks, particularly in the coarse aggregate-cement matrix contact zone. This phenomenon, in turn, diminishes the cracking of the specimens containing nanosilica at high temperatures and improves the mechanical strength.

The titania pulp—a semi product received from the industrial production of titania white—was submitted for the thermal heating at 400–600 °C under Ar and H₂ to obtain TiO₂ with different structure and oxygen surface defects. Heating of titania in H₂ atmosphere accelerated dehydration and crystallisation of TiO₂ compared to heating in Ar. TiO₂ prepared at 500 and 600 °C under H₂ had some oxygen vacancies and Ti³⁺ centres (electron traps), whereas TiO₂ obtained at 450 °C under H₂ exhibited some hole traps centres. The presence of oxygen vacancies induced adsorption of atmospheric CO₂. It was evidenced, that ethylene reacted with TiO₂ after UV irradiation. Formic acid was identified on TiO₂ surface as the reaction product of ethylene oxidation. Hydroxyl radicals were involved in complete mineralisation of ethylene. TiO₂ prepared at 500 °C under H₂ was poorly active because some active sites for coordination of ethylene molecules were occupied by CO₂. The most active samples were TiO₂ with high quantity of OH terminal groups. At 50 °C, the physically adsorbed water molecules on titania surface were desorbed, and then photocatalytic decomposition of ethylene was more efficient. TiO₂ with high quantity of chemisorbed OH groups was very active for ethylene decomposition. The acidic surface of TiO₂ enhances its hydroxylation. Therefore, it is stated that TiO₂ having acidic active sites can be an excellent photocatalyst for ethylene decomposition under UV light.

Acetaldehyde decomposition was performed under heating at a temperature range of 25–125 °C and UV irradiation on TiO₂ doped by metallic Ni powder and TiO₂ supported on nickel foam. The process was carried out in a high-temperature reaction chamber, “The Praying Mantis^TM”, with simultaneous in situ FTIR measurements and UV irradiation. Ni powder was added to TiO₂ in the quantity of 0.5 to 5.0 wt%. The photothermal measurements of acetaldehyde decomposition indicated that the highest yield of acetaldehyde conversion on TiO₂ and UV irradiation was obtained at 75 °C. The doping of nickel to TiO₂ did not increase its photocatalytic activity. Contrary to that, the application of nickel foam as a support for TiO₂ appeared to be highly advantageous because it increased the decomposition of acetaldehyde from 31 to 52% at 25 °C, and then to 85% at 100 °C in comparison with TiO₂ itself. At the same time, the mineralization of acetaldehyde to CO₂ doubled in the presence of nickel foam. However, oxidized nickel foam used as support for TiO₂ was detrimental. Most likely, different mechanisms of electron transfer between Ni–TiO₂ and NiO-TiO₂ occurred. The application of nickel foam greatly enhanced the separation of free carriers in TiO₂. As a consequence, high yields from the photocatalytic reactions were obtained.

Preparation of TiO₂ using the hydrothermal treatment in NH₄OH solution and subsequent thermal heating at 500–700 °C in Ar was performed in order to introduce some titania surface defects. The highest amount of oxygen vacancies and Ti³⁺ surface defects were observed for a sample heat-treated at 500 °C. The presence of these surface defects enhanced photocatalytic properties of titania towards the deactivation of two bacteria species, E. coli and S. epidermidis, under artificial solar lamp irradiation. Further modification of TiO₂ was targeted towards the doping of Cu species. Cu doping was realized through the impregnation of the titania surface by Cu species supplied from various copper salts in an aqueous solution and the subsequent heating at 500 °C in Ar. The following precursors were used as a source of Cu: CuSO₄, CuNO₃ or Cu(CH₃COO)₂. Cu doping was performed for raw TiO₂ after a hydrothermal process with and without NH₄OH addition. The obtained results indicate that Cu species were deposited on the titania surface defects in the case of reduced TiO₂, but on the TiO₂ without NH₄OH modification, Cu species were attached through the titania adsorbed hydroxyl groups. Cu doping on TiO₂ increased the absorption of light in the visible range. Rapid inactivation of E. coli within 30 min was obtained for the ammonia-reduced TiO₂ heated at 500 °C and TiO₂ doped with Cu from CuSO₄ solution. Photocatalytic deactivation of S. epidermidis was greatly enhanced through Cu doping on TiO₂. Impregnation of TiO₂ with CuSO₄ was the most effective for inactivation of both E. coli and S. epidermidis.

TiO₂ was placed in heat-treatment at the temperature of 400–500 °C under flow of hydrogen gas in order to introduce some titania surface defects. It was observed that hole centers in TiO₂ were created during its heat treatment up to 450 °C, whereas at 500 °C some Ti³⁺ electron surface defects appeared. The type of titania surface defects had a great impact on the mechanism of acetaldehyde decomposition under irradiation of artificial visible light. Formation of O^•− defects improved both acetaldehyde decomposition and mineralization due to the increased oxidation of adsorbed acetaldehyde molecules by holes. Contrary to that, the presence of electron traps and oxygen vacancies in titania (Ti³⁺ centers) was detrimental for its photocatalytic properties towards acetaldehyde decomposition. It was proved that transformation of acetaldehyde on the TiO₂ with Ti³⁺ defects proceeded through formation of butene complexes, similar as on rutile-type TiO₂. Formed acetic acid, upon further oxidation of butene complexes, was strongly bound with the titania surface and showed high stability under photocatalytic process. Therefore, titania sample heat-treated with H₂ at 500 °C showed much lower photocatalytic activity than that prepared at 450 °C. This study indicated the great impact of titania surface defects (hole traps) in the oxidation of acetaldehyde and opposed one in the case of defects in the form of Ti³⁺ and oxygen vacancies. Oxidation abilities of TiO₂ seem to be important in the photocatalytic decomposition of volatile organic compounds (VOCs) such as acetaldehyde.