Translation. Region: Russian Federal
Source: Novosibirsk State University – Novosibirsk State University –
Scientists from NSU have patented a catalyst for photooxidation of carbon monoxide (CO) under the influence of radiation of a wide spectral range. It effectively cleans gas-air mixtures and air from carbon monoxide at room temperature and is activated not only by ultraviolet radiation, like traditional photocatalysts used in this field, but also by natural light and by room light sources. The new catalyst also prevents the formation of carbon monoxide as a by-product during photocatalytic oxidation of a number of organic pollutants. In addition, it is capable of destroying chemicals, macromolecules, including DNA and RNA, and inactivating viruses and bacteria. Such a combined catalyst can be used in air purification systems and used as a photoactive coating for walls and other surfaces in office and residential premises. The authors of the invention emphasize that they created it in order to protect the environment and human health, since carbon monoxide is one of the most common pollutants.
— As a result of human activity, (350–600) 106 tons of CO enter the atmosphere annually, with more than half of this amount (56–62%) coming from motor vehicles, since the CO content in the exhaust gases of mobile vehicles can reach 12%. Carbon monoxide is dangerous for humans, since when its molecules interact with blood hemoglobin, carboxyhemoglobin is formed, which blocks the processes of oxygen transport and cellular respiration. A person’s presence in a closed space with a CO concentration in the air of more than 1250 mg/m3 for an hour leads to death. Fortunately, the concentrations of these pollutants in residential and work areas are usually low, but air purification is still necessary, because prolonged contact with these substances can lead to a deterioration in people’s well-being and harm their health, — the research fellow said. Scientific and Educational Center of the Institute of Chemical Technologies (INHIT) NSU, leading researcher of the photo- and electrocatalysis group of the Institute of Catalysis SB RAS Dmitry Selishchev.
Different approaches are used to solve the problem of removing molecular impurities from gas-air mixtures, but the most effective way to clean the air from small concentrations of pollutants under room conditions is photocatalytic oxidation. This oxidation method is based on the fact that under the action of light quanta with an energy exceeding the width of the forbidden zone of the semiconductor photocatalyst, electron-hole pairs are formed in the volume of the photocatalyst. The resulting electron and hole can migrate to the surface of the photocatalyst and participate in oxidation-reduction reactions with adsorbed compounds. This ensures a high oxidation rate even in the case of small concentrations of pollutants under room conditions.
— The photocatalytic method is one of the effective ways to clean indoor air from various types of pollutants. It is based on the use of special substances (photocatalysts), which, under the influence of light, provide oxidation of various types of pollutants, as a result of which they are transformed from the original state, when they are capable of harming human health, into harmless substances, such as, for example, carbon dioxide and water. This method is very effective in cleaning the air from organic types of pollutants. Traditional photocatalysts practically do not solve the problem of its purification from carbon monoxide, showing very low efficiency. To increase it to the desired level, we carried out a number of modifications of the photocatalysts we had previously developed by applying nanoparticles of noble metals – platinum and palladium – to their surface. These particles provide intermediate adsorption of carbon monoxide molecules, due to which the rate of their oxidation significantly increases, — explained Dmitry Selishchev.
The most famous semiconductor photocatalyst is titanium dioxide, which is inexpensive and virtually non-toxic, but at the same time allows for the complete oxidation of virtually any substance due to the formation of highly reactive particles. The main disadvantage of titanium dioxide as a photocatalyst is its fairly large band gap (3.0–3.2 eV), as a result of which it is activated only by ultraviolet radiation and is unable to absorb visible light quanta, which limits its area of application.
As Dmitry Selishchev explained, the most active photocatalysts are based on nanocrystalline titanium dioxide of a certain phase composition. In order to shift its spectrum of action along the wavelength scale to the visible light region, they proposed a synthesis method that provides for the introduction of additional nitrogen impurities, which lead to the appearance of additional energy levels and thus reduce the width of the forbidden zone. In this case, the catalyst is activated under the influence of light radiation of lower energy, i.e. with a longer wavelength.
— First-generation photocatalytic air purifiers were based on ultraviolet sources, such as low-pressure mercury lamps. Currently, mercury light sources are completely banned in a number of countries, while in other countries, restrictions are being introduced with the prospect of completely abandoning their use. Therefore, there was a need to transfer catalytic air purifiers from ultraviolet radiation sources to more efficient and safe, affordable LED sources. We set ourselves the task of creating a catalyst capable of activation in a wide spectral range. Other researchers had previously worked on it, using different approaches. We proposed combining the main advantages of titanium oxide catalysts, which provide high efficiency, with a modification of their structure by introducing a nitrogen impurity into it through the use of certain reagents and treatments, — explained Dmitry Selishchev.
The modified catalyst obtained in this way can be distinguished from its predecessor by color. The original catalyst is a white powder. This color indicates that it does not absorb, but reflects visible light. The modified catalyst turns yellow during manipulations. This means that it absorbs visible spectrum radiation. Such a catalyst is activated not only by visible light, but also by ultraviolet radiation, and can be used in new generations of devices for cleaning air from molecular impurities.
Another important disadvantage of titanium dioxide, like other traditional semiconductor photocatalysts, is its low adsorption capacity for carbon monoxide molecules, resulting in a low rate of photocatalytic oxidation and a low rate of air purification from impurities of this type. This is also important because CO can be formed as a by-product during photocatalytic oxidation of organic pollutants, especially aromatic compounds. This can result in secondary air pollution.
Since CO molecules are poorly sorbed on the surface, it was necessary to create additional centers for their absorption, as well as centers for the transfer of photogenerated charges. For this purpose, nanoparticles of noble metals – platinum and palladium – were introduced into the composition of the new photocatalyst, using certain reagents that ensure uniform distribution of nanoparticles over the surface of the photocatalyst.
— The new catalyst will find application in air purification systems. There are several large manufacturers on the Russian market who are interested in using a new type of catalyst to improve the efficiency of their products. NSU scientists work in cooperation with them and offer their developments for use in creating new generations of air purifiers. Also, the researchers of our laboratory are working on creating self-cleaning coatings for various materials and surfaces. These coatings consist of a photoactive component and binders. We intend to use the modernized catalyst in this area as well, in order to continuously provide passive cleaning of premises from carbon monoxide. Currently, testing of a wall coating based on a catalyst for photooxidation of carbon monoxide is underway in laboratory conditions, — said Dmitry Selishchev.
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