Titanium dioxide (TiO2) is a naturally occurring compound that has been used as a white pigment (titanium white) since ancient times. TiO2 is well known in the scientific community today mainly because of its demonstrated photocatalytic activity – that is, its ability to accelerate chemical reactions by absorbing light. The energy of sunlight can be used to initiate chemical reactions, such as splitting water into oxygen and hydrogen, converting carbon dioxide pollution into synthetic fuel, or breaking down toxic compounds in air, water, and soil. Due to its unique properties, titanium dioxide can break down water and ambient air pollutants using photocatalysis.
In a study conducted at Uppsala University in Sweden, it was shown that titanium dioxide (TiO2) as a semiconductor in photocatalysis process can be used to clean exhaust gas from sulfur compounds (mainly SO2).
The largest source of SO2 emissions to the atmosphere is the burning of fossil fuels by power plants and other industrial facilities. Less significant sources of SO2 emissions are industrial processes such as metal ore processing, the burning of sulfur-containing fuels by ships, construction machinery and other vehicles, and from natural sources such as volcanoes.
Swedish researchers have studied the surface properties of TiO2 and its interaction with sulfur dioxide (SO2). Monocrystalline rutile TiO2 (110) was used as a model surface to study basic surface interactions with SO2 using infrared reflectance-absorption spectroscopy (IRRAS). This is a technique by which the type of substance adsorbed on the surface, as well as its adsorbate structure, can be measured. This is achieved by reflecting linearly polarized infrared light from the surface at different azimuth angles and comparing the signal intensity after adsorption. The molecules then combine differently with the reflected light depending on the azimuth angle and the type of polarization used. The IRRAS technique provides unique information about the chemical composition, bond strength and orientation of molecules on surfaces.
Density functional theory (DFT) was also used as a complementary analysis tool in the study of photocatalytic reactions. This combination of the experimental part with the theoretical part enabled a detailed description of the reaction course and additionally showed how the basic reactivity of the TiO2 surface changes under the influence of light absorption.
The combination of theory and experimental results enabled a unique mapping of the interaction between SO2 and single crystalline rutile TiO2(110) in an ultra-high vacuum environment. With these results, it was again possible to analyze the data.
Sulfur dioxide (SO2) first formed sulfite-like surface forms and then reacted with free oxygen atoms and water to form surface-bound sulfate forms. These molecules, due to their acidic properties, enhance photocatalytic activity as well as induce hydrophilic (water-loving) properties.
In their earlier studies, the Swedish researchers showed that surface forms are formed after ultraviolet light irradiation of atmospheric oxygen at elevated temperatures. In contrast, their latest findings suggest that they can also form in the presence of water on an oxygen-deficient surface.