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Adsorption and reactions of sulfur dioxide on TiO2 surfaces: Fundamental studies on single crystals and nanoparticles
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.ORCID iD: 0000-0001-9589-7073
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Titanium dioxide (TiO2) is a material that is well-known among researchers in environmental science but is perhaps less well-known by the general public. It is commonly used in white paint because of its ability to scatter visible light, and for this reason it is manufactured at a large industrial scale. This is not the reason it has become famous, however, although the large abundance makes it interesting for use in industrial applications. It is famous for its ability to catalyze chemical reactions through the absorption of light. This process is called photocatalysis and it enables both uphill (photosynthetic) and downhill (photocatalytic) reactions since energy is transferred to the reacting molecules during the light absorption. The generation of H2gas from water and the formation of methane or methanol from carbon dioxide and water are two examples of photosynthetic reactions that are possible using TiO2. It can also be used for the degradation of pollutants, toxic compounds and bacteria, as well as for a number of technological applications, such as self-cleaning windows, solar cells and air purification filters.

This thesis work has primarily been focused on investigating the surface properties of TiO2and its interaction with sulfur dioxide (SO2). Single crystalline rutile TiO2(110) has been used as a model surface to study fundamental surface interactions with SO2by infrared reflection-absorption spectroscopy (IRRAS). This is a technique that can measure the type of species adsorbed on a surface and also its adsorbate structure. This is achieved by reflecting linearly polarized infrared light from a surface at different azimuth angles and comparing the signal intensity after adsorption. The molecules will then couple differently with the reflected light depending on the incident azimuth angle and the type of polarization used, which gives information about how the molecules are oriented on the surface. Work has also been done to study the structural modifications of TiO2nanoparticles after reduction through vacuum annealing. The induced changes of the crystal structure and lattice vibrations have been measured with X-ray diffraction and Raman spectroscopy, while the surface chemical properties have been studied via the adsorption of SO2. This was used as a probe molecule to detect the presence of surface defects, since SO2is known to react at defect sites while not binding to the stoichiometric surface at room temperature. The photo-reaction of SO2during UV illumination was also for the first time studied in an oxygen-free atmosphere. 

This work has provided new information about the fundamental surface interactions of SO2with single crystalline rutile TiO2(110). It has also provided valuable information about the defect distribution in reduced TiO2nanoparticles after preparation by vacuum annealing.

Place, publisher, year, edition, pages
Uppsala: Uppsala universitet, 2019. , p. 71
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:uu:diva-379974OAI: oai:DiVA.org:uu-379974DiVA, id: diva2:1298128
Presentation
2019-04-12, 2005, Ångström, Lägerhyddsvägen 1, 751 21, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2019-03-28 Created: 2019-03-21 Last updated: 2019-03-28Bibliographically approved
List of papers
1. SO2 adsorption on rutile TiO2(110): An infrared reflection-absorption spectroscopy and density functional theory study
Open this publication in new window or tab >>SO2 adsorption on rutile TiO2(110): An infrared reflection-absorption spectroscopy and density functional theory study
2018 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 677, p. 46-51Article in journal (Refereed) Published
Abstract [en]

The adsorption of SO2 on single crystalline TiO2(110) has been investigated by means of polarized infrared reflection-absorption spectroscopy (IRRAS) experiments and density functional theory (DFT) calculations. IR absorption bands were detected at 1324 cm(-1) and 985 cm(-1) with p-polarized light incident along both the [110] and [001] crystallographic directions at 123 K. When the temperature was increased to 153 K, the peak at 1324 cm(-1) disappears, while a new, weak band appears at 995 cm(-1). Simultaneously, a band at 995 cm(-1) also emerges with s-polarized light along the [110] direction. Based on the symmetry properties of the IRRAS spectra and accompanying ab initio simulations of the spectra employing a three layer model (vacuum-adsorbate-substrate), it is shown that the low temperature absorption IRRAS bands can be attributed to an SO3-like adsorbate structure. This is also the most stable adsorption structure (E-ad = - 0.58 eV) on the stoichiometric surface. The combined IRRAS and DFT results show that the band appearing at 995 cm(-1) is associated with a surface sulfite specie which is stabilized by residual surface water. The DFT calculations also revealed that a stable adsorption structure exists on a reduced TiO2 surface, where SO2 binds strongly to an oxygen vacancy site. It is suggested that this is an intermediate that form surface sulfate upon further reactions with water, although it was not observed on the stoichiometric surface studied in this work.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2018
Keywords
SO2, TiO2(110), SO3, Infrared reflection absorption spectroscopy, Density functional theory
National Category
Physical Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-369047 (URN)10.1016/j.susc.2018.05.016 (DOI)000447478700008 ()
Funder
Swedish Research Council, 2015 -04757
Available from: 2018-12-12 Created: 2018-12-12 Last updated: 2019-03-21Bibliographically approved

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