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Co-adsorption of oxygen and formic acid on rutile TiO2 (110) studied by infrared reflection-absorption spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.ORCID iD: 12751850400
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.ORCID iD: 0000-0003-0296-5247
2017 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 663, p. 47-55Article in journal (Refereed) Published
Abstract [en]

Adsorption of formic acid and co-adsorption with oxygen have been investigated on the rutile TiO2(110) surface using p- and s-polarized infrared reflection-absorption spectroscopy (IRRAS) at O2 exposures between 45 L to 8100 L and at temperatures between 273 K and 343 K. On the clean surface formic acid dissociates into a formate ion (formate) and a proton. Formate binds to two five-fold coordinated Ti atoms in the troughs along the [001] direction, and the proton binds to neighboring bridging O atoms. Exposure of adsorbed formate to O2 leads to a decrease in the asymmetric νas(OCO) band at 1532 cm−1 and to the concomitant formation of a new vibration band at 1516 cm−1. From the s-and p-polarized IRRAS measurements performed at different O2 exposures, surface pre-treatments and substrate temperatures, and by comparisons with previous reports, we conclude that the new species is a bidentate surface hydrogen carbonate, which is formed by reaction between formate and oxygen adatoms on the surface. The σv reflection plane of the surface hydrogen carbonate molecule is oriented along the [001] direction, i.e. the same direction as the adsorbed formate molecule. On the clean TiO2(110) surface exposed to O2 prior to formic acid adsorption, similar results are obtained. The reaction rate to form surface hydrogen carbonate from formate is found to follow first-order kinetics, with an apparent activation energy of Er=0.25 eV.

Place, publisher, year, edition, pages
2017. Vol. 663, p. 47-55
National Category
Engineering and Technology Condensed Matter Physics
Research subject
Engineering Science with specialization in Materials Science
Identifiers
URN: urn:nbn:se:uu:diva-332994DOI: 10.1016/j.susc.2017.04.012ISI: 000405043300007OAI: oai:DiVA.org:uu-332994DiVA, id: diva2:1162049
Funder
Swedish Research Council, 2010-3514Available from: 2017-12-01 Created: 2017-12-01 Last updated: 2018-10-03Bibliographically approved
In thesis
1. Infrared spectroscopy studies of adsorption and photochemistry on TiO2 surfaces: From single crystals to nanostructured materials
Open this publication in new window or tab >>Infrared spectroscopy studies of adsorption and photochemistry on TiO2 surfaces: From single crystals to nanostructured materials
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

TiO2 based photocatalysis is a green nanotechnology that can be used for removal of pollutants from water and air, as well as making synthetic fuels from water and carbon dioxide. Said photocatalysis has received major research interests during the last decades. Despite these efforts, many elementary processes that occur on the photocatalyst surface are not fully understood and, therefore, limit our ability to purposefully manufacture more efficient photocatalytic materials. The objective of this thesis is to provide new understanding at a molecular level of important adsorbate species on the TiO2 surfaces.

Fundamental properties of adsorption and photochemistry of primarily formic acid on different TiO2 surfaces, ranging from single crystals to nanoparticles, have been studied using infrared spectroscopy. A method to simulate IR spectra have been developed and, combined with experimental data, has been proven to be a powerful tool to identify different adsorbate geometries on the surface. In the presence of oxygen, a thermally activated and irreversible reaction between formate and oxygen adatoms takes place on the single crystal rutile (110) surface to yield hydrogen bicarbonate surface complexes. For disordered single crystal surfaces, the adsorption geometry of formate changes due to exposure of Ti3+ atoms on the surface, and the adsorption spectra shows resemblances with that observed for formate adsorption on nanocrystalline surfaces.

Illumination with UV light results in small changes of the formate coverage on the disordered single crystal and nanocrystalline rutile surfaces, whereas on the rutile (110) surface only miniscule changes in formate coverages are seen. This is due to the lack of oxygen electron acceptors and OH/H2O electron donors in the vacuum environment, which results in a much lower degradation rate compared to measurements made at ambient conditions. Furthermore, it is shown that the coordination of the formate molecule on various TiO2 surfaces has a profound effect on the photocatalytic degradation rate, with bidentate coordinated formate molecules being most resilient towards oxidation.

The results presented here shows that additional insight in the processes on the TiO2 photocatalyst surface can be obtained by combining spectroscopic studies of single crystals and nanocrystalline films and that it is possible to unravel adsorption geometries on surfaces by combining experimental and simulated IR spectra.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 117
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1726
National Category
Nano Technology Materials Engineering
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-362326 (URN)978-91-513-0456-4 (ISBN)
Public defence
2018-11-21, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2018-10-29 Created: 2018-10-03 Last updated: 2018-11-19

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The full text will be freely available from 2019-04-28 00:00
Available from 2019-04-28 00:00

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Österlund, Lars

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