Surface studies of photocatalytic reactions of methanol on TiO2 anatase

Zusammenfassung in deutscher SpracheTitelübersetzung des Autors: Oberflächenphysikalische Untersuchungen der Photokatalyse von Methanol auf TiO2 AnatasDie photokatalytische Reaktion von Methanol auf der (101) Oberfläche von TiO2 Anatas wurde mittels Rastertunnelmikroskopie (STM), Temperatur-programmierter Desorption (TPD) und Röntgenphotoelektronenspektroskopie (XPS) untersucht. Bei einer niedrigen Oberflächenbedeckung zeigt Methanol keine photokatalytische Aktivität. Bei einer höheren Oberflächenbedeckung wurde Methylformiat als Produkt erhalten. Methoxy wurde von uns mittels einer Reaktion von Methanol und entweder vordosiertem Sauerstoff oder Hydroxygruppen erzeugt. Durch die Kombination von TPD und STM konnten wir Formaldehyd und Methanol als Reaktionsprodukte von UV bestrahltem Methoxy identifizieren. Letzteres Produkt deutet auf eine Rückreaktion von Methoxy zu Methanol hin.The photocatalytic reaction of methanol on the TiO2 anatase (101) surface was studied under UV irradiation by means of scanning tunneling microscopy (STM), temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). While low coverages of pure methanol showed no photocatalytic activity, at higher coverages methyl formate appeared as a secondary product. We created methoxy via a reaction of methanol with either predosed oxygen or terminal OH-groups. From the combination of TPD and STM measurements, we successfully identified formaldehyde and methanol as a secondary product of UV irradiated methoxy. The latter product suggests a reverse reaction from methoxy back to methanol.9

Following the Reduction of Oxygen on TiO₂ Anatase (101) Step by Step

We have investigated the reaction between O2 and H2O, coadsorbed on the (101) surface of a reduced TiO2 anatase single crystal by scanning tunneling microscopy, density functional theory, temperature-programmed desorption, and X-ray photoelectron spectroscopy. While water adsorbs molecularly on the anatase (101) surface, the reaction with O2 results in water dissociation and formation of terminal OH groups. We show that these terminal OHs are the final and stable reaction product on reduced anatase. We identify OOH as a metastable intermediate in the reaction. The water dissociation reaction runs as long as the surface can transfer enough electrons to the adsorbed species; the energy balance and activation barriers for the individual reaction steps are discussed, depending on the number of electrons available. Our results indicate that the presence of donor dopants can significantly reduce activation barriers for oxygen reduction on anatase

Methanol on Anatase TiO₂ (101): Mechanistic Insights into Photocatalysis

The photoactivity of methanol adsorbed on the anatase TiO2 (101) surface was studied by a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations. Isolated methanol molecules adsorbed at the anatase (101) surface show a negligible photoactivity. Two ways of methanol activation were found. First, methoxy groups formed by reaction of methanol with coadsorbed O2 molecules or terminal OH groups are photoactive, and they turn into formaldehyde upon UV illumination. The methoxy species show an unusual C 1s core-level shift of 1.4 eV compared to methanol; their chemical assignment was verified by DFT calculations with inclusion of final-state effects. The second way of methanol activation opens at methanol coverages above 0.5 monolayer (ML), and methyl formate is produced in this reaction pathway. The adsorption of methanol in the coverage regime from 0 to 2 ML is described in detail; it is key for understanding the photocatalytic behavior at high coverages. There, a hydrogen-bonding network is established in the adsorbed methanol layer, and consequently, methanol dissociation becomes energetically more favorable. DFT calculations show that dissociation of the methanol molecule is always the key requirement for hole transfer from the substrate to the adsorbed methanol. We show that the hydrogen-bonding network established in the methanol layer dramatically changes the kinetics of proton transfer during the photoreaction

Following the Reduction of Oxygen on TiO₂ Anatase (101) Step by Step

We have investigated the reaction between O2 and H2O, coadsorbed on the (101) surface of a reduced TiO2 anatase single crystal by scanning tunneling microscopy, density functional theory, temperature-programmed desorption, and X-ray photoelectron spectroscopy. While water adsorbs molecularly on the anatase (101) surface, the reaction with O2 results in water dissociation and formation of terminal OH groups. We show that these terminal OHs are the final and stable reaction product on reduced anatase. We identify OOH as a metastable intermediate in the reaction. The water dissociation reaction runs as long as the surface can transfer enough electrons to the adsorbed species; the energy balance and activation barriers for the individual reaction steps are discussed, depending on the number of electrons available. Our results indicate that the presence of donor dopants can significantly reduce activation barriers for oxygen reduction on anatase

Formaldehyde Adsorption on the Anatase TiO 2

Formaldehyde (CH2O) adsorption on the anatase TiO2(101) surface was studied with a combination of experimental and theoretical methods. Scanning tunneling microscopy, noncontact atomic force microscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy were employed on the experimental side. Density functional theory was used to calculate formaldehyde adsorption configurations and energy barriers for transitions between them. At low coverages (<0.25 monolayer), CH2O binds via its oxygen atom to the surface 5- coordinated Ti atoms Ti5c (monodentate configuration). At higher coverages, many adsorption configurations with comparable adsorption energies coexist, including a bidentate configuration and paraformaldehyde chains. The adsorption energies of all possible adsorption configurations lie in the range from 0.6 to 0.8 eV. Upon annealing, all formaldehyde molecules desorb below room temperature; no other reaction products were detected

Following the Reduction of Oxygen on TiO₂ Anatase (101) Step by Step

We have investigated the reaction between O₂ and H₂O, coadsorbed on the (101) surface of a reduced TiO₂ anatase single crystal by scanning tunneling microscopy, density functional theory, temperature-programmed desorption, and X-ray photoelectron spectroscopy. While water adsorbs molecularly on the anatase (101) surface, the reaction with O₂ results in water dissociation and formation of terminal OH groups. We show that these terminal OHs are the final and stable reaction product on reduced anatase. We identify OOH as a metastable intermediate in the reaction. The water dissociation reaction runs as long as the surface can transfer enough electrons to the adsorbed species; the energy balance and activation barriers for the individual reaction steps are discussed, depending on the number of electrons available. Our results indicate that the presence of donor dopants can significantly reduce activation barriers for oxygen reduction on anatase

Teaching Languages for Specific and Academic Purposes in Higher Education - English, Deutsch, Italiano

The international symposium “Teaching Languages for Specific and Academic Purposes” aimed to bring together university instructors and researchers of LSP and LAP in order to share and explore approaches, methods, and practices which have emerged within diverse contexts. Topics include needs analyses, TBLT, curriculum development, collaborative reflective writing, correction codes, specific communication skills and competences, institutional challenges, technology and digital literacies, word lists, reading skills, textbooks, L1 transfer, academic writing styles in L1 versus L2, assessment, and motivation