RESUMO
As the conduction band edge of rutile is close to the reduction potential of hydrogen, there is a long-lasting discussion on whether molecular hydrogen can be evolved from this semiconductor. Our study on methanol photoreforming in the ultra-high vacuum reveals that photocatalysts comprising a TiO2(110) single crystal decorated with platinum clusters indeed enable the evolution of H2. This is attributed to a new type of mechanism, in which the co-catalyst acts as a recombination center for hydrogen and not as a reduction site of a photoreaction. This mechanism is an alternative pathway to the commonly used mechanism derived from photoelectrochemistry and must particularly be considered for systems, in which reducible semiconductors enable the surface diffusion of hydrogen species.
RESUMO
The thermal and photochemical mechanistic pathways for tertiary alcohols on the rutile TiO2(110)-surface are studied with the example of tert-butanol. While the thermal reaction is known to yield isobutene, the photochemical ejection of a methyl radical is observed at 100 K. The C-C scission, which is accompanied by the formation of acetone, is the only photochemical reaction pathway at this temperature and can be attributed to the reaction of photoholes that are created upon UV-light illumination at the surface of the n-type semiconductor. At 293 K the selectivity of the reaction changes, as isobutene is additionally formed photochemically. A comparison of the kinetics of the different reactions reveals further insights. Together with the quantitative evaluation of the reaction products at low temperatures and the comparison of the reaction pathways at different temperatures it is demonstrated how thermal effects can influence the selectivity of the reactions in photocatalysis.
RESUMO
In this work, ethanol is used as a chemical probe to study the passivation of molecular beam epitaxy-grown GaN(0001) by surface oxidation. With a high degree of oxidation, no reaction from ethanol to acetaldehyde in temperature-programmed desorption experiments is observed. The acetaldehyde formation is attributed to a mechanism based on α-H abstraction from the dissociatively bound alcohol molecule. The reactivity is related to negatively charged surface states, which are removed upon oxidation of the GaN(0001) surface. This is compared with the Ga2O3(2¯01) single crystal surface, which is found to be inert for the acetaldehyde production. These results offer a toolbox to explore the surface chemistry of nitrides and oxynitrides on an atomic scale and relate their intrinsic activity to systems under ambient atmosphere.
RESUMO
Based on three different molecules dosed on a Pt(111) single crystal the selectivity and sensitivity of REMPI-TPD in UHV is investigated for a potential application in heterogeneous catalysis. It is shown that the two structural isomers ethylbenzene and p-xylene can be discriminated by REMPI in a standard TPD experiment. The latter is not possible for the ionization with electrons in a Q-MS. It is further demonstrated by benzene TPD studies that the sensitivity of the REMPI-TOF-MS is comparable to commercial EI-Q-MS solutions and enables the detection of less than 0.6% molecules of a monolayer.
RESUMO
In this work we present a stoichiometric reaction mechanism for the photocatalytic ethanol oxidation on TiO2(110). The reaction products are analyzed either under reaction conditions or after irradiation at lower temperatures. Water is identified as a quantitative by-product, which resides in a defect site. These water molecules cause a blocking of the defect sites which results in poisoning of the catalyst. By different preparation techniques of the TiO2(110) surface, the role of surface defects is further elucidated and the role of molecular oxygen is investigated. Based on the investigation, a complete photochemical reaction mechanism is given, which provides insights into general photon driven oxidation mechanisms on TiO2.