Oxidation catalysts are modeled by oxide single crystals, thin oxide films, as well as supported oxide nanoparticles. We characterize the surface of those materials using a variety of surface sensitive techniques including scanning tunneling microscopy and spectroscopy, photoelectron spectroscopy, infrared spectroscopy, and thermal desorption spectroscopy. We find temperature dependent structural transformations from V 2 O 5 (001) to V 2 O 3 (0001) via V 6 O 13 (001). V 2 O 3 (0001) is found to be vanadyl terminated in an oxygen ambient and it loses the vanadyl termination after electron bombardment. It is shown that the concentration of vanadyl groups controls the selectivity of the methanol oxydehydrogenation towards formaldehyde. A proposal for the mechanism is made. The results on single crystalline thin films are compared with similar measurements on deposited vanadia nanoparticles. The experimental results are correlated with theoretical calculations and models.

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Vanadium oxide thin films were grown on Au(111) by the oxidation of vapor-deposited V layers with 50 mbar of oxygen. The structure, composition, and thermal stability of the films have been investigated with scanning tunneling microscopy, low energy electron diffraction, photoemission spectroscopy, near-edge X-ray absorption fine structure, and temperature-programmed desorption. Well-ordered V2O5(001) thin films containing very few point defects have been obtained. Although the films have the tendency to grow in large crystallites and “dewet” the interface layer, growth by multiple steps of V deposition and oxidation precludes this problem and leads to flat films having a surface with a low density of steps. The films are composed of rather large (∼20 nm), single crystalline, and (001)-oriented V2O5 domains which show some azimuthal disorder between themselves. The X-ray-induced surface reduction of the V2O5(001) films was investigated with STM. O vacancies do not form randomly on the surface but rather appear as pairs or rows, indicating a concerted reduction process. Upon heating in UHV, the films are stable up to 500 °C, and they start sublimating above this temperature. Significant thermally induced reduction of the films only occurs above 560 °C. Comparison between these results and published studies emphasizes the influence of surface contamination and beam damage on the thermal reduction of V2O5.

Hydroxy-mediated methoxy formation or stabilization is probably an important process in many methanol adsorption systems. Hydrogen atoms originating from the scission of the methanol O-H bond react with the substrate and form water. This process may result 1) in the production of additional surface defects as reactive centers for methoxy formation and 2) in the stabilization of methoxy groups by suppression of methanol formation.

Well ordered V 2 O 3 (0001) layers may be grown on Au (111) surfaces. These films are terminated by a layer of vanadyl groups which may be removed by irradiation with electrons, leading to a surface terminated by vanadium atoms. We present a study of methanol adsorption on vanadyl terminated and vanadium terminated surfaces as well as on weakly reduced surfaces with a limited density of vanadyl oxygen vacancies produced by electron irradiation. Different experimental methods and density functional theory are employed. For vanadyl terminated V 2 O 3 (0001) only molecular methanol adsorption was found to occur whereas methanol reacts to form formaldehyde, methane, and water on vanadium terminated and on weakly reduced V 2 O 3 (0001). In both cases a methoxy intermediate was detected on the surface. For weakly reduced surfaces it could be shown that the density of methoxy groups formed after methanol adsorption at low temperature is twice as high as the density of electron induced vanadyl oxygen vacancies on the surface which we attribute to the formation of additional vacancies via the reaction of hydroxy groups to form water which desorbs below room temperature. Density functional theory confirms this picture and identifies a methanol mediated hydrogen transfer path as being responsible for the formation of surface hydroxy groups and water. At higher temperature the methoxy groups react to form methane, formaldehyde, and some more water. The methane formation reaction consumes hydrogen atoms split off from methoxy groups in the course of the formaldehyde production process as well as hydrogen atoms still being on the surface after being produced at low temperature in the course of the methanol ? methoxy ? H reaction.

MoO₃ layers on Au(111) were prepared via oxidation of molybdenum at elevated temperature in an atmosphere of 50 mbar of O₂. Three different types of oxide structures were identified. Up to monolayer oxide coverage a structure with a c(4 × 2) unit cell relative to the Au(111) unit cell forms. This structure was previously identified as being similar to a monolayer of α-MoO₃.(1) At larger coverages of up to two layers an oxide with a 11.6 Å × 5 Å rectangular unit cell appears. Further increase in the coverage leads to the occurrence of crystallites of regular α-MoO3 with a very small density of defects. These crystallites grow with the (010) plane parallel to the substrate surface and with random azimuthal orientation leading to rings in the LEED pattern. With increasing layer thickness the crystallites start to coalesce until finally a closed film forms. These layers sublimate at temperatures between about 670 and 770 K with the MoO₃ aggregates sublimating at lower temperature than the bilayer and monolayer films

The identification of oxygen sites at metal oxide surfaces and the characterization of their properties is of great importance for an understanding of the catalytic activity of such materials and, thus, for a rational design of efficient and selective catalysts. In the case of the clean MoO 3 (010) surface we show that an unambiguous discrimination of the different reactive oxygen sites can be obtained by angle-resolved Near-Edge X-Ray Absorption Fine Structure (NEXAFS) combined with Density-Functional-Theory (DFT) based spectrum analyses for different photon polarization directions. In particular, we are able to unequivocally discriminate the characteristic spectral signatures of singly coordinated molybdenyl oxygen covering the topmost molybdenum layers from those of other oxygen centers that have very similar local environment and only differ by their spatial orientation in the crystal. Theoretical predictions are also successfully used to identify and interpret characteristic features in the NEXAFS spectra that arise from specific vacancy sites present at oxygen deficient surfaces.

The growth of vanadium oxide ultrathin films on Au(111) using physical vapor deposition of V and oxidation under 50 mbar of oxygen has been investigated with scanning tunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure analysis. Depending on the coverage, two different well-ordered coincidence monolayer structures with a V ∼5+ oxidation state are observed. These arrangements do not correspond to any known bulk structures, and they are determined by the interaction between the oxide layer and the Au(111) surface, a finding that highlights the ability of vanadium oxides to adopt different structures. After completion of the first layer, a two-layer thick film which wets the substrate and has a surface structure similar to that of V 6 O 13 (001) is formed. This film does not exhibit a well-defined azimuthal orientation, which leads to ringlike structures in the low-energy electron diffraction pattern. The formation of the V 6 O 13 (001)-like film is shown to precede the nucleation and growth of V 2 O 5 (001) crystallites at higher thicknesses. This finding is in contrast with the common assumption that V 2 O 5 islands start to grow as soon as the monolayer is completed.