Elucidating Surface Structure with Action Spectroscopy

Surface Action Spectroscopy, a vibrational spectroscopy method developed in recent years at the Fritz Haber Institute is employed for structure determination of clean and H2O-dosed (111) magnetite surfaces. Surface structural information is revealed by using the microscopic surface vibrations as a fingerprint of the surface structure. Such vibrations involve just the topmost atomic layers, and therefore the structural information is truly surface related. Our results strongly support the view that regular Fe3O4(111)/Pt(111) is terminated by the so-called Fetet1 termination, that the biphase termination of Fe3O4(111)/Pt(111) consists of FeO and Fe3O4(111) terminated areas, and we show that the method can differentiate between different water structures in H2O-derived adsorbate layers on Fe3O4(111)/Pt(111). With this, we conclude that the method is a capable new member in the set of techniques providing crucial information to elucidate surface structures. The method does not rely on translational symmetry and can therefore also be applied to systems which are not well ordered. Even an application to rough surfaces is possible

Surface oxygen Vacancies on Reduced Co₃O₄(100): Superoxide Formation and Ultra-Low-Temperature CO Oxidation

The activation of molecular oxygen is a fundamental step in almost all catalytic oxidation reactions.We have studied this topic and the role of surface vacancies for Co3O4(100) films with a synergistic combination of experimental and theoretical methods. We show that the as-prepared surface is Blayer terminated and that mild reduction produces oxygen single and double vacancies in this layer. Oxygen adsorption experiments clearly reveal different superoxide species below room temperature. The superoxide desorbs below ca. 120 K from a vacancy-free surface and is not active for CO oxidation while superoxide on a surface with oxygen vacancies is stable up to ca. 270 K and can oxidize CO already at the low temperature of 120 K. The vacancies are not refilled by oxygen from the superoxide, which makes them suitable for long-term operation. Our joint experimental/theoretical effort highlights the relevance of surface vacancies in catalytic oxidation reactions

Surface oxygen Vacancies on Reduced Co₃O₄(100): Superoxide Formation and Ultra-Low-Temperature CO Oxidation

The activation of molecular oxygen is a fundamental step in almost all catalytic oxidation reactions.We have studied this topic and the role of surface vacancies for Co3O4(100) films with a synergistic combination of experimental and theoretical methods. We show that the as-prepared surface is Blayer terminated and that mild reduction produces oxygen single and double vacancies in this layer. Oxygen adsorption experiments clearly reveal different superoxide species below room temperature. The superoxide desorbs below ca. 120 K from a vacancy-free surface and is not active for CO oxidation while superoxide on a surface with oxygen vacancies is stable up to ca. 270 K and can oxidize CO already at the low temperature of 120 K. The vacancies are not refilled by oxygen from the superoxide, which makes them suitable for long-term operation. Our joint experimental/theoretical effort highlights the relevance of surface vacancies in catalytic oxidation reactions

Surface action spectroscopy with rare gas messenger atoms

Action spectroscopy with inert gas messengers is commonly used for the characterization of aggregates in the gas phase. The messengers, often rare gas atoms or D2 molecules, are attached to the gas phase aggregates at low temperature. Vibrational spectra of the aggregates are measured via detection of inert gas desorption following a vibrational excitation by variable-energy infrared light. We have constructed an apparatus for the application of action spectroscopy to surfaces of solids with the aim of establishing a new method for the vibrational spectroscopy of surfaces and deposited clusters. Experiments performed for neon covered V2O3(0001) show that this method can provide information about surface vibrations. Besides the surface sensitive channel, there is also a bulk sensitive one as demonstrated with the example of CeO2(111) thin film data. Unlike infrared reflection absorption spectroscopy, normalization to a reference spectrum is not required for action spectroscopy data, and unlike high resolution electron energy loss spectroscopy, the action spectroscopy method does not suffer from moderate resolution nor from multiple excitations. Selective decoration of specific surface features with messenger atoms may be utilized to focus the spectroscopic information onto these features

CO oxidation on palladium field emitter: kinetic oscillations and bistability

CO oxidation on Pd field emitter was studied by means of field electron microscopy (FEM) in a temperature range between 350 and 450 K. The complicated phenomena of kinetic oscillations in the reaction rate and bistability were found to be well reflected in the electron emission current. Subsurface oxygen, the driving species of the oscillations, was found to form preferentially on the {110} regions from where the oscillations start. Under the reaction conditions, the CO dissociation was unavoidable and it was found to affect the reaction kinetics. Sometimes, the imaging process was complicated by the formation of subsurface oxygen that leads to the appearance of very bright regions and by the CO dissociation process that leads to partial poisoning of the surface by the formed carbon

Field ion microscopy study of CO oxidation on palladium field emitter: field effects and imaging mechanism

Field ion microscopy (FIM) was applied in an attempt to monitor the kinetic oscillations in the CO oxidation rate. However, major difficulties arise due to field effects that enhance the CO dissociation leading to rapid poisoning of the surface with the formed carbon. On the other hand, it was found that the ionization of molecular oxygen that acts as the imaging gas under reaction conditions, occurs preferentially on oxygen adsorption sites in agreement with the mechanism of resonant field ionization. The imaging mechanism based on local work function influences on field ionization efficiency was discussed in view of the data obtained and was shown to be incorrect

V-doped TiO2(110) : quantitative structure determination using energy scanned photoelectron diffraction

The surface structure of a novel vanadium-titanium dioxide epitaxial film (Ti1-xVxO2, x ~ 0.2) has been explored using V4+ 2p and Ti4+ 2p energy-scanned photoelectron diffraction (PhD). The determined structure is a rutile TiO2(110)-like surface, with V atoms substitutionally replacing some Ti atoms. The results show no evidence for significant preferential occupation by V atoms of any specific surface or sub-surface sites. LEED shows a (1x2) reconstruction to be present on the surface, and the PhD simulations do favour this being the dominant surface termination, although the reliability factor for simulations for a (1x1) termination falls just within the variance of the value for the preferred (1x2) structure. The V3+ and Ti3+ species were observed to occupy the same sites as the V4+ and Ti4+ species; V5+ species do not appear to occupy a single well-defined structural site

Méthode de calcul des densités et des enthalpies de cristaux ioniques moléculaires

Performances of energetic materials depend on their density, their solid-state of formation and their stoechiometry. The aim of this works is the development of model to estimate the density and the solid-state of formation of energetic salts, X-N+ NO[-3], from their developed formulate. Model using additive volumes for charges groupes is developed to predict crystal densities of molecular salts. On the one hand, the group contribution method of Ammon and Mitchell is extended molecular salts. On the other hand, a less empirical contribution method is presented. Solid state-heat of formation of a molecular salts depend on the anthalpie of the isolated ions and the cohesive energy of the crystal salts. The estimate the cohesive energy two approach are developed : a semi-empirical model using descriptors computed on isolated ions and the crystal packing of virtual crystal salts of ions. Moreover, implementation of the symetry-constrained in the self consistent charges density functional tight binbing model is developped. This routine may riffine virtual salts structure obtained by packing model. The last part shows applications of previous developed models to the prediction of the solid state of formation of nitrate salts.La densité et l'enthalpie de formation sont nécessaires pour calculer les performances propulsives de ces sels énergétiques envisagées comme constituants des propergols. Cette étude a pour objet la mise au point de méthodes de calcul pour évaluer la densité et l'enthalpie de formation de sels de nitrates énergétiques de type X-N+ NO[3-] à partir de la formule semi-developpée du composé. La densité est calculée à l'aide de modèles basés sur l'additivité des volumes des groupes molaires qui composent ces sels de nitrates. D'une part, la méthode de contribution de groupes de Ammon-Mitchell est étendue aux composés ioniques. D'autre part, une nouvelle méthode incrémentale moins empirique est mise au point. L'enthalpie de formation des sels organiques est obtenue par différence entre l'enthalpie des ions isolés qui composent le cristal et son enthalpie de sublimation. Cette dernière dépdend de l'énergie de cohésion du cristal et d'un terme dépendant de la température. Concernant l'énergie de cohésion, deux approches sont développées : d'une part une méthode semi-empirique rapide utilisant des descripteurs calculés à partir de la géométrie des ions isolés et d'autre part, une méthode basée sur la génération explicite de cristaux virtuels par empilement moléculaires. De plus l'implémentation de contraintes de symétrie dans le code de physique du solide SCC-DFTB est réalisée. Ce code ainsi optimisé pourrait permettre de raffiner la géométrie des structures cristallines obtenues par empilement. Enfin, une dernière partie présente l'application de ces méthodes au calcul de l'enthalpie de formation de sels de nitrates.TOURS-BU Sciences Pharmacie (372612104) / SudocSudocFranceF

Oscillating Catalytic CO Oxidation on a Platinum Field Emitter Tip: Determination of a Reactive Phase Diagram by Field Electron Microscopy

A [111]-oriented Pt field emitter tip is used to study bistability and oscillations of CO oxidation by field electron microscopy (FEM) under low-pressure conditions (pO2 = 4 × 10−4 Torr, pCO = 8 × 10−8 to 4 × 10−5 Torr) in the temperature range 240 to 450 K. A kinetic phase diagram can be established, obtained on an FEM level with ≍2 nm lateral resolution. Below T ≍ 300 K a region of bistability prevails, and above T ≍ 300 K an oscillatory regime is obtained, thus yielding a cross-shaped phase diagram. These phenomena are surface plane specific and visible at the {011} planes of Pt and their surroundings