Bernard Weber

Molecular beam study of the mechanism of carbon monoxide oxidation on platinum and isolation of elementary steps

Abstract Carbon monoxide oxidation is catalyzed by platinum even at room temperature. Attack of adsorbed oxygen proceeds through two mechanisms: an Eley-Rideal reaction involving carbon monoxide in the gas phase (or loosely adsorbed molecules) and a Langmuir-Hinshelwood reaction between strongly adsorbed species. Use of the molecular beam technique enabled us to isolate these two reactions and to determine their rate constants. The first reaction is not activated while the activation energy for the second is around 22 kcal/mol.

Etude par spectroscopies d'electrons, emission ionique secondaire et reactivite chimique des modes d'adsorption du monoxyde d'azote et de la vapeur d'eau sur rhenium

Abstract The adsorption of nitric oxide as well as water is characterized, on rhenium (strongly oriented parallel to (0001) plane) by both dissociative and molecular adsorption. For nitric oxide, two types of molecular states have been detected (stable till around 500 and 750 K), while for water, decomposition into oxygen (or OH) and hydrogen atoms occurs at much lower temperatures (below 300 K). Analysis of the adsorption states, using spectroscopic methods (XPS, AES, SIMS) leads to an interpretation of temperature programmed desorption spectra, based on repulsive interaction models and competitive or independent reactions, and to an extension of our previous model of the attack of transition metals by oxygen to the gasification of rhenium by these two oxygenated molecules.

États d'adsorption, vitesses d'adsorption, de désorption de l'oxygène sur rhénium et mécanismes d'atomisatton, d'oxydation à haute température et basse pression

Abstract We have studied interactions of oxygen with rhenium at low pressures (10 −4 to 10 −6 torr) in an ultra high vacuum apparatus where all desorption products, including instable and condensible species are analyzed by mass spectrometry. The chemical reactions depend on the surface coverage which is fixed by an equilibrium between the rate of adsorption and the rates of desorption steps, varying with temperature and pressure. We have measured these rates, independently, or directly (pressure variations after a flash, thermal desorption). When the coverage is lower than 0.7, the main reaction is oxygen atomization. For higher values, desorption of atomic oxygen is progressively replaced by desorption of rhenium trioxide, ReO 3 , main product of reaction. The following mechanism, based on the kinetics of the elementary steps, experimentally isolated, gives a quantitative picture of all the phenomena, for pressures between 10 −4 to 10 −6 torr, and sample temperatures varying from 1500 to 2300°K: (a) dissociative adsorption of oxygen; (b) desorption of atomic oxygen; (c) desorption of trioxide, only formed by surface reaction . The different rates depend on coverage, frequency factors and binding or activation energies. Optimization of the results in stationary conditions give the values of these parameters. The agreement with the values obtained directly is excellent.

Éléments d'un modèle d'interactions oxygène sous basse pression-métaux de transition à haute température

Abstract This paper presents a general model for the interaction of low pressure oxygen with transition metals at high temperatures, when all products are volatile. It is based on the following major hypotheses: (a) adsorption of molecular oxygen is the only direct process between the gas phase and the surface; (b) all chemical reactions occur in the adsorbed layer. This model is in agreement not only with our experimental results on platinum and rhenium but also with those of other authors on different metals (W, Mo. Ta, Nb). It becomes identical to Stickneys quasi-equilibrium theory when assuming that desorption activated complexes are identical to molecules moving freely in a plane parallel to the surface.

Interactions du chlore sous basse pression avec le tantale à haute température

Abstract We have studied interactions of chlorine under low pressure with tantalum at high temperatures in an ultra high vacuum apparatus where all desorption products, including unstable and condensible species are analyzed by mass spectrometry. At low chlorine coverage, atomization, characterized by a zero coverage reactive probability of 1 is the only observable process. When the coverage increases, desorption of atomic chlorine is progressively replaced by desorption of tantalum tetrachloride TaCl 4 . All the experimental results are consistent with a model, previously proposed for the oxygen-transition metals system. This model is based on the following elementary steps: (a) dissociative adsorption of chlorine; (b) desorption of atomic chlorine; (c) desorption of tetrachloride, only formed by surface reaction.

Growth of platinum on rhenium and evolution of the interface under thermal treatment

Abstract Platinum has been evaporated onto a rhenium ribbon at room temperature, and the growth of the platinum film and its subsequent evolution under heating followed using Auger and photoelectron (XPS and UPS) spectroscopy as well as work function measurements and thermal desorption mass spectroscopy. These measurements indicate a layer-by-layer growth mechanism at room temperature; the first layer has properties attributed to platinum-rhenium bonding, whereas beyond around two layers the surface is characteristic of pure platinum. Annealing the sample near 1400 K causes an interdiffusion of the two metals, although there appears to be a stabilization of the surface at a composition of approximately Pt04Re06. This stabilization cannot be explained in terms of the bulk phase diagram and so appears to be a uniquely surface effect which may provide a method for the preparation of Pt Re “surface alloys” requiring greatly reduced quantities of platinum.

Interactions du rhénium à haute température avec N2O sous basse pression

Abstract Starting at room temperature, N20 adsorption on rhenium proceeds dissociatively. Oxygen atoms remain on the surface while nitrogen molecules are desorbed. The overall process is characterized by an initial sticking coefficient value equal to 0.3 at 298 °K. In stationary conditions, and in a higher temperature range (> 1200°K) rhenium trioxide and oxygen atoms are the reaction products, depending on oxygen coverage on the surface. When the oxygen coverage is low, atomization, characterized by a reactive sticking probability of 0.2 is the only observable process. All the results are consistent with a model, previously proposed for the system oxygen-rhenium and oxygen-transition metals. The main differences in reaction rates between rhenium and oxygen or N 2 O are interpreted in terms of saturation coverages.

Étude par spectrométrie de masse en vision directe des interactions sous basse pression du pentachlorure de tantale avec le tantale à haute température

Abstract We have studied the kinetics of the pyrolysis of tantalum pentachloride at low pressure and high temperature using a tantalum ribbon placed in an ultra high vacuum apparatus; all of the reaction products were detected by means of line-of-sight mass spectrometry. For low chlorine surface coverages the reaction product is atomic chlorine, along with the formation of a tantalum deposit on the surface. At higher surface coverages tantalum tetrachloride is observed corresponding to an attack of the substrate. Together, these results were interpreted in terms of a theoretical model developed previously for the interactions between transition metals and oxygen containing molecules, then chlorine, and experimentally verified in all these cases. This model can be summarized by the existence of three steps: (i) the dissociative adsorption of the incident molecule, (ii) the desorption of atomic chlorine, and (iii) the desorption of the tetrachloride formed by surface reaction favored at high surface coverages.

Ségrégation superficielle de rhénium induite par l'oxygèneàtravers des couches minces de platine

After a layer by layer growth of platinum or rhenium, oxygen adsorption induces changes in surface composition, at temperatures where they do not occur without oxygen.

Phase transition of tungsten trioxide - αWO3 orthorhombic → α′WO3 tetragonal - and observation of its influence upon the oxidation kinetics by IR spectrometry

Abstract IR spectrometry is used as a method of (in situ) surface investigation. This technique has been used here to follow the tungsten oxidation during reactions performed under 1.3 Pa of dry oxygen pressure and at different temperatures in the range 800–1100 K. At a fixed temperature Tox, the spectrometer records the evolution of the normal and spectral emissivity, ϵλ, θ = 0, during the formation of the WO3 oxide layer from the first stage (clean metal surface) until measurements have shown a modification in the radiative behaviour of WO3 during its growth when this growth is performed above a critical temperature Tc, between 1000 and 1010 K. To understand this discontinuity, we had to study more precisely the oxidation kinetics of this refractory material in this pressure and temperature range by additional measurements; electron microscopy, X-ray diffraction, measurements of oxidation rates. These observations presented here have enabled us to confirm and locate more precisely this phase transition and to understand its influence on the oxidation kinetics and the radiative behaviour of this oxide versus the reaction temperature.