Jean Fusy

Interaction of H2O with basal and stepped faces of rhenium: An ESDIAD study

The adsorption and dissociation of water has been examined between 80 and 500 K on two different Re surfaces — Re(0001) and a stepped vicinal Re[16(0001) × 2(1101)] — using TDS, LEED, AES and ESDIAD (Electron Stimulated Desorption Ion Angular Distribution). Above 250 K, the adsorption is dissociative on both surfaces but the extent of dissociation is larger on a stepped than on a basal face. During adsorption, evolution of hydrogen is observed after an induction period. This phenomenon is related to the formation of OH radicals. The adsorption on Re(0001) is molecular at 80 K. Several steps could be observed: adsorption of isolated molecules, formation of bilayer clusters ordered in two (3 × 3)R30° structures and finally the formation of a multilayer. Situations of non-uniform coverage of the surface have been found in which multilayer patches, bilayer clusters and uncovered areas are simultaneously present. Three molecular water desorption peaks can be observed. Peaks A (180 K) and B (150 K) are produced by desorption from the first bilayer and peak C (140–150 K) is related to the desorption from the multilayer. Furthermore, molecules in the first bilayer are partly dissociated as shown by desorption of hydrogen between 250 and 450 K. ESDIAD and LEED experiments have shown that desorption, manifested by peak A, occurs after a reconstruction of the bilayer, ordered in two (2 × 2) structures, resulting from a displacement of water molecules from the on-top sites to three fold sites. The latter molecules dissociate between 190 and 240 K giving OHads + Hads. Dissociation of OH would occur only above 360 K. On stepped Re dissociative adsorption occurs on steps, even at 80 K, while adsorption remains molecular on terraces. This might be accounted for by the lowering of the activation barrier of the dissociation on steps.

Preparation and characterisation of iron oxide films deposited on MgO(100)

Abstract Thin films of two iron oxides (4–6 nm), FeO and Fe 3 O 4 , have been prepared on MgO(100) substrates under vacuum by evaporating iron in the presence of oxygen. Structures of the oxides were determined by RHEED transmission patterns and LEED. The Fe(2 p ) and Fe(3 p ) XPS and Fe(M 2,3 VV) Auger spectra were used to characterise the chemical environments of iron in these two oxides. It has been shown that the Wustite formed under these conditions is very unstable and can only be preserved for a very short time as a thin film. In contrast, magnetite is very stable at room temperature even when exposed to the laboratory atmosphere. After heating Fe 3 O 4 at 700 K, magnesium coming from the substrate is detected by AES.

Coadsorption de l'oxygène et du monoxyde de carbone sur des surfaces de rhénium

Abstract Temperature programmed desorption ( 2.65 K sec ) has been used to study carbon monoxide and mixed layers of carbon monoxide and oxygen on rhenium ribbons, strongly oriented parallel to the (0001) plane. Four binding states, populated in decreasing energy have been detected. Interpretation of the results on β states agrees qualitatively with Kings model postulating dissociation of carbon monoxide molecules and a repulsive interaction energy between carbon and oxygen atoms. However, in the coadsorbed layers studies, it is shown that all the oxygen atoms do not play a part in the recombination process, during desorption, and that when oxygen is adsorbed after carbon monoxide, a displacement reaction occurs, due to apparent transfer from β states towards molecular α states. Optimization of the results on pure carbon monoxide layers leads to an interactional energy ω, equal to 3 kcal mole , and is only possible if is assumed that β states are formed on alternatively filled and empty rows.

Growth and reactivity of evaporated platinum films on Cu(111): a study by AES, RHEED and adsorption of carbon monoxide and xenon

Abstract Ultra thin platinum films evaporated on Cu(111) at 100 K and at room temperature have been investigated by AES, RHEED, temperature programmed desorption (TPD) of carbon monoxide and photoemission of adsorbed xenon (PAX). A layer-by-layer growth mechanism was evidenced up to at least 5 ML Pt. In the first Pt monolayer, the PtPt bond distances are strained ∼7% beyond the equilibrium bond distances found for bulk platinum. CO TPD is very sensitive to the coverage of deposited Pt: during the first Pt monolayer built-up the disappearance of Cu adsorption sites is observed and new features related to platinum are observed; at monolayer coverage, it appears that molecular CO is more weakly bound than on bulk Pt, the maximum of the main desorption peak is lowered by about 120–150 K; a 2 ML Pt deposit gives rise to CO TPD spectra looking like CO TPD spectra of bulk platinum. The binding energy of the Xe5p 1 2 levels used in PAX experiments as a local probe for the work function shows that the surface properties during the built-up of the first monolayer of platinum remain close to those of Cu(111). Adsorption of xenon and of carbon monoxide on 1 ML Pt present behaviors very close to those observed on the surface of a bulk PtCu alloy. An interpretation in terms of strong modifications of the electronic structure due to PtCu interactions is proposed.

Oxygen adsorption on Pt(110)(1 × 2) at 100 K

Abstract Oxygen adsorption on Pt(110) has been studied at 100 K by work function measurements, photoemission of adsorbed xenon, electron induced desorption and thermal desorption spectroscopy. The adsorption process takes place in two steps. In the first one oxygen is adsorbed in the valleys of the reconstructed surface along the (110) direction. The molecularly adsorbed species is bound to the surface with a binding energy of 12 kcal/mol. In the second step oxygen molecules are adsorbed on the (111) microfacets with a binding energy of 10 kcal/mol.

Core level binding energy shifts of rhenium surface atoms for a clean and oxygenated surface

Abstract The 4 f 5 2 core level of rhenium metal has been studied with synchrotron radiation from LURE, Orsay, France. A shift of −0.1 eV has been found between the surface and the bulk core levels. With oxygen adsorption the formation of three new core levels has been observed but no oxide formation was detected at room temperature. Heating at 700°C under high exposure (600 L) leads to the formation of ReO.

Oxygène adsorbé sur platine; résultats expérimentaux et modèle d'interprétation

Abstract Isotopic exchange and programmed thermal desorption have been used to study the problem of the nature of the adsorbed states of oxygen on platinum. On strongly oriented ribbons (100 face), five adsorption states can be identified. At low temperature (120 K), the adsorption is partially molecular. Above 300 K, oxygen adsorbs completely dissociatively. The multiplicity of desorption peaks cannot be explained by the existence of distinct planes or adsorption sites. A model with two types of lateral interactions has been applied to take account of the experimental results. It is in agreement with recent result on (100) vicinal surfaces.

Étude de l'adsorption de l'oxygène sur ruban de rhénium par mesure des variations du travail d'extraction électronique

Abstract Adsorption and work function changes have been followed on a recrystallized rhenium ribbon [preferentially oriented with (0001) plane parallel to the surface] in an ultrahigh vacuum apparatus. Gas pressure was monitored by mass spectrometry and work function variations measured with the diode method in the retarding potential range, using an electron gun. Despite changes in saturation coverages, work function differences at saturation, and sticking coefficient with the adsorption temperature of the sample, curves giving the surface potential with coverage are characterized by a common part till a critical coverage ( 5 × 10 14 atoms cm 2 ), in the 300–850 K range. Comparison with LEED, thermal desorption and electron induced desorption results of the literature is in agreement with an adsorption induced surface rearrangement, above this critical coverage.

Water adsorption on Pt(110)

Abstract Water adsorption on Pt(110) has been studied by electron stimulated desorption (ESD), electron stimulated desorption ion angular distribution (ESDIAD), thermal desorption spectroscopy (TDS), and work function (ΔΦ) measurements. Between 100 and 200 K water is molecularly adsorbed. The first fractions are adsorbed as small clusters before an ice layer starts to build up.

Étude de l'interaction à température ordinaire du monoxyde de carbone gazeux avec de l'oxygène adsorbé sur platine polycristallin

Abstract We have studied at 300 K and under low pressures the adsorption and reactivity of gaseous carbon monoxide on platinum with preadsorbed oxygen. The reaction is first order dependent and evidently proceeds through strongly adsorbed oxygen and weakly bond carbon monoxide species interactions. Three tentative interpretations are given in order to explain the independence of the reaction rate on the oxygen coverage in a large domain. One of them, purely geometrical, is of course inadequate at low coverages: the two others attribute the main reactivity to the α or β states, observed when using temperature programmed desorption.