M. Alnot

A study of the adsorption of several oxygen-containing molecules (O2, CO, NO, H2O) on Re(0001) by XPS, UPS and temperature programmed desorption

Abstract We have studied the adsorption of oxygen, carbon monoxide, nitric oxide, and water vapour on Re(0001), using X-ray and ultra-violet photo electron spectroscopies (XPS and UPS) and temperature-programmed desorption. As on polycrystalline rhenium, adsorbed oxygen is completely dissociated, even at room temperature. Furthermore, the formation of a superficial oxide at room temperature seems probable. Carbon monoxide is almost completely molecularly adsorbed, only a very small fraction being dissociatively adsorbed in a single β- state. However, an attractive interaction still exists between the adsorbed atoms in this β- state. Nitric oxide is adsorbed in a dissociated β 2 state and a molecular β 1 state. The population is smaller than on polycrystalline rhenium, corresponding to half a monolayer. Mathematical treatment of the desorption spectra allowed us to determine the activation energy for desorption of nitrogen resulting from the decomposition of adsorbed species. These quantities were found to be similar to those measured for polycrystalline rhenium.

XPS study of the sorption of Hg(II) onto pyrite FeS2

In this study, the sorption of Hg(II) onto pyrite was investigated as a function of the aqueous solution pH. X-ray photoelectron spectroscopy (XPS) was used to characterize the surfaces, and to identify the adsorbed species when possible. After 12 h of equilibration at a given initial pH (between 6 and 10.5), slabs of pyrite were in contact with Hg(II) solution (210 μM, constant ionic strength fixed with 0.01 M NaNO 3 ) for 24 h. The final pH ranged between 4.1 and 3.6. The solid samples were then dried and transferred into the XPS machine for surface analyses (Hg 4f, S 2p, O 1s and Fe 2p levels). Taking advantage of the differential charge effect when the surface was partly covered with Fe(III) oxyhydroxides, it was shown that Hg could be sorbed onto both pyritic zones and oxidized patches. On pyritic zones where no charge effect was observed, the Hg 4f 7/2 level was pointed out at 100.7 eV, ruling the presence of Hg(0) at the surface out. The S 2p level analyses clearly showed that the main component was S 2 2- , with some traces of polysulphur. Neither S(II) nor S(VI) were detected in any experiment, excluding the formation of HgS, sulphate and thiosulphate. Most observations were understood as the formation of two surface complexes between mercury and either the pyritic functional groups or the oxyhydroxide sites.

X-ray photoelectron spectroscopy and Raman spectroscopy investigations of amorphous SixC1x(H) coatings obtained by chemical vapour deposition from thermally labile organosilicon compounds

Abstract Thermal decomposition of organosilicon molecules was carried out in order to obtain amorphous coatings with variable silicon and carbon contents. We selected four molecules: two cyclic molecules, sila-5-spiro[4.4] nona-2,7-diene (I A ) and dimethyl-2,7-sila-5-spiro[4.4]nona-2,7-diene (I B ), and two non-cyclic molecules, tetrapropargylsilane (II A ) and tetraethynylsilane (II B ). Classical chemical vapour deposition (CVD) apparatus was used and the nature of various influences was studied. The coatings were investigated by X-ray photoelectron spectroscopy and micro Raman spectrometer analysis. The coatings have the general formula Si x C 1− x (H), where x varies from 0.24 to 0.82 (depending on the molecule and the decomposition temperature). The binding energies of the core electrons in the Si 2s and C 1s levels ( E b (Si 2s) ≈ 152.4 eV; E b (C 1s) ≈ 284.2 eV) are shifted relative to those for an SiC standard sample ( E b (Si 2s) = 151.6 eV; E b (C 1s) = 283.7 eV). Nevertheless, the Raman spectra display two peaks (800 and 960 cm −1 ) corresponding to SiC after annealing of the deposits. The correlations between the properties of the solid phase (coating) and the gas phase (decomposition byproducts) in the CVD reaction permit an understanding of the decomposition. The coating composition is related to the structure of the starting molecule.

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.

Catalytic activation of cobalt induced by oxidizing treatments in the methanation of carbon dioxide

A ribbon of Co catalyzing CO2 hydrogenation at atmospheric pressure and temperatures ranging from 200 to 500 °C has been taken as an example for studying the activating effect of pre-oxidation upon the activity of metals in hydrogenation reactions. The chemical state of the Co surface could be determined at any time by direct and quick transfer of the sample from the reactor into the ultrahigh vacuum chamber of an electron spectrometer. After cleaning and prolonged exposure to H2 (15 h, 1 bar, 500 °C) the sample displayed no visible activity up to 500 °C despite the absence of any visible contaminant on its surface. Activities ranging over several orders of magnitude could be induced in the metal by adequate pre-oxidations according to their severity. A large part of this activation was transient and its progressive decay required periods of time which were longer the more severe the pre-oxidation and the lower the temperature of reaction. The activity at any time after treatment was shown to depend not only on the oxidation but also on the subsequent reduction, since an increase of the rate of reduction resulted in a corresponding though transient increase of the activity. Reduction of most of the surface region was shown to occur very quickly, whereas deeper layers continued to reduce during the progressive decay of the activity. No visible contamination occurred during deactivation. The activation is interpreted in terms of the creation of surface defects by the alternate oxidations and reductions whereas the deactivation is considered to originate in the thermal restructuring of the surface.

CeO2 thin films on Si(100) obtained by pulsed laser deposition

Abstract Thin films of CeO2 have been deposited on single crystal silicon (100) samples at room temperature by using pulsed laser deposition (PLD). The study of ion–molecule reactions occurring in the microplasma (by laser microprobe coupled with mass spectrometry (LMMS)) allows the understanding of the oxygen deficiency observed in films by X-ray photoelectron spectroscopy (XPS).

Auger electron spectroscopy, X-ray photoelectron spectroscopy, work function measurements and photoemission of adsorbed xenon on thin films of Pt-Re(111) alloys

Abstract Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and measurements of the macroscopic work function and the photoemission of adsorbed xenon (PAX) were used to investigate the deposition of 0–15 monolayers of platinum onto an Re(0001) single crystal and subsequent annealing above 1000 K. During deposition, the macroscopic work function reached the bulk value after deposition of 1 monolayer. The PAX and LEED studies indicated that Pt(111) grows epitaxially on Re(0001), which is consistent with the small difference in lattice parameters. Annealing led to the formation of a homogeneous alloy whose work function varied linearly with the platinum content.

Deposition by laser ablation and characterization of titanium dioxide films on polyethylene-terephthalate

Abstract Thin titanium dioxide films have been deposited on polyethylene-terephthalate (PET) by laser ablation deposition technique. The experiments were performed with a quadrupled Nd:YAG laser (266 nm) and an ArF excimer laser (193 nm), at room temperature and without any oxygen addition during the ablation. The as-prepared films have been characterized by various techniques: Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD), X-Ray Microprobe, Laser Microprobe Mass Spectrometry (LMMS), and ellipsometry. For this last technique the titanium dioxide films were deposited onto a silicon substrate. The thicknesses of the films are ranging between 100 and 150 nm. Their surfaces are smooth and the presence of some holes (diameter

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.