Ph. Parent

Adsorption of acrylic acid on aluminium at 300 K: a multi-spectroscopic study

Abstract The interface between acrylic acid (CH 2 = CHue5f8COOH) and polycristalline aluminium at 300 K has been studied by different spectroscopies to determine the adsorption mechanism. X-ray photoemission spectroscopy (XPS) probed the occupied states (core levels) of the system, near edge X-ray absorption fine structure (NEXAFS) with synchrotron beam specified the unoccupied ones and reflection absorption infrared spectroscopy (RAIRS) measured the vibration modes of this interface. All the data clearly show that acrylic acid is dissociated upon adsorption on aluminium at 300 K, leading to the formation of saturated aliphatic chains directly linked to the metallic surface (Cue5f8Al bond) and of aluminium oxide islands onto which adsorption of aliphatic carboxylate occurs.

X-ray absorption study of the electronic structure of tungsten and molybdenum oxides on the O K-edge

Magnetron sputtered amorphous thin films a-WO3, a-MoO3 and doped a-WO3:Ir have been studied by X-ray absorption spectroscopy on the oxygen K-edge in comparison with crystalline oxides as monoclinic m-WO3, orthorhombic α-MoO3, cubic Na0.6WO3, layered-type hexagonal h-WO3 and WO3·H2O, having variable electronic and atomic structure. The changes in the XANES ranging 10–15 eV above the absorption edge are interpreted based on the known band-structure calculations. The high-energy features are related to the multiple-scattering processes (EXAFS) at the nearest atoms.

Study of the electronic structure of rhenium and tungsten oxides on the O K-edge

Oxygen K-edge x-ray absorption spectra were studied on the electrochromic amorphous thin film a-WO3 in the comparison with crystalline oxides having variable electronic (d0, d1, d2) and atomic structure: monoclinic m-WO3 (insulator - d0), cubic Na0.6WO3 (metal - d1), cubic ReO3 (metal - d1), layered-type hexagonal h-WO3, WO3H2O and with intercalated HxReO3 (metal - d2), HxWO3 oxides having a metal-isulating transition. The changes in the XANES range 10–15 eV above the absorption edge are interpreted based on the known band-structure calculations. The high-energy features are related to the multiple-scattering processes (EXAFS) on the nearest atoms. The intensity of the feature at 550–560 eV is attributed for the first time to the value of the metal-oxygen-metal bond angle.

XAFS analysis of the low symmetry octahedral molybdenum and tungsten oxides

Abstract Comparative XAFS analysis of the main isostructural crystalline phases of W(Mo)O3 oxides and W(Mo)O3·nH2O hydrates has been done at the W L3 and Mo K edges. These compounds have (1) 3D perovskite-type (PT) structure (m-WO3); (2) 2D PT structure (WO3·H2O and MoO3·nH2O (n = 1,2)); (3) 2D hexagonal-type structure (WO3·1/3H2O) and (4) 2D double layered structure (α-MoO3). The detailed analysis of X-ray absorption fine structure (XAFS) in the low-symmetry structures shows that in order to simulate the experimental spectrum, it is sufficient to take into account single-scattering contributions in the range from 1.7 to ∼ 5.0 A and a number of multiple-scattering paths from nearly linear atomic chains in the first and second shells. The results obtained by XAFS are only in partial agreement with known X-ray diffraction data (XRD). The strong deviation of the short-range order, detected by XAFS, from the one, given by XRD, has been found in W(Mo)O3·nH2O.

Cl2p, O1s PSD–NEXAFS study of the adsorption of HCl on ice: a direct experimental evidence of the HCl ionization

Abstract We have used NEXAFS and photo-stimulated desorption (PSD)–NEXAFS spectroscopy at the O1s and at the Cl2p edges to study H 2 O and HCl ices, as well as the adsorption of HCl on water ice. The PSD–NEXAFS provides a clear fingerprint of the dangling OH bond at the water ice surface. We evidence the H–Cl fragmentation at the 2 p → σ ∗ (H–Cl) excitation of condensed HCl. Last, we directly observe the ionization of HCl when adsorbed on H 2 O ice, both at the surface and after diffusion in the bulk of ice.

XAFS study of short range order in the heavily disordered MoxW1−wO3 oxides

We have done comparative XAFS analysis on the MoK and WL 3 edges of polycrystalline Mo x W 1-x O 3 solid solutions. It was found that their structures are closely related to corner-shared WO 3 -type for x<0.9 while α-MoO 3 -type structure is present for x≥0.9. The obtained structural parameters for the first shell around metal ions allow to distinguish several structural phase transitions due to the change of composition. A set of subshells within the first shell was found, and they are in good agreement with Raman data

Influence of the focusing effect on XAFS in ReO3, WO3−x and FeF3

Abstract The role of the focusing effect in the formation of X-ray absorption fine structure (XAFS) is considered for ReO3, non-stoichiometric tungsten oxides WO3−x and FeF3 having the perovskite-type structure. Two cases are mainly discussed: (1) an admixture of rhenium and tungsten L2-edge XAFS in ReO3 and WO3−x crystals to the one above rhenium and tungsten L1-edge and (2) high-order superfocusing effect in Fe0ue5f8F1ue5f8Fe2ue5f8F3ue5f8Fe4 atomic chain in iron K-edge XAFS of FeF3 which is analysed using an ab initio multiple-scattering approach.

In situ XAFS study of phase transitions and hydrogen intercalation in WO3-MoO3 system

Abstract The WO 3 -MoO 3 system has several reconstructive and displacive phase transitions which can be induced by temperature and/or by change of stoichiometry or valence state of metal ions under hydrogen intercalation. We have studied in situ the local electronic and structural changes arising in WO 3 -MoO 3 system with hydrogen intercalation and temperature. In situ XAFS measurements at the Mo K and W L 3 edges in H y Mo x W 1− x O 3 compounds, performed at different hydrogen intercalation level and temperatures (from RT to 500°C), allowed us to determine the rearrangement in the first and second coordination shells of Mo and W cations during phase transitions. It was found that hydrogen insertion leads, depending on W/Mo ratio, to the appearance of localized (mixed valence) or delocalized electronic states with different Mo and W surrounding.

Adsorption study of ethylene derivatives on Pt(111) at 95 K as probed by NEXAFS

Abstract The interactions of unsaturated molecules with a metallic surface is of great interest both from a theoretical point of view (charge transfer mechanisms, surface reactivity, etc.) and from a practical point of view (catalysis, adhesion, corrosion, etc.). We have recently shown that the substitution of one or two hydrogen atoms by CH3 groups on ehtylene (butene, propylene) has no effect on the interaction mode of the molecule on Pt(111): chemisorption with the formation of a di-σ bond. We report in this paper some C K-edge NEXAFS experiments performed on substituted ethylene adsorbed on Pt(111) at 95 K where one hydrogen atom is replaced by a chemical function (acid, aldehyde). Acrylic acid (CH2 ue5fb CHue5f8COOH) is strongly chemisorbed on Pt(111) at 95 K (di-σ bond) whereas acrolein (CH2 ue5fb CHue5f8COH) is π-bonded. These results combined with XPS and UPS data reveal that the differences in the interactions come from differences in molecular orbitals conjugations which make possible (or impossible) an overlapping of the molecular orbitals with the metal d band.

MS-Xα approach of near edge X-ray absorption spectra of the acrylonitrile molecule (CH2CHCN)

Abstract The C and N K-edge NEXAFS spectra of oriented acrylonitrile multilayers have been recorded at normal and grazing incidence. Calculated spectra using the multiple scattering Xα approach (MS-Xα) are used to interpret the different resonances observed from each excited atom of the molecule.