J.C. Bertolini

Vibrational EELS studies of CO chemisorption on clean and carbided (111), (100) and (110) nickel surfaces

Abstract Room temperature adsorption of CO on bare and carbided (111), (100) and (110) nickel surfaces has been studied by vibrational electron energy loss spectroscopy (EELS) and thermal desorption. On the clean (100) and (110) surfaces two configurations of CO adsorbed species, namely “terminal” and bridge bonded CO, are observed simultaneously. On Ni(111), only two-fold sites are involved. The presence of superficial carbon lowers markedly the bond strength of CO on Ni(111)C and Ni(110)C surfaces, while no adsorption has been detected on the Ni(100)C surface. Moreover, on the carbided Ni(110)C surface, the adsorption mode for adsorbed CO is changed with respect to the clean surface; only “terminal” CO is then observed.

Crotonaldehyde and methylcrotonaldehyde hydrogenation over Pt(111) and Pt80Fe20(111) single crystals

Abstract Hydrogenation reactions of crotonaldehyde and methylcrotonaldehyde have been carried out on Pt(111) and Pt 80 Fe 20 (111) single crystals. The reactions have been performed in the gaseous phase with a large excess of hydrogen. A noticeable enhancement of activity has been observed by alloying the platinum with iron. The additional methyl group present in the methylcrotonaldehyde greatly increases the selectivity toward formation of the unsaturated alcohol, while only a slight increase of such selectivity is induced by alloying. The results have been compared with the previous experimental results on crotonaldehyde hydrogenation on Pt/SiO 2 catalysts and cinnamaldehyde hydrogenation on Pt and Pt 80 Fe 20 particles supported on carbon.

Co stretching vibration of carbon monoxide adsorbed on nickel(111) studied by high resolution electron loss spectroscopy

Abstract The frequency of the v -CO stretching vibration measured by HRELS has been followed as a function of the CO coverage and in the presence of coadsorbed hydrocarbons on the Ni(111) face. The v -CO frequency shifts continuously from 225 meV (1814 cm −1 ) to 237 meV (1911 cm −1 ) when the CO coverage increases from 0 to 0.41. Coadsorption of electron donor molecules, such as ethylene and benzene, generates a significant lowering of the v -CO frequency. Results are discussed in terms of the back donation of metallic electrons into the 2π ∗ antibonding orbitals of CO, the dipole-dipole coupling and the coordination number of the CO adsorbed molecules. The back donation is found to play the major role in the range of coverage explored but we cannot exclude some contribution of a dipole-dipole coupling effect.

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Abstract Studies of benzene (C 6 H 6 and C 6 D 6 ) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c (4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.

Alloying effect on the adsorption properties of Pd50Cu50{111} single crystal surface

The surface composition of the Pd50Cu50{111} single crystal as measured by LEIS corresponds to Pd45Cu55 in the very first layer. This small Cu segregation did not induce a special surface structure as observed by LEED. The classical (1 × 1) LEED pattern was observed both after annealing and during gas adsorption. The chemical properties of Pd-Cu alloy have been investigated by adsorption of CO, NO and H2 and also by XPS. The amount of CO adsorbed on the surface at 160 K is at least four times higher than for NO or H2. Furthermore, the adsorption energies decrease on Pd and increase significantly on Cu in alloy with respect to adsorption on pure metals. The significant shifts of core and valence bands observed by XPS and, moreover, the change in adsorption energies of CO and NO were attributed to a strong electronic interaction between Pd and Cu upon alloying. This Pd-Cu interaction has been interpreted in terms of bond formation between the almost full valence “sd” band of Pd and the resonant “dsp” band of Cu near the Fermi level. An explanation using back-donation capabilities of the Pd-Cu alloy surface to CO and NO is also given to support the formation of a hybridized molecular orbital.

PtxNi1−x(111) alloy surfaces: structure and composition in relation to some catalytic properties

The surface structure inside the (111) face of two substitutional alloys, Pt0.5Ni0.5 and Pt0.78Ni0.22 is investigated by LEED. In both cases we find an important compositional oscillation across the three outermost layers. The concerned Pt concentrations (at%) are 88 ± 2, 9 ± 5 and 65 ± 10 for Pt0.5Ni0.5 and 99 ± 1, 30 ± 5 and 87 ± 10 for Pt0.78Ni0.22. This oscillation is a new and important element to be accounted for in the discussion of the specific chemisorptive and catalytic properties, particularly as regards the hydrogenation of 1,3-butadiene.

Relationship between vibrational states of O-Ni systems and their superficial structure on (100) face of nickel single crystal

Abstract High resolution energy loss spectra of 4 eV electrons reflected in the specular direction from Ni(100) surface clean or covered by the ordered structures obtained in the different stages of the metal oxidation, are analysed with reference to LEED patterns. At room temperature, the successive p(2 × 2) and c(2 × 2) structures associated with the chemisorption of oxygen have been observed without modification of the energy loss spectra, in respect of the clean nickel surface. Surface phonons are known to occur in the case of the c(2 × 2)S ordered layer and their absence in the case of Ni-O corresponding system is discussed. After short exposures to oxygen between 200 to 500° C, the surface exhibits a so called “intermediate oxide”. It is identified by its hexagonal unit mesh (∼5 A) with two equivalent orientations along the [100] and [110] directions of the substrate and its vibrational spectra characterized by a loss peak at − 112.5 meV (± 2.5 meV). Subsequent exposures to oxygen lead to the formation of the (100) face of NiO (in epitaxy on the Ni(100) face) accurately identified by its LEED pattern. The obtained typical multiple loss spectra with spacing 67.5 meV (± 15 meV) reveal a scattering of low energy electrons by long wavelength optical phonons associated to the oxide. The characteristic energy loss (67.5 meV) is in relative good agreement with the energy of the Fuchs-Kliewer surface phonon calculated from the optical constants of the nickel oxide.

Pt10Ni90(111) single crystal alloy: Determination of the surface composition by AES, XPS and ISS

Abstract The surface composition of the Pt 10 Ni 90 (111) single crystal alloy has been determined from AES, XPS and ISS experiments. The clean surface is largely enriched with platinum: 30–40% in the top layer instead of 10% in the bulk. This enrichment concerns mainly the ultimate surface layer and appears to be only slightly dependent on the sample temperature.

A comparative study of 1,3-butadiene and 1-butene chemisorbed on Pt(111), and Pd(111)

Abstract The adsorption of 1,3-butadiene and 1-butene on Pt(111), and Pd(111) was studied by near edge X-ray absorption fine structure (NEXAFS). It is found that at 95 K, 1-butene is π-bonded to Pd(111), while it is di-σ bonded to Pt(111). At 95 K, 1,3-butadiene is loosely bonded on both Pd(111) and Pt(111). However, at 300 K, 1,3-butadiene adsorption differs, depending on the substrate. A di-σ bonding is observed on Pd(111) while on Pt(111), a di-σ interaction, keeping one central carbon-carbon double bond, is proposed. In the butadiene molecule half of the carbon atoms undergo a large rehybridization from sp 2 to sp 3 , as evidenced by high resolution electron energy loss spectroscopy (HREELS). These differences between 1,3-butadiene adsorption on Pt(111) and Pd(111) are discussed in terms of differences in catalytic activities and selectivities in the 1,3-butadiene hydrogenation reaction.

C2H4 adsorption on the Ni(111) face at room temperature: Vibrations of surface complexes studied by electron energy loss spectroscopy

Abstract Ethylene adsorption at room temperature on the Ni(111) surface has been followed by vibrational high resolution electron energy loss spectroscopy coupled with LEED, work function changes and thermodesorption measurements. The vibrational spectra of adsorbed species exhibit loss peaks at 365, 151, 85, 60 and 39 meV (274, 147, 79 and 39 meV with C 2 D 4 ). The similarity of these spectra with those obtained after short exposure to acetylene suggests a dehydrogenating adsorption well confirmed by thermodesorption data: C 2 H 4 gas → C 2 H 2ads + 2 H ads . From the observed v C-C frequencies (151 and 147 meV for the respective C 2 H 2 and C 2 D 2 adsorbed species) one can deduce a residual force constant ƒ C-C ≅ 5.25 mdyn/A. By comparison to gaseous molecules, this value suggests a near sp 3 hybridization state of the C atoms confirmed by the position of the v C-H stretching mode (365 meV). Taking in account these results, a triangular surface chemisorption site for the C 2 H 2 species with two σ bonds and one π bond to three Ni atoms is suggested. Frequency analysis of this model allows to fit the experimental loss energies for reasonable force constant values, the C 2 H 2 residue lying flat on the triangular site with a bent H-C-C-H arrangement (C-C-H angle ≅ 155°).