B. Tardy

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.

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.

Electronic properties of supported Pd aggregates in relation with their reactivity for 1,3-butadiene hydrogenation

The electronic and catalytic properties of supported Pd particles have been studied by measurements of XPS core level shifts and activity for 1,3-butadiene hydrogenation reaction respectively. For samples prepared by atom beam deposition on carbonaceous supports the TOF at steady state decreases rapidly with decreasing particle size in the range of particle diameter 2.8 nm-1.2 nm. For these small particles, the Pd 3d core level shows an upwards shift and a larger half width compared to the values for massive Pd samples. For the Pd/SiO2 particles of similar size, both the 3d core level energy and the TOF are comparable to these of the massive Pd systems.

Surface reactivity of platinum-nickel single crystal alloys: Carbon monoxide adsorption

Abstract Carbon monoxide adsorption has been studied on (111) faces of single-crystal alloys — Pt10Ni90, Pt50Ni50 and Pt78Ni22 — by vibrational EELS and TDS measurements. On the nickel rich sample, on which the outer layer is notably enriched in platinum with respect to the nominal bulk concentration, multi-bonded nickel sites and on-top platinum sites for CO chemisorption are evidenced. The multi-bonded sites are preferentially filled. In the platinum rich samples, which exhibit a quasi complete platinum surface layer, linearly bonded species are observed first; multi-bonded CO adspecies appear only at higher coverage. Whatever the considered alloy sample the metal-CO bond strengths are weaker than on pure metals, as demonstrated by the lower values of both the metal-CO stretching mode energy and the temperature of the thermodesorption mass-28 peak. That weakening is more pronounced on the Pt50Ni50 sample.

Carbon on Pt(111): Characterization and influence on the chemisorptive properties

Abstract Fully dehydrogenated carbon deposits in the monolayer range have been characterized on the Pt(111) face by LEED, Auger and Δφ observations. These techniques pointed to the graphitic character of such deposits at least beyond about 0.5 monolayer of carbon. The gradual deactivation of the Pt surface towards CO or C6H6 adsorption with increasing carbon coverage has been explained by the nucleation and the growth of numerous inactive graphitic islands. The loss of chemisorptive properties only occurs for a complete carbon monolayer which may be viewed as a passive film masking the metal surface. Some weakening in the binding energy of CO, C6H6 and H2 has been evidenced on a surface partially covered with carbon. This additional effect of carbon has been discussed mainly in relation with Δφ measurements.

On the reactivity of 2D Pd surface alloys obtained by surface segregation or deposition technique

Abstract Two-dimensional Pd surface alloys can be obtained by two different methods: either by surface segregation or by deposition followed by controlled annealing. For a similar Pd surface concentration, Pd-Ni samples, prepared by these two ways, exhibit similar activity towards the hydrogenation of 1,3-butadiene and are more effective than pure Pd. Different UHV techniques allow us to think that the same catalytic behaviour is due to the same chemical composition and atomic arrangement at the very surface. Moreover, XPS measurements show similar electronic properties of the active site which might be a Pd pair modified by surrounding Ni atoms.

Chemisorptive properties of Pt-Ni(111) single crystal alloys with high platinum content: C2H4 and C6H6 adsorption

The chemisorption of ethylene and benzene has been investigated on Pt78 Ni22 and Pt50Ni50(111) single crystal alloys by vibrational electron energy loss spectroscopy and thermal desorption spectroscopy. These alloys exhibit a strong platinum segregation leading to a quasicomplete platinum layer on the surface. On Pt78Ni22(111), the hydrocarbonated surface complexes formed after C2H4 and c6h6 adsorption show the same main features as these recorded on Pt(111). However, evidence for more weak interactions with the metal surface is obtained. On Pt50Ni50(111), no C2H4 and c6h6 adsorption has been obtained, even at 180 K. These results evidence the specific behaviour of the superficial layers of quasi-pure platinum supported on the bulk alloys with respect to pure platinum: a decrease of chemisorption strengths is observed after alloying, which is much more pronounced on the Pt50Ni50 sample.

Carbon monoxide adsorption on a Pt80Fe20(111) single-crystal alloy

It has been previously shown that the topmost layer of the Pt80Fe20(111) single-crystal alloy is characterized by a large Pt surface segregation and two types of Pt atoms. Carbon monoxide adsorption has been studied on that surface by TDS, HREELS and work-function changes at 120 K. HREELS indicates that on-top CO is the prevalent adsorbed species, although bridge-bonded CO is also present near saturation. The general shape of work-function changes exhibits a similar behaviour on the alloy and on Pt(111). TDS spectra show two main peaks which have been related to the two kinds of surface Pt atoms. It is proposed that the presence of iron atoms in the subsurface induces the observed decrease in the CO binding energy.

Sizeable deactivation effect for the 1,3-butadiene hydrogenation on vapor-deposited Pd aggregates on graphite

The catalytic activity of Pd aggregates vapor-deposited on graphite has been measured in the 1,3-butadiene hydrogenation. It is generally decreasing with particle size. But, such a decrease is largely depending upon the hydrogen and the hydrocarbon pressure. Indeed, with low hydrocarbon pressure (about 1 Torr) and in presence of large excess of hydrogen (Ph2/Pc4h6=100) the activity of the smaller aggregates is comparable to that of larger particles. The different behaviour is consequently attributed to a size dependent deactivation. Such a deactivation could be due to carbon deposits generated by dehydrogenation of the diene admolecules. This behaviour is tentatively related with marked differences observed by XPS on the smaller aggregates, both with respect to the core level energies and to the density of states into the valence band.