J. Massardier

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.

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.

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.

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.

Bulk and surface core level binding energies in the Pt80Fe20(111) alloy

Core level photoemission spectroscopy of the Pt 4f72 level is used to investigate the surface and bulk Pt environments in the ordered catalytic alloy Pt80Fe20. The results obtained support a LEED model of Pt segregation to the surface. This implies the presence of two Pt surface sites.

Pt50Ni50(110) alloy: Chemical surface reactivity with respect to the pure components

Abstract Two surface states have been evidenced on PtNi(110) depending on the equilibration temperature. Different behavior is observed both with respect to CO chemisorption and mild hydrogenation of 1.3-butadiene. The low temperature equilibrated sample looks like the PtNi(111). but the high temperature stabilised sample shows different surface electronic properties and an enhanced reactivity.

On the large Pd surface segregation and reactivity of Pd1Fe99 and Pd5Fe95 dilute alloys

Abstract The surface composition of Pd1Fe99 and Pd5Fe95 was determined using low energy ion spectroscopy (LEIS). After stabilization by annealing of the samples at 870 K, the Pd enrichment at the surface was found to be very important, since it reaches values as large as 55 and 65 at% respectively for the Pd1Fe99 and Pd5Fe95 alloys. The performances in catalysis of such alloy surfaces were measured for the 1,3-butadiene hydrogenation reaction (at 300 K and 35 Torr for the hydrogen pressure). The activity ((2.1−9.3) × 1015 molecules cm−2 s−1) and selectivity into butenes (∼ 1) of such Pd-rich surfaces were found to be of the same order of magnitude as for (111) and (110) Pd single crystal faces.