M. Abon

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.

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.

Synergistic alloying behaviour of Pd50Cu50 single crystals upon adsorption and co-adsorption of CO and NO

Abstract We have studied the surface reactivity of the Pd 50 Cu 50 (111) and (110) single crystals in the adsorption and the co-adsorption of CO and NO at low temperature. The bond strengths of CO and No on Pd and Cu in the alloy are significantly different from those observed on pure metals. Carbon monoxide and nitric oxide are more tightly bound to Cu and less tightly bound to Pd than to the pure components. The calculated NO coverages on PdCu samples are approximately four times lower than for CO due to the dissociation of a fraction of NO on the surfaces. However, the CO or NO adsorption did not suggest an important difference of reactivity between the two surfaces studied. In the co-adsorption experiments, the total surface coverage of CO and NO on Pd 50 Cu 50 (111) and (110) appears quite dependent on the order in which the gases are adsorbed in the successive adsorptions. The pre-saturation with NO leads to molecular adsorbed states and irreversible dissociated species which are not removed upon an exposure to CO. The subsequent CO co-adsorption induced only a slight replacement of molecular NO by CO and the amount of adsorbed CO is quite low ( θ CO =0.06 ML) as compared to the coverage measured with CO alone ( θ CO =0.40 ML). The pre-saturation with CO gives a different behaviour, the subsequent co-adsorption of NO leads to the replacement of CO by NO up to the saturation coverage found for NO alone ( θ NO =0.10 ML). Finally, when NO is adsorbed on a partially CO covered surface, NO displaces CO up to around 0.15 ML but the total amount of surface species (0.30 ML) is lower than the amount (0.55 ML) of pre-adsorbed CO, indicating a strong displacement of CO by NO. The competitive adsorption of CO and NO on PdCu surfaces is discussed in terms of dissociation of NO at low temperature which partially inhibits the further adsorption of CO on the PdCu alloy surfaces.

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.

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.

Photoemission of adsorbed xenon on PtxNi1−x(111) single crystal alloy surfaces

Abstract Photoemission of adsorbed xenon and work function measurements have been performed on three fcc, (111) oriented, PtNi single crystal alloys showing a strong platinum surface segregation. For Pt-rich alloys, the first layer has been found a quasi-homogeneous layer but different from a pure Pt(111) surface. No platinum islands have been observed. The local and the macroscopic work function are nearly the same and do not change linearly with the alloy composition but remain nearly constant and close to 5.4 eV, a value significantly lower than that on a pure Pt(111) face (5.95 eV).

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.

Work function changes associated with hydrocarbon fragments on the Pt(111) face

Hydrocarbon fragments left on a Pt(111) face in the course of the thermal decomposition of unsaturated and aromatic hydrocarbons have been characterized by work function (Δφ), thermal desorption and vibrational EELS measurements. It is concluded that the different hydrocarbon parent molecules leave the same kind of electropositive CH-like species.

Hydrogen chemisorption on Pt80Fe20(111) studied by TDS and UPS

Abstract Hydrogen adsorption on Pt 80 Fe 20 (111) has been investigated mainly by TDS and UPS. It has been compared to hydrogen adsorption on Pt(111) as the composition of the first layer of this alloy is almost pure platinum due to a strong surface segregation. It appears that the valence band of both clean surfaces is different and that the work function is lowered by 0.2 eV on the alloy compared to Pt(111). The work function changes induced by hydrogen adsorption are of the same sign and the same order of magnitude on both surfaces. Finally the activation energy for desorption of hydrogen is lower by about 2 kcal/mol on the alloy. Results are discussed in relation with the catalytic properties of this alloy in hydrogenation reactions.

Work function changes with sulfur coverage on Pt(111)

Abstract Work function changes (Δφ) with sulfur coverage have been measured on Pt(111). The sulfur overlayer has been also characterized by LEED, AES and TDS techniques. An unexpected Δφ decrease, which correlates with the growth of a p (2×2) structure, has been recorded. At higher sulfur coverage, a reverse in the sign of Δφ has been observed with a (√3×√3) R 30° structure. It is proposed that the recent model of Shustorovich and Baetzold could account for these sulfur-induced Δφ changes. It is also concluded that the sulfur adsorption bond on platinum must be essentially covalent.