P. Légaré

Isomerization and hydrogenolysis of C6 alkanes on PtAl2O3 catalysts and Pt single-crystal faces

Abstract The contact reactions of C 6 hydrocarbons (isomerization, dehydrocyclization, hydrocracking) were studied under the same experimental conditions, on platinum-alumina catalysts of low or high dispersion, on Pt(557), Pt(119), Pt(111) surfaces, and on a polycrystalline foil of platinum. The experiments on these well-characterized surfaces were performed up to atmospheric pressure in an isolation cell housed within the main UHV chamber equipped with LEED, AES, and ELS facilities. The reaction mixture was analyzed by GLC and part of it was used for mass spectrometric location of the 13 C in each molecule. The results have shown that isomerization by bond-shift or cyclic mechanisms and hydrogenolysis take place on single-crystal faces of platinum, simulating very well the data from PtAl 2 O 3 catalysts of large size aggregates of Pt. Isomerization by bond shift is much more important on stepped surfaces. The contributions of the relative percentages of bond-shift and cyclic mechanisms are discussed as a function of different models of adsorbed species evolving from one site to multisites of chemisorption, differences in local density of states of the edges, and differences in the electronic charges of the carbon atoms in the hydrocarbons. These models are correlated to the classical ones of Anderson, Rooney, and Gault. The peculiar properties of the highly dispersed 0.2% PtAl 2 O 3 catalyst were never simulated by single crystals of platinum.

Interaction of oxygen and hydrogen with palladium

Abstract The interaction of oxygen and hydrogen with Pd(111) and polycrystalline palladium was studied in a wide domain of temperatures and pressures with low energy electron diffraction, Auger electron spectroscopy (AES), and photoelectron spectroscopy (XPS and UPS). It was shown that the exact knowledge of the influence of possible contaminants (S, C, O) is necessary to prove unambiguously the origin of the structures induced by the adsorbates. Results on the electronic structure of palladium are presented with special attention on surface effects. The different steps of the formation of palladium oxide were characterized, from weak interactions to reconstruction leading to a bulk compound. The study of the Pd-H system confirmed the existence of supplementary states in the bottom of the metal band.

Studies on PtNi alloys: AES and photoemission determination of the surface composition and interaction with oxygen and hydrogen

Abstract Three PtNi alloys with 20, 40 and 70 at% Ni were studied by Auger and photoelectron spectroscopies. The clean surfaces were found to be enriched in platinum in an amount increasing with increasing platinum concentration in the bulk. On the contrary oxidation treatments resulted in a nickel surface segregation which was only slightly offset after reduction. Chemical shifts characteristic of oxidation were observed on XPS peaks of both platinum and nickel under experimental conditions where pure platinum could not be oxidized.

Auger electron spectroscopy study of chemisorption-induced segregation in a PdAu alloy

Abstract The chemisorption of oxygen has been studied by Auger electron spectroscopy at 500 °C and pressures varying from 10 −7 to 1 Torr on a 22 atom % PdAu alloy. A quantitative method has been used for calculating the Pd and Au surface concentrations. For the clean surface it agrees well with the bulk alloy composition even after ion bombardments and annealing. Significant Pd surface enrichment due to oxygen chemisorption has been observed. This result may provide an explanation for the influence of preoxidation treatments on the catalytic behavior of such alloys. Hydrogen chemisorption on the surface after oxygen treatment causes the oxygen and thus palladium surface concentrations to increase further.

Interaction of oxygen and hydrogen with Pd-Au alloys: An AES and XPS study

Abstract Pd-Au alloys of three different concentrations have been studied with Auger Electron Spectroscopy and Photoelectron Spectroscopy. A quantitative method shows that the decontaminated surfaces possess the same composition as the bulk. Interaction of oxygen up to 600°C with gold rich alloys (>85%) is very weak. It starts around 300°C on the other two alloys and results in a significant palladium surface enrichment. On the 60 at% Pd-Au alloy, at 500–580°C, the surface may be completely covered with PdO. A hydrogen treatment at 350°C leads to a complete reduction of the oxide without reequilibration of the surface.

Shapes and shifts in the oxygen Auger spectra

Abstract The oxidation of silicon and platinum single crystal faces, of polycrystalline supported catalysts and of some alloy surfaces has been studied by AES and as far as possible by LEED. A comparison of the oxygen Auger spectra obtained during the oxidation process with those found on oxides has been made; it shows that the modification of the fine structure of the oxygen Auger peaks gives some information about the binding state of oxygen. Two different structures, which compete one with the other, are described. In one case, a spectrum where three lines dominate is obtained; in the other case, a “quasi-atomic” spectrum characterized by five features is observed: multiplet splitting in the two-hole final state is predominant. Besides these differences in the fine structure of the Auger spectra one can notice shifts of several eV for the main feature. They have been correlated with the various observed LEED patterns. Physisorption, chemisorption, solution of oxygen in the metal lattice, growth of ordered or amorphous oxides are the different possibilities which are discussed.

Photoemission study of Pt adlayers on Ni(111)

Abstract The growth of Pt on Ni(111) was followed up to 1.7 monolayers by photoelectron spectroscopy. During the growth, the binding energy of the Pt 4f peaks remained almost constant and close to that of platinum bulk. This reveals that a high Pt-Ni interaction is attained at least in the submonolayer range. The direct consequence of this observation is that the Pt-Pt bond is weakened and that a distorted layer is formed. We show that this is consistent with results from other authors concerning Pt/Ni interfaces as well as Pt-Ni alloys.

Characterization of surface structure dependence for interactions of oxygen with well-defined platinum surfaces

Abstract The interaction of O 2 with clean stepped and smooth surfaces of platinum is studied by LEED, AES and ELS for gas pressures of 10 −8 to 760 Tort and crystal temperatures of 20°C to 600°C. A strong surface structure dependence upon the adsorption of OZ is shown by LEED. During the oxidation process we observed modification of the fine structure of the oxygen Auger peaks which gives information about the binding state of oxygen. ELS spectra changes upon O 2 adsorption are correlated to UPS studies. Working in the high pressure range also makes dissolution into the metal become an important phenomenon which induces the formation of platinum oxide “islands”, as detected by LEED, AES and XPS.

Interaction of polycrystalline platinum and a platinum-silicon alloy with oxygen: An XPS study

Abstract The interaction of polycrystalline platinum and a Pt-Si (3 at% silicon) alloy with oxygen at atmospheric pressure was studied in the range of 600 to 1100 K. No shift on the Pt 4f peaks was detected. The O Is peaks obtained on the two samples enabled us to distinguish between Pt-O and Si-O interaction. Two peaks at 531.6 and 530.0 eV were attributed respectively to chemisorbed oxygen and subsurface oxygen, the latter being likely in solution in the first layers of platinum. The oxidation of silicon induced O 1s transitions at 532.1 and 533.4 eV.

On the interaction of oxygen with Pt single crystals; LEED study of step coalescence

Abstract Several single crystal faces of platinum, after decontamination, are exposed to oxygen pressures from 10−9 to 10−6 Torr in a range of temperatures between 25 to 800°C. The adsorption is studied by LEED and Auger spectroscopy. Oxygen is adsorbed on all platinum surfaces. The existence of defects like steps, terraces or ledges strongly facilitates the dissociation and the adsorption of oxygen. On Pt(111) stepped surfaces, two types of (2×2) structures are obtained: one at room temperature, very sensitive to the electron beam, disappears on heating at 100°C in UHV. A more stable (2×2) is observed at 400°C. Under these experimental conditions, a step coalescence and a terrace broadening is induced by oxygen depending on the terrace orientation. For example, the terrace width and the step height are doubled on the Pt[6(111)×(100)].