Y. Berthier

Hydrogenation of 3-methyl-butenal on Pt(110); comparison with Pt(111)

The gas-phase hydrogenation of 3-methyl-butenal or prenal, was studied, at temperature equal to 353 K, on the (110) plane of platinum and compared to previous results obtained on Pt(111). The selectivity was found to be very structure sensitive. This has been ascribed to steric factors which determine when the unsaturated bond is on a substituted carbon atom. The combination of the use of well characterized surfaces as catalysts, and of the determination of the kinetics of the reaction led us to propose a Langmuir-Hinshelwood type mechanism. The nature of the reaction intermediates, di-σ or tetra-σ activated adsorbed species, depends upon the reactant pressure and upon the geometry of the surface. This catalytic study on model surfaces is of particular interest in understanding the selectivity of real catalysts whose atomic arrangement is possibly defined and controlled.

1,3-Butadiene hydrogenation on single crystals of platinum. II. Sulfur poisoning of Pt(110)

Abstract The sulfur deactivation of Pt(110) for the 1,3-butadiene hydrogenation and H 2 D 2 equilibration reactions at 323–423 K and ≈400 Torr pressure were investigated. It was observed that each sulfur atom poisons one dissociation site for hydrogen without changing the mechanisms of the reactions occurring on the sites remaining free of sulfur. An isotherm of sulfur adsorption was obtained at 373 K during the hydrogenation reaction. This curve indicates that the butadiene molecules adsorbed under the steady-state conditions of hydrogenation significantly decrease the binding energy of sulfur on platinum (15–20%). As a general conclusion of this study, it appears that the deactivation behavior of sulfur for a catalytic reaction involving unsaturated hydrocarbon depends on the adsorption strength of the hydrocarbon and on its concentration on the surface.

Hydrogenation of acetylene on palladium

Abstract The activity of palladium towards acetylene hydrogenation is affected by the formation of a palladium hydride. Hydrogenations have been carried out on a polycrystalline sample of palladium pretreated by two different procedures in order to compare the resulting activities; formation of a superficial hydride appears to be a necessary primary step to ensure good activity of the catalyst. The selectivity to ethylene is always very high, around 90% The hydrogenation proceeds through consumption of hydrogen from the hydride phase.

AES, XPS, and TDS study of the adsorption and desorption of NH3 on ultra-thin chromium oxide films formed on chromium single crystal surfaces

Abstract Ultra-thin films of Cr 2 O 3 have been produced by oxidation of Cr(110) surfaces in O 2 (pO 2 =3×10 −8 mbar) at 190°C. The adsorption and desorption of NH 3 on the thin Cr 2 O 3 films have been studied by LEED, AES, XPS, and TDS. The kinetics of ammonia adsorption was investigated by AES and adsorbed NH 3 states on chromium oxide surfaces were identified by XPS. After exposure to NH 3 , N 1s signals were observed at 398.6 and 400.7 eV. These results were interpreted by the existence of two types of adsorbed species resulting from the interaction of NH 3 with acid sites of the chromium oxide surface. Accordingly, two desorption peaks corresponding to NH 2 and NH 3 were observed by TDS (at 150°C).

Hydrogenation of olefins on platinum

Hydrogenation of buta-1,3-diene was studied on the three low-index planes of platinum under quasi-atmospheric pressure and hydrogenation of 2-methyl-buta-1,3-diene was investigated on Pt(111) in the same conditions. Kinetic results and especially selectivity were analyzed and compared to each other. Special attention has been given to the adsorbed species and their relative abundance on the surface. A mechanism has been proposed which explains the variations observed in the studied systems. In no case is the reaction governed by the dissociation of hydrogen and the selectivity depends on the stability of the hydrocarbon species on the surface.

NH3 probing of the surface acidity of passive films on chromium

The acid–base properties of passive films formed on chromium have been investigated with a probe molecule. The adsorption of NH3, a Lewis base, has been studied by X-ray photoelectron spectroscopy (XPS) on passive films formed on Cr(1 1 0) in 0.5 M H2SO4. A single N 1s signal resulted from the exposure of the passive film to gaseous ammonia (25°C, pNH3=20mbar). The comparison with NH3 probing of dry oxide films reveals that the oxide surface reactivity is markedly reduced by the layer of OH. The Lewis acid sites of the oxide surface are not directly available on the passive film. The data indicate that NH3 reacts with Bronsted acid sites consisting of hydrogen termination of the hydroxyl or hydroxide groups on the passive film surface. The implications of these findings for the interactions of chloride ions with passive films (passivity breakdown and initiation of pitting) are discussed.

A reaction between co-adsorbed sulfur and butadiene on the Pt(111) surface. Evidence for alkylthiol formation in presence of hydrogen

The desorption of methylthiol and propylthiol has been observed, induced by the co-adsorption of hydrogen, butadiene and sulfur on Pt(111). This phenomenon goes with increase of the total amount of sulfur adsorbed on the surface by a factor of 20%.

Hydrogen, hydrogen sulfide and hydrocarbon coadsorption on Pt(111) and Pt(110): Selective formation of alkylthiols

Abstract Simultaneous adsorption of diolefine, hydrogen, and hydrogen sulfide on platinum single crystals induces mercaptan and alkane formation. The nature, the amount and the temperature of desorption of these products depend on the size and configuration of the diolefine and on the structure of the surface. Mechanisms have been proposed to interpret the observed surface reactions. A selective number of compounds are selectively produced when H2S reacts firstly with the diolefine; this occurs with C4H6 on both (111) and (110) orientations and with C5H8 on Pt(110). But hydrogen is the first to react with C5H8 on Pt(111), leading thus to a whole series of mercaptans and saturated hydrocarbons.

Hydrogenation of isoprene on Pt(111)

Abstract The hydrogenation of isoprene (2-methyl-1,3-butadiene) was investigated on Pt(111) under quasi-atmospheric pressure and over a temperature range of 200 K. The surface was characterized by LEED and AES. Special attention has been given to the rate-determining adsorbed species; their relative abundances on the surface, governed by temperature and pressure parameters, showed evidence for their competitive influence on kinetics. However, only temperature affected product selectivity. As for sulfur poisoning, adsorption site blocking and hence mobility inhibition for adsorbed molecules can explain the shape of the deactivation curves.

Hydrogenation of Methyl-3, Butenal on Polycrystalline Platinum

Summary The activity and selectivity of a C 5 unsaturated aldehyde (prenal) has been investigated in gaseous phase on a platinum polycrystalline sample. The aim of this work was the study of the respective effects of the pretreatment of the sample by oxygen and of the presence of a small partial pressure of H 2 S in the gas phase. The results gave us a better understanding of the surface mechanism and proved the importance of the chemisorption mode of the molecule.