R. Ducros

A study of the adsorption of several oxygen-containing molecules (O2, CO, NO, H2O) on Re(0001) by XPS, UPS and temperature programmed desorption

Abstract We have studied the adsorption of oxygen, carbon monoxide, nitric oxide, and water vapour on Re(0001), using X-ray and ultra-violet photo electron spectroscopies (XPS and UPS) and temperature-programmed desorption. As on polycrystalline rhenium, adsorbed oxygen is completely dissociated, even at room temperature. Furthermore, the formation of a superficial oxide at room temperature seems probable. Carbon monoxide is almost completely molecularly adsorbed, only a very small fraction being dissociatively adsorbed in a single β- state. However, an attractive interaction still exists between the adsorbed atoms in this β- state. Nitric oxide is adsorbed in a dissociated β 2 state and a molecular β 1 state. The population is smaller than on polycrystalline rhenium, corresponding to half a monolayer. Mathematical treatment of the desorption spectra allowed us to determine the activation energy for desorption of nitrogen resulting from the decomposition of adsorbed species. These quantities were found to be similar to those measured for polycrystalline rhenium.

The adsorption of carbon monoxide on rhenium: Basal (0001) and stepped 14 (0001) × (101̄1) planes

Abstract The influence of surface defects on the adsorption of CO by rhenium is investigated using LEED, AES and linear temperature programmed desorption. On both surfaces, thermal desorption reveals two adsorption states, the lower temperature α state being resolved into two substates, and one β state, all desorbing with first order kinetics. The α state is unaffected by the surface texture, its maximum population being the same on both surfaces, around 4 × 1014 molecules cm−2, similar to the value found for poly crystalline rhenium. On the other hand, the β state is strongly dependent on surface structure. On Re(0001) a maximum of 4 × 1013 molecules cm−2 was found, and 2 × 1014 molecules cm−2 on the stepped surface. The adsorption is activated and can be increased, by heating to 550 K, to 2 × 1014 molecules cm−2 on the basal plane and 3.5 × 1014 molecules cm−2 on the stepped surface. Ordered structures are now seen in LEED. Comparison of these results with previous results from polycrystalline rhenium indicate that the dissociation of β-CO on the latter surface must occur at defects other than steps.

Oxygen adsorption on Pt(110)(1 × 2) at 100 K

Abstract Oxygen adsorption on Pt(110) has been studied at 100 K by work function measurements, photoemission of adsorbed xenon, electron induced desorption and thermal desorption spectroscopy. The adsorption process takes place in two steps. In the first one oxygen is adsorbed in the valleys of the reconstructed surface along the (110) direction. The molecularly adsorbed species is bound to the surface with a binding energy of 12 kcal/mol. In the second step oxygen molecules are adsorbed on the (111) microfacets with a binding energy of 10 kcal/mol.

Core level binding energy shifts of rhenium surface atoms for a clean and oxygenated surface

Abstract The 4 f 5 2 core level of rhenium metal has been studied with synchrotron radiation from LURE, Orsay, France. A shift of −0.1 eV has been found between the surface and the bulk core levels. With oxygen adsorption the formation of three new core levels has been observed but no oxide formation was detected at room temperature. Heating at 700°C under high exposure (600 L) leads to the formation of ReO.

Hydrogen and deuterium adsorption on polycrystalline rhenium surfaces; effect of carbon and oxygen coadsorption

Abstract Hydrogen adsorption has been studied on polycrystalline rhenium surfaces (films and ribbons) by uptake measurements and temperature programmed desorption. The total amount adsorbed Q and the sticking coefficient σ are lower on films than on ribbons. An isotopic effect has been observed on both surfaces where the sticking coefficient σ for D 2 is lower than the sticking coefficient for H 2 but the total amount adsorbed is the same for H 2 and D 2 on each surface: Q H 2 ribbon = Q D 2 ribbon = 6 × 10 14 atoms cm 2 , Q H 2 film = Q D 2 film = 3.5 × 10 14 atoms cm 2 , σ H 2 ribbon = 0.4, σ H 2 film = 0.1, σ D 2 film = 0.08. On ribbons thermal desorption spectra show that a part of the hydrogen is adsorbed as atoms with 67 kcal/mole for the binding energy with the surface at zero coverage, the other part is adsorbed as molecules with a binding energy of 27.5 kcal/mole Coadsorption of hydrogen with carbon or oxygen shows that these states probably correspond to different crystallographic sites on the surface.

Nitric oxide chemisorption on Re(0001) at 100 and 300 K: Evolution of the adsorbed layer during annealing from 100 to 700 K; XPS, UPS, TDS and work function measurements

At 100 K NO is molecularly adsorbed on Re(0001). Bridge bonded and linear species have been identified by XPS and UPS measurements. Moreover a weakly bonded species reversibly adsorbed at 100 K has been found, but not precisely identified. As the temperature of the surface is increased a complex transformation of the layer occurs: the weakly bonded molecules are probably transformed into a more strongly bonded state and desorb between 100 and 300 K. One part of the linear species desorbs between 300 and 500 K giving the α2 molecular state, the other part dissociates and desorbs between 600 and 700 K giving the β1 nitrogen molecules. In the same temperature range the bridge bonded molecules dissociate into nitrogen and oxygen atoms, but nitrogen desorbs into the gas phase between 700 and 1100 K as β2 and β3 states with a second order process. Oxygen is adsorbed as atoms and desorbs at higher temperature. If adsorption takes place at room temperature, NO is mainly dissociated and nitrogen desorbs as β2 and β3 states with a second order process.

Water adsorption on Pt(110)

Abstract Water adsorption on Pt(110) has been studied by electron stimulated desorption (ESD), electron stimulated desorption ion angular distribution (ESDIAD), thermal desorption spectroscopy (TDS), and work function (ΔΦ) measurements. Between 100 and 200 K water is molecularly adsorbed. The first fractions are adsorbed as small clusters before an ice layer starts to build up.

The influence of structural defects on the adsorption of simple molecules (H2, C2H2, C2H4, C2H6, CO, NO) on rhenium single crystals

Abstract Using Thermal Programmed Desorption (TPD), Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES) we have studied the adsorption of hydrogen-containing molecules (H 2 , C 2 H 2 , C 2 H 4 , C 2 H 6 ) and oxygen-containing molecules (CO and NO) on two vicinal planes of the Re(0001) surface. The two surfaces are designated thus: ReS ¦14(0001)(1011)¦, ReS |6(0001)(1671) | . The structural defects have little effect on the adsorption of hydrogen and the hydrocarbons. They are more influential in the case of the oxygen-containing molecules. This is particularly true for CO; on the kink sites the CO molecules can completely dissociate whereas only a partial dissociation is possible on the steps. These results should be viewed in relation to the strong bond energy between carbon and oxygen in a CO molecule of 256 kcal/mole and the great affinity of oxygen for rhenium; E Re − O = 127 kcal/mole.

Water adsorption on Pt(110)(1 × 2)

Abstract Water adsorption on Pt(110)(1 × 2) has been studied at 100 K by thermal desorption spectroscopy, ultraviolet electron spectroscopy, photoemission of adsorbed xenon and electron stimulated desorption ion angular distribution. At this temperature water is molecularly adsorbed as small clusters with a peculiar orientation from the ridges. When oxygen is preadsorbed at 300 K, water is partially dissociated giving hydroxyl radicals which recombine at high temperature.

Oxygen adsorption on Pt(110)(1×2) : surface characterization by photoemission of adsorbed xenon (PAX)

Abstract Oxygen adsorption on Pt (110)(1 × 2) has been studied by photoemission of adsorbed xenon (PAX). The presence of oxygen on the surface does not change the adsorption kinetics on the xenon at 42 K, but the binding energy between the xenon and the substrate is lowered by around 1.5 kcal/mol. The results show that oxygen is first adsorbed in the valleys of the surface, then on the facets.