Showing papers on "Chemisorption published in 1973"

Specificity in Catalytic Hydrogenolysis by Metals

Abstract: Publisher Summary Hydrogenolysis reactions of hydrocarbons involve the rupture of carbon–carbon bonds and the formation of carbon–hydrogen bonds. The reactions are exothermic and are catalyzed by various transition metals. This chapter discusses the catalytic hydrogenolysis on metals. At temperatures much lower than are required for hydrogenolysis, the chemisorption of hydrocarbons on metals is accompanied by evolution of hydrogen. Exchange reactions between paraffins and deuterium to yield deuteroparaffins occur at similar temperatures. These results indicate that carbon–hydrogen bonds are activated more readily than carbon–carbon bonds and that dehydrogenative chemisorption of the hydrocarbon is the initial step in hydrogenolysis. The hydrogen deficient surface species formed from the hydrocarbon then undergoes carbon–carbon bond scission, followed by hydrogenation and desorption to complete the reaction. The chapter describes the specificity of metal catalysts for hydrogenolysis reactions. This includes an extensive comparison of the catalytic activities of various metals and an attempt to relate the resulting “activity patterns” to properties of the metallic state.

X-Ray photoelectron spectroscopy of chromium–oxygen systems

Abstract: X-Ray photoelectron spectroscopy has been used to study a series of chromium–oxygen compounds. It has been shown that the ionisation energy of the chromium 2p electrons is dependent primarily on the oxidation state of the chromium metal ion, but that small perturbations may be attributed to changes in crystal structure and hence the Madelung potential. Multiplet splitting in chromium(III) compounds contributes to peak widths, and the chemisorption of water and oxygen has a marked effect on the observed peak profiles. In addition such chemisorption apparently contributes to the build up of surface charge, thereby complicating the precise determination of binding energies.

Chemisorption of carbon monoxide on tungsten. Part 2.—Lateral interaction model for desorption kinetics

Abstract: Ordered structures formed by the β states of carbon monoxide on single crystal tungsten surfaces imply strong lateral interactions between adatoms. A statistical model, based on the quasi-chemical approach of Fowler and Guggenheim, is used to derive a kinetic equation for the associative desorption of a heteronuclear diatomic molecule, taking into account the existence of lateral interactions between nearest-neighbour adatoms in the overlayer. The model thus provides a link between structural and kinetic studies of chemisorption. It is successfully applied to the β desorption spectra for CO on W reported in Part 1, and a pairwise lateral repulsive interaction energy of 20 kJ mol–1 between C and O adatoms is derived. The kinetic equation is extended to allow for a variation in the C/O adatom ratio, and again the predictions of the model are in close agreement with the experimental CO desorption spectra of Goymour and King from mixed O2+ CO adlayers on W.

Carbon Monoxide Adsorption on Ni(110)

Abstract: The chemisorption of CO on Ni(110) has been studied at temperatures down to −145 °C, by means of LEED, Auger and work function measurements, and flash desorption. Since electron impact caused dissociation of the adsorbed molecules, precautions had to be taken to minimize spurious effects. The LEED patterns showed considerable disorder but could be described approximately by the sequence c(2×2)→(4×2)→c(2×1). These structures suggest that both linear and bridged bonding of CO occurs on Ni(110), and flash desorption revealed two bindings states separated by ∼0.2 eV. The isosteric heat of adsorption for the lower energy state was found to be 1.1 eV. The maximum work function change was 1.6 eV.

Charge Densities and Binding Energies in Hydrogen Chemisorption.

Abstract: : The chemisorption of hydrogen on metals is treated by a self-consistent calculation. Three dimensional charge densities, interaction energies and dipole moments are given as a function of position of the hydrogen adatoms. The results are compared with experiment. The experimental picture of a dissociated adsorbate of small dipole moment is substantiated, and the calculations are consistent with the observation that hydrogen field desorbs together with the surface atoms of the substrate. (Author)

Surface probing of synthetic faujasites by adsorption of carbon dioxide. Part 1.—Infra-red study of carbon dioxide adsorbed on Na-Ca-Y and Na-Mg-Y zeolites

Abstract: Infra-red spectroscopy of carbon dioxide adsorbed on Na-Ca(Mg)-Y faujasites is used to determine the distribution of Ca and Mg ions over the cation sites. Physisorption of CO2 provides a sensitive method for the detection of Ca2+ and Mg2+ in supercage positions. High temperature outgassing of the samples facilitates the migration of bivalent ions from positions in the supercage to the small cavities. Bivalent ions in the dehydrated Y samples are only present in the supercages when the degree of ion exchange is 46 to 48% or higher. For high temperature chemisorption of CO2, the chemisorbed species are closely related to co-ordinated carbonate ions. The concentration of this species does not exceed 2 molecules per unit cell.

Chemisorption of carbon monoxide on tungsten. Part 1.—Desorption spectra and electron stimulated desorption

Abstract: Desorption kinetics and the surface stoichiometry for carbon monoxide on tungsten are reported, with careful attention to the minimising of experimental errors. Desorption spectra at high coverages show an α and two β peaks, the α state desorbing with apparent first order kinetics (activation energy for desorption ∼100 kJ mol–1); the β1 state also with first order kinetics (∼220 kJ mol–1); and the β2 state with higher order kinetics (∼420 kJ mol–1). The isosteric adsorption energy for the β2 state is independent of coverage in this state, within experimental error. It is shown that pumping at room temperature reduces the magnitude of the α state, but not of the β states (contrary to previous results), and it is concluded that α to β conversion during flash desorption is not appreciable. CO2 desorption spectra were recorded from CO on W, but the total amount was ≲ 104 times the amount of CO desorbed. The total β coverage at saturation is 5.2(±0.8)× 1018 molecule m–2. In conjunction with LEED results in the literature, this coverage is shown to be consistent with dissociative adsorption in the β state. This postulate is also consistent with the desorption spectra, as shown in Part 2. Electron stimulated desorption shows that the α state is composed of two sub-states: α1, yielding low energy CO+ ions on bombardment with 100 eV electrons; and α2, yielding high energy O+ ions.

Chemisorption of Ammonia on Silica

Abstract: The interaction of ammonia with silicas prepared by a variety of methods was studied to resolve conflicting reports of the mode of ammonia adsorption and to determine the role of chlorine impuritie...

Molecular orbital investigation of chemisorption. I. Hydrogen on tungsten (100) surface

Abstract: The relative bonding energies of nitrogen chemisorbed at three symmetric sites on a W(100) surface, represented by finite arrays of tungsten atoms [L. W. Anders. R. S. Hansen, and L. S. Bartell, J. Chem. Phys. 59, 5277 (1973)] were obtained by means of the extended Huckel molecular orbital theory (EHMO). The preferred site for nitrogen chemisorption was found to be the five coordination number (5 CN) site or the fourfold site with a tungsten atom below four tungsten atoms surrounding the nitrogen atom. The 5p orbital repulsive energy, in the case of hydrogen chemisorption, could be adequately approximated by the sum over pairs of empirical exponential repulsive terms; in the case of nitrogen chemisorption, this same method was approximately 10% in error at the equilibrium bond distance, and repulsive energies were therefore obtained from calculations including tungsten 5p orbitals but with smaller arrays.

The Chemisorption of Benzene

Abstract: Publisher Summary This chapter describes the chemisorption of the simplest of the aromatic substances, benzene. It is an unsaturated hydrocarbon and may undergo associative chemisorption in a manner not dissimilar to that of olefins and diolefins. However, disruption of the six π-electron system, leading to loss or partial loss of the resonance energy of stabilization, might be so unfavorable as to reduce the likelihood of hydrogen atom addition. Thus, the typical reactions of aliphatic unsaturated hydrocarbons may not be observed. The chapter discusses the experimental work designed to determine the extent and manner of benzene chemisorption by the use of physical methods and the radioisotope carbon-14. Evidence obtained by the use of deuterium as a tracer is examined. Chemisorption of benzene on clean metals in the absence of molecular hydrogen leads to the fission of at least two carbon–hydrogen bonds at the surfaces of the majority of transition elements.