Journal Articles (before-1995)

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    Chemisorption of nitrogen on tungsten studied by auger electron spectroscopy
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (10), 1974) Joyner, Richard W.; Rickman, John; Roberts, M. Wyn
    By monitoring the surface directly by Auger electron spectroscopy (AES), adsorption isotherms for nitrogen on polycrystalline tungsten ribbons have been determined in the temperature range 1200–1400 K. These are not of the Langmuir type, but can be linearized by plotting log P(eq) against coverage, θ, and therefore conform to the Temkin equation. The isosteric heat of adsorption (ΔHads) is initially about 370 kJ mol–1, but falls slightly with increasing coverage. Integral molar entropies of adsorption are determined and shown to fall between those calculated for an immobile layer and a 2-dimensional perfect gas. The cylindrical Auger analyzer has been used in the fast scanning mode to measure absolute rates of desorption and these are shown to be strictly second order below θ= 1.50 × 1014 molecule cm–2. This is taken to indicate that the adsorption of nitrogen is dissociative. The rate constants for desorption yield ΔEdes= 330 ± 50 kJ mol–1 in agreement with the ΔHads values. By considering the measured rates of desorption sticking probabilities have been determined and the mechanism of desorption shown to conform to a simple collision model. The residence times of nitrogen adatoms are calculated and shown to be in accord with the Frenkel equation.
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    Chemisorption of Carbon Monoxide on Tungsten: Part 1.—Desorption Spectra and Electron Stimulated Desorption
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (4), 1973) Goymour, Clarence G.; King, David A.
    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.
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    Chemisorption and Decomposition of Tetramethylsilane over Tungsten and Iron Surfaces
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (2), 1972) Roberts, M W; Ross, J R H
    The adsorption and subsequent decomposition of tetramcthylsilane Si(CH3)4 on evaporated films of tungsten and iron have been investigated. The results are compared with results previously obtained with neopentane C(CH3)4. Si(CH3)4 is rapidly and irreversibly adsorbed on tungsten at 293 K, and gaseous hydrogen and methane arc formed ; further H2 and CH4 arc desorbed on heating the surface to temperatures up to 420 K. Additional information regarding the adsorbed phase has been obtained from deuterium exchange experiments. Similar experiments carried out with iron show that much less dissociation of the Si(CH3)4 occurs on adsorption and on heating the adsorbed layer; no gaseous products arc observed below 320 K. Comparison of both sets of results with those for C(CH3)4 indicates that Si(CH3)4 is more extensively dissociated under all conditions; this is attributed mainly to the weaker C—Si bond. The results suggest that the rate determining step in hydrocracking reactions of hydrocarbons is likely to be hydrocarbon breakdown rather than desorption of products.
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    Chemisorption of Hydrogen on Evaporated Copper Films
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (2), 1972) Alexander, C S; Pritchard, J
    Surface potentials (s.p.) of atomic hydrogen adsorbed at low temperatures on clean copper films show wide variations from one film to another. This is believed to reflect varying proportions of different crystal planes. Initial adsorption with a negative s.p. may be followed by a weaker adsorption with a positive effect. The negative s.p. corresponds to hydrogen which can be in equilibrium with gaseous molecular hydrogen. This spontaneous dissociative chemisorption has been studied between 242 and 337 K at pressures up to 60 Torr, using vibrating capacitor s.p. measurements to follow the equilibrium concentration of adatoms. Isosteric heats (typically 40-50 kJ mol-1) are almost independent of coverage on some films, and a major part of the adsorption can then be described approximately by the Langmuir isotherm for dissociative adsorption.