Browsing by Author "Roberts, M. Wyn"

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Adsorption of Carbon Monoxide on Copper (100) Studied by Photoelectron Spectroscopy and Low Energy Electron Diffraction
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74(3), 1978) Isa, Saadoon A.; Joyner, Richard W.; Roberts, M. Wyn
By combining u.v. and X-ray photoelectron spectroscopy with low energy electron diffraction, Cu(100) has been shown to adsorb carbon monoxide at 295 K. Adsorption, although molecular, has distinctly different spectroscopic characteristics from adsorption at 80 K. The oxygen (1s) binding energy is 1.3 eV lower at 295 K than at 80 K, the carbon 1s binding energy is also lower. The sticking probability is ∼10–6 at 295 K although close to unity at 80 K, and the energy of desorption is greater for the room temperature state. The LEED pattern, (√2 ×√2)R45° is the same as observed at 80 K (for θ < 0.5) but the maximum coverage is substantially smaller at 295 K (θ∼ 0.3) and the diffraction spots more diffuse. At 80 K the photoelectron spectroscopy and LEED data are similar to those observed previously. Changes in the u.v. induced spectra with increasing CO coverage are shown to reflect the onset of LEED “compression structures” rather than the presence of two adsorbed phases of carbon monoxide. We suggest that the existence of two distinct types of CO bonding on copper (100), (at 295 and 80 K) is a consequence of the molecule acting more as an electron acceptor, (“COδ–”) at 295 K and more like an electron donor, (“COδ+”) at 80 K. The presence of a chemisorption level 1.5 eV below the Fermi edge, i.e. in the s-band region of the spectrum, is interpreted as being due to reorganisation of metal s and d electrons which is in keeping with the transition metal-like characteristics of the adsorption at 295 K.
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.
Evidence for the nature of CO adsorbed on nickel from electron spectroscopy
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (10), 1974) Joyner, Richard W.; Roberts, M. Wyn
At 295 K there is clear evidence from the observed X-ray and u.v. induced spectra that CO is adsorbed in a molecular form. The observed spectra are in good agreement with the ionisation potentials expected of electrons in molecular orbitals of CO(g). Changes in both the X-ray photoelectron spectra and u.v. photoelectron spectra on heating to 430 K are interpreted as reflecting thermally induced dissociation of the adsorbed CO.