Browsing by Author "John, Christopher S."

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    Determination of the Active Site and Mechanism for Alkene Isomerization in Cun Exchanged Y-Type Zeolite
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (10), 1977) John, Christopher S.; Leach, H. Frank
    Reactions of CD2[double bond, length as m-dash]CH—CH3, but-1-ene and 3,3-dimethylbut-1-ene have been studied over a Y-type zeolite containing ∼4 CuII per unit cell. Kinetic induction periods, associated with the generation of active sites by CuII reduction, were only observed for the deuterium redistribution reactions of CD2[double bond, length as m-dash]CH—CH3. Nevertheless, arguments are presented to show that such generation of active sites is central to alkene isomerization reactions over CuII exchanged X- and Y-type zeolites. The mechanism of alkene isomerization is shown to require carbonium ions, generated from protons produced following CuII reduction by the alkene.
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    Interaction of Diatomic Molecules with Clean Metal Wire Surfaces Part 2.—Nitrogen on Tungsten
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (06), 1977) Couper, Aitken; John, Christopher S.
    Heats of adsorption, sticking probabilities and temperature programmed desorption spectra are reported for nitrogen on polycrystalline tungsten at 290 and 222 K. The initial heat of adsorption at 290 K was found to be 380 ± 14 kJ mol–1. Three atomic states for nitrogen adsorption on tungsten were found to desorb between 1150 and 1450 K. In addition, a metastable, molecular state of nitrogen was found which desorbed between 200 and 950 K. This metastable state, able to cover an estimated 10 % of the surface at 222 K, was found to dominate the adsorption behaviour of nitrogen on tungsten at 222 K. It had an estimated heat of adsorption of 190 kJ mol–1. The ability to investigate such metastable states is an important feature of the calorimetric technique which has been employed.
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    Interaction of diatomic molecules with clean metal wire surfaces. Part 1.—Hydrogen on tungsten
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (06), 1977) Couper, Aitken; John, Christopher S.
    Heats of adsorption, sticking probabilities and temperature programmed desorption spectra are reported for hydrogen on polycrystalline tungsten at 88 and 290 K. The heats were measured using a fine-wire calorimeter, as invented by J. K. Roberts, but with automatic recording. The initial heat of adsorption onto a clean surface at 290 K was 192 ± 8 kJ mol–1. Three desorption peaks were observed in the range 290–700 K. A discussion of the results is presented in terms of the crystal planes believed to be present at this polycrystalline surface. It is deduced that the heat of adsorption for hydrogen onto the (100) and (110) planes is 137 ± 5 kJ mol–1 and that on these planes there is a repulsive interaction energy between near neighbours of 5.25 ± 0.50 kJ (mol pair)–1. The importance of adatom interaction in this system is emphasised.
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    Role of Breasted Acid Centres for Alkene Double Bond Migration over Alumina at Temperatures above 450 K
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (02), 1978) John, Christopher S.; Tada, Akio; Kennedy, Lois V. F.
    Water was added to alumina samples following their activation in vacuo at both 723 and 1023 K. Reactions of [2Ho]propene and of CD2=CH—CH2 were then studied on these catalysts. Adsorption of water (2.7 x 1018 molecule m-2) at ~300 K is molecular and serves only slightly to poison catalytic activity at ~300 K. Adsorption of water at >453 K occurs dissociatively and completely poisons those sites on the fresh catalyst surface which promote reactions at ~300 K. Reaction of propene occurs on such a water poisoned catalyst at ~483 K and proceeds with incorporation of Db (from D2O) through an associative mechanism involving carbonium ions, in contrast to a dissociative mechanism at a reaction temperature of 300 K on an unpoisoned catalyst. Water “ added-back ” to activated alumina at >453 K is therefore effective in producing Bronsted acid centres on the surface and it is suggested that residual hydroxyls, those left following activation, can also participate in carbonium ion formation from propene at >453 K. Therefore, not only is the activity of alumina strongly influenced by its activation temperature but it is also determined by the temperature used for subsequent alkene catalysis.
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    Sorption of Hydrogen by Palladium and Palladium/Silver Alloy Wires
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (02), 1978) Couper, Aitken; John, Christopher S.
    Heats of adsorption q{8), sticking probabilities and temperature programmed desorption spectra are reported for the adsorption of H2 onto clean wire surfaces of Pd, all as functions of coverage 0. Data, principally from temperature programmed desorption, have also been obtained for the inter action of H2 with two Pd/Ag alloys. The low temperature (104 K) sorption properties of Pd were dominated by the occurrence of absorption which limited the applicability of the technique used to determine