Browsing by Author "Webb, Geoffrey"

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    Hydrogenation of Acetylene over Supported Metal Catalysts: Part 1.—Adsorption of [14C]Acetylene and [14C]Ethylene on Silica Supported Rhodium, Iridium and Palladium and Alumina Supported Palladium
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (1), 1978) Al-Ammar, Asad S.; Webb, Geoffrey
    The adsorption of [14C]ethylene and [14C]acetylene on supported palladium, rhodium and iridium catalysts occurs irreversibly at 298 K in two distinct stages; a non-linear primary region, in which the species are predominantly dissociatively adsorbed, and a linear secondary region. Hydrogenation catalysis is associated with the hydrocarbon species adsorbed on the secondary region. From [14C]carbon monoxide adsorptions it is concluded that the hydrocarbon primary region is associated with the metal, whilst the secondary region probably involves the formation of overlayers on the primary adsorbed species. The co-adsorption of ethylene and acetylene shows that, under acetylene hydrogenation conditions, both are adsorbed at separate sites and undergo hydrogenation independently of each other. The relevance of these observations to the selective hydrogenation of acetylene is discussed.
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    Hydrogenation of Acetylene over Supported Metal Catalysts: Part 2.—[14C] Tracer Study of Deactivation Phenomena
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (3), 1978) Asad, S. Al-Ammar; Webb, Geoffrey
    In the hydrogenation of acetylene over Pd + SiO2, Rh + SiO2, Ir + SiO2 and Pd + Al2O3, the activities of the catalysts progressively decrease to a steady state constant activity. This deactivation is irreversible at 298 K and can only be effected by acetylene + hydrogen reaction mixtures; acetylene alone produces no significant deactivation. The selectivity and overall reaction orders are independent of the catalytic activity. Adsorptions of [14C]C2H2 and [14C]C2H4 show that the deactivation is associated with the progressive build up of permanently retained acetylenic species on the catalyst surface. Three types of adsorbed acetylene are recognised. The results are interpreted in terms of the hydrogenation reaction occurring by a hydrogen-transfer mechanism between a dissociatively adsorbed C2Hx species and associatively adsorbed acetylene, which forms an overlayer on the dissociatively adsorbed acetylene. The permanent retention of acetylenic species may be due to surface polymer formation.