(14C) Tracer Studies of the Hydrogenation of Acetylene Over Supported Metal Catalysts

Al-Ammar, Asad Saleam (1977) (14C) Tracer Studies of the Hydrogenation of Acetylene Over Supported Metal Catalysts. PhD thesis, University of Glasgow.

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Abstract

The adsorption of ethylene, acetylene and carbon monoxide on silica supported rhodium, palladium and iridium, and alumina supported palladium has been studied using a (14C) radiotracer technique. The adsorption isotherms for ethylene and acetylene over each catalyst show two distinct regions; a steep primary region followed by a linear secondary region. In contrast, carbon monoxide adsorption shows only a non-linear primary region, as expected from Langmuir type adsorption. Evidence has been obtained to show that on the primary region acetylene is adsorbed predominantly as a dissociative species of average composition C2Ha, values of a being 1.4 (Pd), 1.6 (Ir) and 1.8 (Rh); these values being constant throughout the entire primary region. It is also suggested that the primary adsorption of ethylene and acetylene occurs directly on the metal, whereas the secondary adsorption probably involves the formation of overlayers on the primary adsorbed species. In the adsorption of acetylene at least three types of surface species are recognised; acetylene which participates in the hydrogenation reaction, which is located on the secondary region, acetylene which does not participate directly in the hydrogenation, but which can be removed by prolonged treatment in hydrogen, and acetylene which is permanently retained on the surface at 298K. These latter two species are located on the primary region. In the hydrogenation of acetylene over each catalyst it has been observed that the activity decreases from reaction to reaction, until eventually a steady limiting activity is attained. On catalysts which had been "run in" to constant activity the adsorption of acetylene and ethylene shows only a limited or no primary region at all. Although the primary adsorbed acetylenic species do not participate directly in the hydrogenation reaction, the actual rate of hydrogenation is nevertheless directly proportional to the fraction of the primary adsorbed species which can be removed by prolonged treatment in hydrogen. These observations are interpreted in terms of a mechanism for acetylene hydrogenation in which the addition of hydrogen to associatively adsorbed acetylene, located on the secondary region involves hydrogen transfer between a dissociatively adsorbed acetylenic species and the associatively adsorbed acetylene. Direct addition of hydrogen to associatively adsorbed acetylene is not thought to occur. From studies of the competitive adsorption of ethylene and acetylene, and from the behaviour of added (14C)-ethylene during acetylene hydrogenation, it is concluded that the adsorption of acetylene and ethylene in a catalytically active form occurs on independent sites, located on the secondary region. It has also been observed that during acetylene hydrogenation, a small amount of ethylene hydrogenation occurs independently, this amount varying from metal to metal. The rate of hydrogenation of ethylene in the presence of acetylene is much lower than in the absence of the latter, although the surface coverages of ethylene are independent of the presence or absence of acetylene. This behaviour is interpreted as showing that the hydrogen availability for ethylene hydrogenation is different in the two cases. With Rh/SiO2 and Ir/SiO2 catalysts the hydrogenation of acetylene was poisoned completely by carbon monoxide, while with Pd/SiO2 carbon monoxide reduce considerably the rate of the hydrogenation reaction. The poisoning effect of carbon monoxide. is interpreted in terms of displacement of adsorbed hydrogen by carbon monoxide.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1977
Depositing User: Enlighten Team
Unique ID: glathesis:1977-78753
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 14:56
Last Modified: 30 Jan 2020 14:56
URI: http://theses.gla.ac.uk/id/eprint/78753

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