Characterization of supported platinum, hydrogenation and H2–D2 equilibration

Abstract

Experimental work on the characterization of platinum supported on alumina by means of a poisoning method and of a pulse technique is described. COS, H₂S, SO₂, C₄H₄S and CS₂ were used as poisons; the hydrogenation of several hydrocarbons and H₂–D₂ equilibration were used as indicators. CS₂ proved to be the compound exhibiting the best poisoning properties. Results of metallic dispersion, considering a stoichiometry S: Me of 1:1, i.e. each molecule of CS₂ blocking two metallic centres are compared with those obtained by H₂–O₂ titration. The agreement between the two methods is better at low metallic concentrations. The number of active centres found for a 32 Å colloidal platinum on alumina catalyst is equal for all the hydrogenation processes studied and for H₂–D₂ equilibration. This is attributed to ethylene, propylene, 1-butene, cyclohexene and benzene hydrogenation and H₂–D₂ equilibration taking place on two metallic centres. The scheme proposed for the hydrogenation implies a mechanism in which the olefin or benzene bonded to one metallic atom interacts with atomic hydrogen adsorbed on one platinum atom. A Bonhoeffer–Farkas mechanism for H₂–D₂ equilibration is assumed.