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.
