Effect of the ensemble size in the hydrogenation of benzene on Pt and Pt‐Au catalysts

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Abstract
Hydrogenation of benzene and cyclohexene has been studied on Pt and four Pt‐Au alloys. The overall behaviour −‐ activity, selfpoisoning and regeneration was similar to that of Ni−Cu alloys. The surface of the two‐phase Pt−Au alloys had, according to the kinetic data a composition of the β‐alloy (Au‐rich phase). Hydrogenation of benzene was 103 to 104 times slower than the hydrogenation of cyclohexene. The most dilute alloys are active in cyclohexene hydrogenation but they are completely inactive in the low temperature hydrogenation of benzene. The main effect of alloying is the decrease in the pre‐exponential term of the rate. The effect is explained in geometric terms: activation of a benzene molecule for the low temperature hydrogenation requires larger ensembles than those for cyclohexene activation and the larger ensembles are missing on the surface of the most dilute alloys.