The initial stages of catalytic activity of mono-metallic Pt/Al2O3 as well as bimetallic Pt-Sn, Pt-Re and Pt-Pb supported on A12O3 have been studied in n-heptane (and also, n-hexane) conversion. Initially all catalysts exhibited strong cracking activity. This could be suppressed by accumulating carbonaceous deposits. Gradually benzene and toluene formation became predominant. This tendency was more pronounced with higher hydrocarbon pressure; even less hydrocarbon ensured a more rapid change with Pt-Sn and Pt-Pb. Pt-Re occupied an intermediate position. A model was developed to explain the activity shifts: accordingly, small concentrations of C1 species produced during hydrogenolysis, could migrate to the support; in higher concentrations, however, they could act as inert additives like Sn. In still higher concentrations they would polymerize and cause catalyst deactivation. This model was supported by radiotracer studies.
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