Intermetallic Pt-Cr clusters in zeolites as models of bimetallic aromatisation and reforming catalysts

The catalytic properties of a series of Pt-Cr/ZSM-5 zeolites with variable Pt/Cr ratios have been studied in propane and ethane hydrogenolysis. Kinetic results demonstrate a strong effect of the chromium additive both on the reaction rates and activation energies. With propane, the reaction rates drop by an order of magnitude and the activation energies increase from 80 to 120-150 kJ mol[sup -1]. The major effect in ethane hydrogenolysis is a dramatic increase of the apparent activation energy from 135 kJ mol[sup -1] (Pt/ZSM-5) to 230-330 kJ mol[sup -1] (Pt-Cr/ZSM-5). The amount of chromium above 0.75 wt% only slightly affects both the reaction rates and activation energies. The role of chromium in suppressing the hydrogenolysis reaction is explained on the basis of structural analysis of Pt-Cr catalysts and segregation of platinum in the surface monolayer of the alloy particles. The platinum-chromium interaction is assumed to reduce the strength of bonding between the hydrocarbon or its fragments and the metal surface and, consequently to reduce the hydrogenolysis activity. The kinetic compensation observed with ethane may be related to Pt-Cr particle redispersion, noted by EXAFS. The behavior of Pt-Cr catalysts in ethane aromatisation depends on the chromium loading. At lower loading (0.75%) the aromatisation selectivity increases significantly, while at higher loadings the aromatics yield drops dramatically. The promotion is attributed to Pt-Cr alloying, while pore blockage by chromia is probably responsible for deactivation.