CHARACTERIZATION AND ACTIVITY IN N-HEXANE REARRANGEMENT REACTIONS OF METALLIC PHASES ON PT-SN AL2O3 CATALYSTS OF DIFFERENT PREPARATIONS/

The platinum-tin interactions in Pt-Sn/Al2O3 catalysts were followed t hrough several characterisation methods and modified by using two prep aration procedures (1.5 wt% Pt, Sn : Pt = 1 : 1): conventional coimpre gnation with H2PtCl6 and SnCl4 (T sample) or by use of the bimetallic precursor [Pt(NH3)(4)]SnCl6, which was synthesised in the support poro sity (N sample). The effects of these interactions on catalytic proper ties were displayed by the activity and selectivity in n-hexane rearra ngement reactions. For both samples platinum and tin are reduced, but they have very different platinum dispersions which are related to dif ferent temperature-programmed reduction profiles: 52% for sample T and 4% for sample N. In situ tin Mossbauer spectroscopy confirms that the majority of tin is reduced, and a minority remains as Sn-II; air trea tment leads to a partial reoxidation of Sn-II to Sn-IV, sample N retai ning more tin as alloy. X-Ray diffraction displays the simultaneous pr esence of PtSn, Pt3Sn and Pt with more alloys on sample N; the co-impr egnated sample, which has a greater platinum phase, shows a better dis persion of tin (XPS data), in accordance with a high interaction with alumina. The catalytic activity was controlled by the platinum phase; for sample T, the influence of the addition of tin is restricted, wher eas the catalyst prepared from the bimetallic precursor exhibits parti cular properties, attributable to the stabilisation of platinum in sma ller ensembles, and the modifying effect of tin was clearly evidenced. The catalytic properties are explained by the distribution and morpho logy of Pt ensembles present on various faces of Pt-Sn alloys. The low er amount of alloys in sample T can be related to a higher initial act ivity in C-5 ring closure whereas the higher amount of these phases on catalyst N is in accord with a higher turnover frequency, and a good selectivity for the formation of olefins which are transformed into C- 6 saturated skeletal isomers in longer runs. The results are supplemen ted by thermodynamic data on the reduction of tin oxides and by the ge ometric properties of the low-index faces of PtSn and Pt3Sn alloys.