Characterisation and activity in n-hexane rearrangement reactions of metallic phases on Pt–Sn/Al2O3 catalysts of different preparations

Abstract

The platinum–tin interactions in Pt–Sn/Al₂O₃ catalysts were followed through several characterisation methods and modified by using two preparation procedures (1.5 wt% Pt, Sn:Pt=1:1): conventional coimpregnation with H₂PtCl₆ and SnCl₄ (T sample) or by use of the bimetallic precursor [Pt(NH₃)₄]SnCl₆, which was synthesised in the support porosity (N sample). The effects of these interactions on catalytic properties were displayed by the activity and selectivity in n-hexane rearrangement reactions. For both samples platinum and tin are reduced, but they have very different platinum dispersions which are related to different temperature-programmed reduction profiles: 52% for sample T and 4% for sample N. Insitu tin Mössbauer spectroscopy confirms that the majority of tin is reduced, and a minority remains as Sn^II; air treatment leads to a partial reoxidation of Sn^II to Sn^IV, sample N retaining more tin as alloy. X-Ray diffraction displays the simultaneous presence of PtSn, Pt₃Sn and Pt with more alloys on sample N; the co-impregnated sample, which has a greater platinum phase, shows a better dispersion 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, whereas the catalyst prepared from the bimetallic precursor exhibits particular properties, attributable to the stabilisation of platinum in smaller ensembles, and the modifying effect of tin was clearly evidenced. The catalytic properties are explained by the distribution and morphology of Pt ensembles present on various faces of Pt–Sn alloys. The lower amount of alloys in sample T can be related to a higher initial activity in C₅ 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 C₆ saturated skeletal isomers in longer runs. The results are supplemented by thermodynamic data on the reduction of tin oxides and by the geometric properties of the low-index faces of PtSn and Pt₃Sn alloys.