In situ EXAFS studies of Pt-Sn/Al2O3 catalysts prepared by a new procedure

In-situ EXAFS experiments have been performed on fresh Pt-Sn/Al2O3 catalysts prepared with a bimetallic precursor [Pt(NH3)(4)][SnCl6]. The nature of the samples and the choice of the reaction conditions was defined by previous catalytic results as to obtain the highest platinum-tin interactions. The direct reduction (H-2; 450 degrees C) of the dried samples was followed by oxidation-reduction stages as to simulate air-aging effects and to test regeneration conditions. The Pt-Sn interactions are shown during the reduction steps, leading to a dispersion of platinum with limited Pt ensembles due to the vicinity of Sn atoms and to the formation of Pt-Sn alloys. EXAFS data analysis leads to the main following results : (i) the first step of the reduction from 50 to 200 degrees C gives Pt clusters containing tin in the core and increasing in size with temperature; (ii) from 200 degrees C, the onset of tin reduction permits the formation of Pt-Sn alloys; (iii) changing the nature of the gas and the temperature give information on the stability of alloys; the alloys are stable only under H-2 atmosphere and are partially destroyed under N-2; an air treatment leads to a partial reoxidation of tin in the alloy phase and of platinum clusters; and (iv) the regeneration stage shows that a part of metallic tin does not participate to the reformation of the Pt-Sn alloys.