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 cat alysts prepared with a bimetallic precursor [Pt(NH3)(4)][SnCl6]. The n ature of the samples and the choice of the reaction conditions was def ined by previous catalytic results as to obtain the highest platinum-t in interactions. The direct reduction (H-2; 450 degrees C) of the drie d samples was followed by oxidation-reduction stages as to simulate ai r-aging effects and to test regeneration conditions. The Pt-Sn interac tions are shown during the reduction steps, leading to a dispersion of platinum with limited Pt ensembles due to the vicinity of Sn atoms an d to the formation of Pt-Sn alloys. EXAFS data analysis leads to the m ain following results : (i) the first step of the reduction from 50 to 200 degrees C gives Pt clusters containing tin in the core and increa sing in size with temperature; (ii) from 200 degrees C, the onset of t in reduction permits the formation of Pt-Sn alloys; (iii) changing the nature of the gas and the temperature give information on the stabili ty 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 reo xidation of tin in the alloy phase and of platinum clusters; and (iv) the regeneration stage shows that a part of metallic tin does not part icipate to the reformation of the Pt-Sn alloys.