EXAFS characterization of Pt, Ir and Pt-Ir on gamma alumina catalysts prepared with new precursors : influence of the nature of the precursor and of the acidic pretreatment of the support

Abstract

The use of K2 PtCl6 or (NH4 )3 IrCl6 as precursors instead of H2 PtCl6 or H2 IrCl6 allows us to study the influence of the acidic pretreatment of the alumina on the aqueous impregnation reaction. In the case of Pt/Al2 O3 catalysts, the maximum of adsorption is reached after a neutral or weakly acidic pretreatment, whereas a more acidic one is needed for the adsorption of iridium. The determination by EXAFS of the local environment of the adsorbed metal after filtration and drying leads to the following adsorption schemes where adsn means adsorption over n sites and ads H+ adsorption neutralized by H+ :[math]The difference between the two precursors can be explained by the non saturation of the adsorption sites in the case of platinum. For the iridium-based catalysts, taking into account the EXAFS analysis, it has been possible to estimate the number of the alumina adsorption sites about 400 µmol. g-1 .Bimetallic Pt-Ir/Al2 O3 reforming catalysts were then prepared from two new bimetallic compounds by formation of the precursor in the porosity of the support : [Ir(NH3 )5 Cl] [PtCl4 ] and [Pt (NH3 )4 ]3 [Ir(Cl)6 ]2 . The EXAFS study at the iridium absorption edge shows : (i) the same iridium coordination after the calcination step for both precursors : Ir Cl1 O6.5 ; (ii) the absence of Ir-metal bonds after the reduction step and air handling of the sample, just as (iii) a lowering of the number of oxygen atoms bonded to iridium.