Preparation and characterization of Pt particles in unidimensional microporous supports

A series of Pt-exchanged molecular sieves was studied by X-ray absorption s pectroscopy (XANES and EXAFS) in order to probe the effect of the structura l and chemical characteristics of selected molecular sieves on the Pt parti cles. For each molecular sieve (LTL, MOR, and AFI) different preparation pr ocedures (ion exchange, wet impregnation and vapor phase impregnation) were applied. For a particular molecular sieve the particle size increased in t he order, ion exchange<vapor impregnation<wet impregnation. The hydrogen ch emisorbed per Pt increases linearly with the fraction of Pt exposed as calc ulated from EXAFS analysis indicating that on the average one hydrogen atom is adsorbed per Pt on dense oxide supports, while Pt in acidic and neutral molecular sieves show an approximate to 25% lower value. For a given dispe rsion of Pt the amount of sorbed hydrogen per Pt is systematically lower fo r Pt supported on acid and neutral molecular sieves than for Pt supported o n dense oxides. For Pt in alkali exchanged LTL, the results suggest that tw o hydrogen atoms are adsorbed per accessible Pt. (C) 1998 Elsevier Science B.V. All rights reserved.