ALLOY FORMATION AND SURFACE SEGREGATION IN ZEOLITE-SUPPORTED PT-PD BIMETALLIC CATALYSTS

We present extended X-ray absorption fine structure (EXAFS) and transm ission electron microscopy (TEM) measurements on three zeolite-support ed bimetallic Pt-Pd catalysts prepared by wet impregnation. Alloy form ation and segregation of Pd to the surface of the small bimetallic par ticles is clearly identified. Ab initio LMTO calculations are used to predict segregation profiles of low-index single-crystal Pt-Pd alloy s urfaces. Pd is found to be enriched at the surface. EMT potentials for Pt and Pd are adjusted in order to give the same segregation profiles at low-index single-crystal surfaces as linear muffin tin orbital (LM TO) calculations. Monte Carlo simulations based on the adjusted EMT po tentials are then used to calculate the equilibrium structure of small bimetallic Pt-Pd particles. Coordination numbers derived from these e quilibrium structures are compared with those derived from EXAFS analy sis, and a good agreement is found.