Alloy formation and surface segregation in zeolite supported Pt-Pd bimetallic catalysts

We present extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) measurements on three zeolite-supported bimetallic Pt - Pd catalysts prepared by wet impregnation. Alloy formation and segregation of Pd to the surface of the small bimetallic particles is clearly identified. Ab initio LMTO calculations are used to predict segregation profiles of low-index single-crystal Pt - Pd alloy surfaces. 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 (LMTO) calculations. Monte Carlo simulations based on the adjusted EMT potentials are then used to calculate the equilibrium structure of small bimetallic Pt - Pd particles. Coordination numbers derived from these equilibrium structures are compared with those derived from EXAFS analysis, and a good agreement is found.