Intermetallic Pt-Cr Clusters in Zeolites as Models of Bimetallic Aromatization and Reforming Catalysts: I. Characterization of Oxidation States, Dispersion, and Local Structure

Abstract

A variety of both structural (EXAFS, XANES, XRD, TEM) and surface-sensitive (XPS, LEISS) techniques have been applied to study the formation, dispersion, composition, and local structure of zeolite-hosted platinum-chromium metal alloys. Both XPS and XANES indicated a two-step reduction of Cr(VI) to Cr(III) and Cr(0) states, while the analysis of EXAFS data demonstrates the formation of platinum-chromium alloy particles after reduction at 823 K. Chromium stabilises small (8-10 Å) platinum particles at lower temperatures (623 K) and when alloyed the particle size increased to 12-20 Å. The Cr/Pt ratio in the alloy is low (<20% chromium), and is independent of the overall Cr/Pt ratio in the ZSM-5, possibly due to space limitations inside the zeolite voids. TEM suggests the formation of some flattened, raft-like metal particles at the external surface of the zeolite. Based on LEISS measurements of surface segregation in a Pt 70Cr 30 alloy film and thermodynamic considerations, a model of the catalyst particles is proposed in which the outermost monolayer consists entirely of platinum and the second shell is enriched in the additive metal, chromium.