Characterization and catalysis of PtCu clusters in NaY

Abstract

The metal clusters in Pt NaY , Cu NaY , Cu NaHY , and PtCu NaY have been characterized by TPR, TPD, XRD, XPS, H 2 chemisorption, and methylcyclopentane (MCP) conversion as a catalytic probe. For Cu NaY samples which had been calcined at 300 °C the reduction proceeds by a two-step mechanism: Formation of Cu + is followed by its reduction to Cu particles located in supercages and on the external surface of the zeolite. For acidic samples, Cu NaHY , a high local concentration of protons prevents complete reduction to Cu 0. Reduction of Pt NaY calcined at 300 °C results in small Pt particles in the supercages. Pt strongly catalyzes the reduction of Cu; the Cu TPR peaks merge with the Pt TPR peaks. This is even the case if initially all Pt ions are located in supercages and all Cu ions in hexagonal prisms. The reduced bimetal particles do not show memory of the original location of the ions prior to reduction. The surface of the particles is enriched in Cu, and the turnover frequency for MCP ring opening is low. The selectivity toward n-hexane, 2-methylpentane, and 3-methylpentane shows characteristic changes with composition and size of the bimetal particles. These results are rationalized in terms of a tentative model based on the geometry of particles and cages.