Formation of bimetallic alloys in naphtha reforming Pt-Ge/Al2O3 catalysts:: an EXAFS study

The formation of Pt-Ge alloys in Pt-Ge/Al2O3 catalysts by high-temperature H-2 reduction was studied using EXAFS spectroscopy and a variety of physical, spectroscopic, and catalytic techniques. Two Pt-Ge/Al2O3 catalysts were prepared by impregnating a high-purity gamma-alumina with hydrochloric solutions containing H2PtCl6 and GeCl4. Reduction of the Pt-Ge/ Al2O3 catalysts at 773 K did not sinter the Pt crystallites but drastically decreased both the H-2 chemisorption on Pt and the catalytic activity for cyclohexane dehydrogenation. EXAFS spectra taken at the Pt Lm-edge revealed the existence of Pt-Ge distances, thereby indicating the formation of alloyed Pt-Ge particles. The reduction of a part of the GeOx species to metallic germanium by the H-2 treatment was confirmed by EXAFS spectra taken at the Ge K-edge. The electronic nature of alloyed Pt-Ge particles was studied by XANES spectroscopy. The intensity of the Pt white line of the Pt-Ge catalyst reduced at 773 K was higher than that of Pt foil, which suggested that Ge acts as an electron withdrawing element in bimetallic Pt-Ge clusters. This interpretation was consistent with results obtained by both the competitive hydrogenation of benzene and toluene, and the infrared spectroscopy of adsorbed CO. Results show that the H-2 treatment of Pt-Ge/Al2O3 catalysts at 773 K reduces the Ge cations in the vicinity of Pt crystallites and induces formation of Pt-Ge alloy particles. In Pt-Ge clusters, Ge increases the electrophilic character of Pt and dilutes the surface Pt atoms thereby decreasing both the dehydrogenation activity and the H-2 chemisorption capacity. (C) 1999 Elsevier Science B.V. All rights reserved.