Formation of bimetallic alloys in naphtha reforming Pt–Ge/Al2O3 catalysts: an EXAFS study Original Research Article

The formation of Pt–Ge alloys in Pt–Ge/Al2O3 catalysts by high-temperature H2 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 γ-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 H2 chemisorption on Pt and the catalytic activity for cyclohexane dehydrogenation. EXAFS spectra taken at the Pt LIII-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 H2 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 H2 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 H2 chemisorption capacity.