X-RAY-DIFFRACTION AND X-RAY PHOTOELECTRON-SPECTROSCOPY STUDY OF THE RU-CU SIO2 SYSTEM PREPARED BY LOW-TEMPERATURE REDUCTION - OCCURRENCE OFA METASTABLE AMORPHOUS OR NANOCRYSTALLINE PHASE/

Bimetallic copper-ruthenium catalysts supported on silica were prepare d by the reduction of the metallic salts in aqueous solution at room t emperature. The concentration of the two metal components was selected to span the entire range of composition. In spite of the known immisc ibility for the copper-ruthenium equilibrium phase diagram, X-Ray Diff raction (XRD) measurements combined with X-ray Photoelectron Spectrosc opy (XPS) data indicate that this method of preparation is able to pro duce nanocrystalline extended solid solutions and/or amorphous metasta ble phases. In the case of ruthenium-rich compositions, the hexagonal close-packed (hcp) ruthenium crystallites are covered by copper atoms which grow with the same hcp sequence of the ruthenium core, For inter mediate compositions a nanocrystalline and/or amorphous phase is obser ved, while in the case of copper-rich samples a single-phase fee exten ded solid solution is found. The surface composition of the samples ap pears systematically enriched with Cu, as obtained from XPS semiquanti tative results. The phenomena of phase separation and growth induced b y thermal annealing at 870 K are also presented and discussed.