X-ray diffraction and x-ray photoelectron spectroscopy study of the Ru-Cu/SiO2 system prepared by low temperature reduction: Occurrence of a metastable amorphous or nanocrystalline phase

Bimetallic copper-ruthenium catalysts supported on silica were prepared by the reduction of the metallic salts in aqueous solution at room temperature. The concentration of the two metal components was selected to span the entire range of composition. In spite of the known immiscibility for the copper-ruthenium equilibrium phase diagram, X-Ray Diffraction (XRD) measurements combined with X-ray Photoelectron Spectroscopy (XPS) data indicate that this method of preparation is able to produce nanocrystalline extended solid solutions and/or amorphous metastable 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 intermediate compositions a nanocrystalline and/or amorphous phase is observed, while in the case of copper-rich samples a single-phase fee extended solid solution is found. The surface composition of the samples appears systematically enriched with Cu, as obtained from XPS semiquantitative results. The phenomena of phase separation and growth induced by thermal annealing at 870 K are also presented and discussed.