EXAFS studies of supported Rh-Sn catalysts for citral hydrogenation

Rh-Sn/SiO2 bimetallic catalysts were prepared by "traditional" coimpregnati on with aqueous solutions of RhCl3 and SnCl2. Their catalytic properties we re used to study the mechanism of tin promotion in the liquid-phase selecti ve hydrogenation of citral. The structural evolution during the activation treatments has been investigated by in situ EXAFS spectroscopy. As a functi on of the activation treatment used, different bimetallic phases have been detected, depending on the extent of the coordination of tin to rhodium ato ms. The parallel use of H-2 chemisorption measurements gave complementary i nformation on the surface composition of the bimetallic particles. A freshl y calcined-reduced sample may be described as a cherry-like structure, whic h is selective for the formation of unsaturated alcohols. On the contrary, an alloy-type phase obtained by simple reduction is totally unselective. The production of unsaturated alcohols starts only after an induction perio d during which the catalyst surface restructures, and tin oxides moieties o n rhodium particle surface are produced. The selective activation of the ca rbonyl bond is attributed to the interaction with the tin oxide moieties pr esent in the bimetallic phase. (C) 1999 Academic Press.