EXAFS studies of supported Rh-Sn catalysts for citral hydrogenation

Rh-Sn/SiO(2) bimetallic catalysts were prepared by "traditional" coimpregnation with aqueous solutions of RhCl(3) and SnCl(2). Their catalytic properties were used to study the mechanism of tin promotion in the liquid-phase selective hydrogenation of citral. The structural evolution during the activation treatments has been investigated by in situ EXAFS spectroscopy. As a function of the activation treatment used, different bimetallic phases have been detected, depending on the extent of the coordination of tin to rhodium atoms. The parallel use of H(2) chemisorption measurements gave complementary information on the surface composition of the bimetallic particles. A freshly calcined-reduced sample may be described as a cherry-like structure, which 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 period during which the catalyst surface restructures, and tin oxides moieties on rhodium particle surface are produced. The selective activation of the carbonyl bond is attributed to the interaction with the tin oxide moieties present in the bimetallic phase. (C) 1999 Academic Press.