Surface organometallic chemistry on metals. II. Characterization of new bimetallic catalysts generated by reaction of Sn(n-C₄H₉)₄ with silica-supported rhodium

Reaction of Sn(n-C₄H₉)₄ with Rh supported on silica results in a new bimetallic RhSn catalyst which is extremely active and selective in the reduction of ethyl acetate to ethanol. Whereas Rh/SiO₂ gives rise to a selectivity for ethanol of 57%, the RhSn catalyst obtained by the organo-metallic route results in a higher activity and a selectivity to ethanol as high as 98%. Above a Sn/Rh value of 0.3, the activity varies linearly with the tin content which suggests that the enhanced catalytic activity is due to a new intermetallic phase. The catalysts have been characterized at various steps of the preparation. The starting reduced catalyst Rh/SiO₂ A with CO exhibits the typical infrared absorption bands of linear and bridged CO. Reaction of oxidized A with Sn(n-C₄H₉)₄ in refluxing heptane occurs mostly between Rh₂O₃ and the organotin compound to give an unreduced Rh^III7z.sbnd;SnR_x bimetallic surface complex B, the existence of which has been suggested from mass balance, STEM, and IR spectroscopy. Reduction of B at 773 K under H₂ leads to bimetallic particles with an average size of 2.2 nm and which do not contain any organic fragment (catalyst C). C chemisorbs only 0.1 H/Rh_t and 0.4 CO/Rh_t which is in sharp contrast with the values obtained with A (1.1 H/Rh_t and 1.1 CO/Rh_t). CO chemisorption on B gives only a single absorption band at 2000 cm^-1 corresponding to linear coordination of CO. The presence of tin has apparently three effects: (i) it decreases significantly the amount of CO and H₂ adsorbed; (ii) it apparently isolates rhodium atoms from their neighbors; (iii) it increases slightly the electron density on rhodium. Redox behavior of the RhSn/SiO₂ toward O₂ and silanol groups of silica has also been observed. With a fully reduced catalyst C, Rh⁽⁰⁾ and Sn⁽⁰⁾ are fully oxidized by O₂ to Rh₂O₃ and SnO₂. Thermal treatment of catalyst C under flowing He results in an oxidation of tin by surface silanol (or adsorbed water) to give a partially oxidized Sn species. H₂ is evolved during this oxidation process. The origin of the high activity and high selectivity (without hydrogenolysis property) of these catalysts is ascribed to the presence of a new catalytic phase in which rhodium atoms are isolated from their neighbors without any "ensemble" able to cleave the CC and CO bonds of ethyl acetate.