ONE-STEP PREPARATION OF HIGHLY DISPERSED SUPPORTED RHODIUM CATALYSTS BY LOW-TEMPERATURE ORGANOMETALLIC CHEMICAL-VAPOR-DEPOSITION

A novel method for preparing supported rhodium catalysts using organom etallic chemical vapor deposition (OMCVD) has been developed, Vapor of a rhodium complex is brought into contact with silica in a fluidized bed in a special reactor designed to work under reduced pressure (50-1 00 Torr). Introduction of small amounts of dihydrogen allows rhodium t o be deposited at low temperature (100 degrees C); small aggregates (1 -2 nm) and high dispersions (0.95-0.70) are obtained. Three convenient precursors, [Rh(mu-Cl)(CO)(2)](2), [Rh(eta(3)-C3H5)(3)], and [Rh(acac )(CO)(2)], have been used, owing to their suitable vapor pressures und er experimental conditions, Physicochemical investigations have shown that such deposits are characterized by pure, crystallized rhodium par ticles, X-ray photoelectron spectroscopy experiments carried out on pl anar supports (substrates) confirmed the presence of Rh(O) on the surf ace; near the interface, Rh(I) and Rh(III) centers have been detected, presumably covalently bound to the support. Several homogeneous as we ll as heterogeneous steps in the reaction mechanism have been shown to be present by monitoring the deposition by mass spectrometry and by i nfrared spectroscopy. These catalysts display a greater activity for h ydrogenation than the corresponding catalysts prepared by the conventi onal impregnation procedure. (C) 1995 Academic Press, Inc.