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 organometallic 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 (50-100 Torr). Introduction of small amounts of dihydrogen allows rhodium to 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 under experimental conditions, Physicochemical investigations have shown that such deposits are characterized by pure, crystallized rhodium particles, X-ray photoelectron spectroscopy experiments carried out on planar supports (substrates) confirmed the presence of Rh(O) on the surface; near the interface, Rh(I) and Rh(III) centers have been detected, presumably covalently bound to the support. Several homogeneous as well as heterogeneous steps in the reaction mechanism have been shown to be present by monitoring the deposition by mass spectrometry and by infrared spectroscopy. These catalysts display a greater activity for hydrogenation than the corresponding catalysts prepared by the conventional impregnation procedure. (C) 1995 Academic Press, Inc.