Four different rhodium precursors, [Rh(COD)H](4), [Rh(COD)(2)]+BF4-, [Rh(COD)Cl](2), and RhCl3.3H(2)O, adsorbed onto silica (SiO2) or onto palladium supported on silica (Pd-SiO2), were examined for their catalytic arene hydrogenation activities. Rates of toluene hydrogenation were compared when the catalysts were pretreated with hydrogen as follows: none, 40degreesC for 24 h. and 200 degreesC for 4 h. With no hydrogen pretreatment, the rhodium precursors adsorbed on Pd-SiO2 showed higher activities than the corresponding precursors on only SiO2. As the hydrogen pretreatment temperature was increased, the activities of the catalysts on SiO2 increased; the highest activity occurred with catalysts pretreated at 200 degreesC. Conversely, milder pretreatments were more effective for the catalysts on Pd-SiO2; higher temperature pretreatments reduced activities. The catalysts were characterized by their activities, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). All of the studies indicate that the active catalytic species is rhodium metal. On silica, H-2 reduction of the rhodium complex to Rh metal under arene hydrogenation conditions was slow in all cases except [Rh(COD)H](4). However, on Pd-SiO2, the palladium accelerated the reduction of all of the Rh complex precursors to rhodium metal under the same conditions. The resulting Rh-Pd-SiO2 bimetallic catalysts exhibited higher activities than catalysts of the same composition prepared by classical methods. (C) 2003 Elsevier Science B.V. All rights reserved.