The evolution of the morphology, phase composition, and activity in benzene hydrogenation of Co/SiO2 catalysts, prepared from cobalt nitrate and porous (390 m(2)/g) or nonporous (35 m(2)/g) silica, upon reduction with hydrogen at 350-900 degreesC have been studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic measurements, oxygen uptake, BET, and hydrogen chemisorption. A rapid decline of the activity of Co/SiO2 catalysts to zero was observed with reduction temperature increasing from 400 to 600 degreesC. This effect could not be simply explained by a sintering of cobalt particles or by the reaction of Co with the support and alloys or compound formation. The TEM, H-2 chemisorption, O-2 uptake, and magnetic measurements revealed that at T-red greater than or equal to 500 degreesC coverage (encapsulation) of the metal with a thin, most probably SiO2, overlayer was likely to be the reason. Both Co/SiO2 catalysts reduced at T-red > 500 degreesC exhibited a higher and growing with reduction temperature resistance to oxidation when exposed to oxygen or air as it was evidenced by oxygen uptake, magnetic data, and decreasing H-2 chemisorption capacity. At higher temperature (greater than or equal to 700 degreesC) the formation of a thicker, easily observable with TEM, SiO2 or SiOx, overlayer covering the Co particles took place. As a consequence, the catalysts reduced at 900 degreesC were nearly insensitive to the exposure to air. Oxygen uptake and magnetic measurements ruled out the hypothesis on the Co-Si solid solution or cobalt silicide formation at least in quantities higher than the reliability limits of the experimental methods used. The sintering of Co particles and the significant growth of the mean size of Co crystallites was observed at reduction temperatures of 800 and 850 degreesC for nonporous and porous silica, respectively. An apparent growth of the cobalt content to 107 and 114% of the initial value was noted after reduction at 700 and 900 degreesC, respectively, for both catalysts studied, probably due to the strong support dehydroxylation and/or partial reduction. (C) 2003 Elsevier Inc. All rights reserved.