A series of NiO-SiO2-Al2O3 catalysts were synthesized by the sol-gel method. The samples were characterized by a wide array of techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), H-2 temperature-programmed reduction (TPR), and temperature-programmed desorption (TPD) of NH3. The nature of Ni species on the support and their interaction with the support was studied. XPS and TPR studies showed that Ni was present as Ni2+ on the support, and that there was no Ni3+ or Ni-0 in the unreduced samples. Two kinds of NiO were observed on the support, one in which the Ni2+ interacts with the support and the other known as "free" NiO-that is, with no metal-support interactions. The reducibility of NiO decreased with increasing interaction with the support. The extent of interaction between the Ni2+ and the support depended on the total Ni content of the sample and its Si/Al ratio. Increasing the Ni content in the material led to an increase in the relative concentration of free NiO, as well as an increase in the particle size of NiO as seen on TEM. An increase in Si content led to a decrease in the relative free NiO concentration and an increase in the total acidity of the sample, measured by NH3-TPD. The impact of these effects on the catalytic activity of the samples was evaluated in the vapor-phase hydrogenation of naphthalene at 1 atm. A correlation was made between the catalytic activities and the relative concentration of free NiO. Hydrogenation of naphthalene proceeds sequentially by formation first of the partially hydrogenated tetralin and then of the fully hydrogenated decalin. The yield of decalin increased with increasing conversion of naphthalene; however, tetralin was the major product of this reaction. Kinetic studies were carried out to investigate the variation in selectivity with conversion of naphthalene. The reaction was best described using a two-site Langmuir-Hinshelwood model. The adsorption constants and of naphthalene, K-N, and tetralin, K-T, were 4.10 x 10(-3) and 1.88 x 10(-3) m(3)/mol, respectively, at 200 degrees C. The low yield of decalin has been attributed to the weak adsorption of tetralin on the active site. (c) 2006 Elsevier Inc. All rights reserved.