Well-defined nanoparticles obtained by modern wet chemical synthesis methods can be very useful building blocks for construction of model heterogeneous catalysts. Here we show the challenges involved in the construction of heterogeneous Ni catalysts from supports and colloidal nanoparticles. Surprisingly, "1 + 1" is not equal to "the best catalyst", following this concept. The thought-provoking lack of catalytic activity of supported Ni NPs ill the hydrogenation of cyclohexene and the low activity of these materials in the steam reforming of ethanol were analyzed in details by TEM, XPS, DRIFTS and CO adsorption experiments. The factors responsible for the low catalytic activity of Ni/SiO2 system were proposed. The removal of the ligands, used in the synthesis of the colloidal Ni nanoparticles, in the post-steps of catalyst preparation is cumbersome. In fact, low temperature treatments are not able to efficiently expose the Ni surface whereas high-temperature treatments lead to particle sintering. In this systematic Study Several aspects of catalyst preparation through impregnation of colloidal nanoparticles are addressed. (C) 2008 Elsevier B.V. All rights reserved.