Platinum nanoparticles (Pt-NPs) of about 3 nm were synthesized by colloidal methods and encapsulated in mesoporous Al2O3 and CeO2-Al2O3 prepared by sol-gel method A new strategy to synthesize Pt clusters of uniform size supported on mixed oxides was achieved and the catalysts were characterized by Fourier transform Infrared spectroscopy of adsorbed CO transmission electron microscopy and X-ray absorption fine structure spectroscopy and tested in steam reforming of methane (SRM) and partial oxidation of methane (POM) Pt-NPs were well dispersed on the supports and there was no significant modification of their size during calcination and under POM conditions In the SRM the CH4 turnover rate on the Pt-NPs/CeO2-Al2O3 catalyst was twice higher than the one on the Pt-NPs/Al2O3 catalyst despite their similar Pt-NP sizes The use of colloidal Pt-NPs to produce nanocatalysts with similar particle size clearly pointed out the important role of the CeO2 in promoting the catalytic activity (C) 2010 Elsevier Inc All rights reserved