The polyhedral Pd and Pt nanocrystals with well-defined morphologies were synthesized via chemical reduction method in an aqueous or polyol system. The catalytic CO oxidation activities over the catalysts with the polyhedral Pd and Pt nanocrystals supported on SiO2 were evaluated. It was found that the CO oxidation activity over the Pd-O/SiO2 catalyst was higher than that over the Pd-C/SiO2 catalyst, and the dominative exposed Pd nanocrystal facet was (111) for the former and (100) for the later catalyst. The Pt-C/SiO2 catalyst loaded with Pt cube nanocrystals mainly exposing (100) facets showed higher CO oxidation activity than the Pt-O/SiO2 catalysts with the (111) plane as the mainly exposed facet. The investigation revealed that the Pd(111) and Pt(100) facets possessed appropriate CO adsorption strength that was responsible for the high activity of CO oxidation, whereas CO adsorption over the Pt(111) was so strong that the CO self-poisoning occurred over the Pt-O/SiO2 catalyst. The CO adsorption on the Pd(100) was so weak that the CO molecules could not be activated over the Pd-C/SiO2 sample. Thus, the CO oxidation over the Pt-O/SiO2 and Pd-C/SiO2 catalysts was suppressed. (C) 2012 Elsevier B. V. All rights reserved.