Supported Pt catalysts were prepared by ion exchange of Pt(II) tetraamine hydroxide [Pt(NH3)(4)(OH)(2)] onto SiO2, Al2O3, and TiO2 and by adsorption of Pt bis-acetylacetonate [Pt(acac)(2)] onto MgO. All the catalysts except Pt/TiO2 were characterized by in situ X-ray absorption fine structure (XAFS) spectroscopy after reduction at 300degreesC in flowing H-2, and the Pt/SiO2 and Pt/MgO catalysts were examined after subsequent exposure to 1% NO/He at 25-300degreesC. NO decomposition pathways were investigated by temperature-programmed reaction spectroscopy (TPRS) using a 1% NO/He feed, and steady-state conversions for direct NO decomposition were measured at 600degreesC. NO decomposition at low temperatures (200-400degreesC) produces N2O and N-2, and in situ XAFS spectroscopy indicates that the supported Pt particles are partially oxidized under these conditions. Over each catalyst, O-2 production begins at similar to350degreesC; N2O production declines above 400degreesC; and N-2 and O-2 are the only detectable NO decomposition products at 600degreesC. Nanometer-sized Pt clusters on SiO2 sinter during heating in 1% NO/He at 300degreesC, whereas larger Pt particles supported on SiO2 and MgO are resistant to sintering under these conditions. The turnover frequency for direct NO decomposition over Pt/SiO2 catalysts is insensitive to Pt particle size. Pt/SiO2, Pt/Al2O3, and Pt/TiO2 catalysts have equivalent NO decomposition activities suggesting the absence of support effects for these metal oxides. In contrast, the NO decomposition activity of Pt supported on strongly basic MgO is significantly lower. (C) 2004 Elsevier B.V. All rights reserved.