Support and particle size effects on direct NO decomposition over platinum

Abstract

Supported Pt catalysts were prepared by ion exchange of Pt(II) tetraamine hydroxide [Pt(NH 3) 4(OH) 2] onto SiO 2, Al 2O 3, and TiO 2 and by adsorption of Pt bis-acetylacetonate [Pt(acac) 2] onto MgO. All the catalysts except Pt/TiO 2 were characterized by in situ X-ray absorption fine structure (XAFS) spectroscopy after reduction at 300 °C in flowing H 2, and the Pt/SiO 2 and Pt/MgO catalysts were examined after subsequent exposure to 1% NO/He at 25–300 °C. 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 600 °C. NO decomposition at low temperatures (200–400 °C) produces N 2O 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 ∼350 °C; N 2O production declines above 400 °C; and N 2 and O 2 are the only detectable NO decomposition products at 600 °C. Nanometer-sized Pt clusters on SiO 2 sinter during heating in 1% NO/He at 300 °C, whereas larger Pt particles supported on SiO 2 and MgO are resistant to sintering under these conditions. The turnover frequency for direct NO decomposition over Pt/SiO 2 catalysts is insensitive to Pt particle size. Pt/SiO 2, Pt/Al 2O 3, and Pt/TiO 2 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.