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The interaction of NO with Pt/SiO 2 and PtAg/Si02 catalysts is investigated by infrared spectroscopy, volumetric chemisorption and mass spectrometry. Room temperature adsorption of NO on the reduced Pt/Si02 catalyst mainly leads to molecular adsorption, accompanied by a small N2 production. Infrared spectra reveal absorption bands at 1780 and 1600 cm j, ascribed to the stretching vibrations of linearly and bridged bonded NO species, respectively. Furthermore, a weak band at 1935 cm 'is observed. This high-frequency band is strongly enhanced when NO is dosed at elevated temperatures and is attributed to NO bound to Pt sites in the neighbourhood of adsorbed oxygen atoms. Reaction of NO with Pt/SiO2 at 400°Cleads to a limited dissociation of NO, producing N2 in the gas phase and leaving oxygen atoms to remain on the catalyst surface. With the PtAg/Si02 catalysts the 1600 cm band is absent, while the activity for NO dissociation has been lowered as compared to Pt/Si02. Alloying of Pt with Ag diminishes the amount of bridged bonded species which seem to precede dissociation, whereas linearly bonded species desorb molecularly at high temperatures. Investigations of the catalysts after oxidation at 400°C in 1 atm 02 show the adsorption of NO to be weaker and non-dissociative even at 400°C.

The interaction of oxygen with a silver catalyst containing isolated silver particles of ca. 70 nm supported on a-Al,O, has been studied using (combined) volumetric adsorption and temperature-programmed desorption measurements after various pretreatments. The amount of oxygen taken up is greatly influenced by the pretreatment. Considerable amounts of oxygen are able to penetrate into the silver lattice. Impurities present in or on the silver particles strongly affect the amount of oxygen taken up at 443 K. It is argued that penetrated oxygen can facilitate the reorganization of the silver surface giving rise to low-index facets. Plate 1. A representative SEM micrograph of the catalyst.

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