The interaction of nitric oxide with platinum and platinum-silver catalysts

Abstract

The interaction of NO with Pt/SiO2 and PtAg/SiO2 catalysts is investigated by infrared spectroscopy, volumetric chemisorption and mass spectrometry. Room temperature adsorption of NO on the reduced Pt/SiO2 catalysts mainly leads to molecular adsorption, accompanied by a small N2 production. Infrared spectra reveal adsorption bands at 1780 and 1600 cm-1, ascribed to the stretching vibrations of linearly and bridged bonded NO species, respectively. Furthermore, a weak band at 1935 cm-1 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 neighborhood of adsorbed oxygen atoms. Reaction of NO with Pt/SiO2 at 400°C leads 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/SiO2 catalysts the 1600 cm-1 band is absent, while the activity for NO dissociation has been lowered as compared to Pt/SiO2. 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 O2 show the adsorption of NO to be weaker and non-dissociative even at 400°C.