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

Abstract

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.

Monodisperse platinum nanoparticles of well-defined shape: synthesis, characterization, catalytic properties and future prospects

“…These catalysts have Ag loadings much greater than those used in this study, but confirm Ag influences olefin hydrogenation on Pt catalysts. Adsorption and infrared studies of probe gases (H 2 , O 2 , CO) on Pt-Ag alloy nanoparticles supported on SiO 2 suggest that Pt does not segregate to the surface under reducing conditions, while oxidizing conditions leads to segregation of Ag to the surface for catalysts with Ag atomic compositions as high as 64% [47]. It is worth noting that the pretreatment of all catalysts in this study included a 1 h oxidation step.…”

Heterogeneous hydrogenation catalysts

“…187 New procedures for the preparation of highly selective bimetallic hydrogenation catalysts with controlled dispersities were developed using recent progress in the field of inorganic synthesis and synthesis of organometallic compounds. 189,190 Higher yield of the target product and higher stability of catalysts are achieved by modification of the electronic properties of palladium by the second metal. This approach was employed to prepare the industrial catalyst LT-279 for the selective hydrogenation of acetylene in an ethane ± ethylene fraction.…”

FTIR Studies of the Reduction of Nitric Oxide by Propene on Pt/ZSM-5 in the Presence of Oxygen

“…IR Spectra of NO Adsorption on Pt/ZSM-5 at 473 K. Figure illustrates the in situ IR spectra of NO adsorbed on Pt/ZSM-5 at 473 K. As shown in Figure , there is an obvious growth of the band at 1909 cm -1 with exposure time. The band around 1900 cm -1 has been previously assigned in the literature to molecular adsorption of NO on a partly oxidized platinum-group metal, for example, on Pt/SiO 2 at 1935 cm -1 , on Rh/Al 2 O 3 at 1910 cm -1 (Rh−NO + ), and on the oxidized Ir site of Ir/Al 2 O 3 at 1950 cm -1 . We therefore assign the 1909 cm -1 band on Pt/ZSM-5 to NO molecularly adsorbed on partially oxidized platinum sites.…”

“…This band has been assigned to the adsorbed NO 2 + on ZSM-5, which may associate with the framework hydroxyls of the zeolite . The other IR bands of low frequencies at 1630 cm -1 in Figure are probably attributed to bridged or bent bound NO species on platinum sites. ,

2 DRFTIR spectra taken at 473 K of adsorbed species on Pt/ZSM-5 after the addition of 5% NO/He for (a) 5 min; (b) 15 min; (c) 30 min; (d) 60 min; and (e) after (d), in He for 30 min.

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