Characterization of Pt–Sn/Sio2 catalysts and the role of Sn in NO–hydrocarbon reactions

Abstract

Pt–Sn/SiO₂ catalysts prepared by selective chemical vapour deposition of Sn(CH₃)₄ to Pt particles supported on SiO₂ have been characterized by extended X-ray absorption fine structure (EXAFS) at both Sn K-edge and Pt LIII-edge, FTIR, transmission electron microscopy (TEM), and H₂ and CO adsorption measurements. Sn atoms were suggested to be located at the first layer of the bimetallic particles until Sn/Pt = 0.26, giving an Sn—Pt bond length of 0.263 nm and Sn—Sn bond length of 0.260 nm. A model structure of the Pt–Sn bimetallic layers was proposed. The Pt–Sn/SiO₂ catalyst was remarkably active for NO dissociation, leaving deposited oxygen atoms at the surface. The deposited oxygen atoms were bonded to the Sn atoms to give Sn—O bonds of length 0.198 nm, which were active for hydrocarbon oxidation reactions. The reactivity of the SnO_x layer was suggested to be promoted by the presence of Pt atoms. The catalytic NO–C₃H₈ and NO–CH₄ reactions proceeded via NO dissociation and successive oxidation of hydrocarbons on Pt–Sn/SiO₂. The reaction rate and selectivity varied with the Sn/Pt ratio. Sn addition increased the N₂ selectivity from 50% for Pt/SiO₂ to 100% for Pt–Sn/SiO₂. N₂ selectivity was related mainly to the oxygen deficiency in the SnO_x layers.