Surface reconstruction of Pt–Sn nanoparticles supported on Al2O3 in the presence of carbon monoxide
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FTIR spectroscopy and volumetric measurements are used to study the adsorption of CO (mainly 1% CO/He) in the temperature range 300–713 K on a 1.2% Pt–2.7% Sn/Al2O3 solid reduced in H2 at 713 K leading from XRD to PtSn bimetallic particles. This reveals the changes in the adsorption properties of the Pt sites for CO adsorption by comparison with monometallic Pt/Al2O3 solids and the stability of the Pt–Sn bimetallic particles in the presence of CO. At 300 K, FTIR spectroscopy shows that the insertion of Sn leads to (a) the total disappearance of the Pt sites forming bridged CO species (ascribed to a geometric effect of Sn) and (b) a significant shift in the position of the IR band of linear CO species on Pt sites from 2066 to 2044 cm−1 on Pt and Pt–Sn particles, respectively, ascribed to different adsorbed species, namely LPt and L1Pt–Sn, respectively. Moreover, it is shown that the insertion of Sn is associated with the decrease in the amount (in µmol/g of platinum) of Pt adsorption sites for CO adsorption. The evolution of the IR band of the L1Pt–Sn CO species with the increase in Ta in isobaric conditions reveals a modification of the surface of the Pt–Sn particles for Ta > ≈ 460 K ascribed to enrichment in Pt° due to a surface reconstruction. The Pt sites of the reconstructed surface are characterized by an IR band at 2057 cm−1 after adsorption of CO at 300 K ascribed to a linear CO species named L2Pt–Sn. The reconstructed surface is stable in the presence of CO in the range 300–713 K and disappears by hydrogen reduction at 713 K. Successive surface reconstruction/hydrogen reduction at 713 K cycles lead to an ageing of the Pt–Sn particles associated with a progressive decrease in the amount of Pt° sites on the freshly prepared and reconstructed Pt–Sn particles. It is shown that the reconstruction of the Pt–Sn particles is probably due to the formation of SnOx species via oxygen species coming mainly from the hydroxyls groups of the support.