Characterization of mono- and bi-metallic platinum catalysts using CO FTIR spectroscopy - Size effects and topological segregation

CO chemisorption on Pt catalysts supported on non-acidic alumina prepared from platinum acetylacetonate has been studied by FTIR spectroscopy. The stretching frequency for isolated CO species is observed at 2032 cm(-1) for highly dispersed Pt samples (dispersion 0.99) and at 2067 cm(-1) for poorly dispersed samples (dispersion 0.09). This frequency shift is in agreement with single-crystal and on supported Pt catalyst data, and may be ascribed to the change of coordination at the Pt chemisorption site. Large shifts, reaching 180 cm(-1) can be observed upon coadsorption of ammonia suggesting the possibility of long-range interactions involving the collective electronic properties. CO adsorption has also been studied on well dispersed bimetallic samples obtained by modifying the well dispersed Pt/Al2O3 catalyst with Sn or Ge. Quite different effects of the two additives were observed: whereas Ge decreased the dipole-dipole coupling in the adsorbed layer, Sn did not. This suggests that Sn segregates at sites of low coordination at the surface of the Pt particles, while Ge is more homogeneously distributed. The coadsorption of ammonia and CO on the bimetallic particles also shows that the electronic properties of Pt were dramatically modified by Ge and unaffected by Sn.