CHARACTERIZATION OF MONOMETALLIC AND BIMETALLIC PLATINUM CATALYSTS USING CO FTIR SPECTROSCOPY - SIZE EFFECTS AND TOPOLOGICAL SEGREGATION

CO chemisorption on Pt catalysts supported on non-acidic alumina prepa red from platinum acetylacetonate has been studied by FTIR spectroscop y. The stretching frequency for isolated CO species is observed at 203 2 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 catalys t data, and may be ascribed to the change of coordination at the Pt ch emisorption site. Large shifts, reaching 180 cm(-1) can be observed up on coadsorption of ammonia suggesting the possibility of long-range in teractions involving the collective electronic properties. CO adsorpti on has also been studied on well dispersed bimetallic samples obtained by modifying the well dispersed Pt/Al2O3 catalyst with Sn or Ge. Quit e different effects of the two additives were observed: whereas Ge dec reased the dipole-dipole coupling in the adsorbed layer, Sn did not. T his suggests that Sn segregates at sites of low coordination at the su rface of the Pt particles, while Ge is more homogeneously distributed. The coadsorption of ammonia and CO on the bimetallic particles also s hows that the electronic properties of Pt were dramatically modified b y Ge and unaffected by Sn.