Catalytically active gold sites: nanoparticles, borderline sites, clusters, cations, anions? FTIR spectra analysis of 12CO and of 12CO -13CO isotopic mixtures

The analysis of FTIR absorption spectra of CO and of 12CO-13CO isotopic mixtures is reported for gold catalysts supported on a number of oxides. Relationships between the nanostructure and the CO adsorbing properties of the different gold catalysts are discussed. In particular, on titania supported gold, where particles of metallic nature are present, two kinds of surface sites do adsorb CO, the 6- coordinated corners and the 7- coordinated edges, while the terrace sites, 8 coordinated, are not able to adsorb even at 90 K. Moreover, the adsorbing sites are not isolated, but mutually interacting. On zirconia supported gold, the adsorbing sites, mutually interacting, are exposed at the surface of non metallic gold nanoclusters. Almost isolated and negatively charged gold adsorbing nanoclusters have been highlighted on the sample supported on reduced ceria, and, finally, isolated cationic sites on oxidized gold supported on a mixed ceria-titania oxide have been detected.