EFFECTS OF STRUCTURAL DEFECTS AND ALLOYING ON THE FTIR SPECTRA OF CO ADSORBED ON PT ZNO/

The curve fitting analysis of the FTIR spectra of two differently pret reated Pt/ZnO dispersed samples is discussed. Carbon monoxide adsorbed on a photodeposited Pt/ZnO sample mildly reduced in a 0.5% H-2/N-2 mi xture at 493 K shows a main band at 2100 cm(-1), shifting to the red a nd reducing its intensity by decreasing the surface coverage. All the spectra are well fitted by the addition of five Lorentzian bands. Look ing at the literature, the five components, on the basis of their spec troscopic features can be assigned to Pt-X+ sites, Pt-0(111) terrace s ites with coordination number 9, Pt-0(100) terrace sites with coordina tion number 8, and Pt-0 step sites with coordination number 7. The spe ctra of CO adsorbed on the same Pt/ZnO sample reduced in pure H-2 at 5 23 K exhibit a main band at approximate to 2070 cm(-1), at lower frequ ency, significantly less intense and further red-shifting by decreasin g the coverage. In this case the band appears as the superposition of two bands, not completely well fitted by Lorentzian bands. The maximum of these bands appears related to Pt atoms with coordination numbers 6 and 5. It must be stressed that by increasing the temperature of red uction in the case of Pt supported on refractory oxides, usually prepa red by ionic exchange, a sintering and clustering occurs; these proces ses convert almost isolated low coordinated metal sites into terrace s ites and induce a blue-shift of the absorption band. Our mildly reduce d samples, as a consequence of the photoreductive method, are made of well crystallized Pt small particles. By increasing the reduction temp erature, the alloying of Pt with Zn atoms coming from the support occu rs; the opposite frequency shift observed and the change in the shape of the band can be ascribed to the chemical effect of the alloying of platinum with zinc. This hypothesis is confirmed by the analysis of is otopic mixtures experiments.