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 pretreated 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 mixture at 493 K shows a main band at 2100 cm(-1), shifting to the red and reducing its intensity by decreasing the surface coverage. All the spectra are well fitted by the addition of five Lorentzian bands. Looking at the literature, the five components, on the basis of their spectroscopic features can be assigned to Pt-X+ sites, Pt-0(111) terrace sites with coordination number 9, Pt-0(100) terrace sites with coordination number 8, and Pt-0 step sites with coordination number 7. The spectra of CO adsorbed on the same Pt/ZnO sample reduced in pure H-2 at 523 K exhibit a main band at approximate to 2070 cm(-1), at lower frequency, significantly less intense and further red-shifting by decreasing 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 reduction in the case of Pt supported on refractory oxides, usually prepared by ionic exchange, a sintering and clustering occurs; these processes convert almost isolated low coordinated metal sites into terrace sites and induce a blue-shift of the absorption band. Our mildly reduced samples, as a consequence of the photoreductive method, are made of well crystallized Pt small particles. By increasing the reduction temperature, the alloying of Pt with Zn atoms coming from the support occurs; 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 isotopic mixtures experiments.