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.