PT ZNO SYSTEM - IR STUDY OF THE VIBRATIONAL AND ELECTRONIC EFFECTS INDUCED BY HEATING IN CO ATMOSPHERE

Two differently pretreated Pt/ZnO samples were studied with FTIR and HRTEM analysis. HRTEM data show that in mildly reduced samples almost spherical Pt particles are present, while in strongly reduced ones faceted particles of δ-PtZn alloy are formed. The analysis of the IR spectra concerns both the vibrational modes of the adsorbed species and the variations in the IR transparency of the samples, variations related to the electronic properties of the material. IR data indicate that CO adsorption on very mildly reduced samples occurs on stepped Pt particles; on pure H2 reduced ones the surface composition of the metal particles changes from pure Pt to PtZn alloy. Heating in CO atmosphere causes, on mildly prereduced samples, reduction of the ZnO and formation of PtZn particles; on strongly prereduced samples it causes surfaces segregation of Pt on the PtZn alloy. A sensible chemical effect of the alloying on the CO stretching frequency has been put in evidence when Pt surface segregation occurs. The red-shift of the band compared with that of pure Pt indicates that in the alloy the Pt atoms make a bond with CO with a higher backdonation character. The IR transparency changes observed after these treatments have been interpreted as due to changes in the work function of the metallic phase as a consequence of the alloying and of the alloy composition. © 1990.