Chemisorption and FTIR study of bimetallic Pt---Au/SiO₂catalysts

Abstract

Pt/SiO₂, Au/SiO₂, and bimetallic Pt---Au/SiO₂ catalysts were prepared by incipient wetness impregnation of nonporous SiO₂. The catalysts were characterized after reduction in H₂ by static volumetric chemisorption and infrared spectroscopy. For the monometallic and the bimetallic catalysts, H₂, O₂, and CO were used as adsorbates at room temperature. Additionally, for the monometallic Au/SiO₂ catalyst, OZ adsorption at 473 K was also used. Infrared spectra of adsorbed CO were obtained on all catalysts. Addition of gold decreased the uptake of all three adsorbates at room temperature, without significantly influencing the relative amounts of weakly held adsorbed species which could be removed upon pumping for 30 min. Agreeing with CO chemisorption data, the total integrated intensity of the linear CO band decreased with increasing Au content. The IR results indicated a predominantly geometric effect of Au causing a shift of the linear CO band to lower wavenumbers. The shift could be rationalized in terms of decreased dipole-dipole coupling of adsorbed CO species. The results are discussed within the context of a previous investigation using electron microscopy, TPR, and reactivity data. Portions of the bimetallic catalysts were subjected to three high-temperature oxidation/reduction cycles and characterized by static chemisorption of H₂ to investigate the effect of this thermal treatment on Pt dispersion.