Fourier-transform infrared studies of carbon monoxide adsorption on platinum and palladium hydrosols: influence of pH

Abstract

Carbon monoxide adsorption on aqueous platinum and palladium sols has been studied by IR spectroscopy over the pH range 2.0–12.6. IR spectra of the CO-saturated platinum sols at pH ≈ 5.0 show a single asymmetric band at ca. 2070 cm^–1 due to CO adsorbed linearly on platinum metal surface sites. The decrease in ν(CO)_ads as pH is increased has been attributed to a decrease in coverage of adsorbed CO induced by hydroxyl adsorption. At pH < 5.0 the platinum sols are unstable and exhibit broadening of the ν(CO)_ads band associated with aggregation. In contrast, IR spectra of CO-saturated palladium sols at pH ≈ 6.0 exhibit bands due to adsorbed CO on two-fold bridge sites (1950 cm^–1) and to linearly adsorbed CO (2067 cm^–1). The ν(CO)_ads for palladium sols is relatively insensitive to pH over the range 2.0–11.5 but decreases sharply at pH > 11.5. Correlations of ν(CO)_ads with θ suggest that CO island formation is an important aspect of CO adsorption on both the platinum and palladium sols. It was found that CO adsorbed on colloidal platinum at pH < 4.0 does not exhibit similar behaviour to that of CO adsorbed on platinum electrodes in acidic media.