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