Reflectance Spectra of Carbon Monoxide Adsorbed on Alkaline Earth Oxides

Abstract

The adsorption of carbon monoxide at room temperature on well-outgassed specimens of MgO, CaO, SrO and BaO gives rise to a strong absorption spectrum between ν= 15000 and ν= 40000 cm–1. The stronger the outgassing conditions, the more intense is the absorption which can be developed.

The spectrum consists of two main envelopes, one comprising absorption at ν= 29000–40000 cm–1 and the other, more composite in character, at ν= 15000–29000 cm–1. Changes in the CO pressure affect the two spectral regions differently, showing that two different kinds of adsorbed species are involved.

The formation of the new spectrum as CO is dosed is accompanied by a simultaneous decrease in the absorption due to surface states (coordinatively unsaturated surface oxide ions). The effect on the oxide absorption is more pronounced with SrO and CaO than with MgO. The bulk states are unaffected, showing that the process is entirely a surface reaction.

The results are interpreted as an electron donor process in which CO accepts electrons from surface oxide ions and forms anionic clusters in the adsorbed state. The high-ν absorption (ν > 29 000 cm–1) is ascribed to the dimer (CO)2–2, and the low-ν absorption to polymeric oxocarbon anions (CO)xn, where n > 2. The latter are considered to include cyclic, resonance-stabilized structures. The reactivity of the oxide towards CO increases with increase in basic character: on the MgO surface there are relatively few sites of sufficient donor potential to react with CO, but on the more strongly basic CaO and SrO the reaction is much more extensive.