Characterization of oxide surfaces by adsorption of carbon monoxide—a low temperature infrared spectroscopy study

Infrared spectra at low temperatures of CO adsorbed on metal oxides (MgO, γ-Al ₂0 ₃, TiO ₂ (rutile), CeO ₂, α-Cr ₂0 ₃ and SiO ₂-Al ₂0 ₃) are reported. The heat of adsorption is low in these systems which makes CO a rather selective surface probe. The adsorption bond to Lewis acid sites is a σ-donor bond which leads to positive frequency shifts of the carbonyl stretching mode relative to the gas phase I'requency of the free molecule. It is shown that these frequency shifts correlate with the electric field strength of the cation centers, thus providing information on the oxidation state and coordination of these sites. Moreover, heats of adsorption can be determined spectroscopieally, which permits a relative acidity scale to be established.