FTIR indication of CO interaction with O2− ions: A new adsorption form in the gap between chemi- and physisorbed CO

The IR bands around 2140 cm−1 appearing after low temperature (T > 77 K) CO adsorption on oxide and zeolite powders have been, as a rule, assigned to physisorbed CO. However, the bands vary with different samples, which suggests specific adsorption. It is concluded that in these cases CO is polarized by surface oxygen anions because at the conditions at which “physisorbed” CO is formed all cationic sites are already occupied. Studying a titanium oxynitride sample where no physically adsorbed CO was observed proved the hypothesis. Upon oxidation of the sample and creation of surface O2− anions, bands at 2141 and 2136 cm−1 appeared after low-temperature CO adsorption. It was suggested that analysis of the bands around 2140 cm−1 could provide information on the characteristics on the solids, in particular on surface basicity.