CO CLUSTERS ON MGO STUDIED BY LOW-TEMPERATURE UV-VISIBLE DIFFUSE REFLECTANCE SPECTROSCOPY

A silica cell specially designed for low-temperature measurements of UV-visible diffuse reflectance spectra of adsorbed species has been used to study the interaction of CO with well-outgassed MgO. Spectra recorded with the cell jacket cooled to 77 K show that CO reacts rapidly with MgO to produce strong UV absorption at 360 nm and 290 nm. The bands are attributed to adsorbed CO2(2-) and to ketenic CO dimer/trimer anions, respectively. Larger CO-derived anions containing several CO units (telomers) form at room temperature and produce absorption in the visible with lambda-max = 450 nm. The formation of these clusters at ambient temperature is greatly assisted by pre-conditioning in CO at low temperature, confirming that CO2(2-) and ketenic species are precursors of the telomeric CO anions.