Characterization of CO species adsorbed on well-degassed surfaces of polycrystalline MgO at low CO pressures (< 1300 Pa) and at and below room temperature was studied by using temperature-programmed desorption (TPD) and infrared (IR) spectroscopies. Mutual transformation between adsorbed species stable at room temperature, and their reactivities with 0 2 were also investigated. Adsorbed species are classified into five groups, KO, K1 (and K1' ), Kz, K3, and &. Among them species KO and K1 (and K1' ) are stable only below room temperature while species KZ to & are stable above this temperature. Species KO is a linear-type CO monomer with the C atom bonded to surface Mg2+ and has five subspecies depending on its circumstance, while all the other species K1 to & are formed on surface 02-to which the C atom is linked. A chained-type CO monomer, K1, is reversibly transformed at ca. 230 K in the presence of gaseous CO into a linear-type CO monomer, K1'. Species KZ is a chained-type tetramer of CO with a carbonyl CO bond and resonanced bonds. There are two subspecies, K~A and Km, which are formed by the addition of one CO molecule to corresponding trimer species, K~A and K~x , respectively, in the presence of gaseous CO. K~A and Kzx are reversibly transformed into K~A (at 400 K) and K~x (in 300-400 K), respectively, by the elimination of the CO added.There are two more subspecies, K~B and K~c , in species K3, but they have no corresponding KZ subspecies, and hence no transformations occur. All the K3 species seem to be a linear-type trimer with a ketenic group, >C=O=O, and are desorbed in 520-580 K. Species &, though detailed information is not obtained, consists of three subspecies &A, &B, and &c, but no transformations between species K3 and & are observed.
