Spectroscopic study of CO adsorption on CoO-MgO solid solutions. 2. CO adsorption on isolated cobalt ions located on edges and steps

The room-temperature surface chemistry of diluted MgO-CoO solid solutions toward CO is totally accounted for by the selective adsorption of CO on edges and steps whereby the Mg2+, Co2+ and O2- coordinately unsaturated ions primarily react as O2-Mg2+O and/or O2-Co2+O2- triplets to form C2O22- (ketenic-like) and [(CO2)2CoCO]2- (carbenoid-carbonylic) species whose relative concentration depends upon the cobalt oxide mole fraction. Besides these species, also Mg2+-CO (σ-bonded) and Co2+(CO)3 (σ-π-bonded) complexes are contemporarily detected. The simultaneous presence in fixed ratios of both Co2+, Mg2+ and (O2-Mg2+O2-), (O2-Co2+O2-) reactive ions and groupings of ions in the same surface regions is considered as a "fingerprint" manifestation of the "edge chemistry" and has been interpreted on the basis of a simplified geometric (unrelaxed) model of the edge regions. The vibrational assignments of all the surface structures have been made on the basis of a complete set of 12CO13CO isotopic substitution experiments.