Thermal desorption (TD) of l60=C=l8O and l60=C=l6O gases after 180=C=180 adsorption on MgO powders has been observed in the temperature range 350-1000 K.180=C=160; single exchange T on MgO 180=c=180 -160=C=160; double exchange These two exchange reactions were investigated by ab initio calculations with various cluster models including a coordinatively unsaturated oxygen site. The single exchange is found to take place via a bidentate CO, intermediate, and the double one proceeds with CO, migration on the surface.Carbon dioxide is an inert species and the C-0 bond with a bond energy of 191.1 kcal mol-' (8.291 eV) is not broken under mild conditions. As an exception, when it is heated with metals such as Zn and Pd, carbon monoxide is produced. On partly ionic oxide surfaces, it is possible to form carbonate species after adsorption of CO,. On magnetite (Fe,O,) surfaces with C 1802 adsorbed, TD of C l60 l80 (i.e. single oxygen exchange) occurred in the temperature range 200-500 K, suggesting that isotope exchange took place through C 0 3 intermediates.' An inert-inert combination between the typical ionic surface, MgO, and CO, appears not to give any chemical reactions. In fact, only weakly physisorbed CO, species on an MgO(001) surface were observed., Surprisingly, we have found oxygen isotope exchange reactions on defective MgO surfaces by temperature-programmed desorption (TPD) measurement^.^ In this case, the MgO surface is not a catalytic reagent but a reactant. It is of mechanistic interest to examine sites of exchanges on the surface.$The interaction of small molecules such as H, and CO with MgO surfaces was investigated using a molecular orbital (MO) c a l ~u l a t i o n . ~* ~ However, theoretical studies dealing with the oxygen bond interchange between CO, and surfaces have not been known so far. In this work, we present full accounts of those exchange reactions and their plausible mechanisms based on ab initio cluster-model calculations. Particular emphasis is placed on the driving force of the unprecedented double exchange.
Experimental and Computational Methods
ExperimentalAn MgO sample, 'Specpure' grade powders from Johnson-Matthey Chem. Ltd., was preheated at 1150 K in a quartz tube under a high vacuum for more than 10 h. The base pres-
