Dynamic Behavior of Carbonate Species on Metal Oxide Surface: Oxygen Scrambling between Adsorbed Carbon Dioxide and Oxide Surface

The oxygen exchange between surface carbonate species and oxide surfaces was investigated by the temperature-programmed desorption (TPD) of C 18 O 2 adsorbed on metal oxides, MgO, CaO, and ZrO 2 at room temperature. Although desorption plots were quite different for these metal oxides, extensive oxygen exchange was commonly observed. The fraction of C 16 O 2 in the total desorbed CO 2 increased monotonically with desorption temperature, and each desorption peak in the plots for the total CO 2 could not be characterized by the isotopic distribution. The difference in proportion of incorporated lattice oxygen in the total CO 2 at each desorption temperature was small between the low and high concentrations of C 18 O 2 . Among these metal oxides, ZrO 2 showed the highest fraction of C 16 O 2 in the total CO 2 desorbed at 400 °C when C 18 O 2 was adsorbed on these metal oxide with same surface concentration. The value of [C 16 O 18 O] 2 /{[C 16 O 2 ][C 18 O 2 ]} was close to 4 over almost all the temperature region of desorption for all these metal oxides, indicating that the three oxygen atoms composing carbonate were entirely scrambled before desorption as CO 2 . The amount of fixed monodentate carbonate with no exchange of oxygen was quite small. The manner of surface lattice oxygen incorporation into carbon dioxide is discussed on the basis of the mechanism proposed in the previous study. Successive adsorption experiments using 13 CO 2 with MgO demonstrated that the carbonate species switches from the weak basic sites to the strong basic sites during the heating process. It was also suggested that the dynamic behavior of carbonate species varies the coordinative environment and the adsorption strength of basic sites, which results in their further migration and desorption as CO 2 .