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

• Published in: Langmuir 2003-10, Vol.19 (21), p.8793-8800
• Main Authors: Tsuji, Hideto, Okamura-Yoshida, Akie, Shishido, Tetsuya, Hattori, Hideshi
• Format: Article
• Language: English
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la0342666

The oxygen exchange between surface carbonate species and oxide surfaces was investigated by the temperature-programmed desorption (TPD) of C18O2 adsorbed on metal oxides, MgO, CaO, and ZrO2 at room temperature. Although desorption plots were quite different for these metal oxides, extensive oxygen exchange was commonly observed. The fraction of C16O2 in the total desorbed CO2 increased monotonically with desorption temperature, and each desorption peak in the plots for the total CO2 could not be characterized by the isotopic distribution. The difference in proportion of incorporated lattice oxygen in the total CO2 at each desorption temperature was small between the low and high concentrations of C18O2. Among these metal oxides, ZrO2 showed the highest fraction of C16O2 in the total CO2 desorbed at 400 °C when C18O2 was adsorbed on these metal oxide with same surface concentration. The value of [C16O18O]2/{[C16O2][C18O2]} 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 CO2. 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 13CO2 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 CO2.