Kinetics and mechanisms of the reverse Boudouard reaction over metal carbonates in connection with the reactions of solid carbon with the metal carbonates

The decomposition processes of alkali or alkaline earth carbonates with a large excess of carbon, and the reverse Boudouard reaction given by CO2/C --> 2CO over metal carbonates, were compared. The carbonates of Li+, Na+, K+, Cs+, Sr2+ and Ba2+ generated CO exclusively by an intermolecular redox reaction given by CO32- + C --> 2CO + O2-. The reverse Boudouard reaction over these metal carbonates at 700 degrees C proceeded at a steady rate until just before the carbon was completely consumed, and in the cases of Li+, Sr2+ and Ba2+, the rates agreed with the initial rates of the intermolecular redox reaction. On the other hand, the rates over the carbonates of Na+, K+ and Cs+, the oxides of which undergo a disproportionation reaction to produce gas-phase metal and liquid-phase metal peroxide, were much higher than the initial rates of the intermolecular redox reaction. This discrepancy can be explained by the presence of a catalytic process on the metal-covered surface of the silica wool that was used for preventing the highly basic gas-phase metals from escaping.