Microwave-Specific Enhancement of the Carbon-Carbon Dioxide (Boudouard) Reaction

The Boudouard reaction, which is the reaction of carbon and carbon dioxide to produce carbon monoxide, represents a simple and straightforward method for the remediation of carbon dioxide in the environment through reduction: CO2(g) + C(s) (sic) 2CO. However, due to the large positive enthalpy, typically reported to be 172 kJ/mol under standard conditions at 298 K, the equilibrium does not favor CO production until temperatures >700 degrees C, when the entropic term, -T Delta S, begins to dominate and the free energy becomes negative. We have found that, under microwave irradiation to selectively heat the carbon, dramatically different thermodynamics for the reaction are observed. During kinetic studies of the reaction under conditions of flowing CO2, the apparent activation energy dropped from 118.4 kJ/mol under conventional convective heating to 38.5 kJ/mol under microwave irradiation. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction dropped from 183.3 kJ/mol at similar to 1100 K to 33.4 kJ/mol at the same temperature under microwave irradiation. This changes the position of the equilibrium so that the temperature at which CO becomes the major product drops from 643 degrees C in the conventional thermal reaction to 213 degrees C in the microwave. The observed reduction in the apparent enthalpy of the microwave driven reaction, compared to what is determined for the thermal reaction from standard heats of formation, can be thought of as arising from additional energy being put into the carbon by the microwaves, effectively increasing its apparent standard enthalpy. Mechanistically, it is hypothesized that the enhanced reactivity arises from the interaction of CO2 with the steady-state concentration of electron-hole pairs that are present at the surface of the carbon due to the space-charge mechanism, by which microwaves are known to heat carbon. Such a mechanism is unique to microwave-induced heating and, given the effect it has on the thermodynamics of the Boudouard reaction, suggests that its use may yield energy savings in driving the general class of gas-carbon reactions.