Microwaves Improve CO₂ To CO Conversion

The conversion of carbon dioxide to carbon monoxide over a solid carbon substrate, known as the Boudouard reaction, is a key component of coal gasification and smelting processes. Scientists are also interested in harnessing the reaction to remediate excess CO₂ because it’s a greenhouse gas. But the reaction CO₂ + C → 2 CO normally requires temperatures above 600 °C, making it costly. Albert E. Stiegman and colleagues at Florida State University show they can reduce the temperature at which the equilibrium favors CO production to a mere 213 °C by using micro­waves instead of heat sources such as steam (J. Phys. Chem. C 2013, DOI: 10.1021/jp4076965). Although microwaves have been known to increase rates of hetero­geneous gas-phase reactions, the altered thermodynamics based on this type of heating method have not been observed before, the researchers say. Because microwaves rapidly heat the solid carbon substrate while CO₂ gas passes through unheated, this reaction lends itself well to the study of microwave heating mechanisms—determining whether microwaves are just heating the reactants or exerting an irradiation effect. Stiegman’s team posits that microwave heating could be causing the carbon substrate to generate a steady supply of electron-hole pairs with which the CO₂ reacts.