A comparative study of carbon gasification with O₂ and CO₂ by density functional theory calculations

A comparative study of carbon gasification with O₂ and CO₂ was conducted by using density functional theory calculations. It was found that the activation energy and the number of active sites in carbon gasification reactions are significantly affected by both the capacity and manner of gas chemisorption. O₂ has a strong adsorption capacity and the dissociative chemisorption of O₂ is thermodynamically favorable on either bare carbon surface or even isolated edge sites. As a result, a large number of semiquinone and o-quinone oxygen can be formed indicating a significant increase in the number of active sites. Moreover, the weaker o-quinone C-C bonds can also drive the reaction forward at (ca. 30%) lower activation energy. Epoxy oxygen forms under relatively high O₂ pressure, and it can only increase the number of active sites, not further reduce the activation energy. CO₂ has a lower adsorption capacity. Dissociative chemisorption of CO₂ can only occur on two consecutive edge sites and o-quinone oxygen formed from CO₂ chemisorption is negligible, let alone epoxy oxygen. Therefore, CO₂-carbon reaction needs (ca 30%) higher activation energy. Furthermore, the effective active sites are also reduced by the manner of CO₂ chemisorption. A combination of the higher activation energy and the fewer active sites leads to the much lower reaction rate of CO₂-carbon.