The Investigation of the Different Mechanisms of CarbonGasification with O2 and CO2 by Density Function Theorycalculations

摘要

The reaction mechanisms of carbon gasification with O2 and CO2 were investigated by using density function theorycalculations. We found that the activation energy and the number of active sites in carbon gasification reactions aresignificantly affected by both the capacity and manner of gas chemisorption. O2 adsorption is strong and the dissociativechemisorption of O2 is thermodynamically favourable on either bare carbon surface or even isolated edge sites. As a result, alarge number of semiquinone and o-quinone oxygens can be formed, and the weaker o-quinone adjacent C-C bonds can drivethe reaction forward at (ca.30%) lower activation energy than the semiquinone adjacent C-C bonds (without off-planeepoxy). Epoxy oxygen forms under relatively high O2 pressure, and it can only increase the number of active sites, not furtherreduce the activation energy. CO2 adsorption is much weaker. Dissociative chemisorption of CO2 can only occur on twoconsecutive 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 alsoreduced by the manner of CO2 chemisorption. A combination of the higher activation energy and the fewer active sites leadsto the much lower reaction rate of CO2-carbon.