Ab Initio molecular orbital study of the unified mechanism and pathways for gas-carbon reactions

Ab Initio molecular orbital calculations are performed at the B3LYP/6-31G(d)/HF/3-21G(d) calculation level on graphite and oxygen intermediates on and near the edges of graphite. The results are in agreement with experimental data for structural geometry parameters, vibrational frequencies, and, more importantly, bond energies. Three intermediates are studied: semiquinone, carbonyl, and an off-plane, epoxy oxygen. The rate-limiting step for all gasification reactions by oxygen-containing gases is the breakage of C-C bonds to free CO from the semiquinone intermediate. The energy for this C-C bond is near 80 kcal/mol, which is close to the experimental activation energy for the reactions with CO₂ and H₂O. The C-C bond is weakened by 33% by the adjacent epoxy oxygen, to a bond energy of nearly 53 kcal/mol. This extent of weakening in the C-C bond energy coincides with the decrease in the experimental activation energy from 85 kcal/mol for the reactions with CO₂ and H₂O to 58 kcal/mol for the reaction with O₂. The equilibrium constant of dissociative chemisorption of O₂ on edge carbon is several orders of magnitude higher than that of CO₂ and H₂O. Hence the epoxy oxide intermediate only exists in the C + O₂ reaction. The C-C bond breakage that is weakened by epoxy oxygen gives rise to the low-temperature TPD (temperature programmed desorption) peak (near 450°C), whereas the high-temperature peak (near 950°C) is due to that without the epoxy intermediate. Two different reaction pathways are proposed for the C + O₂ and C + CO₂/H₂O reactions. All three oxygen intermediates play important roles in these pathways.