Study on the mechanism of oxidation of nitrogen-containing char by CO₂ based on density functional theory

In order to obtain the NO formation mechanism during the coal combustion, the heterogeneous oxidation of nitrogen-containing char by CO₂ were investigated based on density functional theory. Simplified char models containing pyrrole nitrogen or pyridine nitrogen were selected as the carbonaceous surfaces. Geometric optimizations were carried out at the B3LYP-D3/6-31G(d) level. Energies of optimized geometries were calculated at the B3LYP-D3/def2-TZVP level. The results show that CO₂ oxidation of nitrogen-containing char is composed of three stages: namely CO₂ adsorption, CO desorption and NO desorption. In the reaction of CO₂ heterogeneous oxidation of pyrrole nitrogen-containing char, CO₂ molecules tend to absorb in the C−O−down mode (C−C bonding, N−O bonding) to form a five-membered heterocyclic structure containing nitrogen and oxygen atoms. Then, the surface carbonyl groups and N(O) are formation as the C−O bonds of the original CO₂ molecules in the five-membered ring broken to desorb CO and NO, respectively. The reaction is 401.2 kJ/mol endothermic, and the highest energy barrier is 197.6 kJ/mol. In the reaction of CO₂ heterogeneous oxidation of pyridine nitrogen-containing char, CO₂ molecules tend to form six-membered heterocyclic ring containing nitrogen and oxygen atoms after adsorption in the C−O−down and C−C bonding and C−O bonding mode. And then CO and NO molecules are desorbed. The reaction is 598.6 kJ/mol endothermic, and the energy barrier of rate-determining step is 292.0 kJ/mol.