Analysis of the Reaction between O2 and Nitrogen-Containing Char Using the Density Functional Theory

Quantum chemistry calculations using a reasonably simplified nitrogen-containing char model have been performed to clarify mechanisms for CO and NO desorption during the reaction between molecular oxygen and nitrogen-containing char. The density functional theory at the B3LYP/6-31G(d) level was used to optimize geometries of reactants, products, stable intermediates, and transition states in possible reaction pathways. The first step was chemisorption of molecular oxygen across two adjacent benzene rings on the char surface with 141.6 kJ mol(-1) exothermic. The adsorbed molecular oxygen tends to dissociate in three different possible modes, resulting in CO and NO desorption. Computational results show that the formation of CO and NO can be achieved in three steps: (1) cleavage of corresponding bonds to form open-ring structures, (2) spontaneous ring contraction with the formation of species containing a ketene group or a nitroso group on a five-membered ring, and (3) desorption of CO or NO, regenerating free active sites for further gasification. The number of active sites available for reaction on the carbon surface fluctuates as a result of the open-ring reaction, cyclization reaction, and CO/NO desorption. The overall process for CO and NO desorption from oxidation of nitrogen-containing char by O-2 was exothermic. Energy barriers for NO desorption were lower than those for CO desorption. Nitrogen in char could be preferentially oxidized because of the lower energy barriers.