Theoretical study of the reactions between arsenic and nitrogen oxides during coal combustion

The reactions between arsenic and nitrogen oxides (N₂O, NO₂, and NO) were investigated using density functional theory. The geometries of the reactants, intermediates, transition states, and products in each reaction were optimized. Frequency analysis was applied to verify those geometries, and the authenticity of each transition state was checked using intrinsic reaction coordinate analysis (IRC). The single point energy of each stationary point was calculated at the B2PLYP level, and kinetic analysis was conducted to explore each reaction mechanism in more detail. Results showed that the energy barriers to the reactions of As with N₂O, NO₂, and NO were 78.45, 2.58, and 155.85 kJ mol^-1, respectively. For each reaction, the rate increased as the temperature was increased from 298 to 1800 K. However, temperature had only a tiny impact on the reaction of As with NO₂ due to the low energy barrier involved, and the reaction rate was consistently high (>10¹² cm³ mol^-1 s^-1), which indicates that this reaction occurs readily. On the other hand, the rate of the reaction between As and N₂O or NO increased rapidly between 298 and 900 K, and then increased more gradually upon further increasing the temperature.