The effect of CO on the transformation of arsenic species: A quantum chemistry study

Highlights: • Reaction mechanism of homogeneous and heterogeneous reactions of arsenic oxides with CO were revealed via density functional theory. • The adsorption mode of AsO₂ molecule on the char surface has a great influence on its reduction. • As₂O₃ homogeneous reduction with CO is more likely to take place compared to the heterogeneous reduction. -- Abstract: To explore the effect of CO on the transformation of arsenic species, the reaction mechanism of homogeneous and heterogeneous reactions for arsenic oxides (AsO₂ and As₂O₃) with CO were investigated via density functional theory (DFT). The geometries of reactants, intermediates, transition states and products for each reaction were optimized by using the B3LYP method in conjunction with the 6-31G(d) basis set, and the single-point energy of each structure was calculated at the B2PLYP/Def2-TZVP level. Also, thermodynamic and kinetic analyses were conducted to determine the reaction process. The results showed that the homogeneous reaction of AsO₂ and CO has two channels and a transition state is found in each case. The homogeneous reaction process of As₂O₃ and CO undergoes two transition states and, finally, As₂O₃ is reduced to sub-oxides by CO. Char has a strong adsorption affinity for AsO₂ and As₂O₃ in the presence of CO, and the adsorption mode of the AsO₂ molecule on the char surface has a great influence on its reduction. The activation energy of the homogeneous reduction of As₂O₃ (75.9 kJ mol⁻¹) is lower than the heterogeneous reduction (94.2 kJ mol⁻¹), which suggests that As₂O₃ is more likely to react with CO in the flue gas. The calculation results revealed the mechanism for the influence of CO on arsenic behavior by density functional theory. These results are helpful for a molecular-level understanding of the transformation of arsenic species, which in turn provides a theoretical foundation for the emission and control of arsenic.