Unraveling the morphology and crystal plane dependence of bifunctional MnO₂ catalyst for simultaneous removal of NO and CO at low temperature

It is indeed a challenging problem to simultaneously remove NO and CO from the steel sintering flue gas, in which a bifunctional catalyst has proven to be an efficient solution for removing both pollutants at low temperature. In this study, four different crystalline phases of MnO₂ (α-, β-, γ-, and δ-) catalysts were synthesized via a facile hydrothermal method, and the effects of their crystal structure, morphology, and physicochemical properties on the catalytic performance for NO reduction and CO oxidation were elucidated. The results indicated that γ-MnO₂ catalyst exhibited the best catalytic activity, achieving 90% NO removal efficiency and 82% CO conversion rate at 175 °C. Reaction kinetics confirmed that γ-MnO₂ catalyst exhibited a lower Ea for both NO reduction and CO oxidation compared to α-MnO₂, β-MnO₂ and δ-MnO₂ catalysts. Meanwhile, the interaction of between NH₃-SCR and CO catalytic oxidation reactions over the catalysts was also studied. Intriguingly, it was found that the presence of CO enhanced the catalytic activity of γ-MnO₂ catalyst in the NH₃-SCR reaction. The results of NO + O₂-TPD and in situ DRIFTS experiments revealed that CO contributed to the adsorption and oxidation of NO, thus promoting the L-H pathway over γ-MnO₂ catalyst. Finally, a possible mechanism model for simultaneous removal of NO and CO over γ-MnO₂ catalyst was proposed.