Metal-Doped Mesoporous MnO₂-CeO₂ Catalysts for Low-Temperature Pre-Oxidation of NO to NO₂ in Fast SCR Process

Selective catalytic reduction (SCR) is an effective system for treating nitrogen oxides (NO_x; mainly NO), and fast SCR requires the equimolar reactants of NO and NO₂. This study focused on catalysts for oxidizing 50% of NO to NO₂. A series of catalysts composed of a variety of components, such as mesoporous mMnO₂-nCeO₂ as carrier catalysts (m:n = 9:1 and 7:3) and transition metals (e.g., Fe, Co, Ni, Cu, and Cr), were synthesized and characterized using N₂ adsorption, in situ XRD, TEM, and XPS. All samples had a mesoporous structure with pore size around 8 nm. XPS results demonstrated that addition of cerium ion increased the surface area and provided oxygen vacancy due to the formation of Ce³⁺ within the structure. NO oxidation activity was tested using a feed (205~300 ppm NO and 6% O₂) that simulated typical flue gas conditions. Doped mesoporous mMnO₂–nCeO₂ has higher NO oxidation activity than pristine mMnO₂–nCeO₂. The doped mMnO₂-nCeO₂ catalyzed 50% of NO to NO₂ at between 140 and 200 °C resulting in an equivalent amount of NO and NO₂. Among the transition metals, Cu, Ni, Co, Fe, and Cr have the highest to lowest oxidation activity, respectively. The precatalytic oxidation of NO can potentially be combined with the current SCR system without changes to existing equipment and can be applied to the exhaust gas treatment for de-NO_x.