MnO₂ Nanowire-CeO₂ Nanoparticle Composite Catalysts for the Selective Catalytic Reduction of NO _x with NH₃

MnO_x-based catalysts have been applied to the selective catalytic reduction of NO_x with ammonia (NH₃) owing to their high NO_x removal efficiency and catalytic stability. In general, the fabrication of a variety of nanomaterials in a complex structure requires complicated processes, including heat treatment and a series of cleaning steps. In addition, MnO₂ which has diverse polymorphs, exhibits different catalytic effects depending on its crystalline structure. Among them, synthesizing the ϵ-MnO₂ phase, which functions as a nanocatalyst, has been the most difficult and has hardly been reported. Here, we report the synthesis of heterostructured composite nanocatalysts consisting of ϵ-MnO₂ nanowires (NWs) and CeO₂ nanoparticles (NPs) by applying pulsed currents sequentially. This method drastically simplifies the overall process compared to the conventional techniques. Through X-ray diffraction and transmission electron microscopy, it was confirmed that 2-3 nm of CeO₂ NPs were formed on the surfaces of the ϵ-MnO₂ NWs. The de-NO_x efficiency of the nanocatalysts was analyzed in terms of content variation, specific surface area, and the elemental chemical state of the surface. A ceramic filter containing the nanocatalysts shows a high catalytic activity over the broad operating temperature range 100-400 °C. In the low-temperature region, ϵ-MnO₂ plays a major role in determining the catalytic property, which is consistent with the Brunauer-Emmett-Teller (BET), H₂ temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) results. On the other hand, in the high-temperature region, the efficiency increases gradually as the content of CeO₂ increases. The H₂ TPR, NH₃-temperature-programmed desorption, and XPS patterns reveal why the composite exhibits such superior characteristics in the temperature range mentioned above.