Scale–Activity Relationship of MnO_<i>x</i>-FeO_<i>y</i> Nanocage Catalysts Derived from Prussian Blue Analogues for Low-Temperature NO Reduction: Experimental and DFT Studies

Abstract

Size effects have been recognized to promote the catalytic activity and selectivity of metal oxide particles. So far, limited works and studies are conducted to investigate the size effect of metal oxide with the tailored shape in the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Herein, the MnO_x-FeO_y nanocage catalysts with varied scales (0.25, 0.5, 1, and 2 μm) were synthesized via a Prussian blue analogue (PBA)-derived method and used for NH₃-SCR of NO. By preforming a series of the activity tests over the nanocages with different scales, the NH₃-SCR activity of 0.5 μm MnO_x-FeO_y nanocage catalysts exhibits the highest deNO_x activity in the temperature range of 80-200 °C owing to more preferable physical and chemical properties. It has been demonstrated that there is a strong interaction among Mn and Fe cations in the 0.5 μm MnO_x-FeO_y nanocages. Moreover, the H₂-TPR and XPS analysis prove 0.5 μm nanocages exhibit excellent redox properties, which contribute to the higher conservation of NO_x. Through the DFT studies, it is also demonstrated that the 0.5 μm MnO_x-FeO_y nanocage catalysts could provide more preferable electronic charge, which gives rise to the varied adsorption behavior of the NH₃ species and NO_x species compared to the nanocages with other scales. The in situ DRIFTs were also employed to evaluate the adsorption status of NH₃ with NO_x species over MnO_x-FeO_y nanocage catalysts with varied scales. Finally, the scale-activity relationship of the MnO_x-FeO_y nanocage catalysts and their corresponding activities are also established. The deep insight into the scale-activity relationship of the PBA-derived MnO_x-FeO_y nanocage catalyst paves the way for developing and designing highly efficient Mn-based catalyst at lower temperature.