Rationally Designed Porous MnO_x-FeO_x Nanoneedles for Low-Temperature Selective Catalytic Reduction of NO_x by NH₃

In this work, a novel porous nanoneedlelike MnO_x-FeO_x catalyst (MnO_x-FeO_x nanoneedles) was developed for the first time by rationally heat-treating metal-organic frameworks including MnFe precursor synthesized by hydrothermal method. A counterpart catalyst (MnO_x-FeO_x nanoparticles) without porous nanoneedle structure was also prepared by a similar procedure for comparison. The two catalysts were systematically characterized by scanning and transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, ammonia temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT), and their catalytic activities were evaluated by selective catalytic reduction (SCR) of NO_x by NH₃. The results showed that the rationally designed MnO_x-FeO_x nanoneedles presented outstanding low-temperature NH₃-SCR activity (100% NO_x conversion in a wide temperature window from 120 to 240 °C), high selectivity for N₂ (nearly 100% N₂ selectivity from 60 to 240 °C), and excellent water resistance and stability in comparison with the counterpart MnO_x-FeO_x nanoparticles. The reasons can be attributed not only to the unique porous nanoneedle structure but also to the uniform distribution of MnO_x and FeO_x. More importantly, the desired Mn⁴⁺/Mnⁿ⁺ and O_α/(O_α + O_β) ratios, as well as rich redox sites and abundant strong acid sites on the surface of the porous MnO_x-FeO_x nanoneedles, also contribute to these excellent performances. In situ DRIFT suggested that the NH₃-SCR of NO over MnO_x-FeO_x nanoneedles follows both Eley-Rideal and Langmuir-Hinshelwood mechanisms.