Rationally Designed Porous MnO-FeO Nanoneedles for Low-Temperature Selective Catalytic Reduction of NO by NH.

In this work, a novel porous nanoneedlelike MnO-FeO catalyst (MnO-FeO 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-FeO 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 by NH. The results showed that the rationally designed MnO-FeO nanoneedles presented outstanding low-temperature NH-SCR activity (100% NO conversion in a wide temperature window from 120 to 240 °C), high selectivity for N (nearly 100% N selectivity from 60 to 240 °C), and excellent water resistance and stability in comparison with the counterpart MnO-FeO nanoparticles. The reasons can be attributed not only to the unique porous nanoneedle structure but also to the uniform distribution of MnO and FeO. More importantly, the desired Mn/Mn and O/(O + O) ratios, as well as rich redox sites and abundant strong acid sites on the surface of the porous MnO-FeO nanoneedles, also contribute to these excellent performances. In situ DRIFT suggested that the NH-SCR of NO over MnO-FeO nanoneedles follows both Eley-Rideal and Langmuir-Hinshelwood mechanisms.