Ge Gao

Rationally Designed Porous MnOx–FeOx Nanoneedles for Low-Temperature Selective Catalytic Reduction of NOx by NH3

In this work, a novel porous nanoneedlelike MnOx-FeOx catalyst (MnOx-FeOx nanoneedles) was developed for the first time by rationally heat-treating metal-organic frameworks including MnFe precursor synthesized by hydrothermal method. A counterpart catalyst (MnOx-FeOx 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 NOx by NH3. The results showed that the rationally designed MnOx-FeOx nanoneedles presented outstanding low-temperature NH3-SCR activity (100% NOx 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 MnOx-FeOx nanoparticles. The reasons can be attributed not only to the unique porous nanoneedle structure but also to the uniform distribution of MnOx and FeOx. More importantly, the desired Mn4+/Mnn+ and Oα/(Oα + Oβ) ratios, as well as rich redox sites and abundant strong acid sites on the surface of the porous MnOx-FeOx nanoneedles, also contribute to these excellent performances. In situ DRIFT suggested that the NH3-SCR of NO over MnOx-FeOx nanoneedles follows both Eley-Rideal and Langmuir-Hinshelwood mechanisms.

Mn−Co Mixed Oxide Nanosheets Vertically Anchored on H2Ti3O7 Nanowires: Full Exposure of Active Components Results in Significantly Enhanced Catalytic Performance

The full exposure of active ingredients plays an important role in the enhancement of catalytic performance. In this work, a series of novel catalysts, Mn−Co mixed oxide nanosheets with ultrathin thickness (about 3.5 nm) and different Mn/Co ratios (0.52, 0.69, and 1.52) vertically anchored on a support (H2Ti3O7 nanowires), are rationally developed. This unique structure not only fully exposes the active ingredients of the Mn−Co mixed oxides, but also is very favorable for the diffusion and transfer of gas molecules through the space between these standing nanosheets. As expected, the developed catalysts (MnOx‐CoOy/H2Ti3O7, MnCoTi), especially MnCoTi‐2 with the Mn/Co molar ratio of 0.69, present excellent low‐temperature selective catalytic reduction (SCR) performance, high N2 selectivity, superior water tolerance and stability. The relative turnover frequency (TOF) value over MnCoTi‐2 at 100 °C is as high as 9.25×10−4 s−1 under the gas hourly space velocity (GHSV) of 200 000 h−1, which is rarely reported among Mn‐Ti, Mn−Co, and Mn−Co‐Ti mixed oxide catalysts. The results of in situ diffuse reflectance infrared Fourier transform spectroscopy suggest that the coordinated NH3, NH4+ ions, adsorbed NO2, and bidentate nitrate are the reactive species and the Eley–Rideal and Langmuir–Hinshelwood mechanisms can be simultaneously involved on the surface of the MnCoTi‐2 at a relatively low temperature (90 °C).