Strongly Cooperative Nano-CoO/Co Active Phase in Hierarchically Porous Nitrogen-Doped Carbon Microspheres for Efficient Bifunctional Oxygen Electrocatalysis

The development of highly efficient, non-noble-metal-based bifunctional oxygen electrocatalysts with low cost for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the key for the commercialization of rechargeable metal-air batteries and regenerative fuel cells. In this work, a non-noble-metal-based, hierarchically porous CoO/Co@N–C microsphere electrocatalyst with low cost and earth-abundance was created by directly reducing carbonized glucose-coated urchin-like Co3O4 microspheres through the carbon-thermal processing with NH3 reducing environment. Urchin-like Co3O4 microspheres provided a Co source for the creation of nano-CoO/Co active phase and served as the template without a removal requirement for the creation of hierarchically porous N-doped carbon microsphere matrix with abundant macropores and mesopores for efficient mass diffusion. Among its three major components, CoO played the leading role to provide both ORR and OER active sites, a porous N-doped carbon microsphere matrix mainly provided the conductive catalyst support, and the metallic Co component mainly acted as a “bridge” between CoO and graphic carbon to reduce the electron transfer resistance for the enhancement of its conductivity. Thus, their synergetic effect within its unique structure endowed a superior bifunctional performance toward both ORR and OER to the CoO/Co@N–C electrocatalyst over the commercially available Pt/C and Ir/C electrocatalysts

Synthesis of Superparamagnetic Core–Shell Structure Supported Pd Nanocatalysts for Catalytic Nitrite Reduction with Enhanced Activity, No Detection of Undesirable Product of Ammonium, and Easy Magnetic Separation Capability

Superparamagnetic nanocatalysts could minimize both the external and internal mass transport limitations and neutralize OH^– produced in the reaction more effectively to enhance the catalytic nitrite reduction efficiency with the depressed product selectivity to undesirable ammonium, while possess an easy magnetic separation capability. However, commonly used qusi-monodispersed superparamagnetic Fe₃O₄ nanosphere is not suitable as catalyst support for nitrite reduction because it could reduce the catalytic reaction efficiency and the product selectivity to N₂, and the iron leakage could bring secondary contamination to the treated water. In this study, protective shells of SiO₂, polymethylacrylic acid, and carbon were introduced to synthesize Fe₃O₄@SiO₂/Pd, Fe₃O₄@PMAA/Pd, and Fe₃O₄@C/Pd catalysts for catalytic nitrite reduction. It was found that SiO₂ shell could provide the complete protection to Fe₃O₄ nanosphere core among these shells. Because of its good dispersion, dense structure, and complete protection to Fe₃O₄, the Fe₃O₄@SiO₂/Pd catalyst demonstrated the highest catalytic nitrite reduction activity without the detection of NH₄⁺ produced. Due to this unique structure, the activity of Fe₃O₄@SiO₂/Pd catalysts for nitrite reduction was found to be independent of the Pd nanoparticle size or shape, and their product selectivity was independent of the Pd nanoparticle size, shape, and content. Furthermore, their superparamagnetic nature and high saturation magnetization allowed their easy magnetic separation from treated water, and they also demonstrated a good stability during the subsequent recycling experiment

Synthesis of Mn₃O₄/CeO₂ Hybrid Nanotubes and Their Spontaneous Formation of a Paper-like, Free-Standing Membrane for the Removal of Arsenite from Water

One-dimensional nanomaterials may organize into macrostructures to have hierarchically porous structures, which could not only be easily adopted into various water treatment apparatus to solve the separation issue of nanomaterials from water but also take full advantage of their nanosize effect for enhanced water treatment performance. In this work, a novel template-based process was developed to create Mn₃O₄/CeO₂ hybrid nanotubes, in which a redox reaction happened between the OMS-2 nanowire template and Ce­(NO₃)₃ to create hybrid nanotubes without the template removal process. Both the Ce/Mn ratio and the precipitation agent were found to be critical in the formation of Mn₃O₄/CeO₂ hybrid nanotubes. Because of their relatively large specific surface area, porous structure, high pore volume, and proper surface properties, these Mn₃O₄/CeO₂ hybrid nanotubes demonstrated good As­(III) removal performances in water. These Mn₃O₄/CeO₂ hybrid nanotubes could form paper-like, free-standing membranes spontaneously by a self-assembly process without high temperature treatment, which kept the preferable properties of Mn₃O₄/CeO₂ hybrid nanotubes while avoiding the potential nanomaterial dispersion problem. Thus, they could be readily utilized in commonly used flow-through reactors for water treatment purposes. This approach could be further applied to other material systems to create various hybrid nanotubes for a broad range of technical applications