Sixiang Cai

Design of meso-TiO2@MnOx–CeOx/CNTs with a core–shell structure as DeNOx catalysts: promotion of activity, stability and SO2-tolerance

Developing low-temperature deNOx catalysts with high catalytic activity, SO2-tolerance and stability is highly desirable but remains challenging. Herein, by coating the mesoporous TiO2 layers on carbon nanotubes (CNTs)-supported MnOx and CeOx nanoparticles (NPs), we obtained a core-shell structural deNOx catalyst with high catalytic activity, good SO2-tolerance and enhanced stability. Transmission electron microscopy, X-ray diffraction, N2 sorption, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction and NH3 temperature-programmed desorption have been used to elucidate the structure and surface properties of the obtained catalysts. Both the specific surface area and chemisorbed oxygen species are enhanced by the coating of meso-TiO2 sheaths. The meso-TiO2 sheaths not only enhance the acid strength but also raise acid amounts. Moreover, there is a strong interaction among the manganese oxide, cerium oxide and meso-TiO2 sheaths. Based on these favorable properties, the meso-TiO2 coated catalyst exhibits a higher activity and more extensive operating-temperature window, compared to the uncoated catalyst. In addition, the meso-TiO2 sheaths can serve as an effective barrier to prevent the aggregation of metal oxide NPs during stability testing. As a result, the meso-TiO2 overcoated catalyst exhibits a much better stability than the uncoated one. More importantly, the meso-TiO2 sheaths can not only prevent the generation of ammonium sulfate species from blocking the active sites but also inhibit the formation of manganese sulfate, resulting in a higher SO2-tolerance. These results indicate that the design of a core-shell structure is effective to promote the performance of deNOx catalysts.

Comparative study of 3D ordered macroporous Ce0.75Zr0.2M0.05O2−δ (M = Fe, Cu, Mn, Co) for selective catalytic reduction of NO with NH3

The three dimensional ordered macroporous (3DOM) Ce0.75Zr0.2M0.05O2−δ (M = Fe, Cu, Mn, Co) is synthesized by a colloidal crystal template method for comparative study on selective catalytic reduction (SCR) of NO with NH3. The obtained catalysts are mainly investigated by the measurements of X-ray diffraction (XRD), N2 sorption, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction by hydrogen (H2-TPR), as well as temperature-programmed desorption of ammonia (NH3-TPD). The XRD, XPS and TPR analysis clarified the dopants are effectively doped into the Ce–Zr oxide solid solution, which contribute to the strong synergistic effect between the dopants and Ce–Zr oxides. Moreover, higher Oα/(Oα + Oβ) ratio, improved surface reducibility and enhanced surface acidity are observed after the in situ doping. These facts lead to a better low temperature catalytic performance for the Ce0.75Zr0.2M0.05O2−δ catalysts. Particularly, the Co doped catalysts exhibit the highest active oxygen species, the highest reducibility as well as the strongest NH3 absorption ability, which corresponds to the significant increase of low temperature activity. Meanwhile, the specific surface area and pore volume are increased efficiently by Fe and Mn doping, which broadens the operating temperature window. Moreover, the strong interaction between the dopants and the Ce–Zr oxide solid solution could be ascribed to the good stability of those catalysts doped with Fe, Mn and Co.

Rational design and in situ fabrication of MnO2@NiCo2O4 nanowire arrays on Ni foam as high-performance monolith de-NOx catalysts

In this work, we have rationally designed and originally developed a novel monolith de-NOx catalyst with nickel foam as the carrier and three dimensional hierarchical MnO2@NiCo2O4 core–shell nanowire arrays in situ grown on the surface via a two-step hydrothermal process with a post calcination treatment. The catalysts were systematically examined by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, elemental mapping, ion sputtering thinning, X-ray photoelectron spectroscopy, inductively coupled plasma atomic emission spectroscopy, H2 temperature-programmed reduction, NH3/NO + O2 temperature-programmed desorption measurements and catalytic performance tests. The results indicate that the nanowire is composed of hollow NiCo2O4 spinel as the core and MnO2 nanoparticles as the shell layer. By ingeniously making the hierarchical Ni–Co oxide nanowires as the support for manganese oxides, the MnO2@NiCo2O4@Ni foam catalyst not only takes advantage of the high surface area of Ni–Co nanowires to achieve high loading amounts as well as high dispersion of manganese oxides, but also makes use of the synergistic catalytic effect between Ni, Co and Mn multiple oxides, and exhibits excellent low-temperature catalytic performance in the end. In addition, with the structure and morphology well maintained under long term steady isothermal operation, the catalyst is able to sustain high NO conversion and exhibits superior catalytic cycle stability and good H2O resistance. Considering all these favorable properties, the MnO2@NiCo2O4@Ni foam catalyst could serve as a promising candidate for the monolith de-NOx catalyst at low temperatures and the rational design of in situ synthesis of 3D hierarchical monolith catalysts also puts forward a new way for the development of environmental-friendly and highly active monolith de-NOx catalysts.

Fe2O3 nanoparticles anchored in situ on carbon nanotubes via an ethanol-thermal strategy for the selective catalytic reduction of NO with NH3

Fe2O3 nanoparticles were anchored in situ on carbon nanotubes (CNTs) via an ethanol-thermal route, for the selective catalytic reduction (SCR) of NO with NH3. The structure and surface characteristics of the obtained catalysts were measured by transmission electron microscopy, X-ray diffraction, N2 adsorption–desorption isotherms, Raman, X-ray photoelectron spectroscopy, H2-temperature programmed reduction, and NH3-temperature programmed desorption. Compared with catalysts prepared via impregnation or co-precipitation methods, the synthesized catalyst showed better catalytic activity and a more extensive operating-temperature window. The TEM and XRD results suggested that the iron species was uniformly anchored on the surface of the CNTs. The Raman and XPS results indicated that the catalyst has a relatively higher number of defects, a higher atomic concentration of Fe present on the surface of the CNTs and a higher content of chemisorbed oxygen species. The H2-TPR and NH3-TPD results demonstrated that the catalyst possesses a more powerful reducibility and stronger acid strength than the other two catalysts. Based on the above-mentioned physicochemical properties, the obtained catalyst showed an excellent performance in the SCR of NO to N2 with NH3. Additionally, the catalyst also presented outstanding stability, H2O resistance and SO2 tolerance.