Hongrui Li

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.

Highly dispersed CeO2 on carbon nanotubes for selective catalytic reduction of NO with NH3

Highly dispersed CeO2 on carbon nanotubes (CNTs) is successfully prepared by a pyridine-thermal route for selective catalytic reduction (SCR) of NO with NH3. This catalyst is mainly characterized by the techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction by hydrogen (H2-TPR), temperature-programmed desorption of ammonia (NH3-TPD) and X-ray photoelectron spectroscopy (XPS). The results of the XRD, TEM and TPR analysis show that the CeO2 particles on the CNTs are highly dispersed with a strong interaction between the particles and the CNTs. The NH3-TPD profiles indicate that this catalyst exhibits abundant strong acid sites. Furthermore, the O 1s XPS spectra show that the Oα/(Oα + Oβ) ratio of this catalyst is very high, which can result in more surface oxygen vacancies and therefore favor the NH3-SCR reaction. Compared with the catalysts prepared by impregnation or physical mixture methods, the catalyst prepared by the pyridine-thermal route presents the best NH3-SCR activity in the temperature range of 150–380 °C as well as favourable stability and good SO2 or H2O resistance. More than 90% of NO can be removed in the range of 250–370 °C with a desirable N2 selectivity. Moreover, the NO conversion can be kept at about 97% with the presence of SO2 or H2O at 300 °C. In addition, this catalyst shows a high catalytic activity with a NO conversion remaining constant at ca. 98% during a 16 h continuous run duration at 300 °C. Highly dispersed CeO2 on the CNTs as well as the strong interaction between the particles and the CNTs, the large amounts of strong acid sites and the high Oα/(Oα + Oβ) ratio could be ascribed to the excellent NH3-SCR performance of the catalyst prepared by the pyridine-thermal route.

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.

Enhanced catalytic performance of V2O5–WO3/Fe2O3/TiO2 microspheres for selective catalytic reduction of NO by NH3

The V2O5–WO3/Fe2O3/TiO2 microsphere catalysts were prepared by an impregnation method. The catalytic test results showed that the Fe2O3 additives in V2O5–WO3/Fe2O3/TiO2 improved the NO decomposition in the temperature range of 200–400 °C. The characterization results indicated that the iron oxides mainly existed in the form of Fe2O3, which was beneficial for the oxidation of NO to NO2. Meanwhile, the superior catalytic performance of V2O5–WO3/Fe2O3/TiO2 for the selective catalytic reduction of NO has been attributed to its highly dispersed active species, and lots of surface adsorbed oxygen. Prominently, the activity of V2O5–WO3/Fe2O3/TiO2 microspheres can be recovered after cutting off SO2, and thus these combination metal oxide catalysts show not only high catalytic activities but also good resistance to SO2.

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.

Scale–Activity Relationship of MnOx-FeOy Nanocage Catalysts Derived from Prussian Blue Analogues for Low-Temperature NO Reduction: Experimental and DFT Studies

Size effects have been recognized to promote the catalytic activity and selectivity of metal oxide particles. So far, limited works and studies are conducted to investigate the size effect of metal oxide with the tailored shape in the selective catalytic reduction of NOx with NH3 (NH3-SCR). Herein, the MnOx-FeOy nanocage catalysts with varied scales (0.25, 0.5, 1, and 2 μm) were synthesized via a Prussian blue analogue (PBA)-derived method and used for NH3-SCR of NO. By preforming a series of the activity tests over the nanocages with different scales, the NH3-SCR activity of 0.5 μm MnOx-FeOy nanocage catalysts exhibits the highest deNOx activity in the temperature range of 80-200 °C owing to more preferable physical and chemical properties. It has been demonstrated that there is a strong interaction among Mn and Fe cations in the 0.5 μm MnOx-FeOy nanocages. Moreover, the H2-TPR and XPS analysis prove 0.5 μm nanocages exhibit excellent redox properties, which contribute to the higher conservation of NOx. Through the DFT studies, it is also demonstrated that the 0.5 μm MnOx-FeOy nanocage catalysts could provide more preferable electronic charge, which gives rise to the varied adsorption behavior of the NH3 species and NOx species compared to the nanocages with other scales. The in situ DRIFTs were also employed to evaluate the adsorption status of NH3 with NOx species over MnOx-FeOy nanocage catalysts with varied scales. Finally, the scale-activity relationship of the MnOx-FeOy nanocage catalysts and their corresponding activities are also established. The deep insight into the scale-activity relationship of the PBA-derived MnOx-FeOy nanocage catalyst paves the way for developing and designing highly efficient Mn-based catalyst at lower temperature.

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.

Highly dispersed V2O5/TiO2 modified with transition metals (Cu, Fe, Mn, Co) as efficient catalysts for the selective reduction of NO with NH3

Abstract Different transition metals were used to modify V2O5-based catalysts (M-V, M = Cu, Fe, Mn, Co) on TiO2 via impregnation, for the selective reduction of NO with NH3. The introduced metals induced high dispersion in the vanadium species and the formation of vanadates on the TiO2 support, and increased the amount of surface acid sites and the strength of these acids. The strong acid sites might be responsible for the high N2 selectivity at higher temperatures. Among these catalysts, Cu-V/TiO2 showed the highest activity and N2 selectivity at 225–375 °C. The results of X-ray photoelectron spectroscopy, NH3-temperature-programmed desorption, and in-situ diffuse reflectance infrared Fourier transform spectroscopy suggested that the improved performance was probably due to more active surface oxygen species and increased strong surface acid sites. The outstanding activity, stability, and SO2/H2O durability of Cu-V/TiO2 make it a candidate to be a NOx removal catalyst for stationary flue gas.

Metal–Porphyrin: A Potential Catalyst for Direct Decomposition of N2O by Theoretical Reaction Mechanism Investigation

The adsorption of nitrous oxide (N2O) on metal-porphyrins (metal: Ti, Cr, Fe, Co, Ni, Cu, or Zn) has been theoretically investigated using density functional theory with the M06L functional to explore their use as potential catalysts for the direct decomposition of N2O. Among these metal-porphyrins, Ti-porphyrin is the most active for N2O adsorption in the triplet ground state with the strongest adsorption energy (-13.32 kcal/mol). Ti-porphyrin was then assessed for the direct decomposition of N2O. For the overall reaction mechanism of three N2O molecules on Ti-porphyrin, two plausible catalytic cycles are proposed. Cycle 1 involves the consecutive decomposition of the first two N2O molecules, while cycle 2 is the decomposition of the third N2O molecule. For cycle 1, the activation energies of the first and second N2O decompositions are computed to be 3.77 and 49.99 kcal/mol, respectively. The activation energy for the third N2O decomposition in cycle 2 is 47.79 kcal/mol, which is slightly lower than that of the second activation energy of the first cycle. O2 molecules are released in cycles 1 and 2 as the products of the reaction, which requires endothermic energies of 102.96 and 3.63 kcal/mol, respectively. Therefore, the O2 desorption is mainly released in catalytic cycle 2 of a TiO3-porphyrin intermediate catalyst. In conclusion, regarding the O2 desorption step for the direct decomposition of N2O, the findings would be very useful to guide the search for potential N2O decomposition catalysts in new directions.

In situ preparation of Ni nanoparticles in cerium-modified silica aerogels for coking- and sintering-resistant dry reforming of methane

In this work, highly dispersed Ni nanoparticles confined in nanochannels of cerium-modified silica aerogels were in situ prepared originally via a simple sol–gel route. The obtained materials with good thermal stability were used in the dry reforming of methane. The aerogel catalysts exhibited high catalytic activities and good catalytic stabilities towards the reaction. The improved catalytic activities were closely associated with the advantageous structural properties. The large specific surface area, high Ni dispersion and large pore volume could expose more active species to react with the gaseous reactants. In addition, the confinement effect of the mesopores contributed to the promotion of anti-sintering during the reduction and reaction processes, resulting in longer lifetime stabilities of aerogel catalysts. The addition of Ce played a significant role in suppressing carbon deposition by enhancing the reactive oxygen species on the catalyst surface. Generally, the developed catalysts could be considered as promising coking- and sintering-resistant catalysts for the dry reforming of methane.