Duan Weng

MnOx–CeO2–Al2O3 mixed oxides for soot oxidation: Activity and thermal stability

MnO(x)-CeO(2)-Al(2)O(3) mixed oxides were prepared by impregnating manganese acetate and cerium nitrate on alumina powders using the sol-gel method. The thermal stabilities of MnO(x)-CeO(2) and Al(2)O(3)-modified mixed oxides were evaluated by treating at 800 °C in dry air flow for 20h. The introduction of Al(2)O(3) markedly increases the textural stability of the catalyst with a relatively high dispersion of MnO(x) and CeO(2), remaining a strong synergistic effect between these two oxides. The NO oxidation activity of the ternary oxides experiences a smaller loss after high-temperature calcination, and a low soot oxidation temperature is attained in the presence of NO.

Ceria-based catalysts for soot oxidation: a review

Abstract Developments in ceria-based soot oxidation catalysts, especially during the last decade, are reviewed. Based on the comparisons of the activity, durability and cost-efficiency of different soot oxidation catalysts, four kinds of applicable ceria-based catalysts have been screened out, which are: (1) CexZr1–xO2 catalyst with high cerium content (x

Effects of WOx modification on the activity, adsorption and redox properties of CeO2 catalyst for NOx reduction with ammonia

A series of WO3/CeO2 (WOx/CeO2) catalysts were synthesized by wet impregnation of ammonium metatungstate on a CeO2 support. The resulting solid acid catalysts were characterized by X-ray diffraction (XRD), UV-Vis spectroscopy (UV-Vis), Raman spectroscopy (Raman), in-situ Fourier transform infrared spectroscopy (in-situ FT-IR) of ammonia adsorption, NH3-TPD, H2 temperature-programmed reduction (H2-TPR), NH3/NO oxidation and activity measurements for NOx reduction by NH3 (NH3-SCR). The results show that polytungstate (WOx) species are the main species of tungsten oxide on the surface of ceria. The addition of tungsten oxide enhances the Bronsted acidity of ceria catalysts remarkably and decreases the amount of surface oxygen on ceria, with strong interaction between CeO2 and WOx. As a result, the N2 selectivity of NH3 oxidation and NH3-SCR at high temperatures (> 300°C) is enhanced. Therefore, a wide working temperature window in which NOx conversion exceeds 80% (NOx conversion > 80%) from 200 to 450°C, is achieved over 10 wt.% WOx/CeO2 catalyst. A tentative model of the NH3-SCR reaction route on WOx/CeO2 catalysts is presented.

Rare earth containing catalysts for selective catalytic reduction of NOx with ammonia: A Review

Abstract Increasingly stringent regulations in many countries require effective reduction and control of NOx emissions. To meet these limits, various methods have been exploited, among which the selective catalytic reduction of NOx using ammonia as the reductant (NH3-SCR) is the most favored technology. High catalytic activity, N2 selectivity and resistance to deactivation by sulfur, alkaline metals and hydrothermal conditions are the optimal properties of a successful SCR catalyst. Rare earth oxides, particularly CeO2, have been increasingly used to improve the catalytic activity and resistance to deactivation of deNOx catalysts, both modifying traditional vanadium catalysts, and also developing novel catalysts, especially for low temperature applications. This review summarized the open literature concerning recent research and development progresses in the application of rare earths for NH3-SCR of NOx. Additionally, the roles of rare earths in enhancing the performance of NH3-SCR catalyst were reviewed.

Nitrate storage behavior of Ba/MnOx-CeO2 catalyst and its activity for soot oxidation with heat transfer limitations.

A BaMnCe ternary catalyst was prepared by impregnating barium acetate on MnO(x)-CeO(2) mixed oxides, with the monoxide supported catalysts and the solid solution support as references. The activities of the catalysts for soot oxidation were evaluated in the presence of NO under an energy transference controlled regime. BaMnCe presented the lowest maximal soot oxidation rate temperature at 393 degrees C among the catalysts investigated. Although BaMnCe experienced a loss in the specific surface area and low-temperature redox property due to blocking of the support pores by barium carbonate, its superior soot oxidation activity highlighted the importance of relatively stable bidentate/monodentate nitrates coordinated to Mn(x+) and Ce(x+) sites and more stable ionic barium nitrate. About half of the nitrates stored on this catalyst decomposed within the temperature interval of 350-450 degrees C, and the ignition temperature of soot decreased significantly with involvement of the nitrates or NO(2) released.

Total oxidation of propane on Pt/WOx/Al2O3 catalysts by formation of metastable Ptδ+ species interacted with WOx clusters.

A series of Pt/Al(2)O(3) catalysts with various tungsten oxide loadings were prepared by a stepwise wet impregnation method. The catalysts were characterized by X-ray diffraction, nitrogen physisorption, Raman, UV-vis diffuse reflectance, transmission electron microscopy and infrared spectroscopy of adsorbed probe molecules (CO, NH(3) or C(3)H(8)). The propane oxidation activity of Pt/Al(2)O(3) catalyst is significantly improved by the addition of tungsten oxide. The tungsten oxide overlayer is presented as monomeric/polymeric WO(x) clusters and WO(3) crystals depending on the loading amount. The most active catalyst occurs at an intermediate surface tungsten density corresponding to the maximum of polytungstate species. The electronic interactions between Pt and WO(x) clusters lead to the generation of more reducible Pt(δ+) species which are suggested to be active sites for propane oxidation. Basically, a simple model is proposed involving the initial CH bond activation at the platinum-tungsten oxide interface.

Effects of cerium and vanadium on the activity and selectivity of MnOx-TiO2 catalyst for low-temperature NH3-SCR

Abstract MnO x -TiO 2 , CeO 2 -MnO x -TiO 2 and V 2 O 5 -MnO x -TiO 2 catalysts for low-temperature NH 3 -SCR were prepared by sol-gel method. The results showed that both cerium and vanadium prevented the transformation of anatase TiO 2 to the rutile phase. The addition of vanadium oxide induced the segregation of crystalline Mn 2 O 3 , which contributed little to low-temperature SCR and ammonia oxidation, from the MnO x -TiO 2 solid solutions. However, the selectivity of the V-containing catalyst was almost 100% due to the decreased ammonia consumption and enhanced adsorption capacity of ammonia on Bronsted acid sites at relatively high temperatures. The electron-donating effect of cerium reduced the Mn 4+ /Mn 3+ ratio to some extent, resulting in a decreased activity for ammonia oxidation. This, in combination with the enhanced ammonia adsorption capacity by Ce n + as additional Lewis acid sites, endowed the Ce-doped catalyst a higher N 2 selectivity than MnO x -TiO 2 despite the slightly elevated light-off temperature for NO conversion.

Facile synthesis of hierarchical porous γ-Al2O3 hollow microspheres for water treatment

Hierarchical porous γ-Al2O3 hollow microspheres were synthesized by a modified spray drying method. Ageing the precipitated precursor and spray-drying assisted by NH4Cl salts are considered as two key steps for the synthesis of γ-Al2O3 hollow microspheres. The mechanism of the formation of hierarchical porous γ-Al2O3 hollow microsphere was proposed involving phase transformation from aluminum hydroxide to laminar boehmite during ageing and a following self-assembling process with NH4Cl as the template during spray drying. The meso-/macro-pores in γ-Al2O3 mainly arise from the stacking of the laminar boehmites which are obtained by ageing the precipitated precursors at 90°C. NH4Cl, which was the byproduct from the reaction between AlCl3·6H2O and NH3·H2O, was demonstrated to be an excellent template to act as the core and the barrier for separation of laminar boehmites. No extra NH4Cl was added. The as-synthesized hierarchical porous γ-Al2O3 hollow microsphere presented remarkably higher adsorption capacity, which is thirty times higher adsorption rate for Congo Red than the solid microsphere containing only small mesopores.

Synergistic effect between MnO and CeO2 in the physical mixture: Electronic interaction and NO oxidation activity

Abstract MnO and CeO 2 powders were mechanically mixed by a spatula and by milling to obtain loose-contact and tight-contact mixed oxides, respectively. The monoxides and their physical mixtures were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), Raman, O 2 temperature-programmed desorption (O 2 -TPD), H 2 temperature-programmed reduction (H 2 -TPR) and NO temperature-programmed oxidation (NO-TPO). The MnO x -CeO 2 solid solutions did not form without any calcination process. The oxidation state of manganese tended to increase while the ionic valence of cerium decreased in the mixed oxides, accompanied with the formation of oxygen vacancies. This long-ranged electronic interaction occured more significantly in the tight-contact mixture of MnO and CeO 2 . The formation of more Mn 4+ and oxygen vacancies promoted the catalytic oxidation of NO in an oxygen-rich atmosphere.

Synergistic effects between copper and tungsten on the structural and acidic properties of CuOx/WOx–ZrO2 catalyst

Different amounts of tungsten oxide were introduced to zirconia to form WOx–ZrO2 solid solutions by a coprecipitation method, followed by impregnation with copper nitrate. The received catalysts were characterized by X-ray diffraction, Brunauer–Emmett–Teller surface area, X-ray photoelectron spectroscopy, Raman spectroscopy, diffuse reflectance UV-vis spectroscopy, H2 temperature-programmed reduction and in situFTIR of NH3/NO+O2 adsorption. Higher dispersions of CuOx are obtained on the tungsten-containing catalysts due to the increased surface area of the support. Highly dispersed WOx clusters in tetrahedral coordination are generated when tungsten exceeds its solubility in the solid solutions, and polymerized WOx clusters are formed for further increasing the loading amount of tungsten. The formation of Cun+–O2−–Wn+ bonds at the interface between CuOx and WOx clusters leads to an improved capacity to form nitrite and an enhanced Bronsted acidity, which enhance the activity of the catalyst for NH3-SCR reaction.