High NH3-SCR Activity Research Articles

Structural effects of iron spinel oxides doped with Mn, Co, Ni and Zn on selective catalytic reduction of NO with NH3

The iron spinel oxides doped with Mn, Co, Ni and Zn were studied for selective catalytic reduction (SCR) of NO with NH3. From the TPR and TPD results, the redox properties and surface acidity of metal-doped iron oxides are determined by the structural effects of iron spinel oxides. At high temperatures, it was found that the NH3-related reaction follows the Lewis-acid-inspired E–R mechanism, which involves in the competition between NH3 catalytic reduction with NO (NH3-CR) and NH3 catalytic oxidation to NOx (NH3-CO). The doping of Co, Ni or Zn into iron spinel structures inhibits the redox cycle of Fe3+/Fe2+ between γ-Fe2O3 and Fe3O4 on the surface octahedral sites, resulting in the decrease of NH3 activation and NH3-SCR performance. However, the maghemite-type spinel structure was maintained and the outermost surface coverage of Mn was enhanced after the doping of Mn. Thus, Mn-doped spinel oxide exhibits high NH3-SCR activity but poor N2 selectivity at low temperatures.

A novel Nb–Ce/WO –TiO2 catalyst with high NH3-SCR activity and stability

Abstract A novel Nb–Ce/WOx–TiO2 (NbCe/WTi) catalyst was synthesized by impregnating Ce and Nb salts on WO3–TiO2 powders, which exhibits over 80% NOx conversion at 200–500 °C. NbCe/WTi catalyst owns higher hydrothermal stability and alkali poisoning resistance than V2O5–WO3–TiO2 (VWT). Moreover, the sulfated catalyst can be feasibly regenerated by treatment in air at 650 °C. Ceria exhibits superior redox ability of the catalyst and dispersed NbOx and WOx species provide large amounts of acid sites, facilitating the SCR performance at low and high temperatures.

Lattice oxygen mobility and acidity improvements of NiO–CeO2–ZrO2 catalyst by sulfation for NOx reduction by ammonia

Different amounts of sulfates were introduced to NiO–CeO2–ZrO2 catalyst by a citric-aided sol–gel method for NOx reduction with ammonia. The catalysts were characterized by X-ray diffraction, Raman, H2 temperature programmed-reduction, NH3 temperature-programmed oxidation, DRIFTS of NH3/NO adsorption, oxygen storage capacity and activity measurements for selective catalytic reduction of NOx with ammonia. The results show that the interactions between sulfates and NiO–CeO2–ZrO2 solid solution lead to an enhanced Lewis acidity of the catalyst due to the electron withdrawing effect of sulfates mainly on Nin+. The addition of sulfates decreases the amount of readily available oxygen on the catalyst, but promotes the lattice oxygen mobility by the formation of Nin+OSn+ bond in CeO2–ZrO2 mixed oxides. The ammonia oxidation is inhibited by the reduced amount of surface active oxygen, while the adsorption of ammonia on the catalyst is promoted with the sulfate-derived acid sites. Meanwhile, the oxidation of nitrites to nitrates on the sulfated catalyst at the temperature interval for high NOx conversion is accelerated by the increased lattice oxygen mobility. These effects result in a high NH3-SCR activity and high selectivity to nitrogen over the sulfated NiO–CeO2–ZrO2 catalysts.

An environmentally-benign CeO2-TiO2 catalyst for the selective catalytic reduction of NOx with NH3 in simulated diesel exhaust

A Ce-Ti based (CeO2-TiO2) catalyst prepared by an optimized homogeneous precipitation method showed excellent NH3-SCR activity, high N2 selectivity, broad operation temperature window, and high resistance to space velocity (even under a high gas hourly space velocity (GHSV) of 500,000h−1). Compared with V2O5-WO3/TiO2 and Fe-ZSM-5 catalysts, the CeO2-TiO2 catalyst showed better catalytic performance for NH3-SCR. Under a more realistic condition of simulated diesel engine exhaust, the monolith catalyst of CeO2-TiO2 showed over 90% NOx conversion from 250 to 450°C under a GHSV of 20,000h−1 in the presence of H2O, CO2, and C3H6. The high dispersion of active CeO2 on TiO2 in the process of homogenous precipitation and the synergistic effects between CeO2 and TiO2 in CeO2-TiO2 are important reasons for the high NH3-SCR activity.