Selective Catalytic Reduction Of NO_x With Ammonia On Transition Metal Oxides

Ammonia selective catalytic reduction （NH₃-SCR） remained as one of most efficientstrategies for the control of NO_xemission from stationary sources. This work is mainlyfocusing on the study of NH₃-SCR over various types of transition metal oxides,including vanadium-based catalysts and novel iron-based oxides catalysts.Under different dispersion conditions, tungsten species exhibit altering influences onsurface acidity, redox properties, dispersion of vanadium species and SCR performance.It has been found that the uneven dispersion of tungsten oxide can give rise to thecrystalline WO₃on catalysts surface and a decrease of Br nsted acidic sites. Accordingto the TPR and Raman results, monolayer VO_xspecies are suggested to show a specialtendency to be dispersed in the immediate surrounding of amorphous tungsten oxide onanatase surface. The dispersed forms of VO_xspecies are closely related to the contentsand dispersion conditions of tungsten oxides. At low vanadium contents, the redoxproperties and low-temperature performance are mainly determined by the contents ofV₂O₅in V₂O₅–WO₃/TiO₂catalysts. Monolayer WOxspecies show promoting effects onthe formation of moderately distorted VO_xspecies, which are the well-proved activecenters of SCR reaction. At high vanadium contents, the low-temperature SCRperformance is scarcely influenced by the increasing loadings of V₂O₅and WO₃.Furthermore, surface VO_xspecies would be compressed by the excessive content ofWO₃, resulting in the aggregation of vanadium oxide.The substitution of WO₃by Fe₂O₃in V₂O₅–WO₃/TiO₂catalysts is studied and thephysic-chemical properties V₂O₅/Fe₂O₃–TiO₂catalysts are characterized. The resultsindicate that2wt.%V₂O₅/Fe₂O₃–TiO₂showed excellent SCR performance andH₂O/SO₂resistance in the temperature range of300450°C. On the surface ofV₂O₅/Fe₂O₃–TiO₂catalysts, acidity, redox properties and SCR performance aregenerally influenced by the co-reaction of V Fe Ti.Iron-based oxides have also been demonstrated to be active in SCR reaction, whichis related to doping of metals, surface structure and redox sites. On the surface of ironspinel oxides, the NH₃-related reactions are involved in the competition betweenNH₃-CR and NH₃-CO reaction. It is also indicated that the NH₃-SCR route is assigned to Lewis-acid-inspired E R mechanism on the outermost surface of iron spinel oxides.The catalytic performance of metal-doped iron spinel oxides is structurally determined.With Co, Ni or Zn doped into the spinel structure, the magnetite-type spinel structure ismaintained and the redox cycles of Fe³⁺/Fe²⁺in octahedral sites of outermost surface areinhibited. As a result, the NH₃adsorption, activation and SCR performance areconsequently decreased. Due to the enrichment of Mn on surface and the formationmaghemite-type spinel structure, the Mn-doped spinel oxide shows a improvinglow-temperature activity.