Activity and antitoxic properties of Cr-MnO_x/TiO₂-ZrO₂ for low-temperature selective catalytic reduction of NO

Using a sol-gel prepared TiO₂-ZrO₂ carrier,Cr-MnO_x/TiO₂-ZrO₂composite catalysts were synthesized by a critic acid solution impregnation method. The physical and chemical properties of the catalysts were characterized by X-ray diffraction,specific surface area determination(BET),scanning electron microscopy and X-ray photoelectron spectroscopy. Meanwhile,the lowtemperature catalytic activity and the sulfur and water resistance were evaluated with selective catalytic reduction(SCR) of NO by NH₃. The introduction of Cr element to MnO_xyields a new CrMn_1.5O₄ active phase,in which Mn primarily exists as Mn³⁺ and Mn⁴⁺.The Cr⁵⁺ valence state facilitates the transformation of Mn³⁺ to high oxidation state Mn⁴⁺,which is beneficial to the low temperature SCR reaction. Because of lower first ionization energy and electronegativity,Cr preferentially reacts with SO₄^2- and SO₃^2- than Mn,thus protecting MnO_x not to be sulphatized and resulting in the enhanced antitoxic performance of Cr-MnO_x/TiO₂-ZrO₂. Among the prepared five catalysts with different molar ratios of Cr/(Cr + Mn),Cr(0.4)-MnO_x/TiO₂-ZrO₂ demonstrates the best performance owing to the largest surface area and the best particle dispersion,and a 95.8% denitrification efficiency can be achieved at 180℃.When 5% H₂O and 10^-4 SO₂ are simultaneously added,the denitrification efficiency decreases slowly and stabilizes at 73% after 8h reaction.