Li Or W Modified MnO_x/TiO₂ Catalyst For Selective Catalytic Reduction Of NO With NH₃

Selective catalytic reduction?SCR?of NO_x with NH₃ is currently considered to be the most widely used technique for NO_x removal.V₂O₅-WO₃?MoO₃?/TiO₂ is the most typical and efficient commercial catalyst for SCR of NO_x in a relatively high temperature range of 300°C-400°C.To meet the requirement of the high SCR temperature,the SCR reactor should always be located at the upstream of the purification system before the precipitator and flue gas desulfurization scrubber.As a result,the vanadium-based catalyst is deactivated by SO₂ and fly ash.The relatively low-temperature?<300°C?catalysts,which can efficiently work at the downstream after the precipitator and desulfurization scrubber,become highly desirable to avoid the above problems.Thus researchers have been trying to find new NH₃-SCR catalysts operating in a lower temperature window.It has been reported the monometallic oxides,composite metal oxides,supported metal zeolite catalysts,and found that composite metal oxides catalysts show better activity and selectivity than single metal oxide catalysts in a same preparation method.Li often act as an electron promoter during the preparation of catalysts,while basic metal itself.Consequently,it is able to adjust the redox properties and surface acidity of the catalyst.Firstly,Li and Mn,Fe,Co were used to prepare fixed crystalline form of LiMn₂O₄,LiCoO₂ and LiFeO₂,and the activity of the three catalysts in NH₃-SCR reaction also was tested.It is found that spinel LiMn₂O₄ show good low-temperature activity.Then,the different introduction method of Li into MnO₂ to form the LiMn₂O₄ structure adjusted the surface-acid sites and surface-active oxygen of manganese oxide,resulting in weakened NH₃ adsorption and unselective oxidation at high temperature,further to broaden operating temperature window and enhance the tolerance to sulfur.Meanwhile,LiMn₂O₄ prepared by high temperature solid state reaction method shows the best activity among the three methods.Based on the above studies,LiMn₂O₄/TiO₂,LiMnO_x/TiO₂ and MnO_x/TiO₂ were prepared by lithium manganese oxide and TiO₂ acting as the active phase and supporter respectively.Meanwhlie,the SCR catalyst activity,alkali metal toxicity and tolerance to sulfur were also discussed.It is found that LiMn₂O₄/TiO₂ show better NH₃-SCR activity than LiMnO_x/TiO₂ and MnO_x/TiO₂.The easy electron transfer between manganese cations with different valences within the LiMn₂O₄ spinel lattice with the aid of Li⁺might to be the reason for the better catalytic performance of LiMn₂O₄/TiO₂.Spinel-type manganese oxide in aqueous phase possesses the highest selectivity in Li sieving,and make it difficult for Na to adsorbing on LiMn₂O₄,so that LiMn₂O₄/TiO₂ still shows some better alkali resistance and regeneration than MnO_x/TiO₂.Here,the existence of alkali metal Li can form Li₂SO₄ with SO₂ avoiding transforming the active Mn species into inactive MnSO₄,which might be part of the reason for the better resistance of LiMn₂O₄/TiO₂ to SO₂.Tungsten is a commonly used promoter of SCR catalysts for stabilizing the active phase or supplying extra acid sites,A series of TiO₂ supported MnWO_x catalysts Mn_xW_0.05Ti_0.95-xO_2-??x=0.05,0.1,0.15?were synthesized by self-propagating high-temperature method.Mn enrich towards the surface after tungsten doped,and forming the main active phase MnWO_x,while significantly increasing the specific surface area of the catalyst.Ti_0.85Mn_0.1W_0.05O_2-?show the best NH₃-SCR activity among the catalysts,and the tolerance of sulfur has also been enhanced.