Study On Fe?-Mo?/CNTs Catalysts For Selective Catalytic Reduction Of NO_x With NH₃

To high-efficiently control the emission of NO_x produced by fossil combustion in power plants,the technique of selective catalytic reduction of NO_x with ammonia?NH₃-SCR?has been widely used.High concentrated SO₂ and ash in the fuel gases had heavily inhibited the performance of catalysts,so placing the SCR unit downstream of the desulfurizer and electrostatic precipitators is an optional way to solve this problem.The temperature of fuel gases is lower than 300 oC after desulfurization,implied that the traditional SCR catalysts wellperformed at high temperature do not meet the low temperature requirements anymore.We have already developed a new kind of highly dispersed Fe/CNTs catalysts with good lowtemperature SCR performance by using the unique chemical properties of carbon nanotubes.However,small amount of SO₂ remained after the desulfurization process severely impacts the activity of the catalyst which indicates the investigation of the SO₂ impact on Fe/CNTs and the optimization of Fe/CNTs to improve its resistance to SO₂ and H₂ O are necessary.First,the investigation of SO₂ impact on Fe/CNTs showed that NO_x conversion was inhibited under 275 oC and it was promoted when temperature was over 275 oC.Characterization tests suggested that ferric sulfate was the major sulfate species accumulated on Fe/CNTs,which enhanced the adsorption of NH₃ species while constrained the adsorption of NO_x species.Reaction pathway study implied the adsorption and activation of NO_x species over Fe/CNTs was the key to its excellent low-temperature performance.While this effective reaction pathway was inhibited with SO₂ in the feed gas and lead to the low-temperature activity loss?< 275oC?.Instead,ferric sulfates formed on Fe/CNTs promoted the reaction between adsorbed NH₃ species and gaseous NO through which SCR performance at higher reaction temperature?? 275oC?was promoted and this became the dominating reaction pathway.The transform of dominating reaction route was the reason for the change of de-NO_x performance over Fe/CNTs in presence of SO₂.Second,Fe-Mo/CNTs was prepared by doping molybdenum?Mo?as second active component to widen the active reaction temperature of Fe/CNTs and improve its ability to resist SO₂ and H₂ O.The SCR performance tests showed that Fe-Mo?0.2?/CNTs was the best catalyst with over 90% NO_x conversion at 250-400 oC and over 85% N₂ selectivity.Characterization tests suggested that active components dispersed well on Fe-Mo?0.2?/CNTs.Acidic sites were improved by the addition of Mo and more active oxygen species was detected.This in turn promoted the adsorption and activation of reactants on the surface of Fe-Mo?0.2?/CNTs.Meanwhile,Fe-Mo?0.2?/CNTs exhibited well adaptability to the change of 1-5 vol.% O₂ concentration and high GHSV?46000-62000 h-1?.It also had a perfect resistance to SO₂ by showing with only 3.1% rate of descend of NO_x conversion at 250 oC and the NO_x conversion rarely changed at 300 oC.NO_x conversion over Fe-Mo?0.2?/CNTs declined 45% at 250 oC when some water vapor was added in the reaction gas.