Effects Of H₂O And SO₂on The Low-temperature Selective Catalytic Reduction Of NOx By Ammonia Over MnOx/MWCNTs Catalyst

Selective catalytic reduction （SCR） of NOx with NH₃is one of the most effective fluegas cleaning technologies for stationary sources. Catalysts are crucial for the SCR technology,which determinate the efficiency of the SCR system. However, the commercial catalysts（V₂O₅–WO₃（MoO₃）/TiO₂） are only active within a narrow temperature window of300–400°C, and are prone to be deactivated by high concentration of SO₂or H₂O. So developingcatalysts with high low-temperature SCR activity and resistence to SO₂and H₂O is veryimportant for industrial application, thus study on the SO₂and H₂O deactivation mechanismfor the catalyst is meaningful. Manganese oxides supported on multi-walled carbonnanotubes（MnOx/MWCNTs） catalysts were prepared by method of pore volumeimpregnation using MWCNTs as the catalyst support which was pretreated by oxygendielectric barrier discharge plasma before. The effects of H₂O and SO₂on the catalysts SCRperformance are investigated.Firstly, the effects of water on the NH₃-SCR activity under different water vopourcontents and reaction temperatures are studied. Experimental results showed that H₂O had asignificantly negative effect on the catalytic activity of the catalyst. When the temperaturewas higher than270°C, the effects of water could be negligible. The inhibition was reversible.When H₂O was removed from the feed gas, the SCR activity could be restored to the originallevel. N₂adsorption-desorption, X-ray powder diffraction（XRD）, transient responseexperiments, Raman spectroscopy and in situ Fourier transform infrared（FTIR） spectroscopyresults indicated that the competitive adsorption of H₂O and NH₃on the Lewis acid sites maycontribute to the deactivation of the catalyst.Secondary, the effects of SO₂on the low-temperature SCR activity under different SO₂concentrations and temperatures are also investigated. The catalysts were characterized bythermogravimetric analysis （TGA）, scanning electron microscopy （SEM）, N₂adsorption-desorption, X-ray powder diffraction （XRD）, X-ray photoelectron spectroscopy（XPS）, temperature-programmed reduction/desorption （TPR/TPD）, and Fourier transform infrared spectroscopy （FTIR） measurements to analyze the SO₂poisoning mechanism. Theresults showed that SO₂had an obvious poison-effect on the SCR activity ofMnOx/MWCNTs at low-temperature. The activity decreased more rapidly as the reactiontemperature and SO₂concentration increased. The main reason for catalysts deactivation wasthat the active center atoms were sulfated. The formation of ammonium sulfate on catalystssurface and the inhibiting effect of SO₂on NO adsorption also resulted in the catalystsdeactivation to some extent.At last, the active species and intermediate during the surface reaction were detected bythe in situ FTIR measurement so as to investigate the influence of SO₂on the adsorption ofreaction gases and formation of products, thus analyzing the poisoning mechanism frommolecular and microcosmic aspect. The results showed that the presence of SO₂resulted inthe decrease of coordinated NH₃on Lewis acid site and the increase of NH4+on Br nsted acidsite. Sulfur element on the catalyst surface occupied the adsorption-reaction active sites by theformation of sulfate, causing the decrease of NO adsorption state species. Consequently, itwas difficult to form effective group to participate in the SCR reaction so that the NOxremoval rate declined, thereby explaining the SO₂deactivation mechanism ofMnOx/MWCNTs further.