Selective Catalytic Reduction Of NO_x Over Fe-Ce/Wire-Mesh Honeycomb Catalyst

Selective catalytic reduction with NH3(NH3-SCR) is the most reliable method to remove the NOX from stationary sources due to the low cost and high efficiency. Currently, the commercial catalysts for this process are V2O5-WO3(MoO3)/TiO2catalysts and they are preferentially applied in form of monolithic honeycombs due to their low pressure drop. However, these catalysts commonly suffer from the toxicity of active component to environment, the narrow operation temperature window, the high activity for the oxidation of SO2to SO3and easy to dust blocking. Aiming to develop new DeNOx catalyst, catalyst support and active component were investigated in this paper.Firstly, V2O5-WO3/TiO2/Al2O3/wire-mesh honeycomb catalyst was prepared by a two-step impregnation method. In comparison with ceramic honeycomb catalyst with the same composition, the wire-mesh honeycomb catalyst exhibited good low-temperature activity and resistance to H2O, SO2and dust. During the60h H2O and SO2durability test (600ppm SO2+10%H2O) and40h dust exposure experiment, the wire-mesh honeycomb catalyst could provide nearly92%and90%NOX conversion, respectively. Characteration analyses of fresh and spent catalysts demonstrated that the deposition of sulfate-ammonium salts on catalyst surface was the dominant reason for the catalyst deactivation in the H2O and SO2durability test, and there was less sulfate-ammonium salts deposited on the surface of the wire-mesh honeycomb catalyst than that on the ceramic honeycomb catalyst. For the wire-mesh honeycomb catalyst, the unique three-dimensional structure was unfavorable for the high particle material deposition, which was the reason for the excellent resistance to H2O, SO2and dust.Secondly, Fe doped Ce/TiO2catalyst was prepared by impregnation method. From the activity test results, it was found that the addition of Fe could enhance the low-temperature activity and sulfur-poisoning resistance of Ce/TiO2. The Fe-Ce/TiO2catalyst with Fe/Ti molar ratio of0.2could attain80%NOx conversion at160℃, provide above85%NOx conversion after11h SO2durability test (500ppm SO2) at250℃. The characteration results indicated that the Fe doping to Ce/TiO2could increase the amount of NH4+and adsorbed NO2on catalyst surface, and thus promte the low-temperature SCR activity.Furthermore, the SCR reaction mechanism of the Fe-Ce/TiO2catalyst was studied. In the relatively low temperature range (<200℃), coordinated NH3and ionic NH4+sprcies as well as adsorbed NO2were involved in the SCR reaction following a Langmuir-Hinshelwood mechanism. In the relatively high temperature range (>200℃), coordinated NH3and gas-phase/weakly adsorbed NO predominated on the catalyst surface following Eley-Rideal mechanism.Finally, Fe-Ce/TiO2/Al2O3/wire-mesh honeycomb catalyst was prepared, and its resistance to sulfur-poisoning for different SO2concentrations (100-1000ppm) was investigated. At250℃, the Fe-Ce/TiO2/Al2O3/wire-mesh honeycomb catalyst could provide approximately90%NOX conversion during100h SO2durability test (1000ppm SO2). The characteration results indicated that the sulfation of CeO2occurred preferentially during the SCR reaction in the presence of SO2, and a little sulfate-ammonium salts was formed. The sulfation of CeO2could promote conversion of Ce4+to Ce3+, resulting in the increase of chemisorbed oxygen species which might be the reason for the excellent resistance to sulfur-poisoning of the Fe-Ce/TiO2/Al2O3/wire-mesh honeycomb catalyst. The SCR reaction followed the Eley-Rideal mechanism during the SO2durability test.