Selective Catalytic Reduction And Reaction Mechanism Study Of NO_x With NH₃Over The MnO_x/CNT_s Catalysts

Carbon nanotubes （CNTs）, as one of the new carbon material, have been reported to beapplied in the various fields based on their unique electronic properties and nano-channelstructure, such as high mechanical strength, thermol-chemical stability, confinement effect,and well conductivity, etc. Selective catalytic reduction （SCR） of NO_xwith NH₃is one of themost effective flue gas cleaning technologies for stationary sources, but CNTs as the carrier ofNH₃-SCR catalysts still need to be studied.A series of MnO_x/MWCNTs catalysts were prepared by the different methods. Thecatalytic activity messurement and chemical characteristics were carried out. And then thereaction process on the surface was discussed by in situ DRIFTS analysis.For loading MnO_xparticles on both inside and outside surfaces of CNTs, ethanol wasused as a solvent for manganese acetate because of its low surface tension, which canfacilitate the introduction of metal salt solution into CNT channels by a wet chemical method.For introducing MnO_xparticles only outside the CNT channels, xylene was chosen totemporarily block the CNT channels before decoration of the exterior CNT surface withMnO_x. Deionized water was used as a solvent for manganese acetate, which can preventmanganese acetate from infiltrating the channels. It was found that MnO_xenhanced thecatalytic activity of CNTs in NH₃-SCR, and Mn-both-CNTs played the better activity thanMn-out-CNTs all over the whole temperature range, especially3%Mn-both-CNT_s400in180-300℃. For Mn-both-CNTs, the catalytic activity increased with the increasingtemperature. NO_xconversion can reach nearly100%at250℃over3%Mn-both-CNT_s400.It was found that Mn³⁺and Mn⁴⁺coexisted and the relative ratio between them was verysimilar on the surface between3%Mn-both-CNT_s400and3%Mn-out-CNT_s400, and somolecule adsorbed oxygen and lattice oxygen was. This coexistence and the surface acidfunctional group can increase the catalytic activity. The result of Raman showed that theMn-O band had the distinct red shift when some manganese was confined in the CNTchannels. In addition, the catalyst,3%Mn-both-CNT_s400, showed a two-step reduction process of the MnO_x, which illustrated the stronger interaction between metallic oxides and the innersurface when part of metal oxides was introduced into the CNT channel. It could be easier forthe inside metal to donate more electrons, leading to be reduced easily.DRIFTS found that the reactant was physically absorbed on the catalyst surface at thelow temperature, along with very weak chemical reaction. However, catalytic reaction tookplace and the routine of the SCR reaction on the MnO_x/MWCNTs catalysts mainly belongedto the L-H mechanism at the high temperature, but different intermediate products could bedetected between these two catalysts. One was [NH₄]₂N₂O₂，and another was [NH₂]₂N₂O₂.