Molecular Modeling Study Of Low-temperature SCR Reaction Mechanism Based On Carbon Nanotubes

Nitrogen oxides （NOx） generate in production activities are most from the stationarysources, such as coal-fired power plants. Nowadays, the selective catalytic reduction （SCR）technology performs well in industrial application. Low-temperature SCR could avoid thereform of the original system, reduce production and operational costs, and have higherresistance to dust and SO2. Subsequently, the development of low-temperature SCRtechnology is attractive and become an important direction of De-NOx.At present, most research for the reaction mechanism of low-temperature SCRtechnology adopt the experimental method. However, the results are usually unauthentic asto the research of reaction path, medium product and intermediate state inferred by theexperimental data. Fortunately, the molecular simulation technology could solve thoseproblems. As a result, it’s valuable with its economy, stability and high efficiency.In this paper, the modification mechanism of carbon nanotubes （CNTs） and lowtemperature SCR reaction mechanism based on MnOx/CNTs catalyst system were studiedby the molecular simulation technique.The carbon nanotubes were modified by oxygen dielectric barrier discharge （DBD）plasma in laboratory, so we chose the hydroxyl and carboxyl grafting on （8,0） carbonnanotube as the computational model respectively. The binding energy, energy bandstructure and density of states for the （8,0） carbon nanotube before and after modificationwere studied by density functional theory （DFT）. The results showed that the bindingenergy became smaller, which suggested the dispersion of carbon nanotube surface wasenhanced and the carbon nanotube could exist suitable after modification. In addition,although the Eg of （8,0） carbon nanotube was samll, the top of valence band and the bottomof conduction band were not overlapping the fermi level, which successfully verified thatthe （8,0） carbon nanotube was a semiconductor type. The structure of （8,0） carbon nanotubewas greatly affected by OH and COOH, which increasing the specific surface area. At the same time, the value of Efand the density of states on the Fermi level was significantlyincreased after modification, which proved the oxygen plasma strengthend the activity ofcarbon nanotube and indicated it was an effective modification method.The MnO₂/CNTs catalyst was used to research for low-temperature SCR reactionmechanism in experiment, which worked out the reaction responsed to E-R reactionmechanism. Therefore, density functional theory （DFT） calculations were carried out on（110） MnO₂catalyst surface, which represented by a Mn₇O₁₄cluster, to probe the reactionmechanism for low-temperature SCR of NO by NH₃in accordance with the E-R reactionpath. The computations indicated three main parts made up SCR reaction. Firstly, NH₃wasweakly absorbed to the Mn₇O₁₄cluster, which consistent with the experimental results.Secondly, the gaseous NO reacted with the pre-adsorbed NH₃species to eventually formNH₂NO species with a high activation barrier of15.91kcal/mol; At the same time, theremust be cautioned that only an approximate transition state was obtained for this step.Finally, NH₂NO decomposed to generate N₂and H2O with an energy barrier of41.72kcal/mol. The whole reaction path successfully verified the E-R reaction mechnism oflow-temperature SCR based on MnO₂catalyst raised by experiment.