Morphology Effects Of Manganese Dioxide With Nanostructures And Their Catalytic Performance For Low Temperature NH₃–SCR

Selective catalytic reduction?SCR?of nitrogen oxide?NO_x?with ammonia?NH₃?is the most widely used technology for NO_x removal from stationary sources.Catalysts are the crucial factors in the SCR technology,which directly influence the efficiency of the SCR reaction.The commercial favored catalysts?V₂O₅–WO₃?MoO₃?/TiO₂?show the optimum performance in the temperature range of 300⁴00°C.And they would be deactivated by a high concentration of dust and SO₂,because the SCR system has to be placed in the upstream of the desulfurizer and electrostatic precipitator device.However,if the SCR system was adjusted to be in the final,the temperature of the flue gas would drop to below 200°C.Therefore,it,s important to develop the low-temperature SCR process and active catalysts.Manganese oxides have drawn broad attention as they contain various types of labile oxygen and oxidation states,which are necessary to complete the catalytic cycle in SCR reactions.Cerium oxides have high low-temperature SCR activity due to their unique oxygen storage capacity and excellent redox properties.Moreover,they have a strong ability of protecting the catalysts from sulfur poisoning.The catalytic performance of metal oxides depends on the size distribution and particle morphology,and reducing the size of the materials to the nanoscale can greatly enhance the catalytic performance.A catalyst particle with an anisotropic shape alters the reaction performance by selectively exposing specific crystal facets,which termed as morphology-dependent nanocatalysis.In this paper,a series of MnO₂ catalysts with different nanostructures were prepared by a hydrothermal method,and then applied to study the low-temperature SCR performance and the reaction mechanism.Furthermore,the modification analysis of CeO_x?z?-MnO_y nanosheet catalysts was also taken into account.Firstly,the catalytic performance of MnO₂ catalysts with different nanostructures for low temperature NH₃–SCR was investigated.The results showed that ?-MnO₂ nanosheet performed the highest activity for reduction of NO_x and N₂ selectivity,however ?-Mn O₂ microsphere performed the worst.Characterization results indicated that,the specific surface area was not the main factor affecting the catalytical activities of nanomaterials,which were both determined by the crystal structures and the exposed active crystals.Secondly,?-MnO₂ nanosheet catalysts were selected to study the SCR reaction mechanism.Results indicated that the Br?nsted and Lewis acid sites both existed on the surface of the catalysts.Moreover,NH⁴⁺ species on Br?nsted acid sites and coordinate ammonia species on Lewis acid sites all took participate into the SCR reaction.Furthermore,-NH₂ species which were formed by the dehydrogenation of coordinate ammonia,can not only react with NO adsorption species,but also directly response to the gas-phase NO.Therefore,the routine of the SCR reaction on ?-MnO₂ nanosheet catalysts both belonged to E-R and L-H mechanisms.Furthermore,among all the NO adsorption species,N₂O₄,nitro?M-NO₂?and nitrosyl?NO-?were the main intermediate species.Finally,in order to further optimize the low temperature SCR performance of MnO₂ nanosheet catalysts and improve the ability of sulfur poisoning resistance,the catalysts were modified by doping cerium oxides.The results showed that CeO_x?z?-MnO_y nanosheet catalysts with molar ratio of Ce/?Mn+Ce?0.2 had the highest activity.The addition of cerium oxides decreased the crystallinities of MnO₂,but increased strong acid sites on the surface of the catalysts.Moreover,a high concentration of reactive oxygen species and(Mn³⁺+Mn⁴⁺)strengthened the redox ability.Comparing the performance of MnO₂ nanosheet catalysts and CeO_x?0.2?-MnO_y nanosheet catalysts after sulfur poisoning found,the doping of Ce could slow down the deactivation phenomenon at a certain extent.Because cerium oxides could avoid the sulfating of active center atoms of Mn and retard the production of ammonium sulfate.