Study On Optimization Of Cerium Based Mixed Oxide Catalysts And Their Denitrification Performance

Nitrogen oxides?NO_x?are considered to be one of the major contaminants which could cause acid rain,photochemical smog and greenhouse effects.Their emission has seriously threatened human health and ecosystems.More than 90%of NO_x in the atmosphere comes from fixed source and mobile source.The most promising technology currently used for the removal of nitrogen oxides from stationary sources is the selective catalytic reduction technology using NH₃ as a reductant?NH₃-SCR?because of its high efficiency and low cost.Various catalysts have been developed for the selective catalytic reduction of nitrogen oxides,presentatively transition metal oxides with good redox capabilities.One most widely used category for NO_x SCR is the cerium-based oxide catalyst,in which the redox conversion between Ce³⁺and Ce⁴⁺leads to excellent oxygen storage capacity.Pure cerium oxide is prone to by-products and nitrogen is poorly selective.In order to address such issues,the addition of multiphase components to Ce-based catalysts has been considered to be one of the most promising and most effective approaches.In this thesis,Ce-based catalysts were modified by Zr,W,Mn and other metal elements.A series of Ce-based catalysts were synthesized by co-precipitation method using surfactants as soft templates.By combining characterization methodsofspecificsurfacearea,X-rayphotoelectronspectroscopy,hydrogen temperature-programmed reduction,X-ray diffraction analysis and others,the activity and the physical chemical properties of the catalysts were analyzed and discussed.The obtained results are shown as follows:1.The Ce_0.75W_0.25O_x catalyst derived from the surfactant-assisted method exhibited a NO_x conversion rate of 95%¹00%at a space velocity of 24000 h^-1in a wide temperature range of 175-425°C.Different proportions of W and Ce led to a new complex oxide community.Doping with tungsten did not destroy the lattice structure of CeO₂,and the highly dispersed tungsten species reduced the content of the cerium oxide crystal phase,leading to more defect sites.The amount of active species and acidic sites on the surface were increased,which enhanced the catalytic activity of the catalyst.2.After modification of Ce_0.75W_0.25Mn_0.5O_x catalyst with Mn,the NO_x conversion rate was 100%at 175°C at a space velocity of 30000 h^-1.Meanwhile,within the temperature range below 175°C,the activity of Ce_0.75W_0.25Mn_0.5O_x catalyst was obviously higher than that of Ce_0.75W_0.25O_x catalyst.Doping with manganese overall increased the active manganese species and oxygen species of the catalyst,increased the acid sites on the catalyst surface,and thus significantly increased the catalytic activity of the catalyst.