Preparation And Optimization Of Modified Red Mud-based Catalysts For Selective Catalytic Reduction Of NO_x With NH₃

At present,the total emission of major pollutants in China ranks first in the world,in which nitrogen oxides(NOx)is the main precursor of pollution problems.Ammonia selective catalytic reduction(NH3-SCR)is a highly efficient NOx removal technology,widely used in coal-fired power plant flue gas treatment.In September 2014,the Shandong Provincial Quality Supervision Bureau firstly issued local standard for China's vanadium-free SCR denitration catalyst,which clearly stipulated that the V2O5 material could not be detected,so that the study on non-toxic non-vanadium-based SCR denitration catalyst became a hotspot.Red mud is a kind of solid waste in the process of aluminum smelting,which has good potential for catalyst application,but it has not been effectively integrated.In this paper,four kinds of typical red muds derived from different aluminum plants in China were selected,and the experiment was carried out to simulate the flue gas SCR denitrification reaction.The experimental results were tested under the typical conditions,XRD,BET,and SEM measurements,etc.Based on the optimization of the preparation parameters,the Ce-doped modified red mud catalyst and Zr-optimized Ce-doped catalyst were studied respectively.The effects of the modified treatment on the denitrification activity of the red mud catalyst were investigated,and the influences of reaction conditions on the denitrification activity of the catalyst were discussed,which aimed to develop the low-cost SCR denitration catalyst with good denitrification activity.The influence of preparation process on the denitrification performance of red mud was studied.The results showed that typical red mud is the feasible material for denitration catalyst.Compared with other alkali removal methods,'acid pickling and calcinating' is the best treatment process for original red mud,and the temperature window of the SCR denitrification was obviously widened at high temperature section;Besides,the surface of particle was smooth,while the morphology and distribution of particle was uniform,and the specific surface area of catalyst was increased,leading to the well internal channel connectivity of the particles and higher NH3-SCR denitrification activity.Compared with the catalytic activity of original red mud was obviously decreased at high temperature section,the de-NOx activity of ARM-500 red mud which was acid pickling and calcinating at 500 reached over 70%in the temperature range of 300-400 ?.Based on the above studies,the effect of individual components on the activity of red mud was investigated and the results showed that the SCR activity of Ce/ARM catalysts is much higher than others.Furthermore,the influence of Ce doped on the de-NO,performance of red mud catalyst and its optimum modification were studied.The results suggested that Ce and red mud will interact each other and the optimum doping value is 0.3.Doping with Ce can significantly reduce the crystallinity of a-Fe2O3 in red mud,refine the catalyst grains,and improve the specific surface area of red mud catalyst,thus further broaden the temperature window of de-NOx activity.Finally,based on the above mentioned Ce doped red mud catalyst,Zr was doped,and the influence of Zr doped on the de-NOx performance of Ce0.3/ARM catalyst and its optimum modification were studied.The results showed that the main phase composition of Ce/ARM catalyst will not be changed by Zr doped,and Zr can further refine the average grain size of the catalyst where it was present in an amorphous state.The optimal doping ratio was Ce:Zr=1:1.Ce0.3Zr0.3/ARM catalyst showed that the NOx conversion ratio reached 85%at 250? and its maximum NOx conversion was about 92%at 325?.Meanwhile,more than 80%NOx conversion could be achieved in the temperature range of 250-400 ? and its operation temperature window was obviously widened to both sides.Ce,Zr doped caused catalyst particle size to become smaller and more homogeneous,the surface structure to be more loose,the particles to be relatively independent,and the pore structure has been optimized.specific surface area for catalyst has been improved and more adsorption and reaction active sites were added,leading to higher NH3-SCR de-NO,activity of red mud.The study of the reaction conditions showed that the Ce0.3Zr0.3/ARM prepared in this paper has a certain adaptability with the gas hourly space velocity(GHSV)increasing from 15,000 h-1 to 30,000 h-1.The optimum conditions for the SCR reaction of Ce0.3Zr0.3/ARM catalyst were controlled as follows:GHSV of 30,000 h-1,O2 volume fraction of 3.5%-4%,the ammonia-nitrogen ratio of 1:1.NH3 participates in the reaction in the form of adsorption,while NO participates in the reaction in the form of gaseous or weakly adsorbed states.The adsorption of NH3 and O2 on the surface of Ce0.3Zr0.3/ARM was the key to the de-NOx reaction.Doped Ce can provide sufficient surface oxygen for the red mud catalyst and enable the SCR reaction to proceed,so NOx conversion ratio of the Ce0.3Zr0.3/ARM catalyst prepared by red mud reached 78%in the absence of O2.In addition,the lattice oxygen in the catalyst also participated in the SCR reaction,and further improved the NOx conversion of Ce0.3Zr0.3/ARM.