Activation Mechanism And Performance Optimization Of Red Mud For SCR Denitrification

Nitrogen oxide(NOx)is one of the main air pollutants in China and needs to be treated.Selective Catalytic Reduction(SCR)technology has a high NOx conversion rate.Currently,it's widely used in flue gas denitrification.The core technology of the SCR is the catalyst.The commercial catalysts are vanadium-titanium-based catalysts.This type of catalyst exists some problems,such as high cost,narrow denitration temperature window,and the active component V2O5 easily causes secondary pollution to the environment.Therefore,it's pretty important to develop new type of environment-friendly denitrification catalysts with inexpensive,good denitration activity,widely reaction temperature window.Red mud is a waste produced by the bauxite processing.Our team previous research found that red mud can be used as denitrification catalysts.Using red mud as raw material to develop new catalysts can realize the waste resources reuse,which has a very important social and environmental significance.In this article,the red mud used was from Nanshan Aluminum.The red mud components were treated by the water washing method and the acid washing method.It was explored that the influence of each red mud ingredient on denitrification performance with the help of XRF,XRD,BET,SEM,XPS,NH3-TPD,H2-TPR and FTIR.This provided a theoretical basis for the optimization controlling of red mud compositions.Based on this,the introduction of pickling to adjust the red mud components by controlling the red mud pH value.The effect of the interaction between red mud components on the denitrification performance was studied.Finally,the red mud was treated with ultrasonic technology to explore the effect of ultrasonic on the red mud compositions adjustment process.The main conclusions in this paper are as follows:(1)After water washed,the content of Na2O in red mud was reduced by 38.5%,while CaO has decreased by 63.3%.The specific area and the specific pore volume of the red mud increased by 11.1%and 8.3%.The NOx conversion rate of the red mud catalyst was improved,keeping above 80%between 325-400?.After the acid washed,the aluminosilicate and tricalcium aluminate in the red mud were removed.The content of Na2O and CaO in red mud decreased by 77.9%and 90.0%,while the content of Fe2O3 increased by 28.2%.It was improved the pore structure of the catalyst.The specific surface area and the specific pore volume increased by 80.0%and 34.0%.The acid-washed red mud exhibited better denitrification activity than the water-washed red mud.The catalyst's NOx conversion rate exceeded 90%between 300-430 ?,and its NOx conversion rate reached 100%at 350?.(2)Among several catalysts explored,the CRM-5 catalyst had the highest pseudo-first-order rate constant and initial reaction rate.The CRM-5 showed the best denitrification performance.Its NOx conversion rate reached 100%in the range of 300-450?.The catalyst had the highest Ce3+/Ce4+ratio and surface acid content,which could promote the adsorption and activation of NH3.While the amount of ammonia adsorption increasing was beneficial to improve the catalytic SCR activity.Besides,the specific surface area of the CRM-3 catalyst was the largest among the catalysts,and its area was 72.42 m2·g-1.The high specific surface area was conducive to the dispersion of active components on the catalyst surface.The Fe3+ content on the CRM-3 catalyst surface reached 23.8%,which was 27.9%higher than that of the CRM-5 catalyst.In the range of 275-470?,the CRM-3 NOx conversion rate exceeded 90%.The red mud will generate almandine at pH=4.The almandine wouldn't participate in the SCR reaction progress.The formation of almandine consumed a part of Fe3+ and covered the catalyst surface,which prevented the reaction gas adsorbed.Finally,the catalyst denitration activity was decreased.The DFT calculation results showed that Al can increase the adsorption capacity of Fe2O3 for NH3 and NO,thereby improving the denitrification performance of the catalyst.For the CRM-5 and the CRM-3 catalysts,there were a strong interaction between Al and Fe2O3,which increased the catalyst NOx conversion rate.For the CRM-4 catalyst,the formation of almandine weakened the interaction between Al and Fe2O3,and the catalyst NOx conversion rate was reduced.(3)When the red mud was treated with pickling and ultrasonically,the Fe2O3 content of red mud was significantly increased,while the Na2O and CaO were completely removed.The distribution of red mud's elements on the catalyst surface was uniform,the particles were refined,and the agglomeration status were weakened with ultrasonic treatment.After ultrasonic treatment,the catalyst internal pore structure has been optimized,the specific surface area and the specific pore volume of catalyst have increased by 46.7%and 10.7%,respectively.Meanwhile,it can greatly increase the number of Lewis acid sites and Br(?)nsted acid sites on the catalyst surface.The active temperature window of the catalyst(the temperature range where the NOx conversion rate higher than 90%)widened to the higher and lower temperature.There were optimal values for the time and power with ultrasonic treatment of red mud.In this study,they were 30 min and 70 W,respectively.When the ultrasonic treatment time was longer,particles agglomeration appeared on the catalyst surface,and the effect of ultrasonic optimization was weakened.