Optimization Study On The Performance Of Cu-Fe-Based Catalyst For Low Temperature Selective Catalytic Reduction Of No_x

Low temperature selective catalytic reduction(SCR)of NOx is a technology having broad prospects in flue gas denitrification.However,there are two main problems restricting its industrial application:the low temperature activity and the sulfur poisoning resistance of catalysts for SCR process.Based on the reported reasons for sulfur poisoning of various transition metal oxide catalysts,the influence of sulfation on the catalysts as well as the influence of catalysts on the decomposition of ammonium bisulfate was investigated in this article.A series of modification researches were conducted on the chosen CuOx and FeOx composite materials to enhance the SCR activity and poison resistance of catalysts at low temperatures.The influence of sulfation was investigated by activity tests and characterizations of various transition metal oxide catalysts(MnOx/TiO2,CuOx/TiO2,CeOx/TiO2 and FeOx/TiO2)before and after SO2 pretreament.The results indicated that metal sulfates may form on active metal oxides after SO2 treatment,which led to a significant decline in the redox property of the catalysts.And the low-temperature reaction activity of the surface acid sites was inhibited in spite of the enhancement of surface acidity after sulfation.As a result,the SCR activities of the catalysts at low temperatures decreased dramatically,among which the Fe-and Cu-based catalysts exhibited relatively higher activities after sulfation.The experiments above also indicated that ammonium sulfates may form through NH3 adsorption on the metal sulfates,which were produced during SO2 treatment.The activities of the CuOx/TiO2 and FeOx/TiO2 catalysts at relatively low temperatures were also inhibited after loading ammonium bisulfate(ABS).TG-FTIR results showed that pure ABS decomposed into NH3,SO2 and H2O in one step,while ABS loaded on CuOx/TiO2 and FeOx/TiO2 catalysts decomposed in several steps.The release of NH3 was promoted while the release of SO2 was significantly delayed by the catalysts,which arised from the sulfation of active metal oxides affected by ABS.Several kinds of composite supports were prepared by coprecipitation method.Through comparative study on the low temperature SCR activities and related characterizations,TiO2-ZrO2 was determined to be the more suitable support for Cu-Fe-based catalyst compared with commercial TiO2 as well as the homemade TiO2-Al2O3,TiO2-SiO2 and TiO2-CeO2.From the study on the influence of Zr content in TiO2-ZrO2,it was found that those mesoporous TiO2-ZrO2 which were in amorphous phase owned higher specific surface areas and total pore volumes as well as smaller pore diameters.The strong interaction between TiO2 and ZrO2 contributed to a dramatic increase in the number of surface acid sites and promoted the dispersion of CuOx species as well.With the increase in Zr content,the redox property and the NOx adsorption property of the CuFe/Ti-Zr catalyst were strengthened.Consequently,a sample with Zr content of 0.75 was selected as the best supporter due to the highest low temperature SCR activity of the corresponding catalyst.According to optimization study on the active components loading on the TiO2-ZrO2(0.75)support,the 9Cu9Fe/Ti-Zr catalyst,with 9%CuOx loading and 9%FeOy loading,exihibited over 80%NOx conversion at a wide temperature range of 180?300? and a space velocity of 60,000 h-1?Mechanism study of the optimized 9Cu9Fe/Ti-Zr catalyst indicated that there were almost all Lewis acid sites on the surface of the catalyst,and the SCR reaction followed the E-R mechanism mainly and the L-H mechanism as well.Reasonable choice of space velocity,NH3/NO molar ratio and O2 concentration contributed to enhancement of NOx conversion.There appeared an improvement in the resistance to H2O and SO2 after support modification by compositing TiO2 and ZrO2 for the Cu-Fe-based catalyst.However,H2O and 200 ppm SO2 still deactivated the optimized catalyst reversibly and irreversibly respectively.The SO2 deactivation reasons were analyzed combining with the regeneration and TG results.Sulfation of metal oxides and deposition of ammonium bisulfate both resulted in the fast deactivation of the catalyst at 250?,while the deposition of ammonium bisulfate led to the slow deactivation at 280? and the SCR activity of the poisoned catalyst could be recovered and even enhanced further after heat treament at 400?.What's more,the Cu-Fe/Ti-Zr catalyst was modified by VOx.It was found that the SCR activity at relatively high temperatures as well as the resistance to H2O and SO2 at 250? was enhanced with V additive.Activity of the SO2 poisoned 9Cu9Fe2V/Ti-Zr catalyst could be recovered mostly by heat treatment.The results revealed that V additive contributed to inhibiting the sulfation of active components.