The Effect Of Modified Additives On The Catalytic Activity And Hydrothermal Stability Of The V₂O₅/WO₃-TiO₂ Based NH₃-SCR Catalysts

Driven b y t he pol icy of energy s aving and emission reductions, t he catalytic removal of nitrogen oxides under lean-burn conditions has been receiving much more attention in catalytic research. One of the promising research directions is selectve catalytic r eduction b y am monia ( NH₃-SCR). V₂O₅/WO₃-TiO₂ based N H₃-SCR catalysts exhibit superior NOx removal efficiency and good sulfur resistance. However, problems still exist arising from the poor thermal stability and the potential of vanadia volatilization under c ertain c onditions. T he c atalytic a ctivity and hy drothermal stability of V₂O₅-WO₃-TiO₂ catalysts were investigated. The catalyst modifications by ceria and zirconia were also performed to develop a superior SCR catalyst combined with wide temperature range and good hydrothermal stability. It is necessary for the application of V₂O₅/WO₃-TiO₂ based NH₃-SCR catalysts.V₂O₅/WO₃-TiO₂ catalysts w ith v arious v anadia l oading were p repared b y impregnation m ethod a nd the hydrothermal s tability o f th e c atalysts w as also investigated. It was found that the catalyst exhibits better performance with 1wt% of vanadia loading after aging at 750 oC for 12h in 10wt% H2O/air. Further increase of vanadia content only promotes TiO₂ sintering, phase transition and loss of surface area, resulting in the deactivation of catalysts.The effect of zirconia on the activity and hydrothermal stability of 1wt% vanadia containing catalysts were studied by XRD, XPS, SEM and DRIFT. It was found that zirconia improves the structure stability of the catalysts b y the formation of ZrO₂ particle and ZrxTi1-xO₂ solid solutions. The catalyst with 10 wt% ZrO₂ exhibits the highest NO conversions and N2 selectivity in the temperature range of 150 oC -550 oC among the aged catalysts. On the other hand, zirconia inhibits the low temperature activity of the fresh catalyst. This is mainly due to the decreased amount of Br?nsted acid sites and V5+= O active sites by zirconia.Ceria modification on 1%V₂O₅-10%ZrO₂/WO₃-TiO₂ were also performed, the catalysts were characterized by XRD,XPS,H2-TPD and DRIFT. It was found that ceria could greatly enhance t he l ow t emperature N H₃-SCR p erformance o f t he catalysts. This is due to the presence of Ce3+ and enhanced redox properties by ceria addition. Moreover, the NOx adsorption and its reaction with adsorbed NH₃ are also accecelarated by ceria component. After hydrothermal aging treatment, the decrease of th e lo w te mperature a citivity o f c eria mo dified c atalysts is observed. This is ascribed to the segregation of the active CeO₂ component on the catalyst surface. It is note w orthing t hat i n t he c onditions of 5 % H 2O a nd NO 2/NO= 1, t he a ged 1%V₂O₅-10%CeO₂-10%ZrO₂/WO₃-TiO₂ catalyst could still exhibit over 85% of NO conversion and 98% of N2 selectivity during the temperature range of 200 oC -550 oC.The method for analyzing the loss of V and W component in the catalysts was developed using TG equipment, it was found that the loss of surface active species on the catalyst surface would be accelerated by increasing vanadia loading and in the presence of H2O and O₂ in air.