A Study On Selective Reduction Of NO_x With C₃H₆over Ag/Al₂O₃Catalysts

NO_xis one of the main air pollutants which can cause a series of environmental pollutionsuch as acid rain and photochemical smog.Selective catalytic reduction of NO_xby hydrocarbonis one of the most favored technologies for NO_xemission from mobile source.So far Manyresearchers have found that a variety of catalysts can promote the NO_xreduction under thelean burn conditions,among which,silver on alumnia catalyst has been a major concern becauseof its moderate reaction activity.However,there are still two major problems: medium-lowtemperature activites and sulfur dioxide poisoning. With the above problems in mind,selectivecatalytic reduction of NO_xby propene was systematically investigated by using Ag/Al₂O₃catalysts in this paper.Firstly,several metal oxides were chosen to be dropped into Ag/Al₂O₃,it was found thatTiO₂dropping has great influence on the catalystic activity,the activity was enhanced21.5%from60%at350℃.On the one hand,there was a strong interaction between TiO₂andAl2O3,which was beneficial to the greatly dispersion of active sites,improving adsorptioncapacity of NO_xat low temperature.On the other hand,TiO₂dropping not only could increasethe concentration of active sites and lattice oxygen,but also improve the weak lewis acid oncatalyst surface,thus it would be beneficial to the adsorption of C3H6and its activitation.Secondly,the effect of reaction mechanism resulted from TiO₂dropping over Ag/Al₂O₃catalyst was studied by in situ DRIFTs measurement.The result showed that acetate and enolicspecies were formed by the adsorption of propene on catalyst surface,these species couldparticipate in SCR reaction,enolic species which were combined with silver ion played aimportant role in formation of NCO species.Enolic species were proved to be more active thanacetate species.TiO₂dropping could promote the formation of acetate and enolic species.Thelow concentration of Ag active sites inhibited the formation of enolic species,which caused lowactivity over the Ag/Al₂O₃catalyst at medium-low temperature.There were two major reactionpaths of selective catalytic reduction of NO_xby propene over the Ag/Al₂O₃and Ag/TiO₂-Al₂O₃catalysts:the surface nitrates reacted with acetate to form NCO species,finally,NCO speciesreact with the surface nitrates or NO+O₂to form N₂and CO₂,while in the case of second reaction paths,firstly,propene was activated to enolic,the surface nitrates than reacted withenolic to form NCO species.The process of NO reduction reaction by propene over Ag/Al₂O₃and Ag/TiO₂-Al₂O₃catalysts was mainly from the second reaction paths.Finally,the effect of SO2over the Ag/Al₂O₃and Ag/TiO₂-Al₂O₃catalysts as well as itspoisoning mechanism were studied detailedly.The competitive adsorption of NO and SO2andthe formation of silver sulfate were proved to be the main reason for deactivation of Ag/Al₂O₃catalyst,the addition of TiO₂not only could improve the adsorption of NO and inhibit theadsorption of SO2on catalyst surface, but also inhibit the sulfation of Ag during the catalyticprocess, which could enhance sulfur poisoning performance of the catalysts eventually.