Preparation And Mechanism Of SO₂ Resistant Ce-Based Catalysts For NH₃-SCR Of NO_x

Nitrogen oxides emitted from fossil fuel combustion and motor vehicle exhaust have already greatly affected the atmospheric environment and human health,and may cause environmental problems such as greenhouse effect,acid rain,and photochemical smog.NH3 selective catalytic reduction（SCR）has been considered as one of the most effective methods to control NO_x emissions.The key to this technology lies in the reasonable design of highly effective deNO_x catalyst.The most commonly used vanadium-tungsten-titanium catalysts in the industry exhibited high catalytic activity,but there are still some unavoidable disadvantages.Therefore,the development of NH3-SCR catalysts with high activity,wide temperature window and good SO₂ tolerance performance,and deep understanding of the reaction mechanism become the topic issues for the current research.In this thesie,the SCR catalyst was designed from the aspects of specific crystal plane exposed,active site protection and doping modification.The morphology,redox properties and surface acid sites of the catalyst were systematically studied.The in situ DRIFTs was used to demonstrate the reaction pathways and SO₂ tolerance mechanism of the catalyst.The main contents of this article are listed as follows:（1）The catalytic activity,stability and SO₂ resistance performance of the TiO₂ nanoparticle exposed to the（101）crystal plane loaded CeO₂ catalyst and the TiO₂ nanosheet exposed（001）crystal plane loaded CeO₂ catalyst were compared.The results show that the TiO₂ nanosheet exposed（001）crystal plane loaded CeO₂ catalyst has better catalytic performance,and the surface is more likely to adsorb high-activity species such as bidentate nitrate and gaseous NO₂,which is helpful for catalytic reaction.In addition,the TiO₂ nanosheet exposed（001）crystal plane loaded CeO₂ catalyst has more acidic sites on the surface,the ammonia species mainly exists in the form of coordinated NH3 on the Lewis acid sites of the catalyst,which promotes the adsorption of reactive gases on the catalyst surface and the formation of active intermediate species.When the SO₂ was introduced into the reaction gas,the sulfur species are more easily adsorbed on the surface of the TiO₂ nanoparticle exposed to the（101）crystal plane loaded CeO₂ catalyst,while barely adsorbed on the surface of the TiO₂ nanosheet exposed（001）crystal plane loaded CeO₂ catalyst.Thus,the TiO₂ nanosheet exposed（001）crystal plane loaded CeO₂ catalyst exhibits superior SO₂ tolerance performance than the TiO₂ nanoparticle exposed to the（101）crystal plane loaded CeO₂ catalyst.（2）The halloysite was treated by molten salt method,and then the iron-modified halloysite loaded CeO₂-WO3 catalyst was successfully synthesized by impregnation method and precipitation method.The catalyst showed excellent SCR activity,high N2 selectivity,good stability,high NO_x conversion in the presence of SO₂.The abundant surface active oxygen species and the active metal components of the iron-modified halloysite loaded CeO₂-WO3 catalyst,synergistically catalyzes NO_x into N2 with NH3.In addition,the in situ DRIFTs results show that the active species formed on the surface of the iron-modified halloysite loaded CeO₂-WO3 catalyst are more active,leading to more adsorbed ammonia species and improving the catalytic activity.In the presence of SO₂,due to the protective effect of the deposited Fe species on the surface of the catalyst,the active components in the catalyst are not affected by SO₂,and relatively high catalytic activity can be maintained.（3）The new Fe-CeVO₄ de-NO_x catalyst was obtained by doping CeVO₄ with Fe.The NO_x conversion reaches over 90% in the range of 210-390 °C,and exhibits excellent N2 selectivity,stability and SO₂ tolerance performance.It has been found that iron species are present uniformly in the catalyst,and there is a strong interaction between the active components,making the vanadium species easier to be reduced.In situ DRIFTs characterization showed that the number of Br?nsted acid sites on the surface of the catalyst increased after iron doping,and the contents of active adsorbed increased,which both promoted the SCR reaction.The incorporation of iron effectively inhibits the adsorption of SO₂ on the surface of Fe-CeVO₄ catalyst,reducing the formation of surface sulfate,and maintaining the catalyst with high activity in the presence of SO₂.