Study On Ce-based Catalyst For The Removal Of NO_x And Alkali Resistance

Selecitve catalytic reduction of NOx by NH3(NH3-SCR)is widely applied to remove NOx and the catalyst plays a key role for the technology.In the present research Ce-based catalysts have been studied for the removal of NOx,and sulfated CeO2,Fe2(SO4)3/CeO2,Ce-Ti and 3DOM WO3/Ce-Zr catalysts have been synthesized to improve its activity and alkali resistance.These catalysts exhibited high reaction activity and high alkali metal resistance.Several characterization techniques were employed to study the structure of catalyst and the reaction mechanism.Sulfated CeO2 catalysts were prepared by two kinds of methods,one was prepared by impregnation method,the other was pre-treated by SO2.The maximum NOx conversion over pure CeO2 catalyst was 50%.However,90%NOx conversion was obtained in the temperature range of 300-450? on the sulfated CeO2 catalyst,and it also showed outstanding alkali metal resistance.From the characterization results,there was a strong interaction between Ce and S,which could improve the surface acidity thus promoting the adsorption and activation of NH3 species.After K-poisoning,the acidity of catalysts were decreased slightly,which could prove the catalyst has better resistance to alkali metal poisoning.The adsorbed NH3 species was reactive,but the adsorbed nitrogen oxides are spectator species.Hence,the catalysts were followed E-R(Eley-Rideal)mechanism in the reaction.Fe2(SO4)3/CeO2 and Fe2(SO4)3/TiO2 catalysts were prepared by impregnation method,and their catalytic performance were compared.From the experimental results,Fe2(SO4)3/CeO2 catalyst exhibit better NH3-SCR activity,N2 selectivity and alkali resistance.Over 90%NOx conversion was achieved between 300-450? on the Fe2(SO4)3/CeO2 catalyst.Characterization data suggested the strong interaction between Fe,S,and Ce,which improve the acidity and redox property of catalyst.H2-TPR results showed the reduction peak shifted to lower temperature on the Fe2(SO4)3/CeO2 catalyst after adding alkali metal.However,the peak moved to higher temperature in the case of Fe2(SO4)3/TiO2.This results indicated that the alkali metal had little effect on the redox performance of the Fe2(SO4)3/CeO2 catalyst.Furthermore,the reaction mechanism of the catalyst was studied using in situ DRIFTS.It can be found that more NH3 species adsorbed and activated on the Fe2(SO4)3/CeO2 than on the Fe2(SO4)3/TiO2.Moreover,the rate of reaction between nitrogen oxide and NH3 over the Fe2(SO4)3/CeO2 was also faster than the Fe2(SO4)3/TiO2,which are beneficial for the NH3-SCR performance.A series of Ce-Ti catalysts prepared by hydrothermal method exhibited excellent NH3-SCR activity and outstanding alkali metal resistance.After adding 2wt%K2O to the Ce-Ti and V-W-Ti catalysts,the activity of V-W-Ti catalyst was decreased significantly,but it still remained relatively high activity on the Ce-Ti catalyst.With increasing the molar ratio of Ce/Ti,the structure of the catalyst becomes to be amorphous.After K-poisoning,the crystalline form of catalysts didn't change.Whereas the addition of K2O leads to the slight increase of the diffraction peaks,suggesting that the added K2O leads to an increase in catalyst crystallinity.XPS results showed that there were a strong interaction between Ce and Ti and existed a dual redox cycles(Ce3++Ti4+?Ce4++Ti3+),which promoted the redox property of catalyst.Compare the H2-TPR results of the fresh and alkali poisoned catalysts,the reduction peak position and hydrogen consumption had little changed after alkali doping,which indicated the redox properties of Ce-Ti catalyst was hardly affected by K2O poisoned.The added K2O also seldom affect the adsorption and activation of NH3 and reaction rate over Ce-Ti catalyst,which led to high resistance against alkali metal poisoning of Ce-Ti catalyst.WO3/Ce-Zr catalysts were prepared by colloid crystal template method and co-precipitation method respectively(signed as WCZ(3DOM)and WCZ(CP),respectively).On the 1%K2O/WCZ(3DOM)catalyst,over 85%NOx conversion was obtained in 250-450?,but the 1%K2O/WCZ(CP)was seriously deactivated.After K2O doping on the WCZ(3DOM),BET surface area was declined slightly and pore volume and pore diameter became larger,which would be in favour of maintaining active sites for the reaction.The results indicated that this structure could make three metallic elements distributed better.A strong interaction among W,Ce and Zr resulted in improving redox property.