WO₃/CeO₂ Catalyst And Its NH₃-SCR Denitration Mechanism Based On Morphology Control

Nitrogen oxides(NOx)are one of the major pollutants in the atmosphere.The outline of the "Thirteenth Five-Year Plan" demands that the cumulative increase speed of NOx emissions cannot exceed 15%.At present,the NOx emissions of mobile sources remain high,and the emissions of mobile diesel engines such as ships and locomotives account for the highest proportion.The exhaust gas of mobile diesel engine has the characteristics of wide temperature range(150-500℃),high content of water(H2O)and sulfur(SO2).The most widely used mobile source de-nitration technology is NH3 selective catalytic reduction NOx technology(NH3-SCR),and NH3-SCR catalyst is the core of the technology.In view of the current narrow reaction temperature range of NH3-SCR catalysts and weak resistance to moisture and sulfur,this paper proposes the topic of WO3/CeO2 catalyst and its NH3-SCR de-nitration mechanism based on morphology control.The double modification of CeO2 through WO3 doping and morphology control enhances the surface acid sites and redox sites of the catalyst,thereby achieving the purpose of broadening the reaction temperature window of the catalyst and improving the catalytic activity.WO3 doping can improve the acidic sites and redox sites on the surface of the catalyst.In a series of CeO2 Catalysts with different WO3 doping amount,the catalytic activity and reaction temperature window of the catalysts show a volcanic change with the increase of WO3 doping amount.4%WO3/CeO2 exhibited the best NOx conversion efficiency of above 90%in the temperature range of 150-425℃.However,when SO2 and H2O exist at the same time,it is easy to cause catalyst poisoning and deactivation.An array of analytical techniques shows that 4%WO3/CeO2 has more surface chemical active oxygen,Ce3+ species,high acidity and rich Br(?)nsted acid site,it also shows that the dispersion of WO3 plays an important role in its catalytic activity.Morphological control can effectively improve the surface redox sites of the catalyst and the dispersion of active components.The flower-like CeO2 with better surface properties was selected as the carrier,and the WO3/CeO2(f)CeO2 catalysts with different WO3 doping amount were synthesized.The optimal doping amount drops significantly to 0.1%,and the surface active components are highly dispersed.Its NOx conversion rate in the wide temperature range of 150-450℃ is maintained above 90%and has excellent resistance to SO2+H2O poisoning.The pore structure of the catalyst is blocked gradually until the surface flower-like structure is completely destroyed as the amount of WO3 doped increases.Surface area is greatly reduced.Double modification can effectively improve the surface performance and catalytic activity of the catalyst.The calculation results of X-ray diffraction(XRD)and multi-layer adsorption formula(BET)show that 0.1%WO3/CeO2(f)has smaller particle size and larger specific surface area compared with 0.1%WO3/CeO2,benefitting the adsorption and activation of reactants.XPS results show that 0.1%WO3/CeO2(f)has a large amount of surface chemically active oxygen,promoting fast SCR reaction.Electron spin resonance(ESR)results show that large-scale oxygen vacancy clusters are formed on the surface of 0.1%WO3/CeO2(f),which is conducive to the adsorption and transfer of surface chemically active oxygen,thereby promoting the redox cycle in SCR reactions.The temperature-programmed reduction(H2-TPR)test results show that 0.1%WO3/CeO2(f)has stronger catalytic reduction on the surface,which is conducive to the occurrence of NH3-SCR reaction.The programmed temperature desorption(NH3-TPD)test results show that the surface of 0.1%WO3/CeO2(f)catalyst contains more Br(?)nsted acid sites and Lewis acid sites,which promotes the adsorption and activation of the reactant NH3.The flower-like surface can influence the catalytic reaction mechanism.In situ NH3 adsorption reaction test results showed that the 0.1%WO3/CeO2(f)catalyst had stronger Br(?)nsted acidic sites,and the surface has a weaker oxidation ability to NH3,reserving more surface-adsorbed NH3 for the NH3-SCR reaction,thereby enhancing catalytic activity.The in situ DRIFT test results showed that 0.1%WO3/CeO2 followed the E-R mechanism,while the NH3-SCR reaction mechanism on 0.1%WO3/CeO2(f)has both the E-R mechanism and the L-H mechanism,that is,not only the adsorbed NH3 and gaseous NOx react on the catalyst surface,but also the adsorbed NH3 and adsorbed NOx react with each other.On the surface of 0.1%WO3/CeO2(f),the adsorption NH3 has stronger reactivity,and there are more high-active components in the adsorbed NOx.This indicates that the flower-like support has a strong activation effect on the adsorbed NH3 and NOx,which greatly promotes the catalytic reaction.